Present-day climate forcing and response from black carbon in snow

• Published in: Journal of Geophysical Research
• Other Authors: Flanner, Mark (author), Zender, Charles (author), Randerson, James (author), Rasch, Philip (author)
• Format: Article in Journal/Newspaper
• Language: English
• Published: American Geophysical Union 2007
• Subjects: Sea ice
• Online Access: http://nldr.library.ucar.edu/repository/collections/OSGC-000-000-003-945; https://doi.org/10.1029/2006JD008003

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 (r e), which directly controls snow reflectance and the magnitude of albedo change caused by BC. For a reasonable r e 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&#8315², respectively. Snow forcing from only fossil fuel + biofuel sources is +0.043 W m&#8315² (forcing from only fossil fuels is +0.033 W m&#8315²), 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&#8315², 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 ...