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HomeMy WebLinkAboutBack-Up DocumentsSummary for Policymakers Summary for Policymakers Drafting Authors: Myles Allen (UK), Mustafa Babiker (Sudan), Yang Chen (China), Heleen de Coninck (Netherlands/EU), Sarah Connors (UK), Renee van Diemen (Netherlands), Opha Pauline Dube (Botswana), Kristie L. Ebi (USA), Francois Engelbrecht (South Africa), Marion Ferrat (UK/France), James Ford (UK/Canada), Piers Forster (UK), Sabine Fuss (Germany), Tania Guillen Bolanos (Germany/Nicaragua), Jordan Harold (UK), Ove Hoegh-Guldberg (Australia), Jean -Charles Hourcade (France), Daniel Huppmann (Austria), Daniela Jacob (Germany), Kejun Jiang (China), Tom Gabriel Johansen (Norway), Mikiko Kainuma (Japan), Kiane de Kleijne (Netherlands/EU), Elmar Kriegler (Germany), Debora Ley (Guatemala/Mexico), Diana Liverman (USA), Natalie Mahowald (USA), Valerie Masson-Delmotte (France), J. B. Robin Matthews (UK), Richard Millar (UK), Katja Mintenbeck (Germany), Angela Morelli (Norway/Italy), Wilfran Moufouma-Okia (France/Congo), Luis Mundaca (Sweden/Chile), Maike Nicolai (Germany), Chukwumerije Okereke (UK/Nigeria), Minal Pathak (India), Antony Payne (UK), Roz Pidcock (UK), Anna Pirani (Italy), Elvira Poloczanska (UK/Australia), Hans - Otto Partner (Germany), Aromar Revi (India), Keywan Riahi (Austria), Debra C. Roberts (South Africa), Joeri Rogelj (Austria/Belgium), Joyashree Roy (India), Sonia I. Seneviratne (Switzerland), Priyadarshi R. Shukla (India), James Skea (UK), Raphael Slade (UK), Drew Shindell (USA), Chandni Singh (India), William Solecki (USA), Linda Steg (Netherlands), Michael Taylor (Jamaica), Petra Tschakert (Australia/Austria), Henri Waisman (France), Rachel Warren (UK), Panmao Zhai (China), Kirsten Zickfeld (Canada). This Summary for Policymakers should be cited as: IPCC, 2018: Summary for Policymakers. In: Global Warming of 1.5°C. An 1PCC Special Report on the impacts of global warming of 1.5°C above pre -industrial levels and related global greenhouse gas emission pathways, in the context of strengthening the global response to the threat of climate change, sustainable development, and efforts to eradicate poverty [Masson-Delmotte, V., P. Zhai, H.-0. Partner, D. Roberts, J. Skea, P.R. Shukla, A. Pirani, W. Moufouma-0kia, C. Man, R. Pidcock, S. Connors, J.B.R. Matthews, Y. Chen, X. Zhou, M.I. Gomis, E. Lonnoy, T. Maycock, M. Tignor, and T. Waterfield (eds.)]. In Press. 3 Summary for Policymakers Introduction This Report responds to the invitation for IPCC '... to provide a Special Report in 2018 on the impacts of global warming of 1.5°C above pre -industrial levels and related global greenhouse gas emission pathways' contained in the Decision of the 21 st Conference of Parties of the United Nations Framework Convention on Climate Change to adopt the Paris Agreement.' The IPCC accepted the invitation in April 2016, deciding to prepare this Special Report on the impacts of global warming of 1.5°C above pre -industrial levels and related global greenhouse gas emission pathways, in the context of strengthening the global response to the threat of climate change, sustainable development, and efforts to eradicate poverty. This Summary for Policymakers (SPM) presents the key findings of the Special Report, based on the assessment of the available scientific, technical and socio-economic literature2 relevant to global warming of 1.5°C and for the comparison between global warming of 1.5°C and 2°C above pre -industrial levels. The level of confidence associated with each key finding is reported using the IPCC calibrated language.' The underlying scientific basis of each key finding is indicated by references provided to chapter elements. In the SPM, knowledge gaps are identified associated with the underlying chapters of the Report. A. Understanding Global Warming of 1.5°C° A.1 Human activities are estimated to have caused approximately 1.0°C of global warming' above pre -industrial levels, with a likely range of 0.8°C to 1.2°C. Global warming is likely to reach 1.5°C between 2030 and 2052 if it continues to increase at the current rate. (high confidence) (Figure SPM.1) (1.2) A.1.1 Reflecting the long-term warming trend since pre -industrial times, observed global mean surface temperature (GMST) for the decade 2006-2015 was 0.87°C (likely between 0.75°C and 0.99°C)6 higher than the average over the 1850-1900 period (very high confidence). Estimated anthropogenic global warming matches the level of observed warming to within ±20% (likely range). Estimated anthropogenic global warming is currently increasing at 0.2°C (likely between 0.1°C and 0.3°C) per decade due to past and ongoing emissions (high confidence). {1.2.1, Table 1.1, 1.2.4} A.1.2 Warming greater than the global annual average is being experienced in many land regions and seasons, including two to three times higher in the Arctic. Warming is generally higher over land than over the ocean. (high confidence) {1.2.1, 1.2.2, Figure 1.1, Figure 1.3, 3.3.1, 3.3.2} A.1.3 Trends in intensity and frequency of some climate and weather extremes have been detected over time spans during which about 0.5°C of global warming occurred (medium confidence). This assessment is based on several lines of evidence, including attribution studies for changes in extremes since 1950. {3.3.1, 3.3.2, 3.3.3} 1 Decision 1/CP.21, paragraph 21. 2 The assessment covers literature accepted for publication by 15 May 2018. 3 Each finding is grounded in an evaluation of underlying evidence and agreement. A level of confidence is expressed using five qualifiers: very low, low, medium, high and very high, and typeset in italics, for example, medium confidence. The following terms have been used to indicate the assessed likelihood of an outcome or a result: virtually certain 99-100% probability, very likely 90-100%, likely 66-100%, about as likely as not 33-66%, unlikely 0-33%, very unlikely 0-10%, exceptionally unlikely 0-1 %. Additional terms (extremely likely 95-100%, more likely than not >50-100%, more unlikely than likely 0—<50%, extremely unlikely 0-5%) may also be used when appropriate. Assessed likelihood is typeset in italics, for example, very likely. This is consistent with AR5. 4 See also Box SPM.1: Core Concepts Central to this Special Report. 5 Present level of global warming is defined as the average of a 30-year period centred on 2017 assuming the recent rate of warming continues. 6 This range spans the four available peer -reviewed estimates of the observed GMST change and also accounts for additional uncertainty due to possible short-term natural variability. (1.2.1, Table 1.1) 4 Summary for Policymakers A.2 Warming from anthropogenic emissions from the pre -industrial period to the present will persist for centuries to millennia and will continue to cause further Tong -term changes in the climate system, such as sea level rise, with associated impacts (high confidence), but these emissions alone are unlikely to cause global warming of 1.5°C (medium confidence). (Figure SPM.1) (1.2, 3.3, Figure 1.5) A.2.1 Anthropogenic emissions (including greenhouse gases, aerosols and their precursors) up to the present are unlikely to cause further warming of more than 0.5°C over the next two to three decades (high confidence) or on a century time scale (medium confidence). {1.2.4, Figure 1.5} A.2.2 Reaching and sustaining net zero global anthropogenic CO, emissions and declining net non-0O2 radiative forcing would halt anthropogenic global warming on multi-decadal time scales (high confidence). The maximum temperature reached is then determined by cumulative net global anthropogenic CO, emissions up to the time of net zero CO, emissions (high confidence) and the level of non-0O2 radiative forcing in the decades prior to the time that maximum temperatures are reached (medium confidence). On longer time scales, sustained net negative global anthropogenic CO, emissions and/ or further reductions in non-0O2 radiative forcing may still be required to prevent further warming due to Earth system feedbacks and to reverse ocean acidification (medium confidence) and will be required to minimize sea level rise (high confidence). (Cross -Chapter Box 2 in Chapter 1, 1.2.3, 1.2.4, Figure 1.4, 2.2.1, 2.2.2, 3.4.4.8, 3.4.5.1, 3.6.3.2} A.3 Climate -related risks for natural and human systems are higher for global warming of 1.5°C than at present, but lower than at 2°C (high confidence). These risks depend on the magnitude and rate of warming, geographic location, levels of development and vulnerability, and on the choices and implementation of adaptation and mitigation options (high confidence). (Figure SPM.2) {1.3, 3.3, 3.4, 5.6) A.3.1 Impacts on natural and human systems from global warming have already been observed (high confidence). Many land and ocean ecosystems and some of the services they provide have already changed due to global warming (high confidence). (Figure SPM.2) (1.4, 3.4, 3.5} A.3.2 Future climate -related risks depend on the rate, peak and duration of warming. In the aggregate, they are larger if global warming exceeds 1.5°C before returning to that level by 2100 than if global warming gradually stabilizes at 1.5°C, especially if the peak temperature is high (e.g., about 2°C) (high confidence). Some impacts may be long-lasting or irreversible, such as the loss of some ecosystems (high confidence). {3.2, 3.4.4, 3.6.3, Cross -Chapter Box 8 in Chapter 3} A.3.3 Adaptation and mitigation are already occurring (high confidence). Future climate -related risks would be reduced by the upscaling and acceleration of far-reaching, multilevel and cross-sectoral climate mitigation and by both incremental and transformational adaptation (high confidence). {1.2, 1.3, Table 3.5, 4.2.2, Cross -Chapter Box 9 in Chapter 4, Box 4.2, Box 4.3, Box 4.6, 4.3.1, 4.3.2, 4.3.3, 4.3.4, 4.3.5, 4.4.1, 4.4.4, 4.4.5, 4.5.3} 5 Summary for Policymakers Cumulative emissions of CO2 and future non-0O2 radiative forcing determine the probability of limiting warming to 1.5°C a) Observed global temperature change and modeled responses to stylized anthropogenic emission and forcing pathways Global warming relative to 1850-1900 (°C) 2.r 1.; 10 • Observed monthly global mean surface temperature Estimated anthr000genic warming to date and likely range. 2000 b) Stylized net global CO: emission pathways Billion tonnes CO2 per year (GtCO2/yr) • 20 10 r CO: emissions decline from 2020 to reach net zero in 2055 or 2040 2020 2060 21?0 Faster immediate CO2 emission reductions limit cumulative CO: emissions shown in panel (c). 2017 2020 Likely range of modeled responses to stylized pathways L Global CO: emissions reach net zero in 2055 while net non -CO:: radiative forcing is reduced after 2030 (grey in b. c & d) Faster CO2 reductions (blue in b & c) result in a higher probability of limiting warming to 1.5`C No reduction of net non-0O2 radiative forcing (purple in d) results in a lower probability of limiting warming to 1.5°C 2040 c) Cumulative net CO2 emissions Billion tonnes CO: (GtCO2) 20;0 _ 1 000 / Cumulative CO2 J emissions in pathways reaching net zero in 2055 and 2040 1000 2100 d) Non-0O2 radiative forcing pathways Watts per square metre (W/m2) Non-0O2 radiative forcing reduced after 2030 or not reduced after 2030 1900 202 20b0 ,30 20,0 20,-0 210. Maximum temperature rise is determined by cumulative net CO: emissions and net non-0O2 radiative forcing due to methane, nitrous oxide, aerosols and other anthropogenic forcing agents. Figure SPM.1 I Panel a: Observed monthly global mean surface temperature (GMST, grey line up to 2017, from the HadCRUT4, GISTEMP, Cowtan—Way, and NOAA datasets) change and estimated anthropogenic global warming (solid orange line up to 2017, with orange shading indicating assessed likely range). Orange dashed arrow and horizontal orange error bar show respectively the central estimate and likely range of the time at which 1.5°C is reached if the current rate of warming continues. The grey plume on the right of panel a shows the likely range of warming responses, computed with a simple climate model to a stylized pathway (hypothetical future) in which net CO2 emissions (grey line in panels b and c) decline in a straight line from 2020 to reach net zero in 2055 and net non- COz radiative forcing (grey line in panel d) increases to 2030 and then declines. The blue plume in panel a) shows the response to faster CO2 emissions reductions (blue line in panel b), reaching net zero in 2040, reducing cumulative CO2 emissions (panel c). The purple plume shows the response to net CO2 emissions declining to zero in 2055, with net non-0O2 forcing remaining constant after 2030. The vertical error bars on right of panel a) show the likely ranges (thin lines) and central terciles (33rd — 66th percentiles, thick lines) of the estimated distribution of warming in 2100 under these three stylized pathways. Vertical dotted error bars in panels b, c and d show the likely range of historical annual and cumulative global net CO2 emissions in 2017 (data from the Global Carbon Project) and of net non-0O2 radiative forcing in 2011 from AR5, respectively. Vertical axes in panels c and d are scaled to represent approximately equal effects on GMST. {1.2.1, 1.2.3, 1.2.4, 2.3, Figure 1.2 and Chapter 1 Supplementary Material, Cross -Chapter Box 2 in Chapter 1) 6 Summary for Policymakers B. Projected Climate Change, Potential Impacts and Associated Risks B.1 B.1.1 Climate models project robust' differences in regional climate characteristics between present-day and global warming of 1.5°C,8 and between 1.5°C and 2°C.8 These differences include increases in: mean temperature in most land and ocean regions (high confidence), hot extremes in most inhabited regions (high confidence), heavy precipitation in several regions (medium confidence), and the probability of drought and precipitation deficits in some regions (medium confidence). {3.3} Evidence from attributed changes in some climate and weather extremes for a global warming of about 0.5°C supports the assessment that an additional 0.5°C of warming compared to present is associated with further detectable changes in these extremes (medium confidence). Several regional changes in climate are assessed to occur with global warming up to 1.5°C compared to pre -industrial levels, including warming of extreme temperatures in many regions (high confidence), increases in frequency, intensity, and/or amount of heavy precipitation in several regions (high confidence), and an increase in intensity or frequency of droughts in some regions (medium confidence). {3.2, 3.3.1, 3.3.2, 3.3.3, 3.3.4, Table 3.2) B.1.2 Temperature extremes on land are projected to warm more than GMST (high confidence): extreme hot days in mid -latitudes warm by up to about 3°C at global warming of 1.5°C and about 4°C at 2°C, and extreme cold nights in high latitudes warm by up to about 4.5°C at 1.5°C and about 6°C at 2°C (high confidence). The number of hot days is projected to increase in most land regions, with highest increases in the tropics (high confidence). {3.3.1, 3.3.2, Cross -Chapter Box 8 in Chapter 3) B.1.3 Risks from droughts and precipitation deficits are projected to be higher at 2°C compared to 1.5°C of global warming in some regions (medium confidence). Risks from heavy precipitation events are projected to be higher at 2°C compared to 1.5°C of global warming in several northern hemisphere high -latitude and/or high -elevation regions, eastern Asia and eastern North America (medium confidence). Heavy precipitation associated with tropical cyclones is projected to be higher at 2°C compared to 1.5°C global warming (medium confidence). There is generally low confidence in projected changes in heavy precipitation at 2°C compared to 1.5°C in other regions. Heavy precipitation when aggregated at global scale is projected to be higher at 2°C than at 1.5°C of global warming (medium confidence). As a consequence of heavy precipitation, the fraction of the global land area affected by flood hazards is projected to be larger at 2°C compared to 1.5°C of global warming (medium confidence). {3.3.1, 3.3.3, 3.3.4, 3.3.5, 3.3.6) B.2 By 2100, global mean sea level rise is projected to be around 0.1 metre Tower with global warming of 1.5°C compared to 2°C (medium confidence). Sea level will continue to rise well beyond 2100 (high confidence), and the magnitude and rate of this rise depend on future emission pathways. A slower rate of sea level rise enables greater opportunities for adaptation in the human and ecological systems of small islands, low-lying coastal areas and deltas (medium confidence). {3.3, 3.4, 3.6} B.2.1 Model -based projections of global mean sea level rise (relative to 1986-2005) suggest an indicative range of 0.26 to 0.77 m by 2100 for 1.5°C of global warming, 0.1 m (0.04-0.16 m) less than for a global warming of 2°C (medium confidence). A reduction of 0.1 m in global sea level rise implies that up to 10 million fewer people would be exposed to related risks, based on population in the year 2010 and assuming no adaptation (medium confidence). {3.4.4, 3.4.5, 4.3.2) B.2.2 Sea level rise will continue beyond 2100 even if global warming is limited to 1.5°C in the 21 st century (high confidence). Marine ice sheet instability in Antarctica and/or irreversible loss of the Greenland ice sheet could result in multi -metre rise in sea level over hundreds to thousands of years. These instabilities could be triggered at around 1.5°C to 2°C of global warming (medium confidence). (Figure SPM.2) {3.3.9, 3.4.5, 3.5.2, 3.6.3, Box 3.3) 7 Robust is here used to mean that at least two thirds of climate models show the same sign of changes at the grid point scale, and that differences in large regions are statistically significant. 8 Projected changes in impacts between different levels of global warming are determined with respect to changes in global mean surface air temperature. 7 Summary for Policymakers B.2.3 Increasing warming amplifies the exposure of small islands, low-lying coastal areas and deltas to the risks associated with sea level rise for many human and ecological systems, including increased saltwater intrusion, flooding and damage to infrastructure (high confidence). Risks associated with sea level rise are higher at 2°C compared to 1.5°C. The slower rate of sea level rise at global warming of 1.5°C reduces these risks, enabling greater opportunities for adaptation including managing and restoring natural coastal ecosystems and infrastructure reinforcement (medium confidence). (Figure SPM.2) {3.4.5, Box 3.5} B.3 On land, impacts on biodiversity and ecosystems, including species Toss and extinction, are projected to be lower at 1.5°C of global warming compared to 2°C. Limiting global warming to 1.5°C compared to 2°C is projected to lower the impacts on terrestrial, freshwater and coastal ecosystems and to retain more of their services to humans (high confidence). (Figure SPM.2) {3.4, 3.5, Box 3.4, Box 4.2, Cross -Chapter Box 8 in Chapter 3} B.3.1 Of 105,000 species studied,9 6% of insects, 8% of plants and 4% of vertebrates are projected to lose over half of their climatically determined geographic range for global warming of 1.5°C, compared with 18% of insects, 16% of plants and 8% of vertebrates for global warming of 2°C (medium confidence). Impacts associated with other biodiversity-related risks such as forest fires and the spread of invasive species are lower at 1.5°C compared to 2°C of global warming (high confidence). {3.4.3, 3.5.2) B.3.2 Approximately 4% (interquartile range 2-7%) of the global terrestrial land area is projected to undergo a transformation of ecosystems from one type to another at 1 °C of global warming, compared with 13% (interquartile range 8-20%) at 2°C (medium confidence). This indicates that the area at risk is projected to be approximately 50% lower at 1.5°C compared to 2°C (medium confidence). {3.4.3.1, 3.4.3.5} B.3.3 High -latitude tundra and boreal forests are particularly at risk of climate change -induced degradation and loss, with woody shrubs already encroaching into the tundra (high confidence) and this will proceed with further warming. Limiting global warming to 1.5°C rather than 2°C is projected to prevent the thawing over centuries of a permafrost area in the range of 1.5 to 2.5 million km' (medium confidence). {3.3.2, 3.4.3, 3.5.5} B.4 Limiting global warming to 1.5°C compared to 2°C is projected to reduce increases in ocean temperature as well as associated increases in ocean acidity and decreases in ocean oxygen levels (high confidence). Consequently, limiting global warming to 1.5°C is projected to reduce risks to marine biodiversity, fisheries, and ecosystems, and their functions and services to humans, as illustrated by recent changes to Arctic sea ice and warm -water coral reef ecosystems (high confidence). {3.3, 3.4, 3.5, Box 3.4, Box 3.5) B.4.1 There is high confidence that the probability of a sea ice -free Arctic Ocean during summer is substantially lower at global warming of 1.5°C when compared to 2°C. With 1.5°C of global warming, one sea ice -free Arctic summer is projected per century. This likelihood is increased to at least one per decade with 2°C global warming. Effects of a temperature overshoot are reversible for Arctic sea ice cover on decadal time scales (high confidence). {3.3.8, 3.4.4.7) B.4.2 Global warming of 1.5°C is projected to shift the ranges of many marine species to higher latitudes as well as increase the amount of damage to many ecosystems. It is also expected to drive the loss of coastal resources and reduce the productivity of fisheries and aquaculture (especially at low latitudes). The risks of climate -induced impacts are projected to be higher at 2°C than those at global warming of 1.5°C (high confidence). Coral reefs, for example, are projected to decline by a further 70-90% at 1.5°C (high confidence) with larger losses (>99%) at 2°C (very high confidence). The risk of irreversible loss of many marine and coastal ecosystems increases with global warming, especially at 2°C or more (high confidence). {3.4.4, Box 3.4} 9 Consistent with earlier studies, illustrative numbers were adopted from one recent meta -study. 8 Summary for Policyntakers B.4.3 The level of ocean acidification due to increasing CO, concentrations associated with global warming of 1.5°C is projected to amplify the adverse effects of warming, and even further at 2°C, impacting the growth, development, calcification, survival, and thus abundance of a broad range of species, for example, from algae to fish (high confidence). {3.3.10, 3.4.4} B.4.4 Impacts of climate change in the ocean are increasing risks to fisheries and aquaculture via impacts on the physiology, survivorship, habitat, reproduction, disease incidence, and risk of invasive species (medium confidence) but are projected to be less at 1.5°C of global warming than at 2°C. One global fishery model, for example, projected a decrease in global annual catch for marine fisheries of about 1.5 million tonnes for 1.5°C of global warming compared to a loss of more than 3 million tonnes for 2°C of global warming (medium confidence). {3.4.4, Box 3.4} B.5 Climate -related risks to health, livelihoods, food security, water supply, human security, and economic growth are projected to increase with global warming of 1.5°C and increase further with 2°C. (Figure SPM.2) {3.4, 3.5, 5.2, Box 3.2, Box 3.3, Box 3.5, Box 3.6, Cross -Chapter Box 6 in Chapter 3, Cross -Chapter Box 9 in Chapter 4, Cross -Chapter Box 12 in Chapter 5, 5.2} B.5.1 Populations at disproportionately higher risk of adverse consequences with global warming of 1.5°C and beyond include disadvantaged and vulnerable populations, some indigenous peoples, and local communities dependent on agricultural or coastal livelihoods (high confidence). Regions at disproportionately higher risk include Arctic ecosystems, dryland regions, small island developing states, and Least Developed Countries (high confidence). Poverty and disadvantage are expected to increase in some populations as global warming increases; limiting global warming to 1.5°C, compared with 2°C, could reduce the number of people both exposed to climate -related risks and susceptible to poverty by up to several hundred million by 2050 (medium confidence). {3.4.10, 3.4.11, Box 3.5, Cross -Chapter Box 6 in Chapter 3, Cross -Chapter Box 9 in Chapter 4, Cross -Chapter Box 12 in Chapter 5, 4.2.2.2, 5.2.1, 5.2.2, 5.2.3, 5.6.31 B.5.2 Any increase in global warming is projected to affect human health, with primarily negative consequences (high confidence). Lower risks are projected at 1.5°C than at 2°C for heat -related morbidity and mortality (very high confidence) and for ozone -related mortality if emissions needed for ozone formation remain high (high confidence). Urban heat islands often amplify the impacts of heatwaves in cities (high confidence). Risks from some vector -borne diseases, such as malaria and dengue fever, are projected to increase with warming from 1.5°C to 2°C, including potential shifts in their geographic range (high confidence). {3.4.7, 3.4.8, 3.5.5.8) B.5.3 Limiting warming to 1.5°C compared with 2°C is projected to result in smaller net reductions in yields of maize, rice, wheat, and potentially other cereal crops, particularly in sub-Saharan Africa, Southeast Asia, and Central and South America, and in the CO2-dependent nutritional quality of rice and wheat (high confidence). Reductions in projected food availability are larger at 2°C than at 1.5°C of global warming in the Sahel, southern Africa, the Mediterranean, central Europe, and the Amazon (medium confidence). Livestock are projected to be adversely affected with rising temperatures, depending on the extent of changes in feed quality, spread of diseases, and water resource availability (high confidence). {3.4.6, 3.5.4, 3.5.5, Box 3.1, Cross -Chapter Box 6 in Chapter 3, Cross -Chapter Box 9 in Chapter 4} B.5.4 Depending on future socio-economic conditions, limiting global warming to 1.5°C compared to 2°C may reduce the proportion of the world population exposed to a climate change -induced increase in water stress by up to 50%, although there is considerable variability between regions (medium confidence). Many small island developing states could experience lower water stress as a result of projected changes in aridity when global warming is limited to 1.5°C, as compared to 2°C (medium confidence). {3.3.5, 3.4.2, 3.4.8, 3.5.5, Box 3.2, Box 3.5, Cross -Chapter Box 9 in Chapter 4) B.5.5 Risks to global aggregated economic growth due to climate change impacts are projected to be lower at 1.5°C than at 2°C by the end of this century10 (medium confidence). This excludes the costs of mitigation, adaptation investments and the benefits of adaptation. Countries in the tropics and Southern Hemisphere subtropics are projected to experience the largest impacts on economic growth due to climate change should global warming increase from 1.5°C to 2°C (medium confidence). {3.5.2, 3.5.3} 10 Here, impacts on economic growth refer to changes in gross domestic product (GDP). Many impacts, such as loss of human lives, cultural heritage and ecosystem services, are difficult to value and monetize. 