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. This worsened air pollution would increase the incidence of adverse respiratory and cardiovascular health effects, including premature death. Increased air pollution would also have other environmental consequences, including reduced visibility and damage to agricultural crops and forests.
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.
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.
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.
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. This worsened air pollution would increase the incidence of adverse respiratory and cardiovascular health effects, including premature death. Increased air pollution would also have other environmental consequences, including reduced visibility and damage to agricultural crops and forests.
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.
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.
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.
Virtually Certain | Extremely Likely | Very Likely | Likely | About as Likely as Not | Unlikely | Very Unikely | Extremely Unlikely | Exceptionally Unlikely |
---|---|---|---|---|---|---|---|---|
99%–100% | 95%–100% | 90%–100% | 66%-100% | 33%-66% | 0%-33% | 0%-10% | 0%-5% | 0%-1% |
Very High | Hoch | Medium | Niedrig |
---|---|---|---|
Strong evidence (established theory, multiple sources, consistent results, well documented and accepted methods, etc.), high consensus | Moderate evidence (several sources, some consistency, methods vary and/or documentation limited, etc.), medium consensus | Suggestive evidence (a few sources, limited consistency, models incomplete, methods emerging, etc.), competing schools of thought | Inconclusive evidence (limited sources, extrapolations, inconsistent findings, poor documentation and/or methods not tested, etc.), disagreement or lack of opinions among experts |
Documenting Uncertainty: This assessment relies on two metrics to communicate the degree of certainty in Key Findings. See Guide to this Report for more on assessments of likelihood and confidence.
Unless offset by additional emissions reductions of ozone precursor emissions, there is high confidence that climate change will increase ozone levels over most of the United States, particularly over already polluted areas, thereby worsening the detrimental health and environmental effects due to ozone. The climate penalty results from changes in local weather conditions, including temperature and atmospheric circulation patterns, as well as changes in ozone precursor emissions that are influenced by meteorology. Climate change has already had an influence on ozone concentrations over the United States, offsetting some of the expected ozone benefit from reduced precursor emissions. The magnitude of the climate penalty over the United States could be reduced by mitigating climate change.
Climatic changes, including warmer springs, longer summer dry seasons, and drier soils and vegetation, have already lengthened the wildfire season and increased the frequency of large wildfires. Exposure to wildfire smoke increases the risk of respiratory disease, resulting in adverse impacts to human health. Longer fire seasons and increases in the number of large fires would impair both human health and visibility.
Climate change, specifically rising temperatures and increased carbon dioxide (CO2) concentrations, can influence plant-based allergens, hay fever, and asthma in three ways: by increasing the duration of the pollen season, by increasing the amount of pollen produced by plants, and by altering the degree of allergic reactions to the pollen.
The energy sector, which includes energy production, conversion, and use, accounts for 84% of greenhouse gas (GHG) emissions in the United States as well as 80% of emissions of nitrogen oxides (NOx) and 96% of sulfur dioxide, the major precursor of sulfate aerosol. In addition to reducing future warming, reductions in GHG emissions often result in co-benefits (other positive effects, such as improved air quality) and possibly some negative effects (disbenefits) (Ch. 29: Mitigation). Specifically, mitigating GHG emissions can lower emissions of particulate matter (PM), ozone and PM precursors, and other hazardous pollutants, reducing the risks to human health from air pollution.
