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DOE Progress Reports

Climate Warming Due to Soot and Smoke? Maybe Not.

Submitter: Penner, J. E., University of Michigan

Area of Research: Aerosol Properties

Working Group(s): Aerosol

Journal Reference: Penner, J.E., S.Y. Zhang, and C.C. Chuang, Soot and smoke aerosol may not warm climate, J. Geophys. Res., 108(D21), 4657, doi:10.1029/2003JD003409, 2003.

New research results from the Department of Energy's Atmospheric Radiation Measurement (ARM) Program suggest that fossil fuel soot emissions and biomass smoke may actually have a negligible warming effect and, in some cases, may even result in a net cooling effect. Black carbon is the absorbing component of smoke aerosols that result from the incomplete combustion of various fuels, the most significant sources being fossil fuel combustion and biomass burning. The results of this new study directly contradict a large body of previous work, which claims that both the direct radiative effects of black carbon particles and their indirect effects on cloud properties produce a net atmospheric warming. This, in turn, leads to the conclusion that an observed global temperature increase over the last century has been caused, in part, by increases in the concentration of black carbon aerosols.

In the ARM-sponsored study, the researchers used a chemical transport model coupled to the NCAR (National Center for Atmospheric Research) community climate model. Chemical transport models allow particles and trace gas species to be moved about the atmosphere using wind fields from the climate model. In this study, the black carbon aerosol was allowed to interact fully with clouds and the radiation budget. Land surface processes were permitted to adjust to the new radiative fields, but sea surface temperatures were held fixed. This so-called "relaxed forcing" allows the atmosphere to adjust to short term variations in atmospheric radiation, but not to long-term changes in ocean temperature and circulations. In the study of black carbon effects, changes in atmospheric temperatures and cloud properties offset the direct radiative impacts and, consequently, produced very small changes in the overall modeled climate. In addition to uncertainties due to physical processes, model results showed significant sensitivity to the vertical level of smoke injection.

These new results point to significant uncertainties associated with the effects of soot and smoke on radiative fluxes and climate forcing. Different treatments of the growth of aerosols with increasing humidity, the extent to which black carbon serves as a nucleus for cloud particle growth, and the removal of black carbon by precipitation cause differences in aerosol concentrations and cloud properties. These differences produce different radiative forcings and model sensitivity to black carbon. Determining the correct physical and chemical response of black carbon to atmospheric conditions is a high research priority.