9 Summary for Policymakers B.5.6 Exposure to multiple and compound climate -related risks increases between 1.5°C and 2°C of global warming, with greater proportions of people both so exposed and susceptible to poverty in Africa and Asia (high confidence). For global warming from 1.5°C to 2°C, risks across energy, food, and water sectors could overlap spatially and temporally, creating new and exacerbating current hazards, exposures, and vulnerabilities that could affect increasing numbers of people and regions (medium confidence). {Box 3.5, 3.3.1, 3.4.5.3, 3.4.5.6, 3.4.11, 3.5.4.9} B.5.7 There are multiple lines of evidence that since AR5 the assessed levels of risk increased for four of the five Reasons for Concern (RFCs) for global warming to 2°C (high confidence). The risk transitions by degrees of global warming are now: from high to very high risk between 1.5°C and 2°C for RFC1 (Unique and threatened systems) (high confidence); from moderate to high risk between 1 °C and 1.5°C for RFC2 (Extreme weather events) (medium confidence); from moderate to high risk between 1.5°C and 2°C for RFC3 (Distribution of impacts) (high confidence); from moderate to high risk between 1.5°C and 2.5°C for RFC4 (Global aggregate impacts) (medium confidence); and from moderate to high risk between 1°C and 2.5°C for RFCS (Large-scale singular events) (medium confidence). (Figure SPM.2) {3.4.13; 3.5, 3.5.2) B.6 Most adaptation needs will be lower for global warming of 1.5°C compared to 2°C (high confidence). There are a wide range of adaptation options that can reduce the risks of climate change (high confidence). There are limits to adaptation and adaptive capacity for some human and natural systems at global warming of 1.5°C, with associated losses (medium confidence). The number and availability of adaptation options vary by sector (medium confidence). {Table 3.5, 4.3, 4.5, Cross - Chapter Box 9 in Chapter 4, Cross -Chapter Box 12 in Chapter 5} B.6.1 A wide range of adaptation options are available to reduce the risks to natural and managed ecosystems (e.g., ecosystem - based adaptation, ecosystem restoration and avoided degradation and deforestation, biodiversity management, sustainable aquaculture, and local knowledge and indigenous knowledge), the risks of sea level rise (e.g., coastal defence and hardening), and the risks to health, livelihoods, food, water, and economic growth, especially in rural landscapes (e.g., efficient irrigation, social safety nets, disaster risk management, risk spreading and sharing, and community - based adaptation) and urban areas (e.g., green infrastructure, sustainable land use and planning, and sustainable water management) (medium confidence). {4.3.1, 4.3.2, 4.3.3, 4.3.5, 4.5.3, 4.5.4, 5.3.2, Box 4.2, Box 4.3, Box 4.6, Cross -Chapter Box 9 in Chapter 4}. B.6.2 Adaptation is expected to be more challenging for ecosystems, food and health systems at 2°C of global warming than for 1.5°C (medium confidence). Some vulnerable regions, including small islands and Least Developed Countries, are projected to experience high multiple interrelated climate risks even at global warming of 1.5°C (high confidence). {3.3.1, 3.4.5, Box 3.5, Table 3.5, Cross -Chapter Box 9 in Chapter 4, 5.6, Cross -Chapter Box 12 in Chapter 5, Box 5.3} B.6.3 Limits to adaptive capacity exist at 1.5°C of global warming, become more pronounced at higher levels of warming and vary by sector, with site -specific implications for vulnerable regions, ecosystems and human health (medium confidence). {Cross -Chapter Box 12 in Chapter 5, Box 3.5, Table 3.5} 10 Summary for Policymakers How the level of global warming affects impacts and/or risks associated with the Reasons for Concern (RFCs) and selected natural, managed and human systems Five Reasons For Concern (RFCs) illustrate the impacts and risks of different levels of global warming for people, economies and ecosystems across sectors and regions. Impacts and risks associated with the Reasons for Concern (RFCs) RFC1 Unique and threatened systems r.i RFC2 RFC3 RFC4 RFC5 Extreme Distribution Global Large scale weather of impacts aggregate singular events impacts events 20:: Very high High __..._...._ Moderate Undetectable Level of additional impact/risk due to climate change Impacts and risks for selected natural, managed and human systems E n 15 IVH H . Warm -water Mangroves Small-scale corals low -latitude fisheries Arctic region Terrestrial Coastal F uvia ecosystems flooding flooding yields Confidence level for transition: L=Low, M=Medium, H=High and VH=Very high Crop Tourism Purple indicates very high risks of severe impacts/risks and the presence of significant irreversibility or the persistence of climate -related hazards, combined with limited ability to adapt due to the nature of the hazard or impacts/risks. Red indicates severe and widespread impacts/risks. Yellow indicates that impacts/risks are detectable and attributable to climate change with at least medium confidence. White indicates that no impacts are detectable and attributable to climate change. Heat -related morbidity and mortality Figure SPM.2 Five integrative reasons for concern (RFCs) provide a framework for summarizing key impacts and risks across sectors and regions, and were introduced in the IPCC Third Assessment Report. RFCs illustrate the implications of global warming for people, economies and ecosystems. Impacts and/or risks for each RFC are based on assessment of the new literature that has appeared. As in ARS, this literature was used to make expert judgments to assess the levels of global warming at which levels of impact and/or risk are undetectable, moderate, high or very high. The selection of impacts and risks to natural, managed and human systems in the lower panel is illustrative and is not intended to be fully comprehensive. {3.4, 3.5, 3.5.2.1, 3.5.2.2, 3.5.2.3, 3.5.2.4, 3.5.2.5, 5.4.1, 5.5.3, 5.6.1, Box 3.4} RFC1 Unique and threatened systems: ecological and human systems that have restricted geographic ranges constrained by climate -related conditions and have high endemism or other distinctive properties. Examples include coral reefs, the Arctic and its indigenous people, mountain glaciers and biodiversity hotspots. RFC2 Extreme weather events: risks/impacts to human health, livelihoods, assets and ecosystems from extreme weather events such as heat waves, heavy rain, drought and associated wildfires, and coastal flooding. RFC3 Distribution of impacts: risks/impacts that disproportionately affect particular groups due to uneven distribution of physical climate change hazards, exposure or vulnerability. RFC4 Global aggregate impacts: global monetary damage, global -scale degradation and loss of ecosystems and biodiversity. RFC5 Large-scale singular events: are relatively large, abrupt and sometimes irreversible changes in systems that are caused by global warming. Examples include disintegration of the Greenland and Antarctic ice sheets. 11 Summary for Policymakers C. Emission Pathways and System Transitions Consistent with 1.5°C Global Warming C.1 C.1.1 In model pathways with no or limited overshoot of 1.5°C, global net anthropogenic CO2 emissions decline by about 45% from 2010 levels by 2030 (40-60% interquartile range), reaching net zero around 2050 (2045-2055 interquartile range). For limiting global warming to below 2°C" CO, emissions are projected to decline by about 25% by 2030 in most pathways (10-30% interquartile range) and reach net zero around 2070 (2065-2080 interquartile range). Non-0O2 emissions in pathways that limit global warming to 1.5°C show deep reductions that are similar to those in pathways limiting warming to 2°C. (high confidence) (Figure SPM.3a) (2.1, 2.3, Table 2.4} CO, emissions reductions that limit global warming to 1.5°C with no or limited overshoot can involve different portfolios of mitigation measures, striking different balances between lowering energy and resource intensity, rate of decarbonization, and the reliance on carbon dioxide removal. Different portfolios face different implementation challenges and potential synergies and trade-offs with sustainable development. (high confidence) (Figure SPM.3b) {2.3.2, 2.3.4, 2.4, 2.5.3} C.1.2 Modelled pathways that limit global warming to 1.5°C with no or limited overshoot involve deep reductions in emissions of methane and black carbon (35% or more of both by 2050 relative to 2010). These pathways also reduce most of the cooling aerosols, which partially offsets mitigation effects for two to three decades. Non-0O2 emissions12 can be reduced as a result of broad mitigation measures in the energy sector. In addition, targeted non-0O2 mitigation measures can reduce nitrous oxide and methane from agriculture, methane from the waste sector, some sources of black carbon, and hydrofluorocarbons. High bioenergy demand can increase emissions of nitrous oxide in some 1.5°C pathways, highlighting the importance of appropriate management approaches. Improved air quality resulting from projected reductions in many non-CO2 emissions provide direct and immediate population health benefits in all 1.5°C model pathways. (high confidence) (Figure SPM.3a) {2.2.1, 2.3.3, 2.4.4, 2.5.3, 4.3.6, 5.4.2} C.1.3 Limiting global warming requires limiting the total cumulative global anthropogenic emissions of CO, since the pre- industrial period, that is, staying within a total carbon budget (high confidence).13 By the end of 2017, anthropogenic CO, emissions since the pre -industrial period are estimated to have reduced the total carbon budget for 1.5°C by approximately 2200 ± 320 GtCO2 (medium confidence). The associated remaining budget is being depleted by current emissions of 42 ± 3 GtCO2 per year (high confidence). The choice of the measure of global temperature affects the estimated remaining carbon budget. Using global mean surface air temperature, as in AR5, gives an estimate of the remaining carbon budget of 580 GtCO2 for a 50% probability of limiting warming to 1.5°C, and 420 GtCO2 for a 66% probability (medium confidence).14 Alternatively, using GMST gives estimates of 770 and 570 GtCO2, for 50% and 66% probabilities,15 respectively (medium confidence). Uncertainties in the size of these estimated remaining carbon budgets are substantial and depend on several factors. Uncertainties in the climate response to CO, and non-0O2 emissions contribute ±400 GtCO2 and the level of historic warming contributes ±250 GtCO2 (medium confidence). Potential additional carbon release from future permafrost thawing and methane release from wetlands would reduce budgets by up to 100 GtCO2 over the course of this century and more thereafter (medium confidence). In addition, the level of non-0O2 mitigation in the future could alter the remaining carbon budget by 250 GtCO2 in either direction (medium confidence). (1.2.4, 2.2.2, 2.6.1, Table 2.2, Chapter 2 Supplementary Material} C.1.4 Solar radiation modification (SRM) measures are not included in any of the available assessed pathways. Although some SRM measures may be theoretically effective in reducing an overshoot, they face large uncertainties and knowledge gaps 11 References to pathways limiting global warming to 2°C are based on a 66% probability of staying below 2`C. 12 Non -CO, emissions included in this Report are all anthropogenic emissions other than CO, that result in radiative forcing. These include short-lived climate forcers, such as methane, some fluorinated gases, ozone precursors, aerosols or aerosol precursors, such as black carbon and sulphur dioxide, respectively, as well as long-lived greenhouse gases, such as nitrous oxide or some fluorinated gases. The radiative forcing associated with non -CO, emissions and changes in surface albedo is referred to as non -CO, radiative forcing. (2.2.1} 13 There is a clear scientific basis for a total carbon budget consistent with limiting global warming to 1.5°C. However, neither this total carbon budget nor the fraction of this budget taken up by past emissions were assessed in this Report. 14 Irrespective of the measure of global temperature used, updated understanding and further advances in methods have led to an increase in the estimated remaining carbon budget of about 300 GtCO2 compared to AR5. (medium confidence) (2.2.2} 15 These estimates use observed GMST to 2006-2015 and estimate future temperature changes using near surface air temperatures. 12 Summary for Policymakers as well as substantial risks and institutional and social constraints to deployment related to governance, ethics, and impacts on sustainable development. They also do not mitigate ocean acidification. (medium confidence) (4.3.8, Cross -Chapter Box 10 in Chapter 4} Global emissions pathway characteristics General characteristics of the evolution of anthropogenic net emissions of CO2, and total emissions of methane, black carbon, and nitrous oxide in model pathways that limit global warming to 1.5°C with no or limited overshoot. Net emissions are defined as anthropogenic emissions reduced by anthropogenic removals. Reductions in net emissions can be achieved through different portfolios of mitigation measures illustrated in Figure SPM.3b. Global total net CO2 emissions Billion tonnes of CO2/yr 50 20 ... 10-: 1'0 - -20 In pathways limiting global warming to 1.5°C with no or limited overshoot as well as in pathways with a higher overshoot, CO2 emissions are reduced to net zero globally around 2050. Four illustrative model pathways - - P1 P2 P3 P4 2010 2020 2030 2040 2050 2060 2070 2000 2000 2100 Timing of net zero CO2 Line widths depict the 5-95th percentile and the 25-75th percentile of scenarios Non-0O2 emissions relative to 2010 Emissions of non-0O2 forcers are also reduced or limited in pathways limiting global warming to 1.5°C with no or limited overshoot, but they do not reach zero globally. Methane emissions Black carbon emissions 2;120. Nitrous oxide emissions 202c, 2100 2020 20.0 2050 2080 Pathways limiting global warming to 1.5°C with no or limited overshoot Pathways with higher overshoot _---- --- -- Pathways limiting global warming below2°C (Not shown above) 2100 Figure SPM.3a I Global emissions pathway characteristics. The main panel shows global net anthropogenic CO2 emissions in pathways limiting global warming to 1.5°C with no or limited (less than 0.1 °C) overshoot and pathways with higher overshoot. The shaded area shows the full range for pathways analysed in this Report. The panels on the right show non-0O2 emissions ranges for three compounds with large historical forcing and a substantial portion of emissions coming from sources distinct from those central to CO2 mitigation. Shaded areas in these panels show the 5-95% (light shading) and interquartile (dark shading) ranges of pathways limiting global warming to 1.5°C with no or limited overshoot. Box and whiskers at the bottom of the figure show the timing of pathways reaching global net zero CO2 emission levels, and a comparison with pathways limiting global warming to 2°C with at least 66% probability. Four illustrative model pathways are highlighted in the main panel and are labelled P1, P2, P3 and P4, corresponding to the LED, S1, S2, and S5 pathways assessed in Chapter 2. Descriptions and characteristics of these pathways are available in Figure SPM.3b. (2.1, 2.2, 2.3, Figure 2.5, Figure 2.10, Figure 2.11} 13 Summary for Policymakers Characteristics of four illustrative model pathways Different mitigation strategies can achieve the net emissions reductions that would be required to follow a pathway that limits global warming to 1.5°C with no or limited overshoot. All pathways use Carbon Dioxide Removal (CDR), but the amount varies across pathways, as do the relative contributions of Bioenergy with Carbon Capture and Storage (BECCS) and removals in the Agriculture, Forestry and Other Land Use (AFOLU) sector. This has implications for emissions and several other pathway characteristics. Breakdown of contributions to global net CO2 emissions in four illustrative model pathways Fossil fuel and industry • AFOLU BECCS Billion tonnes CO, per year (GtCO2/yr) Billion tonnes CO, per year (GtCO2/yr) 2020 2050 21:0 P1: A scenario in which social, business and technological innovations result in lower energy demand up to 2050 while living standards rise, especially in the global South. A downsized energy system enables rapid decarbonization of energy supply. Afforestation is the only CDR option considered; neither fossil fuels with CCS nor BECCS are used. Global indicators Pathway classification CO: emission change in 2030 (% rel to 2010) i- in 2050 (96 rel to 2010) Kyoto-GHG emissions* in 2030 (% rel to 2010) in 2050 (% ref to 2010) Final energy demand" in 2030 (% rel to 2010) t- in 2050 (96 rel to 2010) Renewable share in electricity in 2030 (%) • in 2050 (%) Primary energy from coal in 2030 (% rel to 2010) in 2050 (% ref to 2010) from oil in 2030 (% rel to 2010) in 2050 (%rel to 2010) from gas in 2030 (% rel to 2010) in 2050 (95rel to 2010) from nuclear in 2030 (% rel to 2010) in 2050 (% rel to 2010) from biomass in 2030 (% rel to 2010) in 2050 (% rel to 2010) from non -biomass renewables in 2030 (% rel to 2010) L- in 2050 (96 rel to 2010) Cumulative CCS until2100 (GtCO:) I--ofwhich BECCS (GtCO2) Land area of bioenergy crops in 2050 (million km-;) Agricultural CH. emissions in 2030 (% rel to 2010) in 2050 (% rel to 2010) Agricultural N:O emissions in 2030 (% rel to 2010) in 2050 (% rel to 2010) 4''' P2 2020 P2: A scenario with a broad focus on sustainability including energy intensity, human development, economic convergence and international cooperation, as well as shifts towards sustainable and healthy consumption patterns, low -carbon technology innovation, and well -managed land systems with limited societal acceptability for BECCS. P1 No or Limited overshoot -58 -93 -50 -82 -15 -32 60 77 -78 -97 -37 -87 -25 -74 59 150 -11 -16 430 833 0 0 0.2 -24 -33 5 6 Billion tonnes CO, per year (GtCO2/yr) Billion tonnes CO, per year (GtCO2/yr) 2020 2052 2100 P3: A middle-of-the-road scenario in which societal as well as technological development follows historical patterns. Emissions reductions are mainly achieved by changing the way in which energy and products are produced, and to a lesser degree by reductions in demand. P2 No or limited overshoot -47 -95 -49 -89 -5 2 58 81 -61 -77 -13 -50 -20 -53 83 98 0 49 470 1327 348 151 0.9 -48 -69 -26 -26 NOTE: Indicators hove been selected to show global trends identified by the Chopter2 assessment. National and sectorol characteristics con differ substantially from the global trends shown above. P3 No or limited overshoot -41 -91 -35 -78 17 21 48 63 -75 -73 -3 -81 33 21 98 501 36 121 315 878 687 414 2.8 1 -23 15 0 2020 2060 2100 P4: A resource- and energy -intensive scenario in which economic growth and globalization lead to widespread adoption of greenhouse -gas -intensive lifestyles, including high demand for transportation fuels and livestock products. Emissions reductions are mainly achieved through technological means, making strong use of CDR through the deployment of BECCS. P4 Higher overshoot 4 -97 -2 -80 39 44 25 70 -59 -97 86 -32 37 -48 106 468 -1 418 110 1137 1218 1191 7.2 14 2 3 39 Interquartile range No or limited overshoot (-58,-40) (-107,-94) (-51,-39) (-93,-81) (-12,7) (-11,22) (47,65) (69,86) (-78, -59) (-95, -74) (-34,3) (-78,-31) (-26,21) (-56,6) (44,102) (91,190) (29,80) (123,261) (245,436) (576,1299) (550,1017) (364,662) (1.5,3.2) (-30,-11) (-47,-24) (-21,3) (-26,1) • Kyoto -gas emissions ore based on IPCC Second Assessment Report GWP-100 "Changes in energy demand are associated with improvements in energy efficiency and behaviour change 14 Summary for Policymakers Figure SPM.3b Characteristics of four illustrative model pathways in relation to global warming of 1.5°C introduced in Figure SPM.3a. These pathways were selected to show a range of potential mitigation approaches and vary widely in their projected energy and land use, as well as their assumptions about future socio-economic developments, including economic and population growth, equity and sustainability. A breakdown of the global net anthropogenic CO, emissions into the contributions in terms of CO, emissions from fossil fuel and industry; agriculture, forestry and other land use (AFOLU); and bioenergy with carbon capture and storage (BECCS) is shown. AFOLU estimates reported here are not necessarily comparable with countries' estimates. Further characteristics for each of these pathways are listed below each pathway. These pathways illustrate relative global differences in mitigation strategies, but do not represent central estimates, national strategies, and do not indicate requirements. For comparison, the right -most column shows the interquartile ranges across pathways with no or limited overshoot of 1.5°C. Pathways P1, P2, P3 and P4 correspond to the LED, S1, 52 and S5 pathways assessed in Chapter 2 (Figure SPM.3a). {2.2.1, 2.3.1, 2.3.2, 2.3.3, 2.3.4, 2.4.1, 2.4.2, 2.4.4, 2.5.3, Figure 2.5, Figure 2.6, Figure 2.9, Figure 2.10, Figure 2.11, Figure 2.14, Figure 2.15, Figure 2.16, Figure 2.17, Figure 2.24, Figure 2.25, Table 2.4, Table 2.6, Table 2.7, Table 2.9, Table 4.1} C.2 Pathways limiting global warming to 1.5°C with no or limited overshoot would require rapid and far-reaching transitions in energy, land, urban and infrastructure (including transport and buildings), and industrial systems (high confidence). These systems transitions are unprecedented in terms of scale, but not necessarily in terms of speed, and imply deep emissions reductions in all sectors, a wide portfolio of mitigation options and a significant upscaling of investments in those options (medium confidence). {2.3, 2.4, 2.5, 4.2, 4.3, 4.4, 4.5} C.2.1 Pathways that limit global warming to 1.5°C with no or limited overshoot show system changes that are more rapid and pronounced over the next two decades than in 2°C pathways (high confidence). The rates of system changes associated with limiting global warming to 1.5°C with no or limited overshoot have occurred in the past within specific sectors, technologies and spatial contexts, but there is no documented historic precedent for their scale (medium confidence). {2.3.3, 2.3.4, 2.4, 2.5, 4.2.1, 4.2.2, Cross -Chapter Box 11 in Chapter 4} C.2.2 In energy systems, modelled global pathways (considered in the literature) limiting global warming