The case for endangerment In 2009, the U.S. Environmental Protection Agency (EPA) established the so-called “Endangerment Finding.” This defined a suite of six long-lived greenhouse gases as “air pollution.” Such air pollution was anticipated to represent a danger to the health and welfare of current and future generations. Thus, the EPA has the authority to regulate these gases under the rules of the U.S. Clean Air Act. Duffy et al. provide a comprehensive review of the scientific evidence gathered in the years since then. These findings further support and strengthen the basis of the Endangerment Finding. Thus, a compelling case has been made even more compelling with an enormous body of additional data. Science, this issue p. eaat5982

Structured Abstract BACKGROUND The Clean Air Act requires the Environmental Protection Agency (EPA) to regulate air pollutants when the EPA Administrator finds that they “cause, or contribute to, air pollution which may reasonably be anticipated to endanger public health or welfare.” In Massachusetts v. EPA, the U.S. Supreme Court held that the EPA has the authority to regulate greenhouse gases (GHGs) under the Clean Air Act and that the EPA may not refuse to regulate once it has made a finding of endangerment. In December 2009, the EPA released its “Endangerment and Cause or Contribute Findings for Greenhouse Gases under Section 202(a) of the Clean Air Act,” known informally as the Endangerment Finding (EF). The EF found that six long-lived GHGs, in combination, should be defined as “air pollution” under the Clean Air Act and may reasonably be anticipated to endanger the health and welfare of current and future generations. The EF is an essential element of the legal basis for regulating GHG emissions under the Clean Air Act. It provides foundational support for important aspects of U.S. climate policy, including vehicle mileage standards for cars and light trucks and the emissions standards for electricity generation known as the “Clean Power Plan.” The EF was rooted in careful evaluation of observed and projected effects of GHGs, with assessments from the U.S. Global Change Research Program, the Intergovernmental Panel on Climate Change, and the U.S. National Research Council providing primary evidence. The EF was clear that, although many aspects of climate change were still uncertain, the evidence available in 2009 was strong. Since the original EF, scientific information about the causes, historical impacts, and future risks of climate change has continued to accumulate. This Review assesses that new information in the context of the EF. ADVANCES The EF was structured around knowledge related to public health and public welfare, with a primary focus on impacts in the United States. The information on public welfare was grouped into sections on air quality; food production and agriculture; forestry; water resources; sea level rise and coastal areas; energy, infrastructure, and settlements; and ecosystems and wildlife. In this Review, we assess new evidence in the impact areas addressed in the EF, as well as emergent areas that were not addressed in the EF but in which there have been important advances in understanding the risks of climate change. For each area, we characterize changes since the EF in terms of the strength of evidence for a link with anthropogenic climate change, the severity of observed and projected impacts, and the risk of additional categories of impact beyond those considered in the EF. For each of the areas addressed in the EF, the amount, diversity, and sophistication of the evidence has increased markedly, clearly strengthening the case for endangerment (see Fig. 1 in the full article). New evidence about the extent, severity, and interconnectedness of impacts detected to date and projected for the future reinforces the case that climate change endangers the health and welfare of current and future generations. For the sectors analyzed in the 2009 EF, new evidence expands the range of case studies, deepens the understanding of mechanisms, and analyzes the contribution of climate change to particular types of extreme events. In many cases, new evidence points to the risk of impacts that are more severe or widespread than those anticipated in 2009. Further, several categories of climate change impacts, including effects on ocean acidification, violence, national security, and economic well-being, are now supported by such broad evidence that they warrant inclusion in the framing of endangerment. OUTLOOK The EPA Administrator found in 2009 that the EF for six long-lived GHGs was “compellingly” supported by “strong and clear” scientific evidence. Our review of evidence published since the EF shows that the case for endangerment, which was already overwhelming in 2009, is even more strongly justified in 2018. New evidence relevant to the EF. New evidence strengthens the link with anthropogenic climate change (category 1); suggests more severe observed and/or projected impacts (category 2); or identifies new types of risks beyond those considered in the EF (category 3). Examples discussed in this Review include, for category 1, wildfire (left); for category 2, coastal flooding (center); and for category 3, ocean acidification (right). PHOTO (FROM LEFT TO RIGHT): NASA; W. MCNAMEE; L. DEGUIA.

