Climate change will make it harder for any given regulatory approach to reduce ground-level ozone pollution in the future as meteorological conditions become increasingly conducive to forming ozone over most of the United States [Likely, High Confidence]. Unless offset by additional emissions reductions of ozone precursors, these climate-driven increases in ozone will cause premature deaths, hospital visits, lost school days, and acute respiratory symptoms [Likely, High Confidence].

This finding is from chapter 3 of The Impacts of Climate Change on Human Health in the United States: A Scientific Assessment.

Process for developing key messages: The chapter was developed through technical discussions of relevant evidence and expert deliberation by the report authors at several workshops, teleconferences, and email exchanges. The authors considered inputs and comments submitted by the public, the National Academies of Sciences, and Federal agencies. For additional information on the overall report process, see Appendices 2–3.

In addition, the author team held an all-day meeting at the U.S. Environmental Protection Agency National Center for Environmental Assessment in Crystal City, Virginia, on October 15, 2014, to discuss the chapter and develop initial drafts of the Key Findings. A quorum of the authors participated and represented each of the three sections of the chapter—outdoor air quality, aeroallergens, and indoor air quality. These discussions were informed by the results of the literature review as well as the research highlights focused on outdoor air quality and indoor air quality. The team developed Key Finding 2 in response to comments from the National Research Council review panel and the general public.

The Key Findings for outdoor ozone, wildfires, and aeroallergen impacts reflect strong empirical evidence linking changes in climate to these outcomes. When characterizing the human health impacts from outdoor ozone, the team considered the strength of the toxicological, clinical, and epidemiological evidence evaluated in the Ozone Integrated Science Assessment.e00fb4e2-6406-40be-90f8-071dfc43cca3 Because there is increasing evidence that climate change will increase the frequency and intensity of wildfire events, this outcome was included as a key finding, despite the inability to quantify this impact with the available tools and data. Because altered patterns of precipitation and increasing levels of CO₂ are anticipated to promote the level of aeroallergens, this outcome is also included as a Key Finding. Finally, because the empirical evidence linking climate change to indoor air quality was more equivocal, we identified this topic as an emerging issue.

Description of evidence base: The Intergovernmental Panel on Climate Change (IPCC) has concluded that warming of the global climate system is unequivocal and that continued increases in greenhouse gas emissions will cause further temperature increases.f03117be-ccfe-4f88-b70a-ffd4351b8190 dd5b893d-4462-4bb3-9205-67b532919566 At the same time, there is a well-established relationship between measured temperature and monitored peak ozone levels in the United States.552e0a6a-98c6-4d6c-b7ff-fdcc572fa914 1994b6dc-9753-44a1-a1b2-1d1566c39287 Numerous climate and air quality modeling studies have also confirmed that increasing temperatures, along with other changes in meteorological variables, are likely to lead to higher peak ozone levels in the future over the United States,afbd60ab-ba9f-4547-88e3-968bc3a4b949 b4038a28-b14b-4ae8-b783-0de19e3cffdd if ozone precursor emissions remain unchanged.

Risk assessments using concentration–response relationships from the epidemiological literature and modeled air quality data have projected substantial health impacts associated with climate-induced changes in air quality.54a66159-1675-43bb-b5d3-a9b7f283e4de fe6d1c69-790d-46cc-89c4-26dc24585dcf 8e802a4f-d4f1-4f72-a0ae-aefbbece477e c275ae44-75e4-4974-81ea-fe7119474ffb d6e399c7-1efe-4f91-927e-f957965e3aaa d3df6d52-0441-47cc-a939-6d09f57ea48d This literature reports a range of potential changes in ozone-related, non-accidental mortality due to modeled climate change between the present and 2030 or 2050, depending upon the scenario modeled, the climate and air quality models used, and assumptions about the concentration–response function and future populations. Many of the studies suggest that tens to thousands of premature deaths could occur in the future due to climate change impacts on air quality.54a66159-1675-43bb-b5d3-a9b7f283e4de fe6d1c69-790d-46cc-89c4-26dc24585dcf At the same time, hundreds of thousands of days of missed school and hundreds of thousands to millions of cases of acute respiratory symptoms also result from the climate-driven ozone increases in the United States.54a66159-1675-43bb-b5d3-a9b7f283e4de

New information and remaining uncertainties: Climate projections are driven by greenhouse gas emission scenarios, which vary substantially depending on assumptions for economic growth and climate change mitigation policies. There is significant internal variability in the climate system, which leads to additional uncertainties in climate projections, particularly on a regional basis. Ozone concentrations also depend on emissions that are influenced indirectly by climate change (for example, incidence of wildfires, changes in energy use, energy technology choices), which further compounds the uncertainty. Studies projecting human health impacts apply concentration–response relationships from existing epidemiological studies characterizing historical air quality changes; it is unclear how future changes in the relationship between air quality, population exposure, and baseline health may affect the concentration–response relationship. Finally, these studies do not account for the possibility of a physiological interaction between air pollutants and temperature, which could lead to increases or decreases in air pollution-related deaths and illnesses.

Assessment of confidence based on evidence: Given the known relationship between temperature and ozone, as well as the numerous air quality modeling studies that suggest climate-driven meteorological changes will yield conditions more favorable for ozone formation in the future, there is high confidence that ozone levels will likely increase due to climate change, unless offset by reductions in ozone precursor emissions. Based on observed relationships between ozone concentrations and human health responses, there is high confidence that any climate-driven increases in ozone will likely cause additional cases of premature mortality, as well as increasingly frequent cases of hospital visits and lost school days due to respiratory impacts.