Solar heating, reduced snow cover, and warming from carbonaceous particles

Atmospheric aerosols exert a unique influence on the solar radiation budget of snow-covered regions, where surface albedo is extremely high. Carbonaceous particles, an established component of "Arctic haze," can influence snow and ice cover by warming the atmosphere, reducing surface-incident solar energy ("dimming"), and reducing snow reflectance after deposition ("darkening"). We apply a coupled snow-atmosphere radiative transfer model and the NCAR Community Atmosphere Model to study these processes, drawing several conclusions: 1) Nearly all atmospheric particles (those with visible-band single-scatter albedo less than 0.999), including all mixtures of black carbon (BC) and organic matter (OM), increase net solar heating of the atmosphere-snow column. Sulfate exerts only a weak negative forcing over snow. 2) Darkening caused by small concentrations of particles within snow exceeds the loss of absorbed energy from concurrent dimming, thus increasing solar heating of snowpack as well. Over global snow, we estimate 6-fold greater surface forcing from darkening than dimming, caused by BC+OM. This positive forcing reaches a maximum during boreal spring (in seasonally snow-covered regions) and summer (in polar regions), when local insolation becomes intense but large expanses are still snow- and ice-covered. 3) Equilibrium climate experiments suggest that fossil fuel and biofuel emissions of BC+OM induce 95% as much springtime snow cover loss over Eurasia as anthropogenic carbon dioxide, a consequence of strong snow-albedo feedback and large BC+OM emissions from Asia. 4) Of 22 models contributing to the IPCC Fourth Assessment Report, 21 underpredict the rapid warming (0.64 C/decade) observed over springtime Eurasia since 1979. Darkening from BC and mineral dust exerts 3-fold greater forcing on springtime snow over Eurasia (3.9 W/m2) than North America (1.2 W/m2). Inclusion of this forcing significantly improves simulated high-latitude continental warming trends, via reduced snow cover, but low biases persist in rate of spring snow cover decline. Impacts of BC+OM on summer sea-ice extent and polar climate are also explored.