to 1.5°C with no or limited overshoot (for more details see Figure SPM.3b) generally meet energy service demand with lower energy use, including through enhanced energy efficiency, and show faster electrification of energy end use compared to 2°C (high confidence). In 1.5°C pathways with no or limited overshoot, low -emission energy sources are projected to have a higher share, compared with 2°C pathways, particularly before 2050 (high confidence). In 1.5°C pathways with no or limited overshoot, renewables are projected to supply 70-85% (interquartile range) of electricity in 2050 (high confidence). In electricity generation, shares of nuclear and fossil fuels with carbon dioxide capture and storage (CCS) are modelled to increase in most 1.5°C pathways with no or limited overshoot. In modelled 1.5°C pathways with limited or no overshoot, the use of CCS would allow the electricity generation share of gas to be approximately 8% (3-11 % interquartile range) of global electricity in 2050, while the use of coal shows a steep reduction in all pathways and would be reduced to close to 0% (0-2% interquartile range) of electricity (high confidence). While acknowledging the challenges, and differences between the options and national circumstances, political, economic, social and technical feasibility of solar energy, wind energy and electricity storage technologies have substantially improved over the past few years (high confidence). These improvements signal a potential system transition in electricity generation. (Figure SPM.3b) {2.4.1, 2.4.2, Figure 2.1, Table 2.6, Table 2.7, Cross -Chapter Box 6 in Chapter 3, 4.2.1, 4.3.1, 4.3.3, 4.5.21 C.2.3 CO, emissions from industry in pathways limiting global warming to 1.5°C with no or limited overshoot are projected to be about 65-90% (interquartile range) lower in 2050 relative to 2010, as compared to 50-80% for global warming of 2°C (medium confidence). Such reductions can be achieved through combinations of new and existing technologies and practices, including electrification, hydrogen, sustainable bio-based feedstocks, product substitution, and carbon capture, utilization and storage (CCUS). These options are technically proven at various scales but their large-scale deployment may be limited by economic, financial, human capacity and institutional constraints in specific contexts, and specific characteristics of large-scale industrial installations. In industry, emissions reductions by energy and process efficiency by themselves are insufficient for limiting warming to 1.5°C with no or limited overshoot (high confidence). {2.4.3, 4.2.1, Table 4.1, Table 4.3, 4.3.3, 4.3.4, 4.5.2) C.2.4 The urban and infrastructure system transition consistent with limiting global warming to 1.5°C with no or limited overshoot would imply, for example, changes in land and urban planning practices, as well as deeper emissions reductions in transport and buildings compared to pathways that limit global warming below 2°C (medium confidence). Technical measures 15 Summary for Policyrnakers and practices enabling deep emissions reductions include various energy efficiency options. In pathways limiting global warming to 1.5°C with no or limited overshoot, the electricity share of energy demand in buildings would be about 55-75% in 2050 compared to 50-70% in 2050 for 2°C global warming (medium confidence). In the transport sector, the share of low -emission final energy would rise from less than 5% in 2020 to about 35-65% in 2050 compared to 25-45% for 2°C of global warming (medium confidence). Economic, institutional and socio-cultural barriers may inhibit these urban and infrastructure system transitions, depending on national, regional and local circumstances, capabilities and the availability of capital (high confidence). {2.3.4, 2.4.3, 4.2.1, Table 4.1, 4.3.3, 4.5.2} C.2.5 Transitions in global and regional land use are found in all pathways limiting global warming to 1.5°C with no or limited overshoot, but their scale depends on the pursued mitigation portfolio. Model pathways that limit global warming to 1.5°C with no or limited overshoot project a 4 million km2 reduction to a 2.5 million km2 increase of non -pasture agricultural land for food and feed crops and a 0.5-11 million km2 reduction of pasture land, to be converted into a 0-6 million km2 increase of agricultural land for energy crops and a 2 million km2 reduction to 9.5 million km2 increase in forests by 2050 relative to 2010 (medium confidence).16 Land -use transitions of similar magnitude can be observed in modelled 2°C pathways (medium confidence). Such large transitions pose profound challenges for sustainable management of the various demands on land for human settlements, food, livestock feed, fibre, bioenergy, carbon storage, biodiversity and other ecosystem services (high confidence). Mitigation options limiting the demand for land include sustainable intensification of land -use practices, ecosystem restoration and changes towards less resource -intensive diets (high confidence). The implementation of land -based mitigation options would require overcoming socio-economic, institutional, technological, financing and environmental barriers that differ across regions (high confidence). {2.4.4, Figure 2.24, 4.3.2, 4.3.7, 4.5.2, Cross -Chapter Box 7 in Chapter 3) C.2.6 Additional annual average energy -related investments for the period 2016 to 2050 in pathways limiting warming to 1.5°C compared to pathways without new climate policies beyond those in place today are estimated to be around 830 billion USD2010 (range of 150 billion to 1700 billion USD2010 across six models17). This compares to total annual average energy supply investments in 1.5°C pathways of 1460 to 3510 billion USD2010 and total annual average energy demand investments of 640 to 910 billion USD2010 for the period 2016 to 2050. Total energy -related investments increase by about 12% (range of 3% to 24%) in 1.5°C pathways relative to 2°C pathways. Annual investments in low -carbon energy technologies and energy efficiency are upscaled by roughly a factor of six (range of factor of 4 to 10) by 2050 compared to 2015 (medium confidence). {2.5.2, Box 4.8, Figure 2.27} C.2.7 Modelled pathways limiting global warming to 1.5°C with no or limited overshoot project a wide range of global average discounted marginal abatement costs over the 21 st century. They are roughly 3-4 times higher than in pathways limiting global warming to below 2°C (high confidence). The economic literature distinguishes marginal abatement costs from total mitigation costs in the economy. The literature on total mitigation costs of 1.5°C mitigation pathways is limited and was not assessed in this Report. Knowledge gaps remain in the integrated assessment of the economy -wide costs and benefits of mitigation in line with pathways limiting warming to 1.5°C. {2.5.2; 2.6; Figure 2.26} 16 The projected land -use changes presented are not deployed to their upper limits simultaneously in a single pathway. 17 Including two pathways limiting warming to 1.5°C with no or limited overshoot and four pathways with higher overshoot. 16 Summary for Policymakers C.3 All pathways that limit global warming to 1.5°C with limited or no overshoot project the use of carbon dioxide removal (CDR) on the order of 100-1000 GtCO2 over the 21st century. CDR would be used to compensate for residual emissions and, in most cases, achieve net negative emissions to return global warming to 1.5°C following a peak (high confidence). CDR deployment of several hundreds of GtCO2 is subject to multiple feasibility and sustainability constraints (high confidence). Significant near -term emissions reductions and measures to lower energy and land demand can limit CDR deployment to a few hundred GtCO2 without reliance on bioenergy with carbon capture and storage (BECCS) (high confidence). {2.3, 2.4, 3.6.2, 4.3, 5.4} C.3.1 Existing and potential CDR measures include afforestation and reforestation, land restoration and soil carbon sequestration, BECCS, direct air carbon capture and storage (DACCS), enhanced weathering and ocean alkalinization. These differ widely in terms of maturity, potentials, costs, risks, co -benefits and trade-offs (high confidence). To date, only a few published pathways include CDR measures other than afforestation and BECCS. {2.3.4, 3.6.2, 4.3.2, 4.3.7) C.3.2 In pathways limiting global warming to 1.5°C with limited or no overshoot, BECCS deployment is projected to range from 0-1, 0-8, and 0-16 GtCO2 yr' in 2030, 2050, and 2100, respectively, while agriculture, forestry and land -use (AFOLU) related CDR measures are projected to remove 0-5, 1-11, and 1-5 GtCO2 yr-1 in these years (medium confidence). The upper end of these deployment ranges by mid-century exceeds the BECCS potential of up to 5 GtCO2 yr' and afforestation potential of up to 3.6 GtCO2 yr-1 assessed based on recent literature (medium confidence). Some pathways avoid BECCS deployment completely through demand -side measures and greater reliance on AFOLU-related CDR measures (medium confidence). The use of bioenergy can be as high or even higher when BECCS is excluded compared to when it is included due to its potential for replacing fossil fuels across sectors (high confidence). (Figure SPM.3b) (2.3.3, 2.3.4, 2.4.2, 3.6.2, 4.3.1, 4.2.3, 4.3.2, 4.3.7, 4.4.3, Table 2.4} C.3.3 Pathways that overshoot 1.5°C of global warming rely on CDR exceeding residual CO2 emissions later in the century to return to below 1.5°C by 2100, with larger overshoots requiring greater amounts of CDR (Figure SPM.3b) (high confidence). Limitations on the speed, scale, and societal acceptability of CDR deployment hence determine the ability to return global warming to below 1.5°C following an overshoot. Carbon cycle and climate system understanding is still limited about the effectiveness of net negative emissions to reduce temperatures after they peak (high confidence). {2.2, 2.3.4, 2.3.5, 2.6, 4.3.7, 4.5.2, Table 4.11} C.3.4 Most current and potential CDR measures could have significant impacts on land, energy, water or nutrients if deployed at large scale (high confidence). Afforestation and bioenergy may compete with other land uses and may have significant impacts on agricultural and food systems, biodiversity, and other ecosystem functions and services (high confidence). Effective governance is needed to limit such trade-offs and ensure permanence of carbon removal in terrestrial, geological and ocean reservoirs (high confidence). Feasibility and sustainability of CDR use could be enhanced by a portfolio of options deployed at substantial, but lesser scales, rather than a single option at very large scale (high confidence). (Figure SPM.3b) {2.3.4, 2.4.4, 2.5.3, 2.6, 3.6.2, 4.3.2, 4.3.7, 4.5.2, 5.4.1, 5.4.2; Cross -Chapter Boxes 7 and 8 in Chapter 3, Table 4.11, Table 5.3, Figure 5.3) C.3.5 Some AFOLU-related CDR measures such as restoration of natural ecosystems and soil carbon sequestration could provide co -benefits such as improved biodiversity, soil quality, and local food security. If deployed at large scale, they would require governance systems enabling sustainable land management to conserve and protect land carbon stocks and other ecosystem functions and services (medium confidence). (Figure SPM.4) {2.3.3, 2.3.4, 2.4.2, 2.4.4, 3.6.2, 5.4.1, Cross -Chapter Boxes 3 in Chapter 1 and 7 in Chapter 3, 4.3.2, 4.3.7, 4.4.1, 4.5.2, Table 2.4) 17 Summary for Policymakers D. Strengthening the Global Response in the Context of Sustainable Development and Efforts to Eradicate Poverty D.1 D.1.1 Estimates of the global emissions outcome of current nationally stated mitigation ambitions as submitted under the Paris Agreement would lead to global greenhouse gas emissions in 2030 of 52-58 GtCO2eq yr-1 (medium confidence). Pathways reflecting these ambitions would not limit global warming to 1.5°C, even if supplemented by very challenging increases in the scale and ambition of emissions reductions after 2030 (high confidence). Avoiding overshoot and reliance on future large-scale deployment of carbon dioxide removal (CDR) can only be achieved if global CO2 emissions start to decline well before 2030 (high confidence). {1.2, 2.3, 3.3, 3.4, 4.2, 4.4, Cross - Chapter Box 11 in Chapter 4} Pathways that limit global warming to 1.5°C with no or limited overshoot show clear emission reductions by 2030 (high confidence). All but one show a decline in global greenhouse gas emissions to below 35 GtCO2eq yr-1 in 2030, and half of available pathways fall within the 25-30 GtCO2eq yr' range (interquartile range), a 40-50% reduction from 2010 levels (high confidence). Pathways reflecting current nationally stated mitigation ambition until 2030 are broadly consistent with cost-effective pathways that result in a global warming of about 3°C by 2100, with warming continuing afterwards (medium confidence). {2.3.3, 2.3.5, Cross -Chapter Box 11 in Chapter 4, 5.5.3.2} D.1.2 Overshoot trajectories result in higher impacts and associated challenges compared to pathways that limit global warming to 1.5°C with no or limited overshoot (high confidence). Reversing warming after an overshoot of 0.2°C or larger during this century would require upscaling and deployment of CDR at rates and volumes that might not be achievable given considerable implementation challenges (medium confidence). {1.3.3, 2.3.4, 2.3.5, 2.5.1, 3.3, 4.3.7, Cross -Chapter Box 8 in Chapter 3, Cross -Chapter Box 11 in Chapter 4} D.1.3 The lower the emissions in 2030, the lower the challenge in limiting global warming to 1.5°C after 2030 with no or limited overshoot (high confidence). The challenges from delayed actions to reduce greenhouse gas emissions include the risk of cost escalation, lock -in in carbon -emitting infrastructure, stranded assets, and reduced flexibility in future response options in the medium to long term (high confidence). These may increase uneven distributional impacts between countries at different stages of development (medium confidence). {2.3.5, 4.4.5, 5.4.2} D.2 The avoided climate change impacts on sustainable development, eradication of poverty and reducing inequalities would be greater if global warming were limited to 1.5°C rather than 2°C, if mitigation and adaptation synergies are maximized while trade-offs are minimized (high confidence). {1.1, 1.4, 2.5, 3.3, 3.4, 5.2, Table 5.1} D.2.1 Climate change impacts and responses are closely linked to sustainable development which balances social well-being, economic prosperity and environmental protection. The United Nations Sustainable Development Goals (SDGs), adopted in 2015, provide an established framework for assessing the links between global warming of 1.5°C or 2°C and development goals that include poverty eradication, reducing inequalities, and climate action. (high confidence) {Cross -Chapter Box 4 in Chapter 1, 1.4, 5.1} D.2.2 The consideration of ethics and equity can help address the uneven distribution of adverse impacts associated with 1.5°C and higher levels of global warming, as well as those from mitigation and adaptation, particularly for poor and disadvantaged populations, in all societies (high confidence). {1.1.1, 1.1.2, 1.4.3, 2.5.3, 3.4.10, 5.1, 5.2, 5.3. 5.4, Cross - Chapter Box 4 in Chapter 1, Cross -Chapter Boxes 6 and 8 in Chapter 3, and Cross -Chapter Box 12 in Chapter 5} D.2.3 Mitigation and adaptation consistent with limiting global warming to 1.5°C are underpinned by enabling conditions, assessed in this Report across the geophysical, environmental -ecological, technological, economic, socio-cultural and institutional 18 GHG emissions have been aggregated with 100-year GWP values as introduced in the IPCC Second Assessment Report. 18 Summary for Policymakers dimensions of feasibility. Strengthened multilevel governance, institutional capacity, policy instruments, technological innovation and transfer and mobilization of finance, and changes in human behaviour and lifestyles are enabling conditions that enhance the feasibility of mitigation and adaptation options for 1.5°C-consistent systems transitions. (high confidence) {1.4, Cross -Chapter Box 3 in Chapter 1, 2.5.1, 4.4, 4.5, 5.6} D.3 Adaptation options specific to national contexts, if carefully selected together with enabling conditions, will have benefits for sustainable development and poverty reduction with global warming of 1.5°C, although trade-offs are possible (high confidence). (1.4, 4.3, 4.5) D.3.1 Adaptation options that reduce the vulnerability of human and natural systems have many synergies with sustainable development, if well managed, such as ensuring food and water security, reducing disaster risks, improving health conditions, maintaining ecosystem services and reducing poverty and inequality (high confidence). Increasing investment in physical and social infrastructure is a key enabling condition to enhance the resilience and the adaptive capacities of societies. These benefits can occur in most regions with adaptation to 1.5°C of global warming (high confidence). {1.4.3, 4.2.2, 4.3.1, 4.3.2, 4.3.3, 4.3.5, 4.4.1, 4.4.3, 4.5.3, 5.3.1, 5.3.2} D.3.2 Adaptation to 1.5°C global warming can also result in trade-offs or maladaptations with adverse impacts for sustainable development. For example, if poorly designed or implemented, adaptation projects in a range of sectors can increase greenhouse gas emissions and water use, increase gender and social inequality, undermine health conditions, and encroach on natural ecosystems (high confidence). These trade-offs can be reduced by adaptations that include attention to poverty and sustainable development (high confidence). {4.3.2, 4.3.3, 4.5.4, 5.3.2; Cross -Chapter Boxes 6 and 7 in Chapter 3} D.3.3 A mix of adaptation and mitigation options to limit global warming to 1.5°C, implemented in a participatory and integrated manner, can enable rapid, systemic transitions in urban and rural areas (high confidence). These are most effective when aligned with economic and sustainable development, and when local and regional governments and decision makers are supported by national governments (medium confidence). {4.3.2, 4.3.3, 4.4.1, 4.4.2} D.3.4 Adaptation options that also mitigate emissions can provide synergies and cost savings in most sectors and system transitions, such as when land management reduces emissions and disaster risk, or when low -carbon buildings are also designed for efficient cooling. Trade-offs between mitigation and adaptation, when limiting global warming to 1.5°C, such as when bioenergy crops, reforestation or afforestation encroach on land needed for agricultural adaptation, can undermine food security, livelihoods, ecosystem functions and services and other aspects of sustainable development. (high confidence) {3.4.3, 4.3.2, 4.3.4, 4.4.1, 4.5.2, 4.5.3, 4.5.4} D.4 Mitigation options consistent with 1.5°C pathways are associated with multiple synergies and trade- offs across the Sustainable Development Goals (SDGs). While the total number of possible synergies exceeds the number of trade-offs, their net effect will depend on the pace and magnitude of changes, the composition of the mitigation portfolio and the management of the transition. (high confidence) (Figure SPM.4) {2.5, 4.5, 5.4} D.4.1 1.5°C pathways have robust synergies particularly for the SDGs 3 (health), 7 (clean energy), 11 (cities and communities), 12 (responsible consumption and production) and 14 (oceans) (very high confidence). Some 1.5°C pathways show potential trade-offs with mitigation for SDGs 1 (poverty), 2 (hunger), 6 (water) and 7 (energy access), if not managed carefully (high confidence). (Figure SPM.4) {5.4.2; Figure 5.4, Cross -Chapter Boxes 7 and 8 in Chapter 3} D.4.2 1.5°C pathways that include low energy demand (e.g., see P1 in Figure SPM.3a and SPM.3b), low material consumption, and low GHG-intensive food consumption have the most pronounced synergies and the lowest number of trade-offs with respect to sustainable development and the SDGs (high confidence). Such pathways would reduce dependence on CDR. In modelled pathways, sustainable development, eradicating poverty and reducing inequality can support limiting warming to 1.5°C (high confidence). (Figure SPM.3b, Figure SPM.4) {2.4.3, 2.5.1, 2.5.3, Figure 2.4, Figure 2.28, 5.4.1, 5.4.2, Figure 5.4} 19 Summary for Policymakers Indicative linkages between mitigation options and sustainable development using SDGS (The linkages do not show costs and benefits) Mitigation options deployed in each sector can be associated with potential positive effects (synergies) or negative effects (trade-offs) with the Sustainable Development Goals (SDGs). The degree to which this potential is realized will depend on the selected portfolio of mitigation options, mitigation policy design, and local circumstances and context. Particularly in the energy -demand sector, the potential for synergies is larger than for trade-offs. The bars group individually assessed options by level of confidence and take into account the relative strength of the assessed mitigation-SDG connections. Length shows strength of connection The overall size of the coloured bars depict the relative potential for synergies and trade-offs between the sectoral mitigation options and the SDGs. SDG1 No Poverty SDG2 Zero Hunger SDG 3 Good Health and Welt -being SDG 4 Quality Education SDG 5 Gender Equality SDG 6 Clean Water and Sanitation SDG 7 Affordable and Clean Energy SDG 8 Decent Work and Economic Growth SDG 9 Industry, Innovation and Infrastructure SDG 10 Reduced Inequalities SDG 11 Sustainable Cities and Communities SDG 12 Responsible Consumption and Production SDG 14 Life Below Water SDG 15 Life on Land SDG 16 Peace, Justice and Strong Institutions SDG 17 Partnerships for the Goals faith U) 6% 8 16 16 MI I Energy Supply Trade-offs Synergies i ■ i 1 ■ Shades show level of confidence The shades depict the level of confidence of the assessed potential for Trade-offs/Synergies. Very High Energy Demand Trade-offs Synergies I I I 1 I 1 1 Low Land Trade-offs 1 Synergies i i 2 Summary for Policyrnakers Figure SPM.4 I Potential synergies and trade-offs between the sectoral portfolio of climate change mitigation options and the Sustainable Development Goals (SDGs). The SDGs serve as an analytical framework for the assessment of the different sustainable development dimensions, which extend beyond the time frame of the 2030 SDG targets. The assessment is based on literature on mitigation options that are considered relevant for 1.5°C. The assessed strength of the SDG interactions is based on the qualitative and quantitative assessment of individual mitigation options listed in Table 5.2. For each mitigation option, the strength of the SDG-connection as well as the associated confidence of the underlying literature (shades of green and red) was assessed. The strength of positive connections (synergies) and negative connections (trade-offs) across all individual options within a sector (see Table 5.2) are aggregated into sectoral potentials for the whole mitigation portfolio. The (white) areas outside the bars, which indicate no interactions, have low confidence due to the uncertainty and limited number of studies exploring indirect effects. The strength of the connection considers only the effect of mitigation and does not include benefits of avoided impacts. SDG 13 (climate action) is not listed because mitigation is being considered in terms of interactions with SDGs and not vice versa. The bars denote the strength of the connection, and do not consider the strength of the impact on the SDGs. The energy demand sector comprises behavioural responses, fuel switching and efficiency options in the transport, industry and building sector as well as carbon capture options in the industry sector. Options assessed in the energy supply sector comprise biomass and non -biomass renewables, nuclear, carbon capture and storage (CCS) with bioenergy, and CCS with fossil fuels. Options in the land sector comprise agricultural and forest options, sustainable diets and reduced food waste, soil sequestration, livestock and manure management, reduced deforestation, afforestation and reforestation, and responsible sourcing. In addition to this figure, options in the ocean sector are discussed in the underlying report. {5.4, Table 5.2, Figure 5.2} Information about the net impacts of mitigation on sustainable development in 1.5°C pathways is available only for a limited number of SDGs and mitigation options. Only a limited number of studies have assessed the benefits of avoided climate change impacts of 1.5°C pathways for the SDGs, and the co -effects of adaptation for mitigation and the SDGs. The assessment of the indicative mitigation potentials in Figure SPM.4 is a step further from AR5 towards a more comprehensive and