Summary¹

The scientific community has been studying the question of how human-caused emissions of greenhouse gases are affecting the climate for well over a century. Much is known today, drawing upon decades of direct observations of the Earth system and detailed research. In this report, the committee summarizes the latest evidence on whether greenhouse gas (GHG) emissions threaten human health and welfare in the United States.

The impetus for this report was a notice of proposed rulemaking issued by the U.S. Environmental Protection Agency (EPA) indicating its intention to rescind the 2009 action titled “Endangerment and Cause or Contribute Findings for Greenhouse Gases Under Section 202(a) of the Clean Air Act.” Recognizing that significantly more evidence is available today, the National Academies of Sciences, Engineering, and Medicine launched this study to review newly available scientific evidence on the topics considered in the Technical Support Document that EPA prepared in considering whether to make the finding (see Box 1.2 for the Statement of Task). The committee’s report focuses on evidence gathered by the scientific community since the Technical Support Document was published in 2009 and describes supporting evidence, the level of confidence, and areas that are under continuing debate or are unknown.

On the basis of the scientific evidence outlined in the body of this report, the committee reached the following overarching conclusion:

Overarching Conclusion: EPA’s 2009 finding that the human-caused emissions of greenhouse gases threaten human health and welfare was accurate, has stood the test of time, and is now reinforced by even stronger evidence. Today, many of EPA’s conclusions are further supported by longer observational records and multiple new lines of evidence. Moreover, research has uncovered additional risks that were not apparent in 2009.

This overarching conclusion is supported by the following five conclusions:

(1) Emissions of greenhouse gases from human activities are increasing the concentration of these gases in the atmosphere. Human activities—such as the extraction and burning of fossil fuels, cement and chemical production, deforestation, and agricultural activities—emit GHGs, which include carbon dioxide (CO₂), methane (CH₄), nitrous oxide (N₂O), and fluorinated gases (F-gases), to the atmosphere. Total global GHG

emissions continue to increase, even though U.S. emissions of CO₂ have decreased slightly in recent years largely due to changes in energy production and consumption. Multiple lines of evidence show that GHG emissions from human activities are the primary driver of the observed long-term warming trend. No known natural drivers, such as incoming solar radiation or volcanic emissions, can explain observed changes.

(2) Improved observations confirm unequivocally that greenhouse gas emissions are warming Earth’s surface and changing Earth’s climate. Longer records, improved and more robust observational networks, and analytical and methodological advances have strengthened detection of observed changes and their attribution to elevated GHGs. Trends observed include increases in hot extremes and extreme single-day precipitation events, declines in cold extremes, regional shifts in annual precipitation, warming of the Earth’s oceans, a decrease in ocean pH, rising sea levels, and an increase in wildfire severity.

(3) Human-caused emissions of greenhouse gases and resulting climate change harm the health of people in the United States. Climate change intensifies risks to humans from exposures to extreme heat, ground-level ozone, airborne particulate matter, extreme weather events, and airborne allergens, affecting incidence of cardiovascular, respiratory, and other diseases. Climate change has increased exposure to pollutants from wildfire smoke and dust, which has been linked to adverse health effects. The increasing severity of some extreme events has contributed to injury, illness, and death in affected communities. Health impacts related to climate-sensitive infectious diseases—such as those carried by insects and in contaminated water—have increased. New evidence is developing about additional health impacts of climate change, including on mental health, nutrition, immune health, antimicrobial resistance, kidney disease, and negative pregnancy-related outcomes. Groups such as older adults, people with preexisting health conditions or multiple chronic diseases, and outdoor workers are disproportionately susceptible to climate-associated health effects. Even as non-climate factors, including adaptation measures, can help people cope with harmful impacts of climate change, they cannot remove the risk of harm.

(4) Changes in climate resulting from human-caused emissions of greenhouse gases harm the welfare of people in the United States. Climate-driven changes in temperature and precipitation extremes and variability are leading to negative impacts on agricultural crops and livestock, even as technological and other changes have increased agricultural production. Climate change, including increases in climate variability and wildfires, is changing the community composition and function of forest and grassland ecosystems and the services they provide. Climate-related changes in water availability and quality vary across regions in the United States with some regions showing a decline. Climate-related changes in the chemistry and the heat content of the ocean are having negative effects on calcifying organisms and contributing to increases in harmful algal blooms. U.S. energy systems, infrastructure, and many communities are experiencing increasing stress and costs owing to the effects of climate change.

(5) Continued emissions of greenhouse gases from human activities will lead to more climate changes in the United States, with the severity of expected change increasing with every ton of greenhouse gases emitted. Despite successful efforts in many parts of the world to reduce emissions, total global GHG emissions have continued to increase, and additional warming is certain. All climate models—regardless of assumptions about future emissions scenarios or estimates of climate sensitivity—consistently project continued warming in response to future atmospheric GHG increases. Applying fundamental physics of the Earth system leads to the same conclusion. Continued changes in the climate increase the likelihood of passing thresholds in Earth systems that could trigger tipping points or other high-impact climate surprises.

In summary, the committee concludes that the evidence for current and future harm to human health and welfare created by human-caused GHGs is beyond scientific dispute. Much of the understanding of climate change that was uncertain or tentative in 2009 is now resolved, and new threats have been identified. These new threats and the areas of remaining uncertainty are under intensive investigation by the scientific community. The United States faces a future in which climate-induced harm continues to worsen and today’s extremes become tomorrow’s norms.