Springtime Warming and Reduced Snow Cover from Carbonaceous Particles

Boreal spring climate is uniquely susceptible to solar warming mechanisms because it has expansive snow cover and receives relatively strong insolation. Carbonaceous particles can influence snow coverage by warming the atmosphere, reducing surface-incident solar energy ("dimming"), and reducing snow reflectance after deposition ("darkening"). Applying a column radiative transfer model, we show that darkening caused by very small concentrations of absorbing particles within snow exceeds the loss of absorbed energy from concurrent dimming, thereby driving net heating of the snowpack as well as atmosphere. Over global snow, we estimate 6-fold greater surface forcing from darkening than dimming, caused by black carbon (BC) and organic matter (OM). 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. Finally, of 22 climate models contributing to the IPCC Fourth Assessment Report, 21 underpredict the rapid warming (0.64 degrees/decade) observed over springtime Eurasia since 1979. Darkening from natural and anthropogenic sources of 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), and inclusion of this forcing significantly improves simulated continental warming trends. The forcing also reduces, but does not reconcile, the low bias in rate of Eurasian spring snow cover decline exhibited by all models.