Soot climate forcing via snow and ice albedos (original) (raw)

Plausible estimates for the effect of soot on snow and ice albedos (1.5% in the Arctic and 3% in Northern Hemisphere land areas) yield a climate forcing of ؉0.3 W͞m 2 in the Northern Hemisphere. The ''efficacy'' of this forcing is Ϸ2, i.e., for a given forcing it is twice as effective as CO 2 in altering global surface air temperature. This indirect soot forcing may have contributed to global warming of the past century, including the trend toward early springs in the Northern Hemisphere, thinning Arctic sea ice, and melting land ice and permafrost. If, as we suggest, melting ice and sea level rise define the level of dangerous anthropogenic interference with the climate system, then reducing soot emissions, thus restoring snow albedos to pristine high values, would have the double benefit of reducing global warming and raising the global temperature level at which dangerous anthropogenic interference occurs. However, soot contributions to climate change do not alter the conclusion that anthropogenic greenhouse gases have been the main cause of recent global warming and will be the predominant climate forcing in the future. aerosols ͉ air pollution ͉ climate change ͉ sea level T he Intergovernmental Panel on Climate Change (1) estimates the global climate forcing by fossil fuel black carbon (BC) aerosols as 0.2 W͞m 2. Jacobson (2) suggests that the fossil fuel BC forcing is larger, Ϸ0.5 W͞m 2. J.H. and colleagues (3-5) have argued that the total anthropogenic BC forcing, including BC from fossil fuels, biofuels, and outdoor biomass burning, and also including the indirect effects of BC on snow͞ice albedo, is still larger, 0.8 Ϯ 0.4 W͞m 2. Here we estimate the magnitude of one component of the BC climate forcing: its effect on snow͞ice albedo. Several factors complicate evaluation of the BC snow͞albedo climate forcing and dictate the approach we use to estimate the forcing. BC Amount in Snow. BC is highly variable in space and time. A classic study of Arctic sites (6) found average BC (excluding Greenland) of 30 ppbw (parts per billion by weight; equivalent to ng͞g or g͞liter meltwater) in 1983-1984. In contrast, only 4 ppbw was found in 1998 (7) upwind of the drifting SHEBA (Surface Heat Budget of the Arctic Ocean) site on Arctic sea ice (Ϸ76°N, 165°E), although 35 ppbw and more was found in a limited region downwind of the SHEBA site. Differences among measured amounts exceed estimated errors and imply real variability. BC Optical Effects. Classic theory calculates snow͞ice albedo as multiple scattering by BC and ice spheres (8, 9). Empirical data (8) revealed that BC was two to five times more effective in reducing snow albedo than the model indicated. A factor of two was accounted for by use of a more realistic density and absorption cross-section for BC. Still greater absorption per unit BC mass, by perhaps another factor of two, can be obtained with realistic shapes, voids, and degrees of internal mixing of the BC within ice particles (10, 11), but there remains uncertainty and variability of the appropriate absorption cross-section for BC dispersed in snow. BC vs. Soot. Soot is produced by incomplete combustion of carbonaceous material, mainly fossil fuels and biomass. The carbon combustion products are usually classified as BC and organic carbon (OC), but distinctions within the complex car-Abbreviations: BC, black carbon; DAI, dangerous anthropogenic interference; ppbw, parts per billion by weight.