A 2005 National Research Council Report, Climate Science, and our research studies have summarized studies that show that black carbon deposition on snow and sea ice is a major positive radiative forcing; e.g. see
–Page 38 in
National Research Council, 2005: Radiative forcing of climate change: Expanding the concept and addressing uncertainties. Committee on Radiative Forcing Effects on Climate Change, Climate Research Committee, Board on Atmospheric Sciences and Climate, Division on Earth and Life Studies, The National Academies Press, Washington, D.C., 208 pp.
-Strack, J., R.A. Pielke Sr., and G. Liston, 2007: Arctic tundra shrub invasion and soot deposition: Consequences for spring snowmelt and near-surface air temperatures. J. Geophys. Res., accepted.
There is a new paper that provides further evidence on the major role of black carbon deposition on snow and sea ice as a positive radiative forcing.
The paper is
Flanner, M. G., C. S. Zender, J. T. Randerson, and P. J. Rasch (2007), Present-day climate forcing and response from black carbon in snow, J. Geophys. Res., 112, D11202, doi:10.1029/2006JD008003.
The paper was also reported in the media by Henry Fountain with the headline Warming in the Arctic? Blame the Snow. The Dirty Snow, That Is.
The abstract of the Flanner et al paper reads,
We apply our Snow, Ice, and Aerosol Radiative (SNICAR) model, coupled to a general circulation model with prognostic carbon aerosol transport, to improve understanding of climate forcing and response from black carbon (BC) in snow. Building on two previous studies, we account for interannually varying biomass burning BC emissions, snow aging, and aerosol scavenging by snow meltwater. We assess uncertainty in forcing estimates from these factors, as well as BC optical properties and snow cover fraction. BC emissions are the largest source of uncertainty, followed by snow aging. The rate of snow aging determines snowpack effective radius (re), which directly controls snow reflectance and the magnitude of albedo change caused by BC. For a reasonable re range, reflectance reduction from BC varies threefold. Inefficient meltwater scavenging keeps hydrophobic impurities near the surface during melt and enhances forcing. Applying biomass burning BC emission inventories for a strong (1998) and weak (2001) boreal fire year, we estimate global annual mean BC/snow surface radiative forcing from all sources (fossil fuel, biofuel, and biomass burning) of +0.054 (0.007–0.13) and +0.049 (0.007–0.12) W m-2, respectively. Snow forcing from only fossil fuel + biofuel sources is +0.043 W m-2 (forcing from only fossil fuels is +0.033 W m-2), suggesting that the anthropogenic contribution to total forcing is at least 80%. The 1998 global land and sea-ice snowpack absorbed 0.60 and 0.23 W m-2, respectively, because of direct BC/snow forcing. The forcing is maximum coincidentally with snowmelt onset, triggering strong snow-albedo feedback in local springtime. Consequently, the ‘‘efficacy’’ of BC/snow forcing is more than three times greater than forcing by CO2. The 1998 and 2001 land snowmelt rates north of 50 N are 28% and 19% greater in the month preceding maximum melt of control simulations without BC in snow. With climate feedbacks, global annual mean 2-meter air temperature warms 0.15 and 0.10 C, when BC is included in snow, whereas annual arctic warming is 1.61 and 0.50 C. Stronger high latitude climate response in 1998 than 2001 is at least partially caused by boreal fires, which account for nearly all of the 35% biomass burning contribution to 1998 arctic forcing. Efficacy was anomalously large in this experiment, however, and more research is required to elucidate the role of boreal fires, which we suggest have maximum arctic BC/snow forcing potential during April–June. Model BC concentrations in snow agree reasonably well (r = 0.78) with a set of 23 observations from various locations, spanning nearly 4 orders of magnitude. We predict concentrations in excess of 1000 ng g-1 for snow in northeast China, enough to lower snow albedo by more than 0.13. The greatest instantaneous forcing is over the Tibetan Plateau, exceeding 20 W m 2 in some places during spring. These results indicate that snow darkening is an important component of carbon aerosol climate forcing.”
Excerpts from the paper read,
“Global annual mean equilibrium warming resulting from the inclusion of BC in snow is 0.15 and 0.10 C for 1998 and 2001 central experiments, respectively. Annual arctic (66.5–90 N) warming, however, is 1.61 and 0.50 C. Arctic annual mean surface albedo for these two experiments is reduced by 0.047 and 0.017, relative to their control simulations without BC in snow. The 1998 and 2001 land snowmelt rates north of 50 N are 28% and 19% greater in the month preceding maximum melt of control simulations.”
“BC in snowpack can provoke disproportionately large springtime climate response because the forcing tends to coincide with the onset of snowmelt, thus triggering more rapid snow ablation and snow-albedo feedback. The model and methods we develop here could be applied to study snow forcing by other aerosols, including mineral dust, volcanic ash, brown carbon, and marine sediment in sea-ice [Light et al., 1998].”
In terms of the global radiative forcings, the fraction of attribution of observed warming to the radiative forcing of the well-mixed greenhouse gases is reduced by this large radiative forcing by black carbon, as Climate Science has been reporting; e.g. see slide 12 in
Pielke, R.A. Sr., 2006: Regional and Global Climate Forcings. Presented at the Conference on the Earth’s Radiative Energy Budget Related to SORCE, San Juan Islands, Washington, September 20-22, 2006.