The frequency and severity of smog in an area depends least upon the

Changes in the climate affect air pollution levels.8,12,13,14,15,16,17,18,19,20,21,22 Human-caused climate change has the potential to increase ozone levels,1,4 may have already increased ozone pollution in some regions of the United States,3 and has the potential to affect future concentrations of ozone and fine particles ( particulate matter smaller than 2.5 microns in diameter, referred to as PM 2.5).2,7 Climate change and air quality are both affected by, and influence, several factors; these include the levels and types of pollutants emitted, how land is used, the chemistry governing how these pollutants form in the atmosphere, and weather conditions.

Ground-Level Ozone

Ozone levels and subsequent ozone-related health impacts depend on 1) the amount of pollutants emitted that form ozone, and 2) the meteorological conditions that help determine the amount of ozone produced from those emissions. Both of these factors are expected to change in the future. The emissions of pollutants from anthropogenic (of human origin) sources that form ozone (that is, ozone “precursors”) are expected to decrease over the next few decades in the United States.23 However, irrespective of these changes in emissions, climate change will result in meteorological conditions more favorable to forming ozone. Consequently, attaining national air quality standards for ground-level ozone will also be more difficult, as climate changes offset some of the improvements that would otherwise be expected from emissions reductions. This effect is referred to as the “climate penalty.”7,24

Meteorological conditions influencing ozone levels include air temperatures, humidity, cloud cover, precipitation, wind trajectories, and the amount of vertical mixing in the atmosphere.1,2,25,26 Higher temperatures can increase the chemical rates at which ozone is formed and increase ozone precursor emissions from anthropogenic sources and biogenic(vegetative)sources. Lower relative humidity reduces cloud cover and rainfall, promoting the formation of ozone and extending ozone lifetime in the atmosphere. A changing climate will also modify wind patterns across the United States, which will influence local ozone levels. Over much of the country, the worst ozone episodes tend to occur when the local air mass does not change over a period of several days, allowing ozone and ozone precursor emissions to accumulate over time.27,28 Climate change is already increasing the frequency of these types of stagnation events over parts of the United States,3 and further increases are projected.29 Ozone concentrations near the ground are strongly influenced by upward and downward movement of air (“vertical mixing”). For example, high concentrations of ozone near the ground often occur in urban areas when there is downward movement of air associated with high pressure (“subsidence”), reducing the extent to which locally emitted pollutants are diluted in the atmosphere.30 In addition, high concentrations of ozone can occur in some rural areas resulting from downward transport of ozone from the stratosphere or upper troposphere to the ground.31

Ozone ( O3 ) is a compound that occurs naturally in Earth’s atmosphere but is also formed by human activities. In the stratosphere (10–50 kilometers above the Earth’s surface), O3 prevents harmful solar ultraviolet radiation from reaching the Earth’s surface. Near the surface, however, O3 irritates the respiratory system. Ground-level O3, a key component of smog, is formed by chemical interactions between sunlight and pollutants including nitrogen oxides ( NOx ) and volatile organic compounds (VOCs). The emissions leading to O3 formation can result from both human sources (for example, motor vehicles and electric power generation) and natural sources (for example, vegetation and wildfires). Occasionally, O3 that is created naturally in the stratosphere can be mixed downward and contribute to O3 levels near the surface. Once formed, O3 can be transported by the wind before eventually being removed from the atmosphere via chemical reactions or by depositing on the surface.

At any given location, O3 levels are influenced by complex interactions between emissions and meteorological conditions. Generally, higher temperatures, sunnier skies, and lighter winds lead to higher O3 concentrations by increasing the rate of chemical reactions and by decreasing the extent to which pollutants are mixed with “clean” (less polluted) background air.