Abstract We assess scientific evidence that has emerged since the U.S. Environmental Protection Agency’s 2009 Endangerment Finding for six well-mixed greenhouse gases and find that this new evidence lends increased support to the conclusion that these gases pose a danger to public health and welfare. Newly available evidence about a wide range of observed and projected impacts strengthens the association between the risk of some of these impacts and anthropogenic climate change, indicates that some impacts or combinations of impacts have the potential to be more severe than previously understood, and identifies substantial risk of additional impacts through processes and pathways not considered in the Endangerment Finding.

The Clean Air Act (CAA) requires the U.S. Environmental Protection Agency (EPA) to regulate air pollutants when the EPA Administrator finds that they “cause, or contribute to, air pollution which may reasonably be anticipated to endanger public health or welfare” (1). In Massachusetts v. EPA, the U.S. Supreme Court held that the EPA has the authority to regulate greenhouse gases (GHGs) under the CAA and that the EPA may not refuse to regulate these pollutants once it has made a finding of endangerment (2). In this decision, the Supreme Court characterized an endangerment finding on GHGs as a “scientific judgment” about “whether greenhouse gas emissions contribute to climate change.”

The courts have long held that the CAA embraces a precautionary approach to findings of endangerment. For example, the federal court of appeals in Washington, DC, has held that “evidence of potential harm as well as actual harm” meets the endangerment threshold and that the EPA’s degree of certitude may be lower where the hazards are most grave (3). Moreover, public health and welfare are broad concepts under the act, encompassing not only human morbidity and mortality but also effects on soils, water, crops, vegetation, animals, wildlife, weather, and climate (4).

In December 2009, the EPA released its “Endangerment and Cause or Contribute Findings for Greenhouse Gases under Section 202(a) of the Clean Air Act,” known informally as the Endangerment Finding (EF). The EF found that six long-lived GHGs, in combination, should be defined as “air pollution” under the CAA and may reasonably be anticipated to endanger the health and welfare of current and future generations. In addition, the EPA explained that “it is fully reasonable and rational to expect that events occurring outside our borders can affect the U.S. population” (5).

The EF is an essential element of the legal basis for regulating GHG emissions under the CAA. It provides foundational support for important aspects of U.S. climate policy, including vehicle mileage standards for cars and light trucks and the emissions standards for fossil fuel–fired electric utility generating units (the “Clean Power Plan”).

As the DC Circuit held in affirming the EF, the EPA may not decline to find endangerment on the basis of the perceived effectiveness or ineffectiveness of the regulations that may follow in the wake of an endangerment finding or on the basis of predictions about the potential for societal adaptation to climate change (6). The DC Circuit held that arguments to the contrary were “foreclosed by the language of the [Clean Air Act] and the Supreme Court’s decision in Massachusetts v. EPA.” The court also rejected the argument that the EPA must find that the air pollutants it regulates are the dominant source of the harms it identifies, as the act provides that the pollutants being regulated need only contribute to (or, under some provisions of the statute, “significantly” contribute to) (7) harmful air pollution.

The EF was rooted in careful evaluation of the observed and projected effects of GHGs, with assessments from the U.S. Global Change Research Program, the Intergovernmental Panel on Climate Change (IPCC), and the U.S. National Research Council providing primary scientific evidence. The EF was clear that, although many aspects of climate change were still uncertain, the evidence available in 2009 strongly supported the finding. Since the original EF, scientific information about the causes, historical impacts, and future risks of climate change has continued to accumulate. This Review assesses that new information in the context of the EF. We find that the case for endangerment, which was already overwhelming in 2009, is even stronger now.

The EF was structured around knowledge related to public health and public welfare, with a primary focus on effects in the United States. The information on public welfare was grouped into sections on air quality; food production and agriculture; forestry; water resources; sea level rise (SLR) and coastal areas; energy, infrastructure, and settlements; and ecosystems and wildlife. We follow that organization here. In addition, some of the most important advances in understanding the risks of climate change involve sectors or impact types not highlighted in the EF. We summarize the evidence for four of these that are broadly important: ocean acidification, violence and social instability, national security, and economic well-being. We characterize changes since the EF in terms of the strength of the evidence for a link with anthropogenic climate change, the potential severity of observed and projected impacts, and the risks of additional kinds of impacts beyond those considered in the EF (Fig. 1).

