Links of Conventional Air Pollution to Climate, Environmental Impacts and Mitigation

Through shared generation processes and interaction in the atmosphere, tropospheric ozone, aerosols, and other air pollutants are inextricably linked to the problem of climate change. Aerosols and ozone affect the Earth's radiative balance directly, as well as indirectly through interactions with clouds and other climatic processes. Work in this area focuses on evaluating and analyzing these linkages and in clarifying policy questions that hinge on the magnitude of ancillary benefits of greenhouse gas mitigation and climate benefits of urban air pollution control. To address these multiple, interacting concerns, we model the atmospheric chemistry of the urban environment, the economic implications of the joint production of urban pollutants and greenhouse gases, the radiative and climatic properties of pollutants, and the impact these multiple stressors have on ecosystems and on human health.

The Program has pioneered the representation of urban air pollution processes in a global modeling system to investigate how the formation of air pollution within the urban environment affects global concentrations of greenhouse gases (Linking local air pollution to global chemistry and climate).

Would regulation that leads to a reduction of conventional air pollutants, such as ozone and carbonaceous aerosols, have a significant climate benefit? Probably not, because the control of these substances would likely result in a decrease in the formation of reflective aerosols that have a cooling effect. These two opposing forces are likely to largely cancel each other, with only a small overall temperature effect (Effects of air pollution control on climate: Results from an integrated assessment model). But air pollution can have effects well beyond the direct radiative properties of the gases and aerosols they form in the atmosphere. In fact, tropospheric ozone damages vegetation, and thus failure to control ozone could substantially decrease carbon uptake by vegetation with the consequence being a significant feedback to the carbon cycle (Future effects of ozone on carbon sequestration and climate change policy using a global biogeochemical model).

Devising useful climate-change policy is difficult enough if one focuses only on the long-lived greenhouse gases, but that can be misleading. We can't adequately explain recent historical trends in climate without including the influence of aerosols from natural processes as well as those resulting from human activities. The science community has responded to policy-makers by collapsing the climate effects of radiative substances into a single metric of global warming potential (GWP) that helps to convey which substances are the bigger culprits in contributing to global warming. This metric does not, however, extend easily to the shorter-lived substances such as ozone and aerosols. For example, these pollutants have serious consequences for human health (Toward integrated assessment of environmental change: air pollution health effects in the USA), ecoystems (Future effects of ozone on carbon sequestration and climate change policy using a global biogeochemical model) and agriculture (Global economic effects of changes in crops, pasture, and forests due to changing climate, carbon dioxide, and ozone).

Another important complication to be considered is that the impact of aerosols on climate is very unlike that of greenhouse gases. Carbonaecous aerosols operate very differently -- they warm the lower atmosphere where they intercept light, but, unlike greenhouse gases, they also shade and cool the surface. The impact of aerosols can thus tend to stabilize the atmosphere, which affects the convective forces that determine where precipitation falls. In contrast, greenhouse gases actually increase the vertical temperature gradient and thereby act to destabilize the atmosphere, imparting a very different influence on precipitation. So the difficulty of projecting how climate change will affect precipitation thus depends on understanding not only the effects of aerosols and greenhouse gases on clouds and cloud formation. It also involves estimating the potential differential increases in emissions of the various gas and aerosol substances as economies develop, and perhaps control some pollutants while leaving others uncontrolled.