For a given level of emissions of O3 precursors, climate change is generally expected to increase O3 pollution in the future throughout much of the United States, in part due to higher temperatures and more frequent stagnant air conditions.7 Unless offset by additional emissions reductions of ozone precursors, these climate-driven increases in O3 will cause premature deaths, hospital visits, lost school days, and acute respiratory symptoms.14

Aside from the direct meteorological influences, there are also indirect impacts on U.S. ozone levels from other climate-influenced factors. For instance, higher water vapor concentrations due to increased temperatures will increase the natural rate of ozone depletion, particularly in remote areas,32 thus decreasing the baseline level of ozone. Additionally, potential climate-driven increases in nitrogen oxides (NOx) created by lightning or increased exchange of naturally produced ozone in the stratosphere to the troposphere could also affect ozone in those areas of the country most influenced by background ozone concentrations.33 Increased occurrences of wildfires due to climate change can also lead to increased ozone concentrations near the ground.34 

There is natural year-to-year variability in temperature and other meteorological factors that influence ozone levels.7 While global average temperature over 30-year climatic timescales is expected to increase, natural interannual variability will continue to play a significant role in year-to-year changes in temperature.35 Over the next several decades, the influence of climate change on meteorological parameters affecting average levels of ozone is expected to be smaller than the natural interannual variability.36

To address these issues, most assessments of climate impacts on meteorology and associated ozone formation concurrently simulate global and regional chemical transport over multiple years using “coupled” models. This approach can isolate the influence of meteorology in forming ozone from the effect of changes in emissions. The consensus of these model-based assessments is that accelerated rates of photochemical reaction, increased occurrence of stagnation events, and other direct meteorological influences are likely to lead to higher levels of ozone over large portions of the United States.8,14,16,17 At the same time, ozone levels in certain regions are projected to decrease as a result of climate change, likely due to localized increases in cloud cover, precipitation, and/or increased dilution resulting from deeper mixed layers. These climate-driven changes in projected ozone vary by season and location, with climate and air quality models showing the most consistency in ozone increases due to climate change in the northeastern United States.8,37

Generally, ozone levels will likely increase across the United States if ozone precursors are unchanged (see “Research Highlight: Ozone-Related Health Effects”) .4,7,8 This climate penalty for ozone will offset some of the expected health benefits that would otherwise result from the expected ongoing reductions of ozone precursor emissions, and could prompt the need for adaptive measures (for example, additional ozone precursor emissions reductions) to meet national air quality goals.

Air pollution epidemiology studies describe the relationship between a population’s historical exposure to air pollutants and the risk of adverse health outcomes. Populations exposed to ozone air pollution are at greater risk of dying prematurely, being admitted to the hospital for respiratory hospital admissions, being admitted to the emergency department, and suffering from aggravated asthma , among other impacts.38,39,40

Air pollution health impact assessments combine risk estimates from these epidemiology studies with modeled changes in future or historical air quality changes to estimate the number of air-pollution-related premature deaths and illness.41 Future ozone-related human health impacts attributable to climate change are projected to lead to hundreds to thousands of premature deaths, hospital admissions, and cases of acute respiratory illnesses per year in the United States in 2030.14,42,43,44,45,46

Health outcomes that can be attributed to climate change impacts on air pollution are sensitive to a number of factors noted above—including the climate models used to describe meteorological changes (including precipitation and cloud cover), the models simulating air quality levels (including wildfire incidence), the size and distribution of the population exposed, and the health status of that population (which influences their susceptibility to air pollution; see Ch. 1: Introduction).42,47,48,49   Moreover, there is emerging evidence that air pollution can interact with climate-related stressors such as temperature to affect the human physiological response to air pollution.39,42,50,51,52,53,54,55 For example, the risk of dying from exposure to a given level of ozone may increase on warmer days.51

The frequency and severity of smog in an area depends least upon the

Los Angeles, California, May 22, 2012. Unless offset by additional emissions reductions of ozone precursors, climate -driven increases in ozone will cause premature deaths, hospital visits, lost school days, and acute respiratory symptoms.

Importance: Ozone is formed in the atmosphere by photochemical reactions of volatile organic compounds (VOCs) and nitrogen oxides ( NOx ) in the presence of sunlight. Although U.S. air quality policies are projected to reduce VOC and NOx emissions,58 climate change will increase the frequency of regional weather patterns conducive to increasing ground-level ozone, partially offsetting the expected improvements in air quality.