integrated assessment in the future. D.4.3 1.5°C and 2°C modelled pathways often rely on the deployment of large-scale land -related measures like afforestation and bioenergy supply, which, if poorly managed, can compete with food production and hence raise food security concerns (high confidence). The impacts of carbon dioxide removal (CDR) options on SDGs depend on the type of options and the scale of deployment (high confidence). If poorly implemented, CDR options such as BECCS and AFOLU options would lead to trade-offs. Context -relevant design and implementation requires considering people's needs, biodiversity, and other sustainable development dimensions (very high confidence). (Figure SPM.4) {5.4.1.3, Cross -Chapter Box 7 in Chapter 3} D.4.4 Mitigation consistent with 1.5°C pathways creates risks for sustainable development in regions with high dependency on fossil fuels for revenue and employment generation (high confidence). Policies that promote diversification of the economy and the energy sector can address the associated challenges (high confidence). {5.4.1.2, Box 5.2) D.4.5 Redistributive policies across sectors and populations that shield the poor and vulnerable can resolve trade-offs for a range of SDGs, particularly hunger, poverty and energy access. Investment needs for such complementary policies are only a small fraction of the overall mitigation investments in 1.5°C pathways. (high confidence) {2.4.3, 5.4.2, Figure 5.5) D.5 Limiting the risks from global warming of 1.5°C in the context of sustainable development and poverty eradication implies system transitions that can be enabled by an increase of adaptation and mitigation investments, policy instruments, the acceleration of technological innovation and behaviour changes (high confidence). {2.3, 2.4, 2.5, 3.2, 4.2, 4.4, 4.5, 5.2, 5.5, 5.61 D.5.1 Directing finance towards investment in infrastructure for mitigation and adaptation could provide additional resources. This could involve the mobilization of private funds by institutional investors, asset managers and development or investment banks, as well as the provision of public funds. Government policies that lower the risk of low -emission and adaptation investments can facilitate the mobilization of private funds and enhance the effectiveness of other public policies. Studies indicate a number of challenges, including access to finance and mobilization of funds. (high confidence) {2.5.1, 2.5.2, 4.4.5) D.5.2 Adaptation finance consistent with global warming of 1.5°C is difficult to quantify and compare with 2°C. Knowledge gaps include insufficient data to calculate specific climate resilience -enhancing investments from the provision of currently underinvested basic infrastructure. Estimates of the costs of adaptation might be lower at global warming of 1.5°C than for 2°C. Adaptation needs have typically been supported by public sector sources such as national and subnational government budgets, and in developing countries together with support from development assistance, multilateral development banks, and United Nations Framework Convention on Climate Change channels (medium confidence). More recently there is a 21 Summary for Policymakers growing understanding of the scale and increase in non -governmental organizations and private funding in some regions (medium confidence). Barriers include the scale of adaptation financing, limited capacity and access to adaptation finance (medium confidence). {4.4.5, 4.6} D.5.3 Global model pathways limiting global warming to 1.5°C are, projected to involve the annual average investment needs in the energy system of around 2.4 trillion USD2010 between 2016 and 2035, representing about 2.5% of the world GDP (medium confidence). {4.4.5, Box 4.8} D.5.4 Policy tools can help mobilize incremental resources, including through shifting global investments and savings and through market and non -market based instruments as well as accompanying measures to secure the equity of the transition, acknowledging the challenges related with implementation, including those of energy costs, depreciation of assets and impacts on international competition, and utilizing the opportunities to maximize co -benefits (high confidence). {1.3.3, 2.3.4, 2.3.5, 2.5.1, 2.5.2, Cross -Chapter Box 8 in Chapter 3, Cross -Chapter Box 11 in Chapter 4, 4.4.5, 5.5.2} D.5.5 The systems transitions consistent with adapting to and limiting global warming to 1.5°C include the widespread adoption of new and possibly disruptive technologies and practices and enhanced climate -driven innovation. These imply enhanced technological innovation capabilities, including in industry and finance. Both national innovation policies and international cooperation can contribute to the development, commercialization and widespread adoption of mitigation and adaptation technologies. Innovation policies may be more effective when they combine public support for research and development with policy mixes that provide incentives for technology diffusion. (high confidence) {4.4.4, 4.4.5}. D.5.6 Education, information, and community approaches, including those that are informed by indigenous knowledge and local knowledge, can accelerate the wide -scale behaviour changes consistent with adapting to and limiting global warming to 1.5°C. These approaches are more effective when combined with other policies and tailored to the motivations, capabilities and resources of specific actors and contexts (high confidence). Public acceptability can enable or inhibit the implementation of policies and measures to limit global warming to 1.5°C and to adapt to the consequences. Public acceptability depends on the individual's evaluation of expected policy consequences, the perceived fairness of the distribution of these consequences, and perceived fairness of decision procedures (high confidence). {1.1, 1.5, 4.3.5, 4.4.1, 4.4.3, Box 4.3, 5.5.3, 5.6.5} D.6 Sustainable development supports, and often enables, the fundamental societal and systems transitions and transformations that help limit global warming to 1.5°C. Such changes facilitate the pursuit of climate -resilient development pathways that achieve ambitious mitigation and adaptation in conjunction with poverty eradication and efforts to reduce inequalities (high confidence). {Box 1.1, 1.4.3, Figure 5.1, 5.5.3, Box 5.3} D.6.1 Social justice and equity are core aspects of climate -resilient development pathways that aim to limit global warming to 1.5°C as they address challenges and inevitable trade-offs, widen opportunities, and ensure that options, visions, and values are deliberated, between and within countries and communities, without making the poor and disadvantaged worse off (high confidence). {5.5.2, 5.5.3, Box 5.3, Figure 5.1, Figure 5.6, Cross -Chapter Boxes 12 and 13 in Chapter 5} D.6.2 The potential for climate -resilient development pathways differs between and within regions and nations, due to different development contexts and systemic vulnerabilities (very high confidence). Efforts along such pathways to date have been limited (medium confidence) and enhanced efforts would involve strengthened and timely action from all countries and non -state actors (high confidence). {5.5.1, 5.5.3, Figure 5.1} D.6.3 Pathways that are consistent with sustainable development show fewer mitigation and adaptation challenges and are associated with lower mitigation costs. The large majority of modelling studies could not construct pathways characterized by lack of international cooperation, inequality and poverty that were able to limit global warming to 1.5°C. (high confidence) {2.3.1, 2.5.1, 2.5.3, 5.5.2} 22 Summary for Policymakers D.7 Strengthening the capacities for climate action of national and sub -national authorities, civil society, the private sector, indigenous peoples and local communities can support the implementation of ambitious actions implied by limiting global warming to 1.5°C (high confidence). International cooperation can provide an enabling environment for this to be achieved in all countries and for all people, in the context of sustainable development. International cooperation is a critical enabler for developing countries and vulnerable regions (high confidence). (1.4, 2.3, 2.5, 4.2, 4.4, 4.5, 5.3, 5.4, 5.5, 5.6, 5, Box 4.1, Box 4.2, Box 4.7, Box 5.3, Cross -Chapter Box 9 in Chapter 4, Cross -Chapter Box 13 in Chapter 5} D.7.1 Partnerships involving non -state public and private actors, institutional investors, the banking system, civil society and scientific institutions would facilitate actions and responses consistent with limiting global warming to 1.5°C (very high confidence). {1.4, 4.4.1, 4.2.2, 4.4.3, 4.4.5, 4.5.3, 5.4.1, 5.6.2, Box 5.3}. D.7.2 Cooperation on strengthened accountable multilevel governance that includes non -state actors such as industry, civil society and scientific institutions, coordinated sectoral and cross-sectoral policies at various governance levels, gender - sensitive policies, finance including innovative financing, and cooperation on technology development and transfer can ensure participation, transparency, capacity building and learning among different players (high confidence). {2.5.1, 2.5.2, 4.2.2, 4.4.1, 4.4.2, 4.4.3, 4.4.4, 4.4.5, 4.5.3, Cross -Chapter Box 9 in Chapter 4, 5.3.1, 5.5.3, Cross -Chapter Box 13 in Chapter 5, 5.6.1, 5.6.3} D.7.3 International cooperation is a critical enabler for developing countries and vulnerable regions to strengthen their action for the implementation of 1.5°C-consistent climate responses, including through enhancing access to finance and technology and enhancing domestic capacities, taking into account national and local circumstances and needs (high confidence). {2.3.1, 2.5.1, 4.4.1, 4.4.2, 4.4.4, 4.4.5, 5.4.1 5.5.3, 5.6.1, Box 4.1, Box 4.2, Box 4.7}. D.7.4 Collective efforts at all levels, in ways that reflect different circumstances and capabilities, in the pursuit of limiting global warming to 1.5°C, taking into account equity as well as effectiveness, can facilitate strengthening the global response to climate change, achieving sustainable development and eradicating poverty (high confidence). {1.4.2, 2.3.1, 2.5.1, 2.5.2, 2.5.3, 4.2.2, 4.4.1, 4.4.2, 4.4.3, 4.4.4, 4.4.5, 4.5.3, 5.3.1, 5.4.1, 5.5.3, 5.6.1, 5.6.2, 5.6.3) 23 Summary for Policymakers Box SPM.1: Core Concepts Central to this Special Report Global mean surface temperature (GMST): Estimated global average of near -surface air temperatures over land and sea ice, and sea surface temperatures over ice -free ocean regions, with changes normally expressed as departures from a value over a specified reference period. When estimating changes in GMST, near -surface air temperature over both land and oceans are also used.19 {1.2.1.1 } Pre -industrial: The mufti -century period prior to the onset of large-scale industrial activity around 1750. The reference period 1850-1900 is used to approximate pre -industrial GMST. {1.2.1.2} Global warming: The estimated increase in GMST averaged over a 30-year period, or the 30-year period centred on a particular year or decade, expressed relative to pre -industrial levels unless otherwise specified. For 30-year periods that span past and future years, the current multi-decadal warming trend is assumed to continue. (1.2.1) Net zero CO, emissions: Net zero carbon dioxide (CO2) emissions are achieved when anthropogenic CO, emissions are balanced globally by anthropogenic CO, removals over a specified period. Carbon dioxide removal (CDR): Anthropogenic activities removing CO2 from the atmosphere and durably storing it in geological, terrestrial, or ocean reservoirs, or in products. It includes existing and potential anthropogenic enhancement of biological or geochemical sinks and direct air capture and storage, but excludes natural CO, uptake not directly caused by human activities. Total carbon budget: Estimated cumulative net global anthropogenic CO, emissions from the pre -industrial period to the time that anthropogenic CO, emissions reach net zero that would result, at some probability, in limiting global warming to a given level, accounting for the impact of other anthropogenic emissions. {2.2.2} Remaining carbon budget: Estimated cumulative net global anthropogenic CO, emissions from a given start date to the time that anthropogenic CO, emissions reach net zero that would result, at some probability, in limiting global warming to a given level, accounting for the impact of other anthropogenic emissions. (2.2.2) Temperature overshoot: The temporary exceedance of a specified level of global warming. Emission pathways: In this Summary for Policymakers, the modelled trajectories of global anthropogenic emissions over the 21st century are termed emission pathways. Emission pathways are classified by their temperature trajectory over the 21st century: pathways giving at least 50% probability based on current knowledge of limiting global warming to below 1.5°C are classified as 'no overshoot'; those limiting warming to below 1.6°C and returning to 1.5°C by 2100 are classified as '1.5°C limited -overshoot'; while those exceeding 1.6°C but still returning to 1.5°C by 2100 are classified as 'higher -overshoot'. Impacts: Effects of climate change on human and natural systems. Impacts can have beneficial or adverse outcomes for livelihoods, health and well-being, ecosystems and species, services, infrastructure, and economic, social and cultural assets. Risk: The potential for adverse consequences from a climate -related hazard for human and natural systems, resulting from the interactions between the hazard and the vulnerability and exposure of the affected system. Risk integrates the likelihood of exposure to a hazard and the magnitude of its impact. Risk also can describe the potential for adverse consequences of adaptation or mitigation responses to climate change. Climate -resilient development pathways (CRDPs): Trajectories that strengthen sustainable development at multiple scales and efforts to eradicate poverty through equitable societal and systems transitions and transformations while reducing the threat of climate change through ambitious mitigation, adaptation and climate resilience. 19 Past IPCC reports, reflecting the literature, have used a variety of approximately equivalent metrics of GMST change. 24 OOverview Howe Ridge Fire in Montana's Glacier National Park on August 12, 2018. Photo credit: National Park Service. Federal Coordinating Lead Author David Reidmiller, U.S. Global Change Research Program Chapter Lead Alexa Jay, U.S. Global Change Research Program Chapter Authors Christopher W. Avery, U.S. Global Change Research Program Daniel Barrie, National Oceanic and Atmospheric Administration Apurva Dave, U.S. Global Change Research Program Benjamin DeAngelo, National Oceanic and Atmospheric Administration Matthew Dzaugis, U.S. Global Change Research Program Michael Kolian, U.S. Environmental Protection Agency Kristin Lewis, U.S. Global Change Research Program Katie Reeves, U.S. Global Change Research Program Darrell Winner, U.S. Environmental Protection Agency Recommended Citation for Chapter Jay, A., D.R. Reidmiller, C.W. Avery, D. Barrie, B.J. DeAngelo, A. Dave, M. Dzaugis, M. Kolian, K.L.M. Lew- is, K. Reeves, and D. Winner, 2018: Overview. In Impacts, Risks, and Adaptation in the United States: Fourth National Climate Assessment, Volume II [Reidmiller, D.R., C.W. Avery, D.R. Easterling, K.E. Kun- kel, K.L.M. Lewis, T.K. Maycock, and B.C. Stewart (eds.)]. U.S. Global Change Research Program, Wash- ington, DC, USA, pp. 33-71. doi: 10.7930/NCA4.2018.CH1 U.S. Global Change Research Program 33 Fourth National Climate Assessment 1 I Overview Introduction Earth's climate is now changing fast- er than at any point in the history of modern civilization, primarily as a result of human activities. The impacts of global climate change are already being felt in the United States and are pro- jected to intensify in the future —but the severity of future impacts will depend largely on actions taken to reduce green- house gas emissions and to adapt to the changes that will occur. Americans increasingly recognize the risks climate change poses to their everyday lives and livelihoods and are beginning to respond (Figure 1.1). Water managers in the Colorado River Basin have mobilized users to conserve water in response to ongoing drought intensified by higher temperatures, and an extension program in Nebraska is helping ranchers reduce drought and heat risks to their opera- tions. The state of Hawai'i is developing management options to promote coral reef recovery from widespread bleaching events caused by warmer waters that threaten tourism, fisheries, and coastal protection from wind and waves. To ad- dress higher risks of flooding from heavy rainfall, local governments in southern Louisiana are pooling hazard reduction funds, and cities and states in the North- east are investing in more resilient water, energy, and transportation infrastruc- ture. In Alaska, a tribal health organiza- tion is developing adaptation strategies to address physical and mental health challenges driven by climate change and other environmental changes. As Mid- western farmers adopt new management strategies to reduce erosion and nutrient losses caused by heavier rains, forest managers in the Northwest are developing adaptation strategies in response to wild- fire increases that affect human health, water resources, timber production, fish and wildlife, and recreation. After exten- sive hurricane damage fueled in part by a warmer atmosphere and warmer, higher seas, communities in Texas are consid- ering ways to rebuild more resilient infra- structure. In the U.S. Caribbean, govern- ments are developing new frameworks for storm recovery based on lessons learned from the 2017 hurricane season. Climate -related risks will continue to grow without additional action. Decisions made today determine risk exposure for current and future generations and will either broaden or limit options to reduce the negative consequences of climate change. While Americans are responding in ways that can bolster resilience and im- prove livelihoods, neither global efforts to mitigate the causes of climate change nor regional efforts to adapt to the impacts currently approach the scales needed to avoid substantial damages to the U.S. economy, environment, and human health and well-being over the coming decades. U.S. Global Change Research Program 34 Fourth National Climate Assessment 1 1 Overview • Americans Respond to the Impacts of Climate Change Northwest Impact Wildfire increases and associated smoke are affecting human health, water resources, timber production, fish and wildlife, and recreation. Action Federal forests have developed adaptation strategies for climate change that include methods to address increasing wildfire risks. Alaska Impact The physical and mental health of rural Alaskans is increasingly challenged by unpredictable weather and other environmental changes. Action The Alaska Native Tribal Health Consortium's Center for Climate and Health is using novel adaptation strategies to reduce climate -related risks including difficulty in harvesting local foods and more hazardous travel conditions. Northern Great Plains Impact Flash droughts and extreme heat illustrate sustainability challenges for ranching operations, with emergent impacts on rural prosperity and mental health. Action The National Drought Mitigation Center is helping ranchers plan to reduce drought and heat risks to their operations. Impact Drought in the Colorado River basin reduced Lake Mead by over half since 2000, increasing risk of water shortages for cities, farms, and ecosystems. Action Seven U.S. state govemments and U.S. and Mexico federal govemments mobilized users to conserve water, keeping the lake above a critical level. Hawaii and U.S.-Affiliated Pacific Islands Impact The 2015 coral bleaching event resulted in an average mortality of 50% of the coral cover in westem Hawaii alone. Action A state working group generated management options to promote recovery and reduce threats to coral reefs. Midwest Impact Increasing heavy rains are leading to more soil erosion and nutrient loss on Midwestem c opland. Action Iowa State developed a program for using prairie strips in farm fields to reduce soil and nutrient loss while increasing biodiversity. Southern Great Plains Impact Hurricane Harvey's landfall on the Texas coast in 2017 was one of the costliest natural disasters in U.S. history. Action The Governor's Commission to Rebuild Texas was created to support the economic recovery and rebuilding of infrastructure in affected Texas communities. Northeast Impact Water, energy, and transportation infrastructure are affected by snow storms, drought, heat waves, and Flooding. Action Cities and states throughout the region are assessing their vulnerability to climate change and making investments to increase infrastructure resilience. act Flooding in Louisiana is increasing from extreme rainfall. Action The Acadiana Planning Commission in Louisiana is pooling hazard reduction funds to address increasing flood risk. U.S. Caribbean Impact Damages from the 2017 hurricanes have been compounded by the slow recovery of energy, communications, and transportation systems, impacting all social and economic sectors. Action The U.S. Virgin Islands Govemor's Office led a workshop aimed at gathering lessons from the initial hurricane response and establishing a framework for recovery and resilience. Figure 1.1: This map shows climate -related impacts that have occurred in each region since the Third National Climate Assessment in 2014 and response actions that are helping the region address related risks and costs. These examples are illustrative; they are not indicative of which impact is most significant in each region or which response action might be most effective. Source: NCA4 Regional Chapters. U.S. Global Change Research Program 35 Fourth National Climate Assessment 1 1 Overview Climate shapes where and how we live and the environment around us. Natural ecosystems, agricultural systems, water resources, and the benefits they provide to society are adapted to past climate conditions and their natural range of variability. A water manager may use past or current streamflow records to design a dam, a city could issue permits for coastal development based on current flood maps, and an electric utility or a farmer may invest in equipment suited to the current climate, all with the expectation that their investments and management practices will meet future needs. However, the assumption that current and future climate conditions will resemble the recent past is no longer valid (Ch. 28: Adapta- tion, KM 2). Observations collected around the world provide significant, clear, and compelling evidence that global average temperature is much higher, and is rising more rapidly, than anything modern civilization has experienced, with widespread and growing impacts (Figure 1.2) (CSSI�, CIA.1..9). The warming trend observed over the past century can only be explained by the effects that human activities, especially emissions of greenhouse gases, have had on the climate (Ch. 2: Climate, KM 1 and Figure 2.1). Climate change is transforming where and how we live and presents growing challenges to human health and quality of life, the economy, and the natural systems that support us. Risks posed by climate variability and change vary by region and sector and by the vulnerability of people experiencing impacts. Social, economic, and geographic factors shape the exposure of people and communities to climate -related impacts and their capacity to respond. Risks are often highest for those that are already vulner- able, including low-income communities, some communities of color, children, and the elderly (Ch. 14: Human Health, KM 2; Ch. 15: Tribes, KM 1-3; Ch. 28: Adaptation, Introduction). Climate change threatens to exacerbate existing social and economic inequalities that result in higher exposure and sensitivity to extreme weather and climate -related events and other changes (Ch. 11: Urban, KM 1). Marginalized populations may also be affected disproportionately by actions to address the underlying causes and impacts of climate change, if they are not implemented under policies that consider existing inequalities (Ch. 11: Urban, KM 4; Ch. 28: Adaptation, KM 4). This report draws a direct connection between the warming atmosphere and the resulting changes that affect Americans' lives, commu- nities, and livelihoods, now and in the future. It documents vulnerabilities, risks, and impacts associated with natural climate variability and human -caused climate change across the Unit- ed States and provides examples of response actions underway in many communities. It concludes that the evidence of human -caused climate change is overwhelming and continues to strengthen, that the impacts of climate change are intensifying across the country, and that climate -related threats to Americans' physical, social, and economic well-being are rising. These impacts are projected to intensify —but how much they intensify will depend on actions taken to reduce global greenhouse gas emissions and to adapt to the risks from climate change now and in the coming decades (Ch. 28: Adaptation, Introduction; Ch. 29: Mitigation, KM 3 and 4). U.S. Global Change Research Program 36 Fourth National Climate Assessment 1 I Overview Our Changing Climate: Observations, Causes, and Future Change Observed Change Observations from around the world show the widespread effects of increasing greenhouse gas concentrations on Earth's climate. High temperature extremes and heavy precipitation events are increasing. Glaciers and snow cover are shrinking, and sea ice is retreating. Seas are warming, rising, and becoming more acidic, and marine species are moving to new locations toward cooler waters. Flooding is becoming more frequent along the U.S. coast- line. Growing seasons are lengthening, and wildfires are increasing. These and many other changes are clear signs of a warming world (Figure 1.2) (Ch. 2: Climate, Box 2.2; App. 3: Data & Scenarios, see also the USGCRP Indicators and EPA Indicators websites). California Drought Affects Mountain Snowpack California's recent multiyear drought left Tioga Pass in the Sierra Nevada mountain range nearly snowless at the height of winter in January 2015. Photo credit: Bartshe Miller. U.S. Global Change Research Program 37 Fourth National Climate Assessment 0 Heat Wave Season (days) 70 ....