--- - chapter_identifier: air-quality confidence: '

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 PM_2.5 levels over the United States. Ultimately, the actual ozone and PM_2.5 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 PM_2.5 precursors and/or direct emissions will likely lead to improved air quality in the future, thus mitigating adverse climate effects.

' evidence: "

It is well established that air pollutants pose a serious risk to human health and the environment.{{< tbib '5' 'f7ffc8dd-70ec-4779-817a-b2985c0779e7' >}}^,{{}} Short- and long-term exposure to pollutants such as ozone or PM_2.5 results in premature deaths,{{< tbib '8' '2085e6ae-5608-4e91-86c2-36df7baa8fec' >}} hospital and emergency room visits, aggravated asthma,{{< tbib '3' '5ec155e5-8b77-438f-afa9-fbcac4d27690' >}}^,{{}} and shortness of breath.{{< tbib '10' '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.{{< tbib '4' 'b4038a28-b14b-4ae8-b783-0de19e3cffdd' >}}^,{{}}^,{{}}^,{{}}^,{{}} 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.{{< tbib '87' '1994b6dc-9753-44a1-a1b2-1d1566c39287' >}}^,{{}} 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 natural{{< tbib '75' '3ccc0f92-9b21-4012-b368-d66568254a3a' >}} and human sources{{< tbib '77' 'ccd5ec24-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.{{< tbib '3' '5ec155e5-8b77-438f-afa9-fbcac4d27690' >}}^,{{}}^,{{}}^,{{}} The reduced certainty in the response of PM_2.5 concentrations (particulate matter, or PM, less than 2.5 micrometers in diameter) to changing meteorological drivers is the result of the multiple pathways toward PM_2.5 formation and the variable influence of meteorological factors on each of those different pathways.{{< tbib '5' '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 PM_2.5{{< tbib '63' 'fd647847-4da5-4fc8-9488-4b79549d7cf6' >}}^,{{}}^,{{}} and degrading visibility.{{< tbib '54' 'a92b6912-a92c-482b-a8e7-f43d324947e3' >}} Finally, there is ample observational evidence that decreasing ozone and particulate precursor emissions would reduce pollutant levels.{{< tbib '28' '20bac876-62ce-4d20-9041-a7461e1b93fc' >}}^,{{}}

" href: https://data.globalchange.gov/report/nca4/chapter/air-quality/finding/key-message-13-1.yaml identifier: key-message-13-1 ordinal: 1 process: "

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.{{< tbib '3' '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.

" report_identifier: nca4 statement: '

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).

' 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.{{< tbib '90' '8168dfd7-c53f-4e89-ba22-991d6a2179a6' >}}^,{{}}^,{{}}^,{{}}^,{{}} 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.{{< tbib '4' '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 PM_2.5, 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.

" uri: /report/nca4/chapter/air-quality/finding/key-message-13-1 url: ~ - chapter_identifier: air-quality confidence: '

There is high confidence that rising temperatures and earlier spring snowmelt will very likely result in lengthening the wildfire season in portions of the United States, leading to an increased frequency of wildfires and associated smoke. There is very high confidence that increasing exposure to wildfire smoke, which contains particulate matter, will increase adverse health impacts. It is likely that smoke from wildfires will reduce visibility and disrupt outdoor recreational activities.

' evidence: "

Wildfire smoke worsens air quality through its direct emissions to the atmosphere as well as through chemical reactions of those pollutants with sunlight and other pollutants. Exposure to wildfire smoke increases the risk of exacerbating respiratory illnesses in tens of millions of people in vulnerable population groups across the United States.{{< tbib '62' '9a222c75-5ff9-408e-9694-b7bd90a2a0ca' >}} Several studies have indicated that climate change has already led to longer wildfire seasons,{{< tbib '79' 'e1e1f3a0-9fea-4ad2-a3af-575716f9849e' >}} increased frequency of large wildfires,{{< tbib '82' 'd96a729a-a5db-4318-8f52-78f6031b42fd' >}}^,{{}} and increased area of forest burned.{{< tbib '99' 'de4a77df-03ba-4319-a13f-7fdefbb353a5' >}} Additional studies project that climate change will cause wildfire frequency and burned area in North America to increase over the 21st century.{{< tbib '81' 'a29b612b-8c28-4c93-9c18-19314babce89' >}}^,{{}}^,{{}}^,{{}}^,{{}}^,{{}}^,{{}}^,{{}} Increased emissions from wildfires may offset the benefits of large reductions in emissions of PM_2.5 precursors.{{< tbib '54' 'a92b6912-a92c-482b-a8e7-f43d324947e3' >}}^,{{}} There is a broad and consistent evidence base leading to a high confidence conclusion that the increasing impacts of wildfire are very likely. Increases in wildfire smoke events due to climate change would reduce opportunities for outdoor recreational activities (Ch. 22: N. Great Plains, KM 3 and Ch. 24: Northwest, KM 4).