Fig. 1 New evidence since the EF. The columns summarize changes in the amount and implications of new evidence since the EF for each of the impact areas discussed in the EF and four additional impact areas where evidence of climate sensitivity has matured since the EF. An upward-pointing arrow indicates increasing evidence of endangerment. A downward-pointing arrow would indicate decreasing evidence of endangerment. A plain red arrow indicates that the new evidence is abundant and robust. An outlined arrow indicates that the new evidence, in addition, comes from multiple approaches, is derived from independent lines of information, or builds on a new level of mechanistic understanding. The left column refers to confidence in the impacts discussed in the EF. The middle column refers to impact areas that are discussed in the EF but where new evidence points to specific impacts that are fundamentally more severe or pervasive than those discussed in the EF. The right column refers to types of impacts not discussed in the EF.

Our focus is on the evidence for endangerment rather than the potential for adaptation. Although evidence that a risk might be reduced by some future action is certainly relevant for developing an effective portfolio of responses, the DC Circuit has affirmed that such evidence does not change the core question of whether long-lived GHGs endanger public health and welfare (6). In addition, adaptation options are often limited or impose economic costs that reduce adoption (8). Even ambitious adaptation rarely eliminates risk. For 32 specific risks evaluated by the IPCC in its recent special report, the potential for adaptation was assessed as low or very low for 25% of risks at a warming of 1.5°C and 53% of risks at 2°C (9).

One area of scientific progress since the EF is the attribution of extreme weather events (and some of their consequences) to human-caused climate change. This includes observed effects on human health and security, agriculture, and ecosystems (see below), as well as the probability and/or intensity of specific extreme weather events (10, 11). For extreme event attribution in North America, this includes more than 70% of recent record-setting hot, warm, and wet events and ~50% of record-setting dry spells (12), along with the recent California drought (13, 14), the storm-surge flooding during Superstorm Sandy (15) and Hurricane Katrina (16), and heavy precipitation during Hurricane Harvey (17–19). Although the realization of risk is not required for a finding of endangerment, cases where extreme events can be confidently attributed to historical emissions reinforce the understanding that we are already seeing impacts and the risks they bring.

Public health Since the EF, numerous scientific reports, reviews, and assessments have strengthened our understanding of the global health threats posed by climate change [e.g., (20, 21)] (Fig. 1, left column). New evidence validates and deepens the understanding of threats, including increased exposure to extreme heat, reduced air quality, more frequent and/or intense natural hazards, and increased exposure to infectious diseases and aeroallergens. New evidence also highlights additional health-related threats not discussed in the EF, including reduced nutritional security, effects on mental health, and increased risk of population displacement and conflict (Fig. 1, right column). Extreme heat is the most direct health impact (Fig. 2). With future warming, >200 U.S. cities face increased risk of aggregated premature mortality (22). In addition, extreme heat is linked to rising incidence of sleep loss (23), kidney stones (24), low birth weight (25), violence (26), and suicide (27) (Fig. 1, middle column). Fig. 2 The frequency of years from 2080 to 2099 of the RCP8.5 scenario in which the June-July-August (JJA) seasonal temperature equals or exceeds the warmest JJA value in the period from 1986 to 2005. [Adapted from (282)] New studies also strengthen evidence for health impacts via increased exposure to ozone and other air pollutants (28), including smoke from forest fires (29). Likewise, evidence for links among climate change, extreme weather, and climate-related disasters is growing rapidly (30). These events often lead to physical trauma, reduced air quality, infectious disease outbreaks, interruption of health service delivery, undernutrition, and both acute and chronic mental health effects (31). Changes in temperature, precipitation, and soil moisture are also altering habitats, life cycles, and feeding behaviors of vectors for most vector-borne diseases (32), with recent research documenting changes in exposure to malaria (33), dengue (34), West Nile virus (35), and Lyme disease (36), among others. Recent work also reinforces the evidence that increased outbreaks of waterborne (37) and foodborne (38) illnesses are likely to follow increasing temperatures and extreme precipitation. Likewise, recent research reinforces the conclusion that rising temperatures and carbon dioxide (CO 2 ) levels will increase pollen production and lengthen the pollen season for many allergenic plants (39, 40), leading to increased allergic respiratory disease (41). One area of new understanding not covered in the EF is threats to global nutrition. Staple crops grown at 550 parts of CO 2 per million have lower amounts of zinc, iron, and protein than the same cultivars grown at ambient CO 2 (42). These nutrient losses could push hundreds of millions of people into deficiencies of zinc (43), protein (44), and iron (45), in addition to aggravating existing deficiencies in more than one billion people. These effects on nutritional quality exacerbate the impacts of climate change on agricultural yield, discussed below. Together, these effects underscore a substantial headwind in assuring access to nutritious diets for the global population (46). Mental health impacts represent another area of new understanding (47). In particular, increased exposure to climate and weather disasters is associated with posttraumatic stress, anxiety, depression, and suicide (27, 48). Lastly, climate change is increasingly understood to function as a threat magnifier, raising the risk of population displacement and armed conflict (discussed below), which can also amplify risks to human health.