<b>Nolte</b>, C.G., P.D. Dolwick, N. Fann, L.W. Horowitz, V. Naik, R.W. Pinder, T.L. Spero, D.A. Winner, and L.H. Ziska, 2018: Air Quality. In <i>Impacts, Risks, and Adaptation in the United States: Fourth National Climate Assessment, Volume II</i> [Reidmiller, D.R., C.W. Avery, D.R. Easterling, K.E. Kunkel, K.L.M. Lewis, T.K. Maycock, and B.C. Stewart (eds.)]. U.S. Global Change Research Program, Washington, DC, USA, pp. 512–538. doi: 10.7930/NCA4.2018.CH13
Air quality is important for human health, vegetation, and crops as well as aesthetic considerations (such as visibility) that affect appreciation of the natural beauty of national parks and other outdoor spaces. Many of the processes that determine air quality are affected by weather (Figure 13.1). For example, hot, sunny days can increase ozone levels, while stagnant weather conditions can produce high concentrations of both ozone and particulate matter (PM). Ozone and PM are air pollutants that adversely affect human health and are monitored and regulated with national standards. Temperature, wind patterns, cloud cover, and precipitation, as well as the amounts and types of pollutants emitted into the air from human activities and natural sources, all affect air quality (Figure 13.1). Thus, climate-driven changes in weather, human activity, and natural emissions are all expected to impact future air quality across the United States.
These climate effects on air quality are not expected to occur uniformly at all locations. For example, as discussed in Chapter 2: Climate, precipitation is projected to increase in some regions of the country and decrease in other regions. Regions that experience excessive periods of drought and higher temperatures will have increased frequency of wildfires and more windblown dust from soils. At the same time, changes to temperatures and rainfall affect the types of crops that can be grown (Ch. 10: Ag & Rural) and the length of the growing season, the application of fertilizers and pesticides to crops, and ensuing transport and fate of those chemicals into the air, water, and soil. In the future, climate change is expected to alter the demand for heating and cooling of indoor spaces due to changes in temperatures. The resulting shift in fuel types and amounts used will modify the amount and composition of air pollutants emitted. Climate change can also increase the duration of the pollen season and the amount of pollen at some locations, as well as worsen respiratory health impacts due to pollen exposure. Despite the potential variability in regional impacts of climate change, there is evidence that climate change will increase the risk of unhealthy air quality in the future across the Nation in the absence of further air pollution control efforts (for other impacts of climate change on health, see Ch. 14: Human Health).
Since people spend most of their time inside buildings, indoor air quality is important for human health. Indoor air pollutants may come from interior sources or may be transported into buildings with outdoor air. If there are changes in airborne pollutants of outdoor origin, such as ozone, pollen, mold, and PM2.5 (particulate matter less than 2.5 micrometers in diameter), there will be changes in indoor exposures to these contaminants.2,3
There is robust evidence from models and observations that climate change is worsening ozone pollution. The net effect of climate change on PM pollution is less certain than for ozone, but increases in smoke from wildfires and windblown dust from regions affected by drought are expected. The complex interactions of natural variability with changes in climate and emissions pose a significant challenge for air quality management. Some approaches to mitigating climate change could result in large near-term co-benefits for air quality.
Ground-level ozone and particulate matter are common air pollutants that pose a serious risk to human health and the environment.5,6 Short- and long-term exposure to these pollutants results in adverse respiratory and cardiovascular effects,7 including premature deaths,8 hospital and emergency room visits, aggravated asthma,3,9 and shortness of breath.10 Certain population groups, such as the elderly, children, and those with chronic illnesses, are especially susceptible to ozone and PM-related effects.11,12,13
A growing body of evidence indicates the harmful effects of short-term (i.e., daily) exposures to ground-level ozone vary with climate conditions, specifically temperature.14,15,16,17,18 For a given level of ozone, higher temperatures increase the risk of ozone-related premature death.14,19,20,21 However, the risk of premature death is likely to decrease as the prevalence of air conditioning increases, as is expected to occur with rising temperatures.22 The extent to which the growing use of air conditioning will offset climate-induced increases in ozone-related premature death is unknown.