" href: https://data.globalchange.gov/report/nca4/chapter/air-quality/finding/key-message-13-2.yaml identifier: key-message-13-2 ordinal: 2 process: "

" report_identifier: nca4 statement: '

Wildfire smoke degrades air quality, increasing the health risks to tens of millions of people in the United States. More frequent and severe wildfires due to climate change would further diminish air quality, increase incidences of respiratory illness from exposure to wildfire smoke, impair visibility, and disrupt outdoor recreational activities (very likely, high confidence).

' uncertainties: "

Humans affect fire activity in many ways, including increasing ignitions as well as conducting controlled burns and fire suppression activities.{{< tbib '110' '415d7f4d-4e24-4cff-a9aa-c76f30dbeb42' >}}^,{{}} The frequency and severity of wildfire occurrence in the future will be largely determined by forest management practices and climate adaptation measures, which are very uncertain. Housing development practices and changes in the urban–forest interface are also important factors for future wildfire occurrence and for the extent to which associated smoke emissions impair air quality and result in adverse health effects. The composition of the pollutants contained in wildfire smoke and their chemical reactions are highly dependent on a variety of environmental factors, so projecting and quantifying the effects of wildfire smoke on specific pollutants can be particularly challenging. Exposure to wildfire smoke may also increase the risk of cardiovascular illness, but additional data are required to quantify this risk.{{< tbib '62' '9a222c75-5ff9-408e-9694-b7bd90a2a0ca' >}} More accurate forecasting of wildfire smoke events may mitigate health impacts and reduced opportunities for outdoor recreational activities through changes in timing of those activities.

" uri: /report/nca4/chapter/air-quality/finding/key-message-13-2 url: ~ - chapter_identifier: air-quality confidence: '

The scientific literature shows that there is high confidence that changes in climate, including rising temperatures and altered precipitation patterns as well as rising levels of atmospheric CO₂, will increase the concentration, allergenicity, season length, and spatial distribution of a number of aeroallergens. These changes in aeroallergen exposure are, in turn, likely to impact allergic disease.

' evidence: "

Considerable evidence supports the conclusion that climate change and rising levels of CO₂ affect key aspects of aeroallergen biology, including the production, temporal distribution, and potential allergenicity of aeroallergens.{{< tbib '142' '14835bc7-3df6-4fac-9e9a-2863c09e800a' >}}^,{{}}^,{{}}^,{{}}^,{{}} This evidence includes historical trends indicating that climate change has altered seasonal exposure times for allergenic pollen.{{< tbib '113' '1c917926-3eba-452b-bd2b-f9e88b374312' >}} These changes in exposure times are associated with rising CO₂ levels, higher temperatures, changes in precipitation (which can extend the start or duration of pollen release times), and the amount of pollen released, the allergenicity of the pollen, and the spatial distribution of that pollen.{{< tbib '117' '1bc9d76c-14c8-4245-9ccb-1355cdc48d0b' >}}^,{{}}^,{{}}^,{{}}

Specific changes in weather patterns or extremes are also likely to contribute to the exacerbation of allergy symptoms. For example, thunderstorms can induce spikes in aeroallergen concentrations and increase the incidence and severity of asthma and other allergic disease.{{< tbib '148' '713cd919-826b-426d-bf51-761a58ec7f22' >}}^,{{}} However, the specific mechanism for intensification of weather and allergic disease is not entirely understood.