Objective: Project the number and geographic distribution of additional ozone-related illnesses and premature deaths in the contiguous United States due to climate change between 2000 and 2030 under projected U.S. air quality policies.

Method: Climate scenarios from two global climate models (GCMs) using two different emissions pathways (RCP8.5 and RCP6.0) were dynamically downscaled following Otte et al. (2012)79 and used with emissions projections for 2030 and a regional chemical transport model to simulate air quality in the contiguous United States. The resulting changes in ozone in each scenario were then used to compute regional ozone-related health effects attributable to climate change. Ozone-related health impacts were estimated using the environmental Benefits Mapping and Analysis Program–Community Edition (BenMAP–CE). Population exposure was estimated using projected population data from the Integrated Climate and Land Use Scenarios ( ICLUS ). Further details can be found in Fann et al. (2015).14

Interact with the Figure Below

Projected changes in average daily maximum temperature (degrees Fahrenheit), summer average maximum daily 8-hour ozone (parts per billion), and excess ozone-related deaths (incidences per year by county) in the year 2030 relative to the year 2000, following two global climate models and two greenhouse gas concentration pathways, known as Representative Concentration Pathways, or RCPs (see van Vuuren et al. 201149). Each year (2000 and 2030) is represented by 11 years of modeled data for May through September, the traditional ozone season in the United States.

The Higher Emissions maps are based on the National Center for Atmospheric Research/Department of Energy (NCAR/ DOE) Community Earth System Model (CESM) following RCP8.5 (a higher greenhouse gas concentration pathway). The Moderate Emissions maps are based on the National Aeronautics and Space Administration (NASA) Goddard Institute for Space Studies (GISS) ModelE2-R following RCP6.0 (a moderate greenhouse gas concentration pathway).

The leftmost panels are based on dynamically downscaled regional climate using the NCAR Weather Research and Forecasting (WRF) model, the center panels are based on air quality simulations from the U.S. Environmental Protection Agency (EPA) Community Multiscale Air Quality (CMAQ) model, and the rightmost panels are based on the U.S. EPA Environmental Benefits and Mapping Program (BenMAP).

Fann et al. 2015 reports a range of mortality outcomes based on different methods of computing the mortality effects of ozone changes—the changes in the number of deaths shown in the rightmost panels were computed using the method described in Bell et al. 2004.14,38 (Figure source: adapted from Fann et al. 2015)14

Results: The two downscaled GCM projections result in 1°C to 4°C (1.8°F to 7.2°F) increases in average daily maximum temperatures and 1 to 5 parts per billion increases in daily 8-hour maximum ozone in 2030 throughout the contiguous United States. As seen in previous modeling analyses of climate impacts on ozone, the air quality response to climate change can vary substantially by region and across scenarios.22,80 Unless reductions in ozone precursor emissions offset the influence of climate change, this climate penalty of increased ozone concentrations due to climate change would result in tens to thousands of additional ozone-related illnesses and premature deaths per year.

The frequency and severity of smog in an area depends least upon the


Projected change in ozone -related premature deaths from 2000 to 2030 by U.S. region and based on CESM/RCP8.5. Each year (2000 and 2030) is represented by 11 years of modeled data. Ozone-related premature deaths were calculated using the risk coefficient from Bell et al. (2004).38 Boxes indicate 25th, 50th, and 75th percentile change over 11-year sample periods, and vertical lines extend to 1.5 times the interquartile range. U.S. regions follow geopolitical boundaries shown in Figure 3.2. (Figure source: Fann et al. 2015)14

Conclusions: Future climate change will result in higher ozone levels in polluted regions of the contiguous United States. This study isolates the effect of climate change on ozone by using the same emissions of ozone precursors for both 2000-era and 2030-era climate. In addition, this study uses the latest generation of GCM scenarios and represents the most comprehensive analysis of climate-related, ozone-attributable health effects in 2030, and includes not only deaths but also emergency department admissions for asthma, hospital visits for respiratory causes, acute respiratory symptoms, and missed days of school. These results are subject to important uncertainties and limitations. The ozone-climate modeling reflects two scenarios (based on two separate GCMs) considered. Several emissions categories that are important in the formation of ozone and that could be affected by climate, such as motor vehicles, electrical generating units, and wildfires, were left unchanged between the current and future periods. The analysis applied concentration–response relationships from epidemiology studies of historical air pollution episodes; this both implies that the relationship between air pollution and risk will remain constant into the future and that populations will not attempt to reduce their exposure to ozone.

