Black-carbon reduction of snow albedo

Climate models indicate that the reduction of surface albedo caused by black-carbon contamination of snow contributes to global warming and near-worldwide melting of ice1,2. In this study, we generated and characterized pure and black-carbonladen snow in the laboratory and verified that black-carbon contamination appreciably reduces snow albedo at levels that have been found in natural settings1,3,4. Increasing the size of snow grains in our experiments decreased snow albedo and amplified the radiative perturbation of black carbon, which justifies the aging-related positive feedbacks that are included in climate models. Moreover, our data provide an extensive verification of the snow, ice and aerosol radiation model1, which will be included in the next assessment of the Intergovernmental Panel on Climate Change5. Snow is among the most reflective of natural surfaces on Earth. Addition of dark impurities decreases its reflectance, also known as albedo, and increases its absorption of solar energy. In 1950s Soviet Central Asia, following a practice dating back to the time of Alexander the Great, snow was intentionally darkened with coal dust to accelerate glacial melting and increase the supply of irrigation water. In contrast, present-day, impurity-enhanced glacial melting that stems from the pollution of snow with trace amounts of black carbon (BC) is unintended and a cause for concern1,2,6. BC, a main component of microscopic soot particles produced from the burning of diesel, coal and biomass, strongly absorbs solar radiation. Radiation-transfer calculations indicate that seemingly small amounts of BC in snow, of the order of 10–100 parts per billion by mass (ppb), decrease its albedo by 1–5% (refs 2,7,8). When included in climate models, this BC-induced albedo reduction constitutes a positive radiative climate forcing that contributes to global and regional warming because less solar energy is reflected back to space. The globally averaged radiative forcing fromBC contamination of snow is small (0.05Wm−2), but regional forcing over snow-covered regions, such as the Arctic and the Himalayas (0.6 and 3.0Wm−2, respectively), are comparable to the perturbation caused by the accumulation of carbon dioxide in the atmosphere since preindustrial times (1.5Wm−2; ref. 1). Although it may seem surprising that trace amounts of BC in snow can have significant climate effects, there are several, not immediately obvious, contributing factors at play. Compared with other constituents of atmospheric particulate matter that deposit in snow, notably dust, BC absorbs solar radiation most efficiently2. The absorbing efficiency of BC is higher in snow than in the atmosphere because sunlight is scattered more in snow than in air, which increases the probability of interaction with BC (ref. 9). Additionally, the BC–snow forcing ismore impactful than indicated by a direct comparison with the CO2 forcing because, according to climate models1,2, BC warms the planet two to three times more thanCO2 for the same instantaneousWm−2 of forcing1. The greater warming, or efficacy, of the BC–snow forcing is the result of positive feedbacks, including a BC-caused acceleration in the growth of snow grains that further decreases albedo1.