- 50 40 30 20 0 Change in Annual Average Temperature Change in Temperature (°F) -1 0 1 2 3 U.S. Heat Waves 10 o 1.1-11 _ . _tt _ 1960s 1970s 10 0 1910 1980s 1990s 2000s 2010s U.S. Heavy Precipitation 1930 1950 1970 1990 2010 Climate Change Indicators r Temperature OArctic Sea Ice Extent tOHeat Waves A A , A • A A • 1 A , 1 • • , 1 , • 1 • • , • • A , , A A , A • A A IA A A • A i A ♦ • A Hew Prec Wildfire Heating Degree Days Ark Growing ♦ `'"" Cooling Degree Days RP Season Length Change in Western U.S. Snowpack Percent change >80 • 60 to 80 go 40 to 60 • 20 to 40 f.. 011 CO t 8.. 6. 4.. t x cpa, 2 O C U.S. Drought Conditions - Annual average —9-year weighted average!: co 3.5 a) N x 3.0- Li] .°' a E 2.5 cal co 2.0 Arctic Sea Ice Extent 0 (-7, 10 a) ▪ 8 m m U.S. Sea Level 1 I Overview Atmosphere (a—c): (a) Annual average temperatures have increased by 1.8°F across the contiguous United States since the beginning of the 20th century; this figure shows observed change for 1986-2016 (relative to 1901-1960 for the contiguous United States and 1925-1960 for Alaska, Hawai'i, Puerto Rico, and the U.S. Virgin Islands). Alaska is warming faster than any other state and has warmed twice as fast as the global average since the mid-20th century (Ch. 2: Climate, KM 5: Ch. 26: Alaska. Background). (b) The season length of heat waves in many U.S. cities has increased by over 40 days since the 1960s. Hatched bars indicate partially complete decadal data. (c) The relative amount of annual rainfall that comes from large, single -day precipitation events has changed over the past century; since 1910, a larger percentage of land area in the contiguous United States receives precipitation in the form of these intense single -day events. Ice, snow, and water (d—f): (d) Large declines in snowpack in the western United States occurred from 1955 to 2016. (e) While there are a number of ways to measure drought, there is currently no detectable change in long-term U.S. drought statistics using the Palmer Drought Severity Index. (f) Since the early 1980s, the annual minimum sea ice extent (observed in September each year) in the Arctic Ocean has decreased at a rate of 11 %-16% per decade (Ch. 2: Climate, KM 7). Oceans and coasts (g—i): (g) Annual median sea level along the U.S. coast (with land motion removed) has increased by about 9 inches since the early 20th century as oceans have warmed and land ice has melted (Ch. 2: Climate, KM 4). (h) Fish, shellfish, and other marine species along the Northeast coast and in the eastern Bering Sea have, on average, moved northward and to greater depths toward cooler waters since the early 1980s (records start in 1982). (i) Oceans are also currently absorbing more than a quarter of the carbon dioxide emitted to the atmosphere annually by human activities, increasing their acidity (measured by lower pH values; Ch. 2: Climate, KM 3). Land and ecosystems (j—I): (j) The average length of the growing season has increased across the contiguous United States since the early 20th century, meaning that, on average, the last spring frost occurs earlier and the first fall frost arrives later; this map shows changes in growing season length at the state level from 1895 to 2016. (k) Warmer and drier conditions have contributed to an increase in large forest fires in the western United States and Interior Alaska over the past several decades (CSSR, Ch. 8.3). (I) Degree days are defined as the number of degrees by which the average daily temperature is higher than 65°F (cooling degree days) or lower than 65°F (heating degree days) and are used as a proxy for energy demands for cooling or heating buildings. Changes in temperatures indicate that heating needs have decreased and cooling needs have increased in the contiguous United States over the past century. Sources: (a) adapted from Vose et al. 2017, (b) EPA, (c—f and h—I) adapted from EPA 2016, (g and center infographic) EPA and NOAA. Causes of Change Scientists have understood the fundamental physics of climate change for almost 200 years. In the 1850s, researchers demonstrated that carbon dioxide and other naturally occurring greenhouse gases in the atmosphere prevent some of the heat radiating from Earth's surface from escaping to space: this is known as the greenhouse effect. This natural greenhouse effect warms the planet's surface about 60°F above what it would be otherwise, creating a habitat suitable for life. Since the late 19th century, however, humans have released an increasing amount of greenhouse gases into the atmosphere through burning fossil fuels and, to a lesser extent, deforestation and land -use change. As a result, the atmospheric concentration of carbon dioxide, the largest contributor to human -caused warming, has increased by about 40% over the industrial era. This change has intensified the natural greenhouse effect, driving an increase in global surface temperatures and other widespread changes in Earth's climate that are unprece- dented in the history of modern civilization. Global climate is also influenced by natural factors that determine how much of the sun's energy enters and leaves Earth's atmosphere and by natural climate cycles that affect temperatures and weather patterns in the short term, especially regionally (see Ch. 2: Climate, Box 2.1). However, the unambiguous long-term warming trend in global average temperature over the last century cannot be explained by natural factors alone. Greenhouse gas emissions from human activities are the U.S. Global Change Research Program 39 Fourth National Climate Assessment 1 Overview only factors that can account for the observed warming over the last century; there are no credible alternative human or natural explana- tions supported by the observational evidence. Without human activities, the influence of natural factors alone would actually have had a slight cooling effect on global climate over the last 50 years (Ch. 2: Climate, KM 1, Figure 2.1). Future Change Greenhouse gas emissions from human activities will continue to affect Earth's climate for decades and even centuries. Humans are adding carbon dioxide to the atmosphere at a rate far greater than it is removed by natural processes, creating a long-lived reservoir of the gas in the atmosphere and oceans that is driving the climate to a warmer and warmer state. Some of the other greenhouse gases released by human activities, such as methane, are removed from the atmosphere by natural processes more quickly than carbon dioxide; as a result, efforts to cut emissions of these gases could help reduce the rate of global tempera- ture increases over the next few decades. However, longer -term changes in climate will largely be determined by emissions and atmospheric concentrations of carbon dioxide and other longer -lived greenhouse gases (Ch. 2: Climate, KM 2). Climate models representing our understand- ing of historical and current climate conditions are often used to project how our world will change under future conditions (see Ch. 2: Cli- mate, Box 2.7). "Climate" is defined as weather conditions over multiple decades, and climate model projections are generally not designed to capture annual or even decadal variation in climate conditions. Instead, projections are typically used to capture Long-term changes, such as how the climate system will respond to changes in greenhouse gas levels over this century. Scientists test climate models by comparing them to current observations and historical changes. Confidence in these models is based, in part, on how well they reproduce these observed changes. Climate models have proven remarkably accurate in simulating the climate change we have experienced to date, particularly in the past 60 years or so when we have greater confidence in observations (see CSSR, Ch. 4.3,1). The observed signals of a changing climate continue to become stron- ger and clearer over time, giving scientists increased confidence in their findings even since the Third National Climate Assessment was released in 2014. Today, the largest uncertainty in projecting future climate conditions is the level of greenhouse gas emissions going forward. Future global greenhouse gas emissions levels and resulting impacts depend on economic, political, and demographic factors that can be difficult to predict with confidence far into the future. Like previous climate assessments, NCA4 relies on a suite of possible scenarios to evaluate the implications of different climate outcomes and associated impacts throughout the 21st century. These "Representative Con- centration Pathways" (RCPs) capture a range of potential greenhouse gas emissions pathways and associated atmospheric concentration levels through 2100. RCPs drive climate model projections for temperature, precipitation, sea level, and other variables under futures that have either lower or higher greenhouse gas emissions. RCPs are numbered according to changes in radiative forcing by 2100 relative to preindustrial condi- tions: +2.6, +4.5, +6.0, or +8.5 watts per square meter (W/m2). Each RCP leads to a different U.S. Global Change Research Program 40 Fourth National Climate Assessment 1 I Overview Box 1.1: Confidence and Uncertainty in Climate Science Many of the decisions we make every day are based on less -than -perfect knowledge. For example, while GPS-based applications on smartphones can provide a travel -time estimate for our daily drive to work, an unexpected factor like a sudden downpour or fender bender might mean a ride originally estimated to be 20 minutes could actually take longer. Fortunately, even with this uncertainty we are confident that our trip is unlikely to take Tess than 20 minutes or more than half an hour —and we know where we are headed. We have enough information to plan our commute. Uncertainty is also a part of science. A key goal of scientific research is to increase our confidence and reduce the uncertainty in our understanding of the world around us. Even so, there is no expectation that uncertainty can be fully eliminated, just as we do not expect a perfectly accurate estimate for our drive time each day. Studying Earth's climate system is particularly challenging because it integrates many aspects of a complex natural system as well as many human -made systems. Climate scientists find varying ranges of uncertainty in many areas, including observations of climate variables, the analysis and interpretation of those measurements, the development of new observational instruments, and the use of computer -based models of the processes governing Earth's climate system. While there is inherent uncertainty in climate science, there is high confidence in our understanding of the greenhouse effect and the knowledge that human activities are changing the climate in unprecedented ways. There is enough information to make decisions based on that understanding. Where important uncertainties do exist, efforts to quantify and report those uncertainties can help decision - makers plan for a range of possible future outcomes. These efforts also help scientists advance under- standing and ultimately increase confidence in and the usefulness of model projections. Assessments like this one explicitly address scientific uncertainty associated with findings and use specific language to express it to improve relevance to risk analysis and decision -making (see Front Matter and Box 1.2). level of projected global temperature change; higher numbers indicate greater projected temperature change and associated impacts. The higher scenario (RCP8.5) represents a future where annual greenhouse gas emissions increase significantly throughout the 21st century before leveling off by 2100, whereas the other RCPs represent more rapid and substantial mitigation by mid-century, with greater reductions thereafter. Current trends in annual greenhouse gas emissions, globally, are consistent with RCP8.5. Of the two RCPs predominantly referenced throughout this report, the lower sce- nario (RCP4.5) envisions about 85% lower greenhouse gas emissions than the higher scenario (RCP8.5) by the end of the 21st century (see Ch. 2: Climate, Figure 2.2). In some cases, throughout this report, a very low scenario (RCP2.6) that represents more imme- diate, substantial, and sustained emissions reductions is considered. Each RCP could be consistent with a range of underlying socio- economic conditions or policy choices. See the Scenario Products section of Appendix 3 in this report, as well as CSSR Chapters 4.2.1 and 10.2.1 for more detail. The effects of different future greenhouse gas emissions levels on global climate become most evident around 2050, when temperature U.S. Global Change Research Program 41 Fourth National Climate Assessment 1 I Overview Projected Changes in U.S. Annual Average Temperatures Mid-21st Century Lower Scenario (RCP4.5) Higher Scenario (RCP8.5) Late 21 st Century Lower Scenario (RCP4 5) Change in Temperature (°F) Higher Scenario (RCP8 5) Figure 1.3: Annual average temperatures across the United States are projected to increase over this century, with greater changes at higher latitudes as compared to lower latitudes, and under a higher scenario (RCP8.5; right) than under a lower one (RCP4.5; left). This figure shows projected differences in annual average temperatures for mid-century (2036-2065; top) and end of century (2071-2100; bottom) relative to the near present (1986-2015). From Figure 2.4, Ch. 2: Climate (Source: adapted from Vase et al. 2017). (Figure 1.3) (Ch. 2: Climate, Figure 2.2), pre- cipitation, and sea level rise (Figure 1.4) (Ch. 2: Climate, Figure 2.3) projections based on each scenario begin to diverge significantly. With substantial and sustained reductions in greenhouse gas emissions (e.g., consistent with the very low scenario [RCP2.6]), the increase in global annual average temperature relative to preindustrial times could be limited to less than 3.6°F (2°C) (Ch. 2: Climate, Box 2.4; CSSR, Ch. 4.2.1). Without significant greenhouse gas mitigation, the increase in global annual aver- age temperature could reach 9°F or more by the end of this century (Ch. 2: Climate, KM 2). For some aspects of Earth's climate system that take longer to respond to changes in atmo- spheric greenhouse gas concentrations, such as global sea level, some degree of long-term change will be locked in for centuries to come, regardless of the future scenario (see CSSR. Ch. 12.5.3). Early greenhouse gas emissions mitiga- tion can reduce climate impacts in the nearer term (such as reducing the loss of arctic sea ice and the effects on species that use it) and in the longer term by avoiding critical thresholds (such as marine ice sheet instability and the resulting consequences for global sea level and coastal development; Ch. 29: Mitigation, Timing and Magnitude of Action). Annual average temperatures in the United States are projected to continue to increase in the coming decades. Regardless of future scenario, additional increases in temperatures U.S. Global Change Research Program 42 Fourth National Climate Assessment 1 I Overview Projected Relative Sea Level Change in the United States by 2100 Lower Scenario (RCP4.5) Higher Scenario (RCP8.5) Relative Sea Level Change (feet) —6 —4 —2 0 2 4 6 Figure 1.4: The maps show projections of change in relative sea level along the U.S. coast by 2100 (as compared to 2000) under the lower (RCP4.5) and higher (RCP8.5) scenarios (see CSSR, Ch. 12.5). Globally, sea levels will continue to rise from thermal expansion of the ocean and melting of land -based ice masses (such as Greenland, Antarctica, and mountain glaciers). Regionally, however, the amount of sea level rise will not be the same everywhere. Where land is sinking (as along the Gulf of Mexico coastline), relative sea level rise will be higher, and where land is rising (as in parts of Alaska), relative sea level rise will be lower. Changes in ocean circulation (such as the Gulf Stream) and gravity effects due to ice melt will also alter the heights of the ocean regionally. Sea levels are expected to continue to rise along almost all U.S. coastlines, and by 2100, under the higher scenario, coastal flood heights that today cause major damages to infrastructure would become common during high tides nationwide (Ch. 8: Coastal: Scenario Products section in Appendix 3). Source: adapted from CSSR. Figure 12.4. across the contiguous United States of at least 2.3°F relative to 1986-2015 are expected by the middle of this century. As a result, recent record -setting hot years are expected to become common in the near future. By late this century, increases of 2.3°-6.7°F are expected under a lower scenario (RCP4.5) and 5.4°-11.0°F under a higher scenario (RCP8.5) relative to 1986-2015 (Figure 1.3) (Ch. 2: Climate, KM 5, Figure 2.4). Alaska has warmed twice as fast as the global average since the mid-2Oth century; this trend is expected to continue (Ch. 26: Alaska, Background). High temperature extremes, heavy precipitation events, high tide flooding events along the U.S. coastline, ocean acidification and warming, and forest fires in the western United States and Alaska are all projected to continue to increase, while land and sea ice cover, snowpack, and surface soil moisture are expected to continue to decline in the coming decades. These and other changes are expected to increasingly impact water resources, air quality, human health, agriculture, natural ecosystems, energy and transportation infrastructure, and many other natural and human systems that support communities across the country. The severity of these projected impacts, and the risks they present to society, is greater under futures with higher greenhouse gas emissions, especially if limited or no adaptation occurs (Ch. 29: Mitigation; KM 2). U.S. Global Change Research Program 43 Fourth National Climate Assessment 1 ( Overview Box 1.2: Evaluating Risks to Inform Decisions In this report, risks are often defined in a qualitative sense as threats to life, health and safety, the environ- ment, economic well-being, and other things of value to society (Ch. 28: Adaptation, Introduction). In some cases, risks are described in quantitative terms: estimates of how likely a given threat is to occur (probability) and the damages that would result if it did happen (consequences). Climate change is a risk management challenge for society; it presents uncertain —and potentially severe —consequences for natural and human systems across generations. It is characterized by multiple intersecting and uncertain future hazards and, therefore, acts as a risk multiplier that interacts with other stressors to create new risks or to alter existing ones (see Ch. 17: Complex Systems, KM 1). Current and future greenhouse gas emissions, and thus mitigation actions to reduce emissions, will largely determine future climate change impacts and risks to society. Mitigation and adaptation activities can be considered complementary strategies —mitigation efforts can reduce future risks, while adaptation can min- imize the consequences of changes that are already happening as a result of past and present greenhouse gas emissions. Adaptation entails proactive decision -making and investments by individuals, businesses, and governments to counter specific risks from climate change that vary from place to place. Climate risk man- agement includes some familiar attributes and tactics for most businesses and local governments, which often manage or design for a variety of weather -related risks, including coastal and inland storms, heat waves, threats to water availability, droughts, and floods. Measuring risk encompasses both likelihoods and consequences of specific outcomes and involves judg- ments about what is of value, ranking of priorities, and cost —benefit analyses that incorporate the tradeoffs among climate and non -climate related options. This report characterizes specific risks across regions and sectors in an effort to help people assess the risks they face, create and implement a response plan, and monitor and evaluate the efficacy of a given action (see Ch. 28: Adaptation, KM 1, Figure 28.1). Climate Change in the United States: Current and Future Risks Some climate -related impacts, such as increasing health risks from extreme heat, are common to many regions of the United States (Ch. 14: Human Health, KM 1). Others represent more localized risks, such as infrastructure damage caused by thawing of permafrost (long -frozen ground) in Alaska or threats to coral reef ecosystems from warmer and more acidic seas in the U.S. Caribbean, as well as Hawaii and the U.S.