Particulate Matter

Particulate matter (PM) is a complex mixture of solid- or liquid-phase substances in the atmosphere that arise from both natural and human sources. Principal constituents of PM include sulfate, nitrate, ammonium, organic carbon, elemental carbon, sea salt, and dust. These particles (also known as aerosols) can either be directly emitted or can be formed in the atmosphere from gas-phase precursors. PM smaller than 2.5 microns in diameter (PM2.5) is associated with serious chronic and acute health effects, including lung cancer, chronic obstructive pulmonary disease ( COPD ), cardiovascular disease, and asthma development and exacerbation.11 The elderly are particularly sensitive to short-term particle exposure, with a higher risk of hospitalization and death.56,57

As is the case for ozone, atmospheric PM2.5 concentrations depend on emissions and on meteorology. Emissions of sulfur dioxide (SO2), NOx, and black carbon are projected to decline substantially in the United States over the next few decades due to regulatory controls,58,59,60,61 which will lead to reductions in sulfate and nitrate aerosols.

Climate change is expected to alter several meteorological factors that affect PM2.5, including precipitation patterns and humidity, although there is greater consensus regarding the effects of meteorological changes on ozone than on PM2.5.2 Several factors, such as increased humidity, increased stagnation events, and increased biogenic emissions are likely to increase PM2.5 levels, while increases in precipitation, enhanced atmospheric mixing, and other factors could decrease PM2.5 levels.2,8,37,62 Because of the strong influence of changes in precipitation and atmospheric mixing on PM2.5 levels, and because there is more variability in projected changes to those variables, there is no consensus yet on whether meteorological changes will lead to a net increase or decrease in PM2.5 levels in the United States.2,8,17,21,22,62,63

As a result, while it is clear that PM2.5 accounts for most of the health burden of outdoor air pollution in the United States,10 the health effects of climate-induced changes in PM2.5 are poorly quantified. Some studies have found that changes in PM2.5 will be the dominant driver of air quality-related health effects due to climate change,44 while others have suggested a potentially more significant health burden from changes in ozone.50

PM resulting from natural sources (such as plants, wildfires, and dust) is sensitive to daily weather patterns, and those fluctuations can affect the intensity of extreme PM episodes (see also Ch. 4: Extreme Events, Section 4.6).8 Wildfires are a major source of PM, especially in the western United States during summer.64,65,66 Because winds carry PM2.5 and ozone precursor gases, air pollution from wildfires can affect people even far downwind from the fire location.35,67 PM2.5 from wildfires affects human health by increasing the risk of premature death and hospital and emergency department visits.68,69,70

Climate change has already led to an increased frequency of large wildfires, as well as longer durations of individual wildfires and longer wildfire seasons in the western United States.71 Future climate change is projected to increase wildfire risks72,73 and associated emissions, with harmful impacts on health.74 The area burned by wildfires in North America is expected to increase dramatically over the 21st century due to climate change.75,76 By 2050, changes in wildfires in the western United States are projected to result in 40% increases of organic carbon and 20% increases in elemental carbon aerosol concentrations.77 Wildfires may dominate summertime PM2.5 concentrations, offsetting even large reductions in anthropogenic PM2.5 emissions.22

Likewise, dust can be an important constituent of PM, especially in the southwest United States. The severity and spatial extent of drought has been projected to increase as a result of climate change,78 though the impact of increased aridity on airborne dust PM has not been quantified (see Ch. 4. Extreme Events).2