Ocean acidification The removal of anthropogenic CO 2 emissions by air-sea gas exchange and chemical dissolution into the ocean alters the acid-base chemistry of the ocean. Since the EF, scientific understanding of this process and of its possible negative effects on marine life has improved (Fig. 1, right column). Excess CO 2 gas in the ocean reacts with water, resulting in a series of chemical changes that include reductions in pH, carbonate ion (CO 3 2−) concentrations, and the saturation state for carbonate minerals used by many organisms to construct shells and skeletons (209). Such chemical changes are now well documented in the upper ocean. Acidification in coastal waters can be exacerbated by local pollution sources (210). Over the next several decades, trends in near-surface acidification are likely to closely track atmospheric CO 2 trends (211), with acidification hot spots in coastal upwelling systems, the Arctic, and the Southern Ocean (212, 213). Evidence since the EF reveals a wide range of biological responses to elevated CO 2 and ocean acidification (Fig. 1, right column). For all marine species, the effects of current and future ocean acidification must be framed in the context of a rapidly changing ocean environment with multiple human-driven stressors, particularly ocean warming (214). Warming is reducing open-ocean oxygen levels and exacerbating coastal hypoxia driven by excess nutrients (215), the same nutrient pollution that also causes estuarine and coastal acidification. Model and data syntheses indicate that acidification may shift reef systems to net dissolution during the 21st century (216). Coral bleaching from ocean warming is already having negative consequences for biologically rich coral reef ecosystems that provide food, income, and other valuable ecosystem services to >500 million people around the world (217), and the combined effects of warming and acidification are expected to worsen in the future (207). Different kinds of organisms vary substantially in their responses to acidification, with generally negative effects for many mollusks and some plankton to neutral and even positive effects for other species (218). Lower seawater carbonate saturation states reduce calcification and may restrict the geographic habitat for planktonic pteropods (219) that are prey for many fish, marine mammals, and seabirds. Many shellfish, and perhaps some kinds of crustaceans, are vulnerable to acidification, especially in larval and juvenile stages, with possible repercussions for valuable U.S. and international fisheries (220, 221) (Fig. 1, right column). During the mid-2000s, low-pH waters associated with coastal upwelling led to reduced larval survival of Pacific oysters in some U.S. Pacific Northwest shellfish hatcheries, a problem that has been largely addressable so far through adaptive strategies (222). Wild-harvest fisheries may be more at risk, particularly in regions with combined social and ecological vulnerability (223). Less is known about acidification responses in fish, with most studies indicating weak or no effects on growth and reproduction. However, a number of studies report negative effects on fish olfaction and behavior (224). Taken as a whole, acidification will likely exacerbate many of the climate warming effects on marine ecosystems, including shifting species ranges, degrading coral reefs, and expanding low-oxygen zones.

Violence and social instability Since the EF, a number of studies have used historical data to explore whether changes in environmental conditions influence the risk of violence or instability (225). In general, high temperatures and rainfall extremes amplify underlying risks (26) (Fig. 1, right column). These effects are not uniform (226). Many factors, including political institutions (227), income levels (228), and local economic structures (229), play a role in determining the structure of these effects. A robust and generalizable finding is an increased risk of threatening and violent interactions between individuals under hot conditions (Fig. 1, right column). In the United States, exposure to high temperatures is associated with higher rates of domestic violence (230), rape, assault, and murder (231, 232), as well as greater use of threatening behaviors, such as aggressive language in social media posts (233) and horn honking in traffic (234), and higher rates of violent retaliation in sports (235). Emerging evidence also indicates that hot periods elevate the risk that individuals harm themselves, including by suicide (27, 236). U.S. data indicate no evidence of adaptation (27, 232). Effects of temperature [+2.4% per SD (σ)] and rainfall (0.6% per σ) on interpersonal violence are both highly statistically significant, according to a meta-analysis (237). If these responses to historical fluctuations translate to future climate change, warming of 1°C could lead to an increase in national violent crime (rape, assault, and murder) by 0.88% (±0.04%) (238). Under RCP8.5, this trend projects to a warming-caused increase in violent crime of 1.7 to 5.4% by 2080 to 2099. Warming is projected to increase the national suicide rate by 0.6 to 2.6% by 2050 (27). Many studies document a heightened risk of violence between groups of individuals when temperatures are hot and/or rainfall is extreme (26) (Fig. 1, right column). The patterns are similar for organized violence, such as civil conflicts (228, 239), and disorganized violence, such as ethnic riots (240), with highly statistically significant effects of temperature (+11.3% per σ) and rainfall (3.5% per σ, over 2 years) (237). Political instability is heightened in hot periods, even in contexts where political institutions are sufficiently robust to avoid outright violence (Fig. 1, right column). The probability of political leadership changes, through both democratic process (241, 242) and “irregular” conditions (243, 244), rises in warm periods. Coups are more likely in hot years with extreme rainfall in agriculturally dependent countries (245). By degrading economic conditions, climate events may contribute to out-migrations of populations seeking better opportunities. Drought and soil loss in the Dust Bowl induced mass out-migration from the rural Midwest (246), and young working-age individuals left the corn belt during periods of extreme heat in recent decades (247). Likewise, periods of high temperatures have been linked to migration from rural regions of Mexico to the United States (247, 248). Population movements after periods of extreme heat or dryness have been documented in multiple regions (249–251), and high temperatures in agrarian regions elevate international applications for political asylum (252).