-Affiliated Pacific Islands (Ch. 26: Alaska, KM 2; Ch. 20: U.S. Caribbean, KM 2; Ch. 27: Hawai'i & Pacific Islands, KM 4). Risks vary by both a community's exposure to physical climate impacts and by factors that influence its ability to respond to changing conditions and to recover from adverse weath- er and climate -related events such as extreme storms or wildfires (Ch. 14: Human Health, KM 2; Ch. 15: Tribes, State of the Sector, KM 1 and 2: Ch. 28: Adaptation, KM 4). Many places are subject to more than one climate -related impact, such as extreme rain- fall combined with coastal flooding, or drought coupled with extreme heat, wildfire, and flooding. The compounding effects of these impacts result in increased risks to people, infrastructure, and interconnected economic sectors (Ch. 11: Urban, KM I). Impacts affecting U.S. Global Change Research Program 44 Fourth National Climate Assessment 1 Overview interconnected systems can cascade across sectors and regions, creating complex risks and management challenges. For example, changes in the frequency, intensity, extent, and duration of wildfires can result in a higher instance of landslides that disrupt transportation systems and the flow of goods and services within or across regions (Box 1.3). Many observed impacts reveal vulnerabilities in these inter- connected systems that are expected to be exacerbated as climate -related risks intensify. Under a higher scenario (RCP8.5), it is very likely that some impacts, such as the effects of ice sheet disintegration on sea level rise and coastal development, will be irreversible for many thousands of years, and others, such as species extinction, will be permanent (Ch. 7: Ecosystems, KM 1; Ch. 9: Oceans, KFfvt 1; Ch. 29: Mitigation, KM 2). Economy and Infrastructure Without more significant global greenhouse gas mitigation and regional adaptation efforts, climate change is expected to cause substan- tial losses to infrastructure and property and impede the rate of economic growth over this century (Ch. 4: Energy, KM 1; Ch. 8: Coastal, KM 1; Ch. 11: Urban, KM 2; Ch. 12: Transporta- tion, KM 1; Regional Chapters 18-27). Regional economies and industries that depend on natural resources and favorable climate conditions, such as agriculture, tourism, and fisheries, are increasingly vulnerable to impacts driven by climate change (Ch. 7: Ecosystems, KM 3; Ch. 10: Agriculture, KM 1). Reliable and affordable energy supplies, which underpin virtually every sector of the economy, are increasingly at risk from climate change and weather extremes (Ch. 4: Energy, Box 1.3: Interconnected Impacts of Climate Change The impacts of climate change and extreme weather on natural and built systems are often considered from the perspective of individual sectors: how does a changing climate impact water resources, the electric grid, or the food system? None of these sectors, however, exists in isolation. The natural, built, and social systems we rely on are all interconnected, and impacts and management choices within one sector may have cascad- ing effects on the others (Ch. 17: Complex Systems, KM 1). For example, wildfire trends in the western United States are influenced by rising temperatures and changing precipitation patterns, pest populations, and land management practices. As humans have moved closer to forestlands, increased fire suppression practices have reduced natural fires and led to denser vegetation, resulting in fires that are larger and more damaging when they do occur (Figures 1.5 and 1.2k) (Ch. 6: Forests, KM 1). Warmer winters have led to increased pest outbreaks and significant tree kills, with varying feedbacks on wildfire. Increased wildfire driven by climate change is projected to increase costs associated with health effects, loss of homes and other property, wildfire response, and fuel management. Failure to anticipate these interconnected impacts can lead to missed opportunities for effectively managing risks within a single sector and may actually increase risks to other sectors. Planning around wildfire risk and other risks affected by climate change entails the challenge of accounting for all of these influences and how they interact with one another (see Ch. 17: Complex Systems, Box 17.4). U.S. Global Change Research Program 45 Fourth National Climate Assessment 1 I Overview Box 1.3: Interconnected Impacts of Climate Change, continued New to this edition of the NCA, Chapter 17 (Complex Systems) highlights several examples of interconnect- ed impacts and documents how a multisector perspective and joint management of systems can enhance resilience to a changing climate. It is often difficult or impossible to quantify and predict how all relevant pro- cesses and interactions in interconnected systems will respond to climate change. Non -climate influences, such as population changes, add to the challenges of projecting future outcomes (Ch. 17: Complex Systems, M 2 . Despite these challenges, there are opportunities to learn from experience to guide future risk man- agement decisions. Valuable lessons can be learned retrospectively after Superstorm Sandy in 2012, for example, the mayor of New York City initiated a Climate Change Adaptation Task Force that brought together stakeholders from several sectors such as water, transportation, energy, and communications to address the interdependencies among them (Ch. 17: Complex Systems, Box 17,1, KM 3). Buck Fire. mow.. Redwood Comte • Sulphur Fire Tubbs Fire Nuns Fire Patrick Fire 37 Fire Atlas Fire Lobo Fire • McCourtney Fire ,.;Santa Rosa 'Sacramento Stockton San Francisco • San Jose • Fresno Lion Fire • Bakersfield Wildfire at the Wildland—Urban Interface Figure 1.5: Wildfires are increasingly encroaching on American communities, posing threats to lives, critical infrastructure, and property. In October 2017, more than a dozen fires burned through northem California, killing dozens of people and leaving thousands more homeless. Communities distant from the fires were affected by poor air quality as smoke plumes darkened skies and caused the cancellation of school and other activities across the region. (left) A NASA satellite image shows active fires on October 9. 2017. (right) The Tubbs Fire, which burned parts of Napa, Sonoma, and Lake counties, was the most destructive in California's history. It caused an estimated $1.2 billion in damages and destroyed over 5,000 structures, including 5% of the housing stock in the city of Santa Rosa. Image credits: (left) NASA; (right) Master Sgt. David Loeffler, U.S. Air National Guard. KM 1). The impacts of climate change beyond our borders are expected to increasingly affect our trade and economy, including import and export prices and U.S. businesses with overseas operation and supply chains (Box 1.4) (Ch. 16: International, KM 1; Ch. 17: Complex Systems, KM 1). Some aspects of our economy may see slight improvements in a modestly warmer world. However, the continued warming that is projected to occur without significant reductions in global greenhouse gas emissions is expected to cause substantial net damage to the U.S. economy, especially in the absence of increased adaptation efforts. The potential for losses in some sectors could reach hundreds of billions of dollars per year by the end of this century (Ch. 29: Mitigation, KM 2). Existing water, transportation, and energy infrastructure already face challenges from heavy rainfall, inland and coastal flooding, landslides, drought, wildfire, heat waves, and U.S. Global Change Research Program 46 Fourth National Climate Assessment 1 Overview other weather and climate events (Figures 1.5-1.9) (Ch. 11: Urban, KM 2; Ch. 12: Trans- portation, KM 1). Many extreme weather and climate -related events are expected to become more frequent and more intense in a warmer world, creating greater risks of infrastructure disruption and failure that can cascade across economic sectors (Ch. 3: Water, KM 2; Ch. 4: Energy, KM 1; Ch. 11: Urban, KM 3; Ch. 12: Transportation, KM 2). For example, more frequent and severe heat waves and other extreme events in many parts of the United States are expected to increase stresses on the energy system, amplifying the risk of more frequent and longer -lasting power outages and fuel shortages that could affect other critical sectors and systems, such as access to medical care (Ch. 17: Complex Systems, Box 17.5; Ch. 4: Energy, KM 1; Ch. 8: Coastal, KM 1; Ch. 11: Urban, KM 3; Ch. 12: Transportation, KM 3). Current infrastructure is typically designed for historical climate conditions (Ch. 12: Transpor- tation, KM 1) and development patterns —for instance, coastal land use —generally do not account for a changing climate (Ch. 5: Land Changes, State of the Sector), resulting in increasing vulnerability to future risks from weather extremes and climate change (Ch. 11: Urban, KM 2). Infrastructure age and dete- rioration make failure or interrupted service from extreme weather even more likely (Ch. 11: Urban, KM 2). Climate change is expected to increase the costs of maintaining, repairing, and replacing infrastructure, with differences across regions (Ch. 12: Transportation, Regional Summary). Recent extreme events demonstrate the vulnerabilities of interconnected economic sectors to increasing risks from climate change (see Box 1.3). In 2017, Hurricane Harvey dumped an unprecedented amount of rainfall over the greater Houston area, some of which has been attributed to human -induced climate change (Ch. 2: Climate, Box 2.5). Resulting power outages had cascading effects on critical infra- structure facilities such as hospitals and water and wastewater treatment plants. Reduced oil production and refining capacity in the Gulf of Mexico caused price spikes regionally and nationally from actual and anticipated gasoline shortages (Figure 1.6) (Ch. 17: Complex Systems, KM 1). In the U.S. Caribbean, Hurricanes Irma and Maria caused catastrophic damage to infrastructure, including the complete failure of Puerto Rico's power grid and the loss of power throughout the U.S. Virgin Islands, as well as extensive damage to the region's agri- cultural industry. The death toll in Puerto Rico grew in the three months following Maria's landfall on the island due in part to the lack of electricity and potable water as well as access to medical facilities and medical care (Ch. 20: U.S. Caribbean, Box 20.1, KM 5). Climate -related risks to infrastructure, prop- erty, and the economy vary across regions. Along the U.S. coastline, public infrastructure and $1 trillion in national wealth held in coastal real estate are threatened by rising sea levels, higher storm surges, and the ongoing increase in high tide flooding (Figures 1.4 and 1.8) (Ch. 8: Coastal, KM 1). Coastal infrastructure provides critical lifelines to the rest of the country, including energy supplies and access to goods and services from overseas trade; increased damage to coastal facilities is expected to result in cascading costs and national impacts (Ch. 8: Coastal, KM 1; Ch. 4: Energy, State of the Sector, KM 1). High tide flooding is projected to become more disruptive and costlier as its frequency, depth, and inland extent grow in the coming decades. Without significant adaptation measures, many coastal cities in the U.S. Global Change Research Program 47 Fourth National Climate Assessment 1 1 Overview Widespread Impacts from Hurricane Harvey Figure 1.6: Hurricane Harvey led to widespread flooding and knocked out power to 300,000 customers in Texas in 2017, with cascading effects on critical infrastructure facilities such as hospitals, water and wastewater treatment plants, and refineries. The photo shows Port Arthur, Texas, on August 31, 2017—six days after Hurricane Harvey made landfall along the Gulf Coast. From Figure 17.2, Ch. 17: Complex Systems (Photo credit: Staff Sgt. Daniel J. Martinez, U.S. Air National Guard). Flooding at Fort Calhoun Nuclear Power Plant Figure 1.7: Floodwaters from the Missouri River surround the Omaha Public Power District's Fort Calhoun Station, a nuclear power plant just north of Omaha, Nebraska, on June 20, 2011. The flooding was the result of runoff from near -record snowfall totals and record -setting rains in late May and early June. A protective berm holding back the floodwaters from the plant failed, which prompted plant operators to transfer offsite power to onsite emergency diesel generators. Cooling for the reactor temporarily shut down, but spent fuel pools were unaffected. From Figure 22.5, Ch. 22: N. Great Plains (Photo credit: Harry Weddington, U.S. Army Corps of Engineers). Norfolk Naval Base at Risk from Rising Seas Figure 1.8: Low-lying Norfolk, Virginia, houses the world's largest naval base, which supports multiple aircraft carrier groups and is the duty station for thousands of employees. Most of the area around the base lies less than 10 feet above sea level, and local relative sea level is projected to rise between about 2.5 and 11.5 feet by the year 2100 under the Lower and Upper Bound USGCRP sea level rise scenarios, respectively (see Scenario Products section of Appendix 3 for more details on these sea level rise scenarios; see also Ch. 8: Coastal, Case Study "Key Messages in Action —Norfolk, Virginia"). Photo credit: Mass Communication Specialist 1st Class Christopher B. Stoltz, U.S. Navy. U.S. Global Change Research Program 48 Fourth National Climate Assessment 1 I Overview Southeast are expected to experience daily high tide flooding by the end of the century (Ch. 8: Coastal, KM 1; Ch. 19: Southeast, KM 2). Higher sea levels will also cause storm surge from tropical storms to travel farther inland than in the past, impacting more coastal properties and infrastructure (Ch. 8: Coastal: KM 1; Ch. 19: Alaska Guam • • •• • a• • • • i 14 Southeast, KM 2). Oil, natural gas, and electrical infrastructure located along the coasts of the Atlantic Ocean and Gulf of Mexico are at increased risk of damage from rising sea levels and stronger hurricanes; regional disruptions are expected to have national implications (Ch. 4: Energy, State of the Sector, KM 1; Ch. Weather and Climate -Related Impacts on U.S. Military Assets s• •• • • •-•• • • rs • •► • =s • a• s_sob s • 416 • • 4 �•a *is Hawaii s • •_ • i !• • • Defense Assets with Multiple Climate -Related Vulnerabilities s• • • • • • • s• • • • • • ••• • • • 40. • • • • • • • • • • •• • • • • • • • • • • • • • • a •) 4 • • • •i • r • • • fe • • t• e • ._4. • • s • i• �•4 +i , s A,• •i- • t.•+ •' • •• is to 4/ •• • t• •• • • • • •, i • • s • • Puerto Rico 1.•,,54 and the U.S. Virgin Islands Figure 1.9: The Department of Defense (DoD) has significant experience in planning for and managing risk and uncertainty. The effects of climate and extreme weather represent additional risks to incorporate into the Department's various planning and risk management processes. To identify DoD installations with vulnerabilities to climate -related impacts, a preliminary Screening Level Vulnerability Assessment Survey (SLVAS) of DoD sites worldwide was conducted in 2015. The SLVAS responses (shown for the United States; orange dots) yielded a wide range of qualitative information. The highest number of reported effects resulted from drought (782), followed closely by wind (763) and non -storm surge related flooding (706). About 10% of sites indicated being affected by extreme temperatures (351), while flooding due to storm surge (225) and wildfire (210) affected about 6°/a of the sites reporting. The survey responses provide a preliminary qualitative picture of DoD assets currently affected by severe weather events as well as an indication of assets that may be affected by sea level rise in the future. Source: adapted from Department of Defense 2018 (http://www. oea.gov/resource/2018-climate-related-risk-dod-infrastructure-initial-vulnerability-assessment-survey-slvas). U.S. Global Change Research Program 49 Fourth National Climate Assessment 1 1 Overview 18: Northeast, KM 3; Ch. 19: Southeast, KM 2). Hawaii and the U.S.-Affiliated Pacific Islands and the U.S. Caribbean also face high risks to critical infrastructure from coastal flooding, erosion, and storm surge (Ch. 4: Energy, State of the Sector; Ch. 20: U.S. Caribbean, KM 3; Ch. 27: Hawaii & Pacific Islands, KM 3). In the western United States, increasing wildfire is damaging ranches and rangelands as well as property in cities near the wildland-urban interface. Drier conditions are projected to increase the risk of wildfires and damage to property and infrastructure, including energy production and generation assets and the power grid (Ch. 4: Energy, KM 1; Ch. 11: Urban, Regional Summary; Ch. 24: Northwest, KM 3). In Alaska, thawing of permafrost is responsible for severe damage to roads, buildings, and pipelines that will be costly to replace, especially in remote parts of Alaska. Alaska oil and gas operations are vulnerable to thawing permafrost, sea level rise, and increased coastal exposure due to declining sea ice; however, a longer ice -free season may enhance offshore energy operations and trans- port (Ch. 4: Energy, State of the Sector; Ch. 26: Alaska, KM 2 and 5). These impacts are expected to grow with continued warming. U.S. agriculture and the communities it sup- ports are threatened by increases in tempera- tures, drought, heavy precipitation events, and wildfire on rangelands (Figure 1.10) (Ch. 10: Ag & Rural, KM 1 and 2, Case Study "Groundwater Depletion in the Ogallala Aquifer Region"; Ch. 23: S. Great Plains, KM 1, Case Study "The Edwards Aquifer"). Yields of major U.S. crops (such as corn, soybeans, wheat, rice, sorghum, and cotton) are expected to decline over this century as a consequence of increases in temperatures and possibly changes in water availability and disease and pest outbreaks (Ch. Conservation Heavy Rains Figure 1.10: Increasing heavy rains are leading to more soil erosion and nutrient loss on midwestern cropland. Integrating strips of native prairie vegetation into row crops has been shown to reduce soil and nutrient loss while improving biodiversity. The inset shows a close-up example of a prairie vegetation strip. From Figure 21.2, Ch. 21: Midwest (Photo credits: [main photo] Lynn Betts; [inset] Farnaz Kordbacheh). Practices Reduce Impact of 10: Ag & Rural, KM 1). Increases in growing sea- son temperatures in the Midwest are projected to be the largest contributing factor to declines in U.S. agricultural productivity (Ch. 21: Mid- west, KM 1). Climate change is also expected to lead to large-scale shifts in the availability and prices of many agricultural products across the world, with corresponding impacts on U.S. agricultural producers and the U.S. economy (Ch. 16: International, KM 1). Extreme heat poses a significant risk to human health and labor productivity in the agricul- tural, construction, and other outdoor sectors (Ch. 10: Ag & Rural, KM 3). Under a higher scenario (RCP8.5), almost two billion labor hours are projected to be lost annually by 2090 from the impacts of temperature extremes, costing an estimated $160 billion in lost wages (Ch. 14: Human Health, KM 4). States within the Southeast (Ch. 19: Southeast, KM 4) and South- ern Great Plains (Ch. 23: S. Great Plains, KM 4) regions are projected to experience some of the greatest impacts (see Figure 1.21). U.S. Global Change Research Program 50 Fourth National Climate Assessment 1 I Overview Natural Environment and Ecosystem Services Climate change threatens many benefits that the natural environment provides to society: safe and reliable water supplies, clean air, protection from flooding and erosion, and the use of natural resources for economic, recreational, and subsistence activities. Valued aspects of regional heritage and quality of life tied to the natural environment, wildlife, and outdoor recreation will change with the climate, and as a result, future generations can expect to experience and interact with natural systems in ways that are much different than today. Without significant reductions in greenhouse gas emissions, extinctions and transformative impacts on some ecosystems cannot be avoided, with varying impacts on the economic, recreational, and subsistence activities they support. Changes affecting the quality, quantity, and availability of water resources, driven in part by climate change, impact people and the envi- ronment (Ch. 3: Water, KM 1). Dependable and safe water supplies for U.S. Caribbean, Hawaii, and U.S.-Affiliated Pacific Island communities and ecosystems are threatened by rising tem- peratures, sea level rise, saltwater intrusion, and increased risks of drought and flooding (Ch. 3: Water, Regional Summary; Ch. 20: U.S. Caribbean, KM 1; Ch. 27: Hawaii l\ Pacific Islands, KM 1). In the Midwest, the occurrence of conditions that contribute to harmful algal blooms, which can result in restrictions to water usage for drinking and recreation, is expected to increase (Ch. 3: Water, Regional Summary; Ch. 21: Midwest, KM 3). In the Southwest, water supplies for people and nature are decreasing during droughts due in part to climate change. Intensifying droughts, heavier downpours, and reduced snowpack are combining with other stressors such as groundwater depletion to reduce the future reliability of water supplies in the region, with cascading impacts on energy production and other water -dependent sectors (Ch. 3: Water, Regional Summary; Ch. 4: Energy, State of the Sector; Ch. 25: Southwest, KM 5). In the South- ern Great Plains, current drought and project- ed increases in drought length and severity threaten the availability of water for agriculture (Figures 1.11 and 1.12) (Ch. 23: S. Great Plains, KM 1). Reductions in mountain snowpack and shifts in snowmelt timing are expected to reduce hydropower production in the Southwest and the Northwest (Ch. 24: Northwest, KM 3; Ch. 25: Southwest, KM 5). Drought is expected to threaten oil and gas drilling and refining as well as thermoelectric power plants that rely on a steady supply of water for cooling (Ch. 4: Energy, State of the Sector, KM 1; Ch. 22: N. Great Plains, K.M 4; Ch. 23: S. Great Plains, KM 2; Ch. 25: Southwest, KM 5). Tourism, outdoor recreation, and subsis- tence activities are threatened by reduced snowpack, increases in wildfire activity, and Impacts of Drought on Texas Agriculture Figure 1.11: Soybeans in Texas experience the effects of drought in August 2013. During 2010-2015, a multiyear regional drought severely affected agriculture in the Southern Great Plains. One prominent impact was the reduction of irrigation water released for farmers on the Texas coastal plains. Photo credit: Bob Nichols, USDA. U.S. Global Change Research Program 51 Fourth National Climate Assessment 1 I Overview Desalination Plants Can Reduce Impacts from Drought in Texas • Dallas 0 • • • o• Austin San Antonioo • Laredo Corpus Chris • Desalination Plants County Boundaries ti 0 Cities Interstates Brownsville • Houston �• South Padre Island Figure 1.12: Desalination activities in Texas are an important contributor to the state's efforts to meet current and projected water needs for communities, industry, and agriculture. The state's 2017 Water Plan recommended an expansion of desalination to help reduce longer -term risks to water supplies from drought, higher temperatures, and other stressors. There are currently 44 public water supply desalination plants in Texas. From Figure 23.8, Ch. 23: S. Great Plains (Source: adapted from Texas Water Development Board 2017). other stressors affecting ecosystems and natural resources (Figures 1.2d, 1.2k, and 1.13) (Ch. 7: Ecosystems, KM 3). Increasing wildfire frequency (Ch. 19: Southeast, Case Study "Prescribed Fire"), pest and disease outbreaks (Ch. 21: Midwest, Case Study "Adaptation in Forestry"), and other stressors are projected to reduce the ability of U.S. forests to support recreation as well as economic and subsistence activities (Ch. 6: Forests, KM 1 and 2; Ch. 19: Southeast, KM 3; Ch. 21: Midwest, KM 2). Increases in wildfire smoke events driven by climate change are expected to reduce the amount and quality of time spent in outdoor activities (Ch. 13: Air Quality, KM 2; Ch. 24: Northwest, KM 4). Projected declines in snow - pack in the western United States and shifts to more precipitation falling as rain than snow in the cold season in many parts of the central and eastern United States are expected to adversely impact the winter recreation indus- try (Ch. 18: Northeast, KM 1; Ch. 22: N. Great Plains, KM 3; Ch. 24: Northwest, KM 1, Box 24.7). In the Northeast, activities that rely on natural snow and ice cover may not be economically viable by the end of the century without significant reductions in global greenhouse gas emissions (Ch. 18: Northeast, KM 1). Diminished U.S. Global Change Research Program 52 Fourth National Climate Assessment 1 I Overview Razor Clamming on the Washington Coast Figure 1.13: Razor clamming draws crowds on the coast of Washington State. This popular recreation activity is expected to decline due to ocean acidification, harmful algal blooms, warmer temperatures, and habitat degradation. From Figure 24.7, Ch. 24: Northwest (Photo courtesy of Vera Trainer, NOAA). snowpack, increased wildfire, pervasive drought, flooding, ocean acidification, and sea level rise directly threaten the viability of agriculture, fisheries, and forestry enterprises on tribal lands across the United States and impact tribal tourism and recreation sectors (Ch. 15: Tribes, KM 1). Climate change has already had observable impacts on biodiversity and ecosystems throughout the United States that are expected to continue. Many species are shifting their ranges (Figure 1.2h), and changes in the timing of important biological events (such as migration and reproduction) are occurring in response to climate change (Ch. 7: Ecosystems, KM 1). Climate change is also aiding the spread of invasive species (Ch. 21: Midwest, Case Study "Adaptation in Forestry"; Ch. 22: N. Great Plains, Case Study "Crow Nation and the Spread of Invasive Species"), recognized as a major driver of biodiversity loss and substantial ecological and economic costs globally (Ch. 7: Ecosystems, Invasive Species). As environ- mental conditions change further, mismatches between species and the availability of the resources they need to survive are expected to occur (Ch. 7: Ecosystems, KM 2). Without significant reductions in global greenhouse gas emissions, extinctions and transforma- tive impacts on some ecosystems cannot be avoided in the long term (Ch. 9: Oceans, KM 1). While some new opportunities may emerge from ecosystem changes, economic and recreational opportunities and cultural heritage based around historical use of species or natural resources in many areas are at risk (Ch. 7: Ecosystems, KM 3; Ch. 18: Northeast, KM 1 and 2, Box 18.6). Ocean warming and acidification pose high and growing risks for many marine organ- isms, and the impacts of climate change on ocean ecosystems are expected to lead to reductions in important ecosystem services such as aquaculture, fishery productivity, and recreational opportunities (Ch 9: Oceans, KM 2). While climate change impacts on ocean ecosystems are widespread, the scope of ecosystem impacts occurring in tropical and polar areas is greater than anywhere else in the world. Ocean warming is already leading to reductions in vulnerable coral reef and sea ice habitats that support the livelihoods of many communities (Ch. 9: Oceans, KM 1). Decreasing sea ice extent in the Arctic represents a direct loss of important habitat for marine mammals, causing declines in their populations (Figure 1.2f) (Ch. 26: Alaska, Box 26.1). Changes in spring ice melt have affected the ability of coastal communities in Alaska to meet their walrus harvest needs in recent years (Ch. 26: Alaska, KM 1). These changes are expected to continue as sea ice declines further (Ch. 2: Climate, KM 7). In the tropics, ocean warming has already led to widespread coral reef bleaching and/or outbreaks of coral diseases off the coastlines of Puerto Rico, the U.S. Virgin Islands, Florida, and U.S. Global Change Research Program 53 Fourth National Climate Assessment 1 Overview Severe Coral Bleaching Projected for Hawai`i and the U.S.