Abstract
Abstract. The presence of light-absorbing aerosol particles deposited on arctic snow and sea ice influences the surface albedo, causing greater shortwave absorption, warming, and loss of snow and sea ice, lowering the albedo further. The Community Earth System Model version 1 (CESM1) now includes the radiative effects of light-absorbing particles in snow on land and sea ice and in sea ice itself. We investigate the model response to the deposition of black carbon and dust to both snow and sea ice. For these purposes we employ a slab ocean version of CESM1, using the Community Atmosphere Model version 4 (CAM4), run to equilibrium for year 2000 levels of CO2 and fixed aerosol deposition. We construct experiments with and without aerosol deposition, with dust or black carbon deposition alone, and with varying quantities of black carbon and dust to approximate year 1850 and 2000 deposition fluxes. The year 2000 deposition fluxes of both dust and black carbon cause 1–2 °C of surface warming over large areas of the Arctic Ocean and sub-Arctic seas in autumn and winter and in patches of Northern land in every season. Atmospheric circulation changes are a key component of the surface-warming pattern. Arctic sea ice thins by on average about 30 cm. Simulations with year 1850 aerosol deposition are not substantially different from those with year 2000 deposition, given constant levels of CO2. The climatic impact of particulate impurities deposited over land exceeds that of particles deposited over sea ice. Even the surface warming over the sea ice and sea ice thinning depends more upon light-absorbing particles deposited over land. For CO2 doubled relative to year 2000 levels, the climate impact of particulate impurities in snow and sea ice is substantially lower than for the year 2000 equilibrium simulation.
Highlights
Pollutants emitted in mid-latitudes are transported to the Arctic, where they can influence Arctic regional climate and air quality (Shindell et al, 2008)
First we compare equilibrium simulations with and without light-absorbing particulates included in snow and sea ice, leaving prescribed atmospheric aerosols constant, and carbon dioxide levels at a constant year 2000 value
The global annual top of atmosphere (TOA) radiative forcing for Black carbon (BC) and dust combined is F = 0.06 W m−2. This estimate of radiative forcing is solely due to the change in shortwave absorption, and it is computed by running the radiative transfer scheme twice, with and without the changes to albedo due to surface particulates in the snow and sea ice
Summary
Pollutants emitted in mid-latitudes are transported to the Arctic, where they can influence Arctic regional climate and air quality (Shindell et al, 2008). Particulate pollutants are deposited to the surface and incorporated in snow and sea ice in the Arctic. The measurements of BC concentration in snow or sea ice that have been made (e.g., Doherty et al, 2010) help us to constrain the potential magnitude of impact of BC on climate. There are additional fine-scale feedbacks within the snow deposits that amplify the impact of impurities on the overall snow albedo. The presence of impurities accelerates snow aging by lowering the albedo (Warren and Wiscombe, 1980), leading to more absorption and a warmer surface. Measurements indicate that a fraction of the BC is left at the snow surface with melt (rather than washing away), further lowering surface snow albedo and accelerating melt (Conway et al, 1996; Xu et al, 2006; Doherty et al, 2010)
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