Attribution of climate forcing to economic sectors.

A much-cited bar chart provided by the Intergovernmental Panel on Climate Change displays the climate impact, as expressed by radiative forcing in watts per meter squared, of individual chemical species. The organization of the chart reflects the history of atmospheric chemistry, in which investigators typically focused on a single species of interest. However, changes in pollutant emissions and concentrations are a symptom, not a cause, of the primary driver of anthropogenic climate change: human activity. In this paper, we suggest organizing the bar chart according to drivers of change-that is, by economic sector. Climate impacts of tropospheric ozone, fine aerosols, aerosol-cloud interactions, methane, and long-lived greenhouse gases are considered. We quantify the future evolution of the total radiative forcing due to perpetual constant year 2000 emissions by sector, most relevant for the development of climate policy now, and focus on two specific time points, near-term at 2020 and long-term at 2100. Because sector profiles differ greatly, this approach fosters the development of smart climate policy and is useful to identify effective opportunities for rapid mitigation of anthropogenic radiative forcing.

Improved attribution of climate forcing to emissions.

Evaluating multicomponent climate change mitigation strategies requires knowledge of the diverse direct and indirect effects of emissions. Methane, ozone, and aerosols are linked through atmospheric chemistry so that emissions of a single pollutant can affect several species. We calculated atmospheric composition changes, historical radiative forcing, and forcing per unit of emission due to aerosol and tropospheric ozone precursor emissions in a coupled composition-climate model. We found that gas-aerosol interactions substantially alter the relative importance of the various emissions. In particular, methane emissions have a larger impact than that used in current carbon-trading schemes or in the Kyoto Protocol. Thus, assessments of multigas mitigation policies, as well as any separate efforts to mitigate warming from short-lived pollutants, should include gas-aerosol interactions.

Cross influences of ozone and sulfate precursor emissions changes on air quality and climate.

Tropospheric O(3) and sulfate both contribute to air pollution and climate forcing. There is a growing realization that air quality and climate change issues are strongly connected. To date, the importance of the coupling between O(3) and sulfate has not been fully appreciated, and thus regulations treat each pollutant separately. We show that emissions of O(3) precursors can dramatically affect regional sulfate air quality and climate forcing. At 2030 in an A1B future, increased O(3) precursor emissions enhance surface sulfate over India and China by up to 20% because of increased levels of OH and gas-phase SO(2) oxidation rates and add up to 20% to the direct sulfate forcing for that region relative to the present day. Hence, O(3) precursors impose an indirect forcing via sulfate, which is more than twice the direct O(3) forcing itself (compare -0.61 vs. +0.35 W/m(2)). Regulatory policy should consider both air quality and climate and should address O(3) and sulfate simultaneously because of the strong interaction between these species.