-Affiliated Pacific Islands Commonwealth of the Northern Mariana Islands Rota Saipan r Tinian Aguijan 10 mi Guam 10 mi 2 / 3 f\ 4 5 Northwestern Hawaiian Islands cri 7 L! e 9 10 Kure Atoll 1 Midway Atoll2 Pearl and Hermes Atoll' Laysan Island 5 Lisianski Island Maro Reefs j Gardner Pinnacles 7 / French Frigate Shoals / Mokumanamana 9 % Nihoat0 N 330 feet depth 100 mi Kauai Main Hawaiian Islands `V Molokai exct Lanai Kaho`olawe Maui Hawaii American Samoa 10 mi Swains Rose 0fu Olosega 9-4 Ta`0 2030 2032 2034 2036 2038 2040 2042 2044 Figure 1.14: The figure shows the years when severe coral bleaching is projected to occur annually in the Hawai`i and U.S.- Affiliated Pacific Islands region under a higher scenario (RCP8.5). Darker colors indicate earlier projected onset of coral bleaching. Under projected warming of approximately 0.5°F per decade, all nearshore coral reefs in the region will experience annual bleaching before 2050. From Figure 27.10, Ch. 27: Hawaii & Pacific Islands (Source: NOAA). Hawaii and the U.S.-Affiliated Pacific Islands (Ch. 20: U.S. Caribbean, KM 2; Ch. 27: Ilawai`i & Pacific islands, KM 4). By mid-century, wide- spread coral bleaching is projected to occur annually in Hawaii and the U.S.-Affiliated Pacific Islands (Figure 1.14). Bleaching and ocean acidification are expected to result in loss of reef structure, leading to lower fisheries yields and loss of coastal protection and hab- itat, with impacts on tourism and livelihoods U.S. Global Change Research Program 54 Fourth National Climate Assessment 1 Overview in both regions (Ch. 20: L.S. Caribbean, KM 2; Ch. 27: Hawaii & Pacific Islands, KM 4). While some targeted response actions are underway (Figure 1.15), many impacts, including losses of unique coral reef and sea ice ecosystems, can only be avoided by significantly reducing global greenhouse gas emissions, particularly carbon dioxide (Ch. 9: Oceans, KM 1). Human Health and Well -Being Higher temperatures, increasing air quality risks, more frequent and intense extreme weather and climate -related events, increases in coastal flooding, disruption of ecosystem services, and other changes increasingly threaten the health and well-being of the American people, particularly populations that are already vulnerable. Future climate change is expected to further disrupt many areas of life, exacerbating existing challenges and revealing new risks to health and prosperity. Rising temperatures pose a number of threats to human health and quality of life (Figure 1.16). High temperatures in the summer are linked directly to an increased risk of illness and death, particularly among older adults, preg- nant women, and children (Ch. 18: Northeast. Box 18.3). With continued warming, cold -re- lated deaths are projected to decrease and Promoting Coral Reef Recovery Figure 1.15: Examples of coral farming in the U.S. Caribbean and Florida demonstrate different types of structures used for growing fragments from branching corals. Coral farming is a strategy meant to improve the reef community and ecosystem function, including for fish species. The U.S. Caribbean Islands, Florida, Hawaii, and the U.S.-Affiliated Pacific Islands face similar threats from coral bleaching and mortality due to warming ocean surface waters and ocean acidification. Degradation of coral reefs is expected to negatively affect fisheries and the economies that depend on them as habitat is lost in both regions. While coral farming may provide some targeted recovery, current knowledge and efforts are not nearly advanced enough to compensate for projected losses from bleaching and acidification. From Figure 20.11, Ch. 20: U.S. Caribbean (Photo credits: [top left] Carlos Pacheco, U.S. Fish and Wildlife Service; [bottom left] NOAA; [right] Florida Fish and Wildlife). U.S. Global Change Research Program 55 Fourth National Climate Assessment 1 Overview 180- 160 140 ti 0 100- 80 60 Projected Change in Very Hot Days by 2100 in Phoenix, Arizona Annual Number of Days Above 100°F -40- Higher Scenario (RCP8.5) Lower Scenario (RCP4.5) -♦-• Observed Observed (1976-2005) Late 21 st Century (2070-2099) Hydration Stations and Cooling Refuges rrO - + Hydration Station Cooling Refuge Roads Is U.S. Census Urban Area Greater Phoenix Area Figure 1.16: (left) The chart shows the average annual number of days above 100°F in Phoenix, Arizona, for 1976-2005, and projections of the average number of days per year above 100°F through the end of the 21st century (2070-2099) under the lower (RCP4.5) and higher (RCP8.5) scenarios. Dashed lines represent the 5th-95th percentile range of annual observed values. Solid lines represent the 5th-95th percentile range of projected model values. (right) The map shows hydration stations and cooling refuges (cooled indoor locations that provide water and refuge from the heat during the day) in Phoenix in August 2017. Such response measures for high heat events are expected to be needed at greater scales in the coming years if the adverse health effects of more frequent and severe heat waves are to be minimized. Sources: (left) NOAA NCEI, CICS-NC, and LMI; (right) adapted from Southwest Cities Heat Refuges (a project by Arizona State University's Resilient Infrastructure Lab), available at http://www. coolme. today/#phoenix. Data provided by Andrew Fraser and Mikhail Chester, Arizona State University. heat -related deaths are projected to increase. In most regions, the increases in heat -related deaths are expected to outpace the reductions in cold -related deaths (Ch. 14: Human Health, KM 1). Rising temperatures are expected to reduce electricity generation capacity while increasing energy demands and costs, which can in turn lead to power outages and blackouts (Ch. 4: Energy, KM 1; Ch. 11: Urban, Regional Summary, Figure 11.2). These changes strain household budgets, increase people's exposure to heat, and limit delivery of medical and social services. Risks from heat stress are higher for people without access to housing with sufficient insulation or air conditioning (Ch. 11: Urban, KM 1). Changes in temperature and precipitation can increase air quality risks from wildfire and ground -level ozone (smog). Projected increases in wildfire activity due to climate change would further degrade air quality, resulting in increased health risks and impacts on quality of life (Ch. 13: Air Quality, KM 2; Ch. 14: Human Health, KM 1). Unless counteracting efforts to improve air quality are implemented, climate change is expected to worsen ozone pollution across much of the country, with adverse impacts on human health (Figure 1.21) (Ch, 13: Air Quality, KM 1). Earlier spring arrival, warm- er temperatures, changes in precipitation, and higher carbon dioxide concentrations can also increase exposure to airborne pollen allergens. The frequency and severity of allergic illnesses, including asthma and hay fever, are expected to increase as a result of a changing climate (Ch. 13: Air Quality, KM 3). Rising air and water temperatures and changes in extreme weather and climate -related events are expected to increase exposure to waterborne and foodborne diseases, affecting U.S. Global Change Research Program 56 Fourth National Climate Assessment 1 1 Overview food and water safety. The geographic range and distribution of disease -carrying insects and pests are projected to shift as climate changes, which could expose more people in North America to ticks that carry Lyme disease and mosquitoes that transmit viruses such as West Nile, chikungunya, dengue, and Zika (Ch. 14: Human Health, KM 1; Ch. 16: Inter- national, KM 4). Mental health consequences can result from exposure to climate- or extreme weather - related events, some of which are projected to intensify as warming continues (Ch. 14: Human Health, KM 1). Coastal city flooding as a result of sea level rise and hurricanes, for example, can result in forced evacuation, with adverse effects on family and commu- nity stability as well as mental and physical health (Ch. 11: Urban; KM I). In urban areas, disruptions in food supply or safety related to extreme weather or climate -related events are expected to disproportionately impact those who already experience food insecurity (Ch. 11: Urban, KM 3). Indigenous peoples have historical and cultural relationships with ancestral lands, ecosystems, and culturally important species that are threatened by climate change (Ch. 15: Tribes, KM 1; Ch. 19: Southeast, KM 4, Case Study "Mountain Ramps"; Ch. 24: Northwest, KM 5). Climate change is expected to compound existing physical health issues in Indigenous communities, in part due to the loss of tradi- tional foods and practices, and in some cases, the mental stress from permanent community displacement (Ch. 14: Human Health, KM 2; Ch. 15: Tribes, KM 2). Throughout the United States, Indigenous peoples are considering or actively pursuing relocation as an adaptation strategy in response to climate -related disasters, more frequent flooding, loss of land due to erosion, or as livelihoods are compro- mised by ecosystem shifts linked to climate change (Ch. 15: Tribes, KM 3). In Louisiana, a federal grant is being used to relocate the tribal community of Isle de Jean Charles in response to severe land loss, sea level rise, and coastal flooding (Figure 1.17) (Ch. 19: Southeast, KM 2, Case Study "A Lesson Learned for Community Resettlement"). In Alaska, coastal Community Relocation —Isle de Jean Charles, Louisiana Figure 1.17: (left) A federal grant is being used to relocate the tribal community of Isle de Jean Charles, Louisiana, in response to severe land loss, sea level rise, and coastal flooding. From Figure 15.3, Ch. 15: Tribes (Photo credit: Ronald Stine). (right) As part of the resettlement of the tribal community of Isle de Jean Charles, residents are working with the Lowlander Center and the State of Louisiana to finalize a plan that reflects the desires of the community. From Figure 15.4, Ch. 15: Tribes (Photo provided by Louisiana Office of Community Development). U.S. Global Change Research Program 57 Fourth National Climate Assessment 1 I Overview Adaptation Measures in Kivalina, Alaska Figure 1.18: A rock revetment was installed in the Alaska Native Village of Kivalina in 2010 to reduce increasing risks from erosion. A new rock revetment wall has a projected lifespan of 15 to 20 years. From Figure 15.3, Ch. 15: Tribes (Photo credit: ShoreZone. Creative Commons License CC BY 3.0: https://creativecommons.org/licenses/by/3.0/tegalcode). The inset shows a close-up of the rock wall in 2011. Photo credit: U.S. Army Corps of Engineers —Alaska District. Native communities are already experiencing heightened erosion driven by declining sea ice, rising sea levels, and warmer waters (Figure 1.18). Coastal and river erosion and flooding in some cases will require parts of communities, or even entire communities, to relocate to safer terrain (Ch. 26: Alaska, KM 2). Combined with other stressors, sea level rise, coastal storms, and the deterioration of coral reef and mangrove ecosystems put the long-term habitability of coral atolls in the Hawaii and U.S.-Affiliated Pacific Islands region at risk, introducing issues of sovereignty, human and national security, and equity (Ch. 27: Hawaii & Pacific islands, IKM 6). Reducing the Risks of Climate Change Climate change is projected to significantly affect human health, the economy, and the environment in the United States, particularly in futures with high greenhouse gas emissions and limited or no adaptation. Recent findings reinforce the fact that without substantial and sustained reductions in greenhouse gas emis- sions and regional adaptation efforts, there will be substantial and far-reaching changes over the course of the 21st century with negative consequences for a large majority of sectors, particularly towards the end of the century. The impacts and costs of climate change are already being felt in the United States, and changes in the likelihood or severity of some recent extreme weather events can now be U.S. Global Change Research Program 58 Fourth National Climate Assessment 1 I Overview Box 1.4: How Climate Change Around the World Affects the United States The impacts of changing weather and climate patterns beyond U.S. international borders affect those living in the United States, often in complex ways that can generate both challenges and opportunities. The Inter- national chapter (Ch, 6), new to this edition of the NCA, assesses our current understanding of how global climate change, natural variability, and associated extremes are expected to impact —and in some cases are already impacting—U.S. interests both within and outside of our borders. Current and projected climate -related impacts on our economy include increased risks to overseas operations of U.S. businesses, disruption of international supply chains, and shifts in the availability and prices of com- modities. For example, severe flooding in Thailand in 2011 disrupted the supply chains for U.S. electronics manufacturers t C. 36: International, Figure 16.1). U.S. firms are increasingly responding to climate -related risks, including through their financial disclosures and partnerships with environmental groups (Ch. 16: Inter- national, KM 1). Impacts from climate -related events can also undermine U.S. investments in international development by slowing or reversing social and economic progress in developing countries, weakening foreign markets for U.S. exports, and increasing the need for humanitarian assistance and disaster relief efforts. Predictive tools can help vulnerable countries anticipate natural disasters, such as drought, and manage their impacts. For example, the United States and international partners created the Famine Early Warning Systems Network (FEWS NET), which helped avoid severe food shortages in Ethiopia during a historic drought in 2015 (Ch. 16: internationa, KM 2). Natural variability and changes in climate increase risks to our national security by affecting factors that can exacerbate conflict and displacement outside of U.S. borders, such as food and water insecurity and com- modity price shocks. More directly, our national security is impacted by damage to U.S. military assets such as roads, runways, and waterfront infrastructure from extreme weather and climate -related events (Figures 1.8 and 1.9). The U.S. military is working to both fully understand these threats and incorporate projected climate changes into Tong -term planning. For example, the Department of Defense has performed a com- prehensive scenario -driven examination of climate risks from sea level rise to all of its coastal military sites, including atolls in the Pacific Ocean (Ch. 16: International, KM 3). Finally, the impacts of climate change are already affecting the ecosystems that span our Nation's borders and the communities that rely on them. International frameworks for the management of our shared resourc- es continue to be restructured to incorporate risks from these impacts. For example, a joint commission that implements water treaties between the United States and Mexico is exploring adaptive water management strategies that account for the effects of climate change and natural variability on Colorado River water (Ch 16: International, KM 4). attributed with increasingly higher confidence to human -caused warming (see CSSR, Ch. 3). Impacts associated with human health, such as premature deaths due to extreme tempera- tures and poor air quality, are some of the most substantial (Ch. 13: Air Quality, KM 1; Ch. 14: Human Health, KM 1 and 4; Ch 29: Mitigation, KM 2). While many sectors face large economic risks from climate change, other impacts can have significant implications for societal or cultural resources. Further, some impacts will very likely be irreversible for thousands of O.S. Global Change Research Program 59 Fourth National Climate Assessment 1 Overview years, including those to species, such as corals (Ch. 9: Oceans, KM 1; Ch. 27: Hawaii 8 Pacific Islands, KM 4), or that involve the crossing of thresholds, such as the effects of ice sheet disintegration on accelerated sea level rise, leading to widespread effects on coastal development lasting thousands of years (Ch. 29: Mitigation, KM 2). Future impacts and risks from climate change are directly tied to decisions made in the present, both in terms of mitigation to reduce emissions of greenhouse gases (or remove carbon dioxide from the atmosphere) and adaptation to reduce risks from today's changed climate conditions and prepare for future impacts. Mitigation and adaptation activities can be considered complementary strategies —mitigation efforts can reduce future risks, while adaptation actions can minimize the consequences of changes that are already happening as a result of past and present greenhouse gas emissions. Many climate change impacts and economic damages in the United States can be substan- tially reduced through global -scale reductions in greenhouse gas emissions complemented by regional and local adaptation efforts (Ch 29: Mitigation, KM 4). Our understanding of the magnitude and timing of risks that can be avoided varies by sector, region, and assump- tions about how adaptation measures change the exposure and vulnerability of people, live- lihoods, ecosystems, and infrastructure. Acting sooner rather than later generally results in lower costs overall for both adaptation and mitigation efforts and can offer other benefits in the near term (Ch. 29: Mitigation, KM 3). Since the Third National Climate Assessment (NCA3) in 2014, a growing number of states, cities, and businesses have pursued or expanded upon initiatives aimed at reducing greenhouse gas emissions, and the scale of adaptation implementation across the country has increased. However, these efforts do not yet approach the scale needed to avoid sub- stantial damages to the economy, environment, and human health expected over the coming decades (Ch. 28: Adaptation, KM 1; Ch. 29: Mitigation, KM 1 and 2). Mitigation Many activities within the public and private sectors aim for or have the effect of reducing greenhouse gas emissions, such as the increas- ing use of natural gas in place of coal or the expansion of wind and solar energy to generate electricity. Fossil fuel combustion accounts for approximately 85% of total U.S. greenhouse gas emissions, with agriculture, land -cover change, industrial processes, and methane from fossil fuel extraction and processing as well as from waste (including landfills, wastewater treat- ment, and composting) accounting for most of the remainder. A number of efforts exist at the federal level to promote low -carbon energy technologies and to increase soil and forest carbon storage. State, local, and tribal government approaches to mitigating greenhouse gas emissions include comprehensive emissions reduction strategies as well as sector- and technology -specific policies (see Figure 1.19). Since NCA3, private companies have increasingly reported their greenhouse gas emissions, announced emissions reductions targets, implemented actions to achieve those targets, and, in some cases, even put an internal price on carbon. Individuals and other organizations are also making choices every day to reduce their carbon footprints. U.S. Global Change Research Program 60 Fourth National Climate Assessment 1 1 Overview (a) • (b) Mitigation -Related Activities at State and Local Levels 0 Cities Supporting Emissions Reductions (455) Total Number of State -Level Mitigation -Related Activities (out of 30) 1-4 5-7 8-12 13-18 19-25 Total State -Level Mitigation -Related Activities by Type GHG Target / Cap / Pricing Renewable / CCS / Nuclear Transportation Energy Efficiency Non-0O2 Greenhouse Gases Forestry and Land Use 0 50 100 Number of Activities 150 Figure 1.19: (a) The map shows the number of mitigation -related activities at the state level (out of 30 illustrative activities) as well as cities supporting emissions reductions; (b) the chart depicts the type and number of activities by state. Several territories also have a variety of mitigation -related activities, including American Samoa, the Federated States of Micronesia, Guam, Northern Mariana Islands, Puerto Rico, and the U.S. Virgin Islands. From Figure 29.1, Ch. 29: Mitigation (Sources: [a] EPA and ERT, Inc. [b] adapted from America's Pledge 2017). U.S. Global Change Research Program 61 Fourth National Climate Assessment 1 I Overview Market forces and technological change, particularly within the electric power sector, have contributed to a decline in U.S. green- house gas emissions over the past decade. In 2016, U.S. emissions were at their lowest levels since 1994. Power sector emissions were 25% below 2005 levels in 2016, the largest emissions reduction for a sector of the American economy over this time. This decline was in large part due to increases in natural gas and renewable energy generation, as well as enhanced energy efficiency standards and programs (Ch. 4: Energy, KM 2). Given these advances in electricity generation, trans- mission, and distribution, the largest annual sectoral emissions in the United States now come from transportation. As of the writing of this report, business -as -usual (as in, no new policies) projections of U.S. carbon dioxide and other greenhouse gas emissions show flat or declining trajectories over the next decade with a central estimate of about 15% to 20% reduction below 2005 levels by 2025 (Ch. 29: -litigation, KM 1). Recent studies suggest that some of the indi- rect effects of mitigation actions could signifi- cantly reduce —or possibly even completely off- set —the potential costs associated with cutting greenhouse gas emissions. Beyond reduction of climate pollutants, there are many benefits, often immediate, associated with greenhouse gas emissions reductions, such as improving air quality and public health, reducing crop damages from ozone, and increasing energy independence and security through increased reliance on domestic sources of energy (Ch. 13: Air Quality, KM 4; Ch. 29: Mitigation, KM 4). Adaptation Many types of adaptation actions exist, includ- ing changes to business operations, hardening infrastructure against extreme weather, and adjustments to natural resource management strategies. Achieving the benefits of adaptation can require upfront investments to achieve longer -term savings, engaging with different stakeholder interests and values, and planning under uncertainty. In many sectors, adaptation can reduce the cost of climate impacts by more than half (Ch. 28: Adaptation, KM 4; Ch. 29: Mitigation, KM 4). At the time of NCA3's release in 2014, its authors found that risk assessment and plan- ning were underway throughout the United States but that on -the -ground implementation was limited. Since then, the scale and scope of adaptation implementation has increased, including by federal, state, tribal, and local agencies as well as business, academic, and nonprofit organizations (Figure 1.20). While the level of implementation is now higher, it is not yet common nor uniform across the United States, and the scale of implementation for some effects and locations is often considered inadequate to deal with the projected scale of climate change risks. Communities have gener- ally focused on actions that address risks from current climate variability and recent extreme events, such as making buildings and other assets incrementally less sensitive to climate impacts. Fewer communities have focused on actions to address the anticipated scale of future change and emergent threats, such as reducing exposure by preventing building in high -risk locations or retreating from at -risk coastal areas (Ch. 28: Adaptation, KM 1). Many adaptation initiatives can generate economic and social benefits in excess of their costs in both the near and long