Many constituents of the atmosphere that impact public health also play a significant role in influencing climate. Of concern are aerosols, including black carbon, organic carbon, and sulfates. As discussed in Chapter 6, aerosols can have a net cooling effect on climate if they increase the Earth's reflectivity, such as inorganic carbon released during biomass burning, or a net warming effect if they absorb outgoing infrared radi ation, such as the black carbon released during incomplete combustion of diesel fuel and biomass burning. Aerosols are of concern for human health due to their impacts on lung function and on respiratory and cardiac disease (Smith et al., 2009).
Tropospheric ozone is not only a greenhouse gas (GHG); it is also classified as a criteria air pollutant. Ozone is a secondary pollutant formed from the action of sunlight on ozone precursors such as carbon monoxide, nitrogen oxides, and volatile organic compounds (see Chapter 6). Human-caused emissions of ozone precursors have led to large increases in tropospheric ozone over the past century (Marenco et al., 1994; Wang and Jacob, 1998). When increased ozone events occur simultaneously with heat waves, the mortality rate can rise by as much as 175 percent (Filleul et al., 2006). Acute exposure to elevated concentrations of ozone is associated with increased hospital admissions for pneumonia, chronic obstructive pulmonary disease, asthma, allergic rhinitis, and other respiratory diseases, and also with premature mortality (e.g., Bell et al., 2005, 2006; Gryparis et al., 2004; Ito et al., 2005; Levy et al., 2005; Mudway and Kelly, 2000). A National Research Council committee concluded that "the association between short-term changes in ozone concentrations and mortality is generally linear throughout most of the concentration ranges. If there is a threshold, it is probably at a concentration below the current ambient air standard" (NRC, 2008e).
Although projected increases in temperatures across the United States in the decades ahead may raise the occurrence of high ozone concentrations (see Figure 11.5), ozone concentrations also depend on a wide range of other factors, including the rate and amount of ozone precursor emissions, human actions taken to limit ozone precursors, and meteorological factors. For example, extremely hot days tend to be associated with stagnant air circulation patterns that can concentrate ground-level ozone, exacerbating respiratory diseases and short-term reductions in lung function (USGCRP, 2001). Under one scenario of climate change for 50 U.S. cities, the increase in temperature projected to occur by the 2050s due to climate change, and a subsequent rise in tropospheric ozone, could exacerbate ozone-related health effects such as cardiovascular, respiratory, and total mortality, as well as hospital admissions for asthma, chronic obstructive pulmonary disease, and respiratory diseases of the elderly (Bell et al., 2007).
Climate change could also affect local and regional air quality through temperature-induced changes in chemical reaction rates, changes in boundary-layer heights that affect vertical mixing of pollutants, and changes in airflow patterns that govern pollutant transport. Responses to climate change can also affect air quality, most notably through changes in emissions associated with efforts to limit the magnitude of climate change. Sources of uncertainty include the degree of future climate change, future emissions of air pollutants and their precursors, and how population vulnerabil-
ity may change in the future. When precursor emissions are held constant, projections suggest climate change will increase concentrations of tropospheric ozone across many regions, increasing morbidity and mortality (Ebi and McGregor, 2008). Increases in urban ozone pollution alone could be as much as 10 parts per billion (ppb) over the next few decades, which would make it difficult for many cities to meet air quality standards (Ebi and McGregor, 2008; Jacob and Winner, 2009). The evidence is less robust for other air pollutants, although several studies have found increased mortality associated with simultaneous rise in temperature and surface aerosols, including both particulate matter and sulfur dioxide (Hu et al., 2008; Katsouyanni et al., 1997; Smith et al., 2009). However, research is needed to understand how concentrations of these pollutants could change with climate change.
There are several examples of how the health impacts of climate change intersect with ecosystem and agricultural impacts in the context of air quality. For example, higher ozone concentrations would be detrimental not only to human health but also to crop production. Losses in crop yields due to increasing ozone and other climate-related factors over the next two to three decades in some rapidly developing regions are expected to have a major impact on the food supply (see Chapter 10), possibly leading to malnutrition and other negative public health impacts (CCSP, 2008a; Epstein, 2005; Haines and Patz, 2004; The Royal Society, 2009). Another example is that the frequency and intensity of wildfires is enhanced in a warming climate (see Chapter 9), and this would be expected to lead to increases in the atmospheric concentration of fine particulate matter, which would have adverse health consequences (Epstein, 2005; Haines and Patz, 2004).
The potential synergies and trade-offs between climate change policies and public health policies are complex. For example, reducing some aerosols such as organic carbon or sulfates would reduce air pollution-related health impacts but increase the rate of climate change (Forster et al., 2007; see also Chapter 6). Conversely, some of the technologies and policy mechanisms that might be used to control climate change may also be complementary to measures adopted to control air pollution; for example, reducing commuter traffic by encouraging mass transit and carpooling would reduce both transportation-related GHG emissions and ozone precursors. Walking or biking for transportation would have the added benefit of increasing physical activity, potentially lowering the incidence of obesity and its related negative health outcomes. Policies designed to reduce or offset climate change may thus have a variety of intended and unintended consequences on public health, and vice versa.
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