National security Since the EF, the American military and intelligence communities have substantially increased their integration of climate change into national security strategies, policies, and plans. These considerations have been reflected in analyses of the national security implications of climate change by the U.S. Department of Defense, with almost 50 reports considering climate security impacts published between 2010 and 2018 (253) (Fig. 1, right column). The National Intelligence Council (NIC) has warned Congress about the security risks of climate change every year since 2008, after the release of the landmark report by the CNA Military Advisory Board, “National Security and the Threat of Climate Change” (254). The NIC’s “Worldwide Threat Assessment,” which reflects the intelligence community’s consensus on the most substantial risks to national security, in 2018 for the first time included a robust section titled “Environment and climate change,” noting a range of security risks related to environmental concerns (255). The 2018 Defense Authorization Act, signed by President Donald J. Trump, stated that “climate change is a direct threat to the national security of the United States …” (256). During the Trump presidency, 16 military leaders, including Secretary of Defense James Mattis (257), have voiced concerns about climate change and its security implications. Chairman of the Joint Chiefs of Staff General Joe Dunford stated, “Climate change … is very much something that we take into account in our planning as we anticipate when, where and how we may be engaged in the future and what capabilities we should have” (258). New studies strengthen the evidence that climate change causes weather patterns and extreme events that directly harm military installations and readiness through infrastructure damage, loss of utilities, and loss of operational capability (Fig. 1, right column). An SLR of 3.7 feet would threaten 128 military bases (259). Thawing permafrost exposes foundations to damage, whereas the loss of Arctic sea ice causes coastal erosion near critical facilities. Intensifying wildfires threaten facilities, transportation infrastructure, and utility lines. Fire-hazard days and inclement weather suspend outdoor training, and droughts limit the use of live-fire training. Greater storm frequency and strength put a strain on the resources of the defense support of civil authorities at home, as well as on assistance to humanitarian efforts and disaster relief around the world (260). As of 2018, 50% of military installations both at home and abroad had already reported damage due to climate change (260). Droughts or unpredictable rainfall could leave armed forces stationed abroad vulnerable to being disconnected from potable water supplies, a cause for concern given that protecting convoys for the “resupply of fuel and drinking water for troops in-theater costs lives” (261). Climate change increasingly disrupts existing international security dynamics in geostrategic environments (Fig. 1, right column). Reduced Arctic sea-ice extent will open the way for more trade, as well as oil and gas extraction, turning a historically neutral territory into a potential political flashpoint. Moreover, the U.S. military now has to operate in an increasingly open water Arctic region as sea ice retreats. As Secretary of Defense Mattis recently stated, “America’s got to up its game in the Arctic” (262). Both China and Russia have been deepening their Arctic presence through investment and the development of ports. As much as 15 percent of China’s trade value could travel through the Arctic by 2030, and between 20 and 30 percent of Russia’s oil production will come from deposits in the Arctic shelf by 2050 (263). These interests will require further American military and coast guard activity in the region, as well as broader diplomatic and scientific engagement. Indirectly, climate change has a major effect on national security by acting as a “threat multiplier” (254) or “accelerant of instability” (264) (Fig. 1, right column). This means that climate change heightens the risk posed by threats the United States is already facing and, in aggregate, fundamentally alters the security landscape (265). In both the 2010 and 2014 quadrennial defense reviews (264, 266), the Department of Defense emphasized how seriously the military takes this dangerous dynamic, a commitment that receives meaningful redress every year in its annual strategic sustainability performance plans (267). As discussed in other parts of this Review, an expanding body of evidence reinforces how climate change fuels economic and social discontent, and even upheaval. This includes extreme weather events, which raise the risk of humanitarian disasters, conflict, water and food shortages, population migration, labor shortfalls, price shocks, and power outages (255).