Ozone is not directly emitted but is formed in the atmosphere by reactions between nitrogen oxides (NOx) and volatile organic compounds (VOCs). Ozone concentrations depend on emissions of these two precursors as well as weather conditions such as temperature, humidity, cloud cover, and winds.3 These emissions come from a variety of human sources, such as power plants and motor vehicles, and from natural sources, such as forests and wildfires (Figure 13.1). Additionally, ozone concentrations in one region may be influenced by the transport of either precursors or ozone itself from another region.23,24
Ozone levels in the United States are often highest in Southern California and the Northeast Corridor as well as around other large cities like Dallas, Houston, Denver, Phoenix, and Chicago,25 and during extended episodes of extreme heat and sunshine.26 Ozone air quality in the United States has improved dramatically over the past few decades due to NOx and VOC emissions control efforts, despite population and economic growth.27,28,29 Nationally, ozone concentrations have been reduced by 22% over the 1990 to 2016 period.29 Nonetheless, in 2015 nearly 1 in 3 Americans were exposed to ozone values that exceeded the national standard determined by the U.S. Environmental Protection Agency (EPA) to be protective of human health.29 Adverse human health impacts associated with exposure to ground-level ozone include premature death, respiratory hospital admissions, cases of aggravated asthma, lost days of school, and reduced productivity among outdoor workers.30,31,32 Ozone pollution can also damage crops and plant communities, including forests, by reducing photosynthesis.33
Due in part to air pollutant regulations driven by the Clean Air Act, NOx and VOC emissions from human sources should continue to decline over the next few decades.34 These emissions reductions are designed to reduce ozone concentrations so that polluted areas of the country meet air quality standards. However, climate change will also influence future levels of ozone in the United States by altering weather conditions and impacting emissions from human and natural sources. The prevailing evidence strongly suggests that climate change alone introduces a climate penalty (an increase in air pollution resulting from climate change35,36) for ozone over most of the United States from warmer temperatures and increases in natural emissions.3,4,37,38 This climate penalty will partially counteract the continued reductions in emissions of ozone precursors from human activities.
Tiny liquid or solid particles suspended in the atmosphere are known as aerosols or particulate matter (PM). PM includes many different chemical components, such as sulfate, nitrate, organic and black carbon, mineral dust, and sea spray. Unlike ozone, PM can be either directly emitted or formed in the atmosphere. PM2.5 refers to atmospheric PM with a diameter less than 2.5 micrometers. These particles are small enough to be inhaled deeply, and exposure to high concentrations can result in serious health impacts, including premature death, nonfatal heart attacks, and adverse birth outcomes.5,39,40,41 PM2.5 concentrations vary greatly with daily weather conditions,42,43 depending particularly on wind speed (which affects the mixing of pollutants) and precipitation (which removes particles from the air).4 Concentrations of PM2.5 build up during long periods of low wind speeds, and they are reduced when weather fronts move air through a region.4
Wildfires not only emit gases that contribute to ozone formation44,45,46,47,48 but they also are a major source of PM, especially in the western United States during the summer49,50,51,52,53,54,55 and in the Southeast48,56 (see Ch. 6: Forests; Ch. 19: Southeast, Case Study “Prescribed Fire”; Ch. 24: Northwest; and Ch. 25: Southwest). Wildfire smoke can worsen air quality locally,57 with substantial public health impacts in regions with large populations near heavily forested areas.56,58,59,60,61 Exposure to wildfire smoke increases the incidence of respiratory illnesses, including asthma, chronic obstructive pulmonary disease, bronchitis, and pneumonia.62 Smoke can decrease visibility63 and can be transported hundreds of miles downwind, often crossing national boundaries.54,64,65,66,67,68,69
Climate change is expected to impact atmospheric PM concentrations in numerous ways.38,70 Changing weather patterns, including increased stagnation,71,72 altered frequency of weather fronts,73,74 more frequent heavy rain events,43 changing emissions from vegetation75,76 and human sources,77 and increased evaporation of some aerosol components78 will all affect PM concentrations. In addition, more frequent and longer droughts would lengthen the wildfire season79,80,81 and result in larger wildfires82,83 and increased dust emissions in some areas.84 Projections of regional precipitation changes show considerable variation across models and thus remain highly uncertain.85 Accurately assessing how PM2.5 concentrations will respond to the changing climate is difficult due to these complex and highly spatially variable interactions.