Overall, climate change and rising CO₂ levels are likely to increase exposure to aeroallergens and contribute to the severity and prevalence of allergic disease, including asthma.{{< tbib '115' '971ee908-7da0-416e-8b6c-a72984d129ba' >}} There is consistent and compelling evidence that exposure to aeroallergens poses a significant health risk in regard to the occurrence of asthma, hay fever, sinusitis, conjunctivitis, hives, and anaphylaxis.{{< tbib '150' '036ba27d-8341-4f6d-ad66-1288e53dee65' >}}^,{{}}^,{{}}^,{{}} Finally, there is evidence that synergies between aeroallergens and air pollution, especially particulate matter, may increase health risks for individuals who are simultaneously exposed.{{< tbib '154' '7a9fde66-dbc1-4152-bd11-2d68d4e7d66a' >}}^,{{}}^,{{}}

" href: https://data.globalchange.gov/report/nca4/chapter/air-quality/finding/key-message-13-3.yaml identifier: key-message-13-3 ordinal: 3 process: "

" report_identifier: nca4 statement: '

The frequency and severity of allergic illnesses, including asthma and hay fever, are likely to increase as a result of a changing climate. Earlier spring arrival, warmer temperatures, changes in precipitation, and higher carbon dioxide concentrations can increase exposure to airborne pollen allergens. (Likely, High Confidence)

' uncertainties: "

While specific climate- and/or CO₂-induced links to aeroallergen biology are evident, allergic diseases develop in response to complex and multiple interactions, including genetic and nongenetic factors, a developing immune system, environmental exposures (such as ambient air pollution or weather conditions), and socioeconomic and demographic factors. Overall, the role of these factors in eliciting a health response has not been entirely elucidated. However, recent evidence suggests that climate change and aeroallergens are having a discernible impact on public health.{{< tbib '123' 'c9c2ea5f-223f-4594-b182-40b473c6e665' >}}^,{{}}

There are a number of areas where additional information is needed, including regional variation in climate and aeroallergen production; specific links between aeroallergens and related diseases, particularly asthma; the need for standardized approaches to determine exposure times and pollen concentration; and uncertainty regarding the role of CO₂ on allergenicity.

" uri: /report/nca4/chapter/air-quality/finding/key-message-13-3 url: ~ - chapter_identifier: air-quality confidence: '

There is very high confidence that emissions of ozone and PM precursors could be reduced by reducing combustion sources of CO₂. Reducing emissions of ozone and PM precursors would be very likely to reduce ozone and PM pollution, which would very likely result in fewer adverse health effects from air pollution. There is very high confidence that controlling methane emissions would also reduce ozone formation rates, which would also very likely lead to lower ozone levels.

' evidence: '

Decades of experience in air quality management have resulted in a detailed accounting of the largest emission sources of greenhouse gases (GHGs) and precursors of ozone and PM. The cost and effectiveness of emission control technologies for the largest emissions sources are well understood. By combining these emission and control technology data with energy system modeling tools, the potential to achieve benefits to air quality while mitigating GHG emissions under a range of scenarios has been quantified in numerous studies.

' href: https://data.globalchange.gov/report/nca4/chapter/air-quality/finding/key-message-13-4.yaml identifier: key-message-13-4 ordinal: 4 process: "

" report_identifier: nca4 statement: '

Many emission sources of greenhouse gases also emit air pollutants that harm human health. Controlling these common emission sources would both mitigate climate change and have immediate benefits for air quality and human health. Because methane is both a greenhouse gas and an ozone precursor, reductions of methane emissions have the potential to simultaneously mitigate climate change and improve air quality. (Very Likely, Very High Confidence)

' uncertainties: '

A wide range of values have been reported for the magnitude of air quality co-benefits. Much of this variability can be attributed to differences in the mix of co-benefits included in the analysis and the time period under consideration. The largest sources of uncertainty are the cost paths of different energy technologies over time and the extent to which policy choices impact the evolution of these costs and the availability of different energy technologies.

' uri: /report/nca4/chapter/air-quality/finding/key-message-13-4 url: ~