Economic well-being Research on the economic consequences of climate change has advanced substantially since the EF, with important progress on understanding nonagricultural sectors and broad measures of well-being (225, 268) (Fig. 1, right column). In the United States, economic impacts of hot temperatures and changing tropical cyclone environments are clearly documented (238), and growing evidence indicates long-term adverse effects on the labor force (269–271). Other impacts, such as those from water availability or wildfire risks, are thought to be important but remain less well understood (272). Since the EF, new “top-down” analyses of overall macroeconomic performance estimate that warming by an additional 1°C over 75 years can be expected to permanently reduce the U.S. gross domestic product (GDP) by ~3% through direct thermal effects (273) and that the U.S. GDP can be expected to be ~4% greater at 1.5°C than at 2°C above preindustrial temperatures (274) (Fig. 1, right column). The average projected alteration of cyclone activity under “business as usual” may cost the United States the equivalent of 29% of one year of current GDP (in net present value discounted at 3% annually) (275). In one study, the net cumulative market-based cost of thermal effects in RCP8.5 by 2100 should be valued at $4.7 trillion to $10.4 trillion (in net present value discounted at 3% annually) (276). Notably, in some cases these top-down analyses are able to account for both the opportunity costs and benefits of adaptations undertaken by populations as they adjust to new climatic conditions (276). “Bottom-up” analyses examining impacts on individual sectors or industries have key advantages, including capturing the value of nonmarket impacts such as the loss of human life or biodiversity (238). Evidence from combining sector-specific analyses of impacts such as agricultural output (277), the quantity of labor supplied by workers (278), energy demand (167, 279), mortality rates (279), crime rates (232), SLR (280) and tropical cyclone damage (281) suggests U.S. costs equivalent to 1.2% of GDP for each 1°C of warming, with poorer counties experiencing an economic burden roughly five times that of wealthier counties (238) (Fig. 1, right column, and Fig. 4). Fig. 4 Total direct economic damage integrated over agriculture, crime, coastal storms, energy, human mortality, and labor in 2080 to 2099 under a scenario of continued high emissions (RCP8.5). (Left) Damages in the median scenario for each county. Negative damages indicate benefits. (Right) Range of economic damages per year for groupings of U.S. counties, on the basis of income (with 29,000 simulations for each of 3143 counties) as a fraction of county income (white lines, median; boxes, inner 66% of possible outcomes; outer whiskers, inner 90% of possible outcomes). [Adapted from (238)]

Conclusions The EPA Administrator found in 2009 that the EF for six long-lived GHGs was “compellingly” supported by “strong and clear” scientific evidence (5). Since 2009, the amount, diversity, and sophistication of the evidence have increased markedly, clearly strengthening the case for endangerment. New evidence about the extent, severity, and interconnectedness of impacts detected to date and projected for the future reinforces the case that climate change may reasonably be anticipated to endanger the health and welfare of current and future generations. For the sectors analyzed in the 2009 EF, new evidence expands the range of case studies, deepens the understanding of mechanisms, and analyzes the contribution of climate-related extremes. In many cases, new evidence points to the risk of impacts that are more severe or widespread than those anticipated in 2009. Several categories of climate-change impacts, including effects on ocean acidification, violence, national security, and economic well-being, are now supported by such broad evidence that they warrant inclusion in the framing of endangerment. In sum, the EF, fully justified in 2009, is much more strongly justified in 2018.

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Acknowledgements: Funding: N.S.D. was supported by Stanford University. S.C.D. was supported by the University of Virginia Environmental Resilience Institute. S.M. was supported by the NSF through grants NSF-1417700 and NSF-1312402. Competing interests: L.J.M. received consulting fees from the EPA for contributions to the Integrated Science Assessment (ISA) on PM matter and for review of the ozone ISA. S.T. serves on the boards of directors of the ClimateWorks Foundation and the Energy Foundation. Data and materials availability: All data are available in the main text.