Unless offset by additional reductions of ozone precursor emissions, there is high confidence that climate change will increase ozone levels over most of the United States, particularly over already polluted areas,3,86 thereby worsening the detrimental health and environmental effects due to ozone. Although competing meteorological effects determine local ozone levels, temperature is often the largest single driver.87 The climate penalty35,36 results from changes in local weather conditions, including temperature and atmospheric circulation patterns,4,88 as well as changes in ozone precursor emissions that are influenced by meteorology.75,76,77 Climate change has already had an influence on ozone concentrations over the United States, offsetting some of the expected ozone benefit from reduced precursor emissions.89,90 Assessments of climate change impacts on ozone trends are complicated by year-to-year changes in weather conditions91 and require multiple years of model information to estimate the potential range of effects.92 Besides being affected by climate change, future ozone levels in the United States will also be affected greatly by domestic emissions of ozone precursors as well as by international emissions of ozone precursors and global methane levels. Studies suggest that climate change will decrease the sensitivity of regional ozone air quality to intercontinental sources.93
PM2.5 accounts for most of the health impacts due to air pollution in the United States,94 and small changes in average concentrations have large implications for public health. Without consideration of climate effects, concentrations of PM2.5 in the United States are projected to decline through 2040 due to ongoing emissions control efforts.34 PM2.5 is highly sensitive to weather conditions, including temperature, humidity, wind speed, and rainfall. The effects of climate change on the timing, intensity, duration, and frequency of rainfall are highly uncertain, influencing both the removal of PM2.5 from air and the incidence of wildfires and their associated emissions. Accordingly, the net impact of climate-driven weather changes on PM2.5 concentrations is less certain than for ozone.3,4,43,70 However, some studies have indicated that even without considering increased wildfire frequency, climate change will cause a small but important increase in PM2.5 over North America.95,96 The impact of climate change on the PM2.5 contribution from intercontinental sources, which depends strongly on projected changes in precipitation, remains highly uncertain.24
The health impacts of climate-induced changes in air quality may be reduced by various adaptation measures. For example, as local authorities issue air quality alerts, people may reduce their exposure to air pollution by postponing outdoor activities and staying indoors (for further information on the role of adaptation in reducing climate-related health risks, (see Ch. 14: Human Health, KM 3).
The magnitude of the climate penalty over the United States could be reduced by mitigating climate change.1,90,97 For example, Figure 13.2 shows results from one study1 projecting the change in summertime ozone resulting from two different future scenarios (RCP8.5 and RCP4.5) see the Scenario Products section of App. 3 for additional information about these scenarios) at 2050 and 2090, with human emissions of ozone precursors held constant. Due to climate change, ozone is projected to increase over a broad portion of the United States. Mitigating climate change globally (for instance, following RCP4.5 rather than RCP8.5) would reduce the impact on ozone, resulting in fewer adverse health effects, including 500 fewer premature deaths per year due to ozone in 2090.1
Climatic changes, including warmer springs, longer summer dry seasons, and drier soils and vegetation, have already lengthened the wildfire season79,80,81,98 (see Ch. 6: Forests) and increased the frequency of large wildfires.82,83 Human-caused climate change is estimated to have doubled the area of forest burned in the western United States from 1984 to 2015.99 Projections indicate that the wildfire frequency and burned area in North America will continue to increase over the 21st century due to climate change.100,101,102,103,104,105,106
Wildfires and prescribed fires contribute to ozone formation44,107 and are major sources of PM, together comprising about 40% of directly emitted PM2.5 in the United States in 2011.34 Exposure to wildfire smoke increases the risk of respiratory disease and mortality.56,60,62 Longer fire seasons and increases in the number of large fires would impair both human health108 and visibility.54,63 Wildfires are projected to become the principal driver of summertime PM2.5 concentrations, offsetting even large reductions in emissions of PM2.5 precursors.54,109
Opportunities for outdoor recreational activities are also vulnerable to changes in the frequency and intensity of wildfires due to climate change. Climate change-induced increases in wildfire smoke events are likely to reduce the amount and quality of time spent in outdoor activities (see Ch. 22: N. Great Plains, KM 3 and Ch. 24: Northwest, KM 4). More accurate forecasting of smoke events may mitigate some of the negative effects through changes in timing of outdoor activities.