term (Ch. 28: Adaptation, KM 4). Damages to infrastructure, such as road and rail networks, are particularly U.S. Global Change Research Program 62 Fourth National Climate Assessment 1 I Overview Five Adaptation Stages and Progress Awareness Monitoring and Evaluation Leadership, Partnerships, Stakeholder Engagement Implementation Planning Assessment Figure 1.20: Adaptation entails a continuing risk management process. With this approach, individuals and organizations become aware of and assess risks and vulnerabilities from climate and other drivers of change, take actions to reduce those risks, and learn over time. The gray arced lines compare the current status of implementing this process with the status reported by the Third National Climate Assessment in 2014; darker color indicates more activity. From Figure 28.1, Ch. 28: Adaptation (Source: adapted from National Research Council, 2010. Used with permission from the National Academies Press, © 2010, National Academy of Sciences. Image credits, clockwise from top: National Weather Service; USGS; Armando Rodriguez, Miami -Dade County; Dr. Neil Berg, MARISA; Bill Ingalls, NASA). sensitive to adaptation assumptions, with proactive measures that account for future climate risks estimated to be capable of reducing damages by large fractions. More than half of damages to coastal property are estimated to be avoidable through adaptation measures such as shoreline protection and beach replenishment (Ch. 29: Mitigation, KM 4). Considerable guidance is available on actions whose benefits exceed their costs in some sectors (such as adaptation responses to storms and rising seas in coastal zones, to riverine and extreme precipitation flooding, and for agriculture at the farm level), but less so on other actions (such as those aimed at addressing risks to health, biodiversity, and ecosystems services) that may provide signif- icant benefits but are not as well understood (Ch. 28: Adaptation, KM 4). Effective adaptation can also enhance social welfare in many ways that can be difficult to quantify, including improving economic opportunity, health, equity, national security, U.S. Global Change Research Program 63 Fourth National Climate Assessment 1 I Overview education, social connectivity, and sense of place, while safeguarding cultural resources and enhancing environmental quality. Aggre- gating these benefits into a single monetary value is not always the best approach, and more fundamentally, communities may value benefits differently. Considering various outcomes separately in risk management processes can facilitate participatory planning processes and allow for a specific focus on equity. Prioritizing adaptation actions for populations that face higher risks from climate change, including low-income and marginal- ized communities, may prove more equitable and lead, for instance, to improved infrastruc- ture in their communities and increased focus on efforts to promote community resilience that can improve their capacity to prepare for, respond to, and recover from disasters (Ch. 28: Adaptation, KM 4). A significant portion of climate risk can be addressed by integrating climate adaptation into existing investments, policies, and practic- es. Integration of climate adaptation into deci- sion processes has begun in many areas includ- ing financial risk reporting, capital investment planning, engineering standards, military planning, and disaster risk management. A growing number of jurisdictions address cli- mate risk in their land -use, hazard mitigation, capital improvement, and transportation plans, and a small number of cities explicitly link their coastal and hazard mitigation plans using analysis of future climate risks. However, over the course of this century and especially under a higher scenario (RCP8.5), reducing the risks of climate change may require more significant changes to policy and regulations at all scales, community planning, economic and financial systems, technology applications, and ecosys- tems (Ch. 28: Adaptation, KM 5). Some sectors are already taking actions that go beyond integrating climate risk into current practices. Faced with substantial climate - induced changes in the future, including new invasive species and shifting ranges for native species, ecosystem managers have already begun to adopt new approaches such as assisted migration and development of wildlife corridors (Ch. 7: Ecosystems, KM 2). Many mil- lions of Americans live in coastal areas threat- ened by sea level rise; in all but the very lowest sea level rise projections, retreat will become an unavoidable option in some areas along the U.S. coastline (Ch. 8: Coastal, KM 1). The Federal Government has granted funds for the relocation of some communities, including the Biloxi-Chitimacha-Choctaw Tribe from Isle de Jean Charles in Louisiana (Figure 1.17). However, the potential need for millions of people and billions of dollars of coastal infrastructure to be relocated in the future creates challenging legal, financial, and equity issues that have not yet been addressed (Ch. 28: Adaptation, KM 5). In some areas, lack of historical or current data to inform policy decisions can be a limitation to assessments of vulnerabilities and/or effective adaptation planning. For this National Climate Assessment, this was particularly the case for some aspects of the Alaska, U.S. Caribbean, and Hawaii and U.S.-Affiliated Pacific Islands regions. In many instances, relying on Indig- enous knowledges is among the only current means of reconstructing what has happened in the past. To help communities across the United States learn from one another in their efforts to build resilience to a changing climate, this report highlights common climate -related risks and possible response actions across all regions and sectors. U.S. Global Change Research Program 64 Fourth National Climate Assessment 1 I Overview What Has Happened Since the Last National Climate Assessment? Our understanding of and experience with climate science, impacts, risks, and adaptation in the United States have grown significantly sincethe Third National Climate Assessment (NCA3), advancing our knowledge of key processes in the earth system, how human and natural forces are changing them, what the implications are for society, and how we can respond. Key Scientific Advances Detection and Attribution: Significant advances have been made in the attribution of the human influence for individual climate and weather extreme events (see CSSR. Chs. 3, ti.. Extreme Events and Atmospheric Circulation: How climate change may affect specific types of extreme events in the United States and the extent to which atmospheric circula- tion in the midlatitudes is changing or is projected to change, possibly in ways not captured by current climate models, are important areas of research where scientific understanding has advanced (see. C ;S Chs =?. Localized Information: As computing resources have grown, projections of future climate from global models are now being conducted at finer scales (with resolution on the order of 15 miles), providing more realistic characterization of intense weather systems, including hurricanes. For the first time in the NCA process, sea level rise projections incorporate geographic variation based on factors such as local land subsidence, ocean currents, and changes in Earth's gravitational field (see CSSR, Chs. 9 and 12). Ocean and Coastal Waters: Ocean acidification, warming, and oxygen loss are all increas- ing, and scientific understanding of the severity of their impacts is growing. Both oxygen loss and acidification may be magnified in some U.S. coastal waters relative to the global average, raising the risk of serious ecological and economic consequences (see C.SSR, C1s._...,_1:31. Rapid Changes for Ice on Earth: New observations from many different sources confirm that ice loss across the globe is continuing and, in many cases, accelerating. Since NCA3, Antarctica and Greenland have continued to lose ice mass, with mounting evidence that mass loss is accelerating. Observations continue to show declines in the volume of U.S. Global Change Research Program 65 Fourth National Climate Assessment 1 I Overview mountain glaciers around the world. Annual September minimum sea ice extent in the Arctic Ocean has decreased at a rate of 11%-16% per decade since the early 1980s, with accelerating ice loss since 2000. The annual sea ice extent minimum for 2016 was the second lowest on record; the sea ice minimums in 2014 and 2015 were also among the lowest on record (see CSSR, Chs. 1. 11, and 12). Potential Surprises: Both large-scale shifts in the climate system (sometimes called "tip- ping points") and compound extremes have the potential to generate outcomes that are difficult to anticipate and may have high consequences. The more the climate changes, the greater the potential for these surprises (see CSSR, Ch. 15). Extreme Events Climate change is altering the characteristics of many extreme weather and climate -related events. Some extreme events have already become more frequent, intense, widespread, or of longer duration, and many are expected to continue to increase or worsen, presenting substantial challenges for built, agricultural, and natural systems. Some storm types such as hurricanes, tornadoes, and winter storms are also exhibiting changes that have been linked to climate change, although the current state of the science does not yet permit detailed understanding (see CSSR, Executive Summar\). Individual extreme weather and climate - related events —even those that have not been clearly attributed to climate change by scientific analyses —reveal risks to society and vulnerabilities that mirror those we expect in a warmer world. Non -climate stressors (such as land -use changes and shifting demograph- ics) can also amplify the damages associated with extreme events. The National Oceanic and Atmospheric Administration estimates that the United States has experienced 44 billion -dollar weather and climate disasters since 2015 (through April 6, 2018), incurring costs of nearly $400 billion (https://Ltiww.ncdc.noaa.gm ). Hurricanes: The 2017 Atlantic Hurricane season alone is estimated to have caused more than $250 billion in damages and over 250 deaths throughout the U.S. Caribbean, Southeast, and Southern Great Plains. More than 30 inches of rain fell during Hurricane Harvey, affecting 6.9 million people. Hurricane Maria's high winds caused widespread devastation to Puerto Rico's transportation, agriculture, communication, and energy infra- structure. Extreme rainfall of up to 37 inches caused widespread flooding and mudslides across the island. The interruption to commerce and standard living conditions will be sustained for a long period while much of Puerto Rico's infrastructure is rebuilt. Hurricane Irma destroyed 25% of buildings in the Florida Keys. U.S. Global Change Research Program 66 Fourth National Climate Assessment 1 1 Overview Damage from Hurricane Maria in San Juan, Puerto Rico Photo taken during a reconnaissance flight of the island on September 23, 2017. Photo credit: Sgt. Jose Ahiram Diaz - Ramos, Puerto Rico National Guard. Floods: In August 2016, a historic flood resulting from 20 to 30 inches of rainfall over sev- eral days devastated a large area of southern Louisiana, causing over $10 billion in damages and 13 deaths. More than 30,000 people were rescued from floodwaters that damaged or destroyed more than 50,000 homes, 100,000 vehicles, and 20,000 businesses. In June 2016, torrential rainfall caused destructive flooding throughout many West Virginia towns, damaging thousands of homes and businesses and causing considerable loss of life. More than 1,500 roads and bridges were damaged or destroyed. The 2015-2016 El Nino poured 11 days of record -setting rainfall on Hawai`i, causing severe urban flooding. Drought: In 2015, drought conditions caused about $5 billion in damages across the South- west and Northwest, as well as parts of the Northern Great Plains. California experienced the most severe drought conditions. Hundreds of thousands of acres of farmland remained fallow, and excess groundwater pumping was required to irrigate existing agricultural interests. Two years later, in 2017, extreme drought caused $2.5 billion in agricultural damages across the Northern Great Plains. Field crops, including wheat, were severely damaged, and the lack of feed for cattle forced ranchers to sell off livestock. Wildfires: During the summer of 2015, over 10.1 million acres —an area larger than the entire state of Maryland —burned across the United States, surpassing 2006 for the highest U.S. Global Change Research Program 67 Fourth National Climate Assessment 1 1 Overview The Deadly Carr Fire The Carr Fire (as seen over Shasta County, California, on August 4, 2018) damaged or destroyed more than 1,500 structures and resulted in several fatalities. Photo credit: Sgt. Lani O. Pascual, U.S. Army National Guard. annual total of U.S. acreage burned since record keeping began in 1960. These wildfire conditions were exacerbated by the preceding drought conditions in several states. The most extensive wildfires occurred in Alaska, where 5 million acres burned within the state. In Montana, wildfires burned in excess of 1 million acres. The costliest wildfires occurred in California, where more than 2,500 structures were destroyed by the Valley and Butte Fires; insured losses alone exceeded $1 billion. In October 2017, a historic firestorm damaged or destroyed more than 15,000 homes, businesses, and other structures across California (see Figure 1.5). The Tubbs, Atlas, Nuns, and Redwood Valley Fires caused a total of 44 deaths, and their combined destruction represents the costliest wildfire event on record. Tornadoes: In March 2017, a severe tornado outbreak caused damage across much of the Midwest and into the Northeast. Nearly 1 million customers lost power in Michigan alone due to sustained high winds, which affected several states from Illinois to New York. Heat Waves: Honolulu experienced 24 days of record -setting heat during the 2015-2016 El Nino event. As a result, the local energy utility issued emergency public service announce- ments to curtail escalating air conditioning use that threatened the electrical grid. U.S. Global Change Research Program 68 Fourth National Climate Assessment 1 I Overview New Aspects of This Report Hundreds of states, counties, cities, businesses, universities, and other entities are implementing actions that build resilience to climate -related impacts and risks, while also aiming to reduce greenhouse gas emissions. Many of these actions have been informed by new climate -related tools and products developed through the U.S. Global Change Research Program (USGCRP) since NCA3 (see Appendix 3: Scenario Products and Data Tnnok); we briefly highlight a few of them here. In addition, several structural changes have been introduced to the report and new methods used in response to stakeholder needs for more localized information and to address key gaps identified in NCA3. The Third National Climate Assessment remains a valuable and relevant resource —this report expands upon our knowledge and experience as presented four years ago. Climate Science Special Report: Early in the development of NCA4, experts and Adminis- tration officials recognized that conducting a comprehensive physical science assessment (Volume I) in advance of an impacts assessment (Volume II) would allow one to inform the other. The Climate Science Special Report, released in November 2017, is Volume I of NCA4 and represents the most thorough and up-to-date assessment of climate science in the United States and underpins the findings of this report; its findings are summarized in Chapter 2 (Our Changing Climate). See the "Key Scientific Advances" section in this box and Box 2.3 in Chapter 2 for more detail. Scenario Products: As described in more detail in Appen- dix 3 (Data Tools & Scenario Products), federal interagency groups developed a suite of high -resolution scenario products that span a range of plausible future changes in key environmental variables through at Least 2100. These USGCRP scenario products help ensure consistency across the report and improve the ability to synthesize across chapters. Where possible, authors have used these scenario products to frame uncertainty in future climate as it relates to the risks that are the focus of their chapters. In addition, the Indicators Interagency Work- ing Group has developed an Indicators platform that uses observations or calculations to monitor conditions or trends in the earth system, just as businesses might use the unem- ployment index as an indicator of economic conditions (see Figure 1.2 and https://www. globalchange.ov/browse/indicators). VI Fesearch�Progrlol am CLIMATE SCIENCE Fourth National Climate A,sossmert I volume I U.S. Global Change Research Program 69 Fourth National Climate Assessment 1 I Overview Localized Information: With the increased focus on local and regional information in NCA4, USGCRP agencies developed two additional products that not only inform this assessment but can serve as valuable decision -support tools. The first are the State Cli- mate Summaries —a peer -reviewed collection of climate change information covering all ten NCA4 regions at the state level. In addition to standard data on observed and projected climate change, each State Climate Summary contains state -specific changes and their related impacts as well as a suite of complementary graphics (stateclimatesummaries. globalchange.gov). The second product is the U.S. Climate Resilience Toolkit (https:// toolkit.climate.gov/), which offers data -driven tools, infoi illation, and subject -matter expertise from across the Federal Government in one easy -to -use location, so Americans are better able to understand the climate -related risks and opportunities impacting their communities and can make more informed decisions on how to respond. In particular, the case studies showcase examples of climate change impacts and accompanying response actions that complement those presented in Figure 1.1 and allow communities to learn how to build resilience from one another. New Chapters: In response to public feedback on NCA3 and input solicited in the early stages of this assessment, a number of significant structural changes have been made. Most fundamentally, the balance of the report's focus has shifted from national -level chapters to regional chapters in response to a growing desire for more localized infor- mation on impacts. Building on this theme, the Great Plains chapter has been split into Northern and Southern chapters (Chapters 22 and 23) along the Kansas -Nebraska border. In addition, the U.S. Caribbean is now featured as a separate region in this report (Chapter 20), focusing on the unique impacts, risks, and response capabilities in Puerto Rico and the U.S. Virgin Islands. Public input also requested greater international context in the report, which has been addressed through two new additions. A new chapter focuses on topics including the effects of climate change on U.S. trade and businesses, national security, and U.S. humani- tarian assistance and disaster relief (Chapter 16). A new international appendix (Appendix 4) presents a number of illustrative examples of how other countries have conducted national climate assessments, putting our own effort into a global context. Given recent scientific advances, some emerging topics warranted a more visible platform in NCA4. A new chapter on Air Quality (Chapter 13) examines how traditional air pollutants are affected by climate change. A new chapter on Sector Interactions, Multiple Stressors, and Complex Systems (Chapter 17) evaluates climate -related risks to interconnected human and natural systems that are increasingly vulnerable to cascading impacts and highlights advances in analyzing how these systems will interact with and respond to a changing environment (see Box 1.3). U.S. Global Change Research Program 70 Fourth National Climate Assessment 1 I Overview Integrating Economics: This report, to a much greater degree than previous National Climate Assessments, includes broader and more systematic quantification of climate change impacts in economic terms. While this is an emerging body of literature that is not yet reflected in each of the 10 NCA regions, it represents a valuable advancement in our understanding of the financial costs and benefits of climate change impacts. Figure 1.21 provides an illustration of the type of economic information that is integrated throughout this report. It shows the financial damages avoided under a lower scenario (RCP4.5) versus a higher scenario (RCP8.5). New Economic Impact Studies Billions of Dollars 200 150 100 50 Annual Economic Damages in 2090 RCP8 5 RCP4 5 Labor Projected annual cost savings due to global mitigation RCP8.5 RCP4.5 Air Quality Labor Change in Hours Worked (%) -5 -4 -3 -2 -1 0 Air Quality Change in Daily 8-hour Ozone Concentrations (ppb) -3 -2 -1 -0.5 0.5 1 2 3 4 5 Figure 1.21: Annual economic impact estimates are shown for labor and air quality. The bar graph on the left shows national annual damages in 2090 (in billions of 2015 dollars) for a higher scenario (RCP8.5) and lower scenario (RCP4.5); the difference between the height of the RCP8.5 and RCP4.5 bars for a given category represents an estimate of the economic benefit to the United States from global mitigation action. For these two categories, damage estimates do not consider costs or benefits of new adaptation actions to reduce impacts, and they do not include Alaska, Hawai'i and U.S.-Affiliated Pacific Islands, or the U.S. Caribbean. The maps on the right show regional variation in annual impacts projected under the higher scenario (RCP8.5) in 2090. The map on the top shows the percent change in hours worked in high -risk industries as compared to the period 2003-2007. The hours lost result in economic damages: for example, $28 billion per year in the Southern Great Plains. The map on the bottom is the change in summer -average maximum daily 8-hour ozone concentrations (ppb) at ground -level as compared to the period 1995-2005. These changes in ozone concentrations result in premature deaths: for example, an additional 910 premature deaths each year in the Midwest. Source: EPA, 2017. Multi -Model Framework for Quantitative Sectoral Impacts Analysis: A Technical Report for the Fourth National Climate Assessment. U.S. Environmental Protection Agency.. EPA 430-R-17-001. U.S. Global Change Research Program 71 Fourth National Climate Assessment WAY POLICIES 2018 STATE SCORECARD O ACEEE Table 8.2017 net incremental electricity savings by state State 2017 net incremental savings (MWh) % of 2016 retail Score sales (7 pts.) Vermont State 2017 net incremental savings (MWh) %of 2016 retail Score sales (7 pts.) 183,722 3.33% 7 Pennsylvania Rhode Island 797,448 0.55% 1.5 232,032 3.08% 7 New Jersey Massachusetts 1,374,066 2.57% 7 Californiat 5,062,747 1.97% 6.5 Connecticut 469,822 1.62% 5.5 Michigan 413,344 0.55% 1.5 New Mexico 120,404 0.52% 1.5 Montanat 71,689 0.51% 1.5 Kentuckyt 311,552 0.42% 1 1,545,158 1.48% 5 Oklahoma Hawaiit 254,425 0.41% 1 136,847 1.45% 5 Indiana*t Washingtont 1,195,606 1.35% 4.5 Illinois 1,885,000 1.34% 4.5 Arizonat 1,040,031 1.33% 4.5 Minnesotat 868,973 1.31% 4.5 Oregont 424,127 0.41% 1 South Carolina*t 304,919 0.38% 1 Wyomingt 46,274 0.28% 0.5 Nebraskat 75,953 0.25% 0.5 South Dakota 29,937 0.25% 0.5 574,167 1.21% 4 Georgiat New York 328,147 0.24% 0.5 1,722,962 1.17% 4 West Virginia Maryland 69,770 0.22% 0.5 594,234 0.97% 3 Mississippit Idahot 222,307 0.96% 3 Oh iot 99,873 0.20% 0.5 Texast 800,893 0.20% 0.5 1,448,198 0.96% 3 Tennessee* Colorado 189,930 0.19% 0.5 483,500 0.88% 3 Delaware lowat 421,963 0.87% 2.5 Mainet 97,322 0.85% 2.5 Uta h 254,907 0.84% 2.5 Missouri 615,564 0.78% 2.5 District of Columbia 85,613 0.75% 2.5 New Hampshiret 12,564 0.11% 0 Virginia*t 99,557 0.09% 0 Floridat 207,106 0.09% 0 Alabama* 49,988 0.06% 0 Louisiana 45,514 0.05% 0 North Dakota*t 1,761 0.01% 0 77,740 0.71% 2 Alaska* Arkansas 346 0.01% 0 319,788 0.69% 2 Kansas*t North Carolinat 440 0.00% 0 928,922 0.69% 2 US total Wisconsin 27,274,908 0.72% 460,743 0.66% 2 Median Nevadat 217,014 0.60% 2 254,907 0.6696 Savings data are from public service commission staff as listed in Appendix A, unless noted otherwise. Sales data are from EIA Form 861M (2017b). * States for which we did not have 2017 savings data were scored on 2016 state -reported savings or EIA-reported 2016 savings. t At least a portion of savings reported as gross. We adjusted the gross portion by a net -to -gross factor of 0.856 to make it comparable with net savings figures reported by other states. 28