Science to Support a Unified Policy on Climate Change and Air Quality
Emissions of pollutants into the atmosphere from human activities affect both the quality of the air we breathe and Earth's climate. However, policies are typically designed with the goal of limiting damage to one or the other, but largely treat the air quality and climate effects separately. For example, the United States Enviornmental Protection Agency regulates several pollutants affecting air quality but does not currently regulate carbon dioxide, the primary driver of modern climate change. On the other hand, the international Kyoto Protocol covers greenhouse gases such as carbon dioxide but does not include shorter-lived pollutants such as ozone and particulates which also affect climate.
To provide information that could help guide policy-makers concerned with these issues, we recently studied the joint response of air quality and radiative forcing, an indicator of climate change, to emissions reductions. We examined the impact of decreasing emissions from particular economic sectors in North America and in developing Asia. To test the uncertainty in our results, we compared the response in two composition/climate models, the NASA Goddard Institute for Space Studies ModelE and the NCAR Community Atmospheric Model.
The results show that when considering climate change over the next 20-30 years, taking into account the influence of short-lived pollutants dramatically changes the relative benefit of emissions reductions from a particular activity or region. For example, when one considers only long-lived gases (carbon dioxide and methane), reduced emissions from industry and power generation have a much larger impact than reduced emissions from surface transportation in North America (Figure 1). But when short-lived pollutants are included, the situation is reversed. This occurs because transportation in North America produces a substantial amount of black carbon (soot) and ozone (a main ingredient in smog), both of which warm climate, while power generation leads to a large amount of sulfate particles, which cool climate even as they also lead to acid rain and damage human health. Little sulfur is present in North American fuels, so transportation does not have a large cooling effect there.
In developing Asia, however, the sulfur content of fuels is much larger, so the impact of sulfate particles offsets the effect of changes in soot and ozone from transportation, leading to a net effect from short-lived pollutants that is quite small. Hence, reductions in emissions from surface transportation in North America have a much larger near-term climate change mitigation effect than do such reductions in developing Asia, although both would lead to air quality improvements.
The two models find very similar patterns of decreased surface level particulate concentrations (Figure 2). Because breathing particulates can lead to respiratory problems and even premature mortality, these reductions would have substantial benefits to human health. We find that across-the-board emissions reductions in domestic fuel burning in developing Asia and in surface transportation in North America are likely to offer the greatest potential for substantial, simultaneous improvement in local air quality and near-term mitigation of global climate change.
The economic benefit of better air quality, from improved human health and increased yields from agriculture and forestry, can offset costs associated with emissions reductions. Thus, a strategy with a dual focus on both air quality and climate change mitigation may present a unique opportunity to engage parties and nations not yet fully committed to climate change mitigation for its own sake.
Shindell, D., J.-F. Lamarque, N. Unger, D. Koch, G. Faluvegi, S. Bauer, and H. Teich, 2008: Climate forcing and air quality change due to regional emissions reductions by economic sector. Atmos. Chem. Phys. 8, 7101-7113.