Forests are actively managed, and the frequency and severity of wildfire occurrence in the future will not be determined solely by climate factors. Humans affect fire activity in many ways, including increasing ignitions and conducting controlled burns and fire suppression.110,111 Forest management decisions may outweigh the impacts of climate change on both forest ecosystems and air quality.112
Climate change, specifically rising temperatures and increased CO2 concentrations, can influence plant-based allergens, hay fever, and asthma in three ways: by increasing the duration of the pollen season, by increasing the amount of pollen produced by plants, and by altering the degree of allergic reactions to pollen.
Seasonally, airborne allergen (aeroallergen) exposure in the United States begins with the release of tree pollen in the spring. Between the 1950s and the early 2000s, warming winters and earlier arrival of springs have resulted in earlier flowering of oak trees.113 Projected increases in CO2 induce earlier and greater seasonal pollen production in pine trees114 and oak trees.115 For summer pollen producers, such as weeds and grasses, the effect of warming temperatures on earlier flowering is less evident. However, the allergen content of timothy grass pollen increases with concurrent increases in ozone and CO2.116 For common ragweed, the primary fall aeroallergen, greenhouse studies simulating increased temperature and CO2 concentrations resulted in earlier flowering, greater floral numbers, increased pollen production, and enhanced allergen content of the pollen.117,118,119,120 Regional and continental studies indicate that ragweed growth and pollen production increase with urban-induced increases in temperature and CO2. Ragweed pollen season exposure varies as a function of latitude and delayed autumnal frosts in North America.119,121 In addition to pollen, aeroallergens are also generated by molds. Plants are often affected, since they can serve as hosts for fungi. For example, projected end-of-century CO2 concentrations would substantially increase the number of allergenic spores produced from timothy grass.122
Although warming temperatures and rising CO2 levels clearly increase aeroallergen prevalence, the link between exposure and health impacts is less well established. However, hay fever prevalence has been associated with exposure to annual and seasonal extreme heat events.123 Furthermore, climate-induced changes in oak pollen are projected to increase the number of asthma-related emergency department visits in the Northeast, Southwest, and Midwest.115
The energy sector, which includes energy production, conversion, and use, accounts for 84% of greenhouse gas (GHG) emissions124 as well as 80% of emissions of NOx and 96% of sulfur dioxide, the major precursor of sulfate aerosol.125 In addition to reducing future warming, reductions in GHG emissions often result in co-benefits (other positive effects, such as improved air quality) and possibly some negative effects (disbenefits) (Ch. 29: Mitigation). Specifically, mitigating GHGs can lower emissions of PM, ozone and PM precursors, and other hazardous pollutants, reducing the risks to human health from air pollution.97,126,127,128,129,130 However, the magnitude of air quality co-benefits depends on a number of factors. Areas with higher levels of air pollution have more potential for air quality co-benefits compared to areas where emission controls have been enacted and air pollution levels have been reduced.131 Different approaches to GHG mitigation yield different reductions, or in some cases, increases in ozone and PM precursors.132 For example, diesel vehicles emit less GHGs than gasoline-powered vehicles, but without correctly operating pollution-control devices, diesel vehicles emit more particles and ozone precursors and thus contribute more to air quality human health risks.133
In addition to co-benefits from sources that emit multiple pollutants, mitigating individual GHGs could yield co-benefits. For example, methane is both a GHG and a slowly reactive ozone precursor that contributes to global background surface ozone concentrations. Some monitoring stations in remote parts of the western United States have recorded rising ozone concentrations, resulting in part from increased global methane levels.90 The magnitude of the human health benefit of lowering ozone levels via methane mitigation is substantial and is similar in value to the climate change benefits.134,135 Additionally, PM influences climate on local to global scales by affecting the radiation balance of the Earth,23,136 so controlling emissions of PM and its precursors would not only yield direct human health benefits via reduced exposure but also avoid or minimize local meteorological conditions that lead to a buildup of pollutants.137
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.3 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.