finding 13.1 : key-message-13-1

More than 100 million people in the United States live in communities where air pollution exceeds health-based air quality standards. Unless counteracting efforts to improve air quality are implemented, climate change will worsen existing air pollution levels (likely, high confidence). This worsened air pollution would increase the incidence of adverse respiratory and cardiovascular health effects, including premature death (high confidence). Increased air pollution would also have other environmental consequences, including reduced visibility and damage to agricultural crops and forests (likely, very high confidence).

This finding is from chapter 13 of Impacts, Risks, and Adaptation in the United States: The Fourth National Climate Assessment, Volume II.

Process for developing key messages:

Due to limited resources and requirements imposed by the Federal Advisory Committee Act, the decision was made that this chapter would be developed using an all-federal author team. The author team was selected based on expertise in climate change impacts on air quality; several of the chapter authors were authors of the “Air Quality Impacts” chapter of the U.S. Global Change Research Program’s (USGCRP) Climate and Health Assessment.5ec155e5-8b77-438f-afa9-fbcac4d27690 This chapter was developed through technical discussions of relevant evidence and expert deliberation by the report authors via weekly teleconferences and email exchanges. The authors considered inputs and comments submitted by the public; the National Academies of Sciences, Engineering, and Medicine; and federal agencies.

Description of evidence base:

It is well established that air pollutants pose a serious risk to human health and the environment.f7ffc8dd-70ec-4779-817a-b2985c0779e7 e00fb4e2-6406-40be-90f8-071dfc43cca3 Short- and long-term exposure to pollutants such as ozone or PM2.5 results in premature deaths,2085e6ae-5608-4e91-86c2-36df7baa8fec hospital and emergency room visits, aggravated asthma,5ec155e5-8b77-438f-afa9-fbcac4d27690 328b7923-3257-4326-804c-c2ee79a023d5 and shortness of breath.d3f82686-12ef-4e1e-9a15-cf89236a53a8 Numerous air quality modeling studies have assessed the potential impacts of a changing climate on future ozone and particulate matter levels in the United States.b4038a28-b14b-4ae8-b783-0de19e3cffdd 5c599d79-a94a-40bc-862e-d599de07b599 afbd60ab-ba9f-4547-88e3-968bc3a4b949 2bd16a59-d347-4fb4-9ff7-701e0c32ab60 a19a16db-8155-45a3-83f0-357064ec254a These studies examine simulations conducted with a broad ensemble of global and regional climate models under various potential climate scenarios. For ozone, these model assessments consistently project higher future levels commensurate with warmer climates, independent of varying individual model assumptions. This model consensus strengthens confidence in the projected signal. Additionally, well-established data analyses have shown a strong positive correlation between temperature and ozone at many locations in the United States.1994b6dc-9753-44a1-a1b2-1d1566c39287 552e0a6a-98c6-4d6c-b7ff-fdcc572fa914 Although competing meteorological effects determine local ozone levels, temperature is often the single largest meteorological driver. This present-day signal also bolsters confidence in the conclusion that warmer climates will be associated with higher ozone. There are also modeling and observational studies that demonstrate that ozone precursor emissions from natural3ccc0f92-9b21-4012-b368-d66568254a3a and human sourcesccd5ec24-c9a9-4e7d-9ae4-b51314ef0e03 increase with temperature. In aggregate, the consistency in the ozone response to past and projected future climate across a large volume of analyses provides high confidence that ozone air pollution will likely be worsened in a warmer climate. For particulate matter, the model assessments exhibit greater variability in terms of future concentration differences projected to result from meteorological changes in a warmer climate.5ec155e5-8b77-438f-afa9-fbcac4d27690 b4038a28-b14b-4ae8-b783-0de19e3cffdd 0dcee9aa-9230-420c-a1a9-c02510c6df0f 2bd16a59-d347-4fb4-9ff7-701e0c32ab60 The reduced certainty in the response of PM2.5 concentrations (particulate matter, or PM, less than 2.5 micrometers in diameter) to changing meteorological drivers is the result of the multiple pathways toward PM2.5 formation and the variable influence of meteorological factors on each of those different pathways.f7ffc8dd-70ec-4779-817a-b2985c0779e7 Most of these model assessments have not considered the impact of changes in PM from changes in wildfires or windblown dust because they are difficult to quantify. Studies that have included projections of future wildfire incidences have concluded that climate-driven increases in wildfire activity are likely, with wildfires becoming an increasingly important source of PM2.5fd647847-4da5-4fc8-9488-4b79549d7cf6 60bb2b8e-5a2d-46f4-8474-84a1256ca392 c644739f-2708-4c5b-ba4d-a9dd0a50d3dc and degrading visibility.a92b6912-a92c-482b-a8e7-f43d324947e3 Finally, there is ample observational evidence that decreasing ozone and particulate precursor emissions would reduce pollutant levels.20bac876-62ce-4d20-9041-a7461e1b93fc 1bc5d64e-b530-4b8b-a6b5-19f40499ee94

New information and remaining uncertainties:

Model simulations of future air quality indicate that climate warming generally increases ground-level ozone across the United States (see Figure 13.2), but results differ spatially and in the magnitude of the projected signal.8168dfd7-c53f-4e89-ba22-991d6a2179a6 342dcbb2-a2cf-49d9-9f2f-46562422c410 fc7459fe-8533-4a00-8ba8-2ce2783c3103 4f1b9afd-efc6-4f29-82ca-032786eb9a9c 1d7245db-8407-4ca3-9db8-175ce77f2b10 Because meteorological influences on ozone formation can vary to some degree by location (for example, wind direction may be paramount in locations affected primarily by ozone transport), a few areas may experience lower ozone levels.b4038a28-b14b-4ae8-b783-0de19e3cffdd Future ozone levels over the United States will depend not only on the severity of the climate change impacts on meteorology favorable for ozone accumulation but also on any measures to reduce ozone precursor emissions, introducing further uncertainty. Even larger uncertainties exist with respect to the climate impacts on PM2.5, where the future concentrations will depend on changes in a suite of meteorological factors, which in some cases (for example, precipitation) are more difficult to quantify.

Assessment of confidence based on evidence:

There is high confidence that rising temperatures will likely increase future ozone levels in many parts of the United States in response to climate change. There is greater uncertainty that a warmer climate will increase future PM2.5 levels over the United States. Ultimately, the actual ozone and PM2.5 changes between the present and the future at any given location will depend on the local climate impacts on meteorology and pollutant emission controls in that region. There is very high confidence that reducing ozone precursor emissions and PM2.5 precursors and/or direct emissions will likely lead to improved air quality in the future, thus mitigating adverse climate effects.

References :

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