Carbonaceous particles can influence snow coverage by warming the atmosphere; reducing surface-incident solar energy (dimming); and reducing snow reflectance after deposition (darkening).

The researchers applied a range of models and observations to explore impacts of these processes on the springtime boreal climate, which is already susceptible to solar warming mechanisms because it has expansive snow cover and receives relatively strong insolation. Among their conclusions:

1. Nearly all atmospheric particles (those with visible-band single-scatter albedo less than 0.999), including all mixtures of BC and OM, increase net solar heating of the atmosphere-snow column.

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 (positive net surface forcing). Over global snow, the team estimated 6-fold greater surface forcing from darkening than dimming, caused by BC+OM.

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 climate models contributing to the IPCC Fourth Assessment Report, 21 underpredict the rapid warming (0.64 °C decade^-1) observed over springtime Eurasia since 1979. Darkening from natural and anthropogenic sources of BC and mineral dust exerts 3-fold greater forcing on spring-time snow over Eurasia (3.9 Wm^-2) than North America (1.2 Wm^-2). Inclusion of this forcing significantly improves simulated continental warming trends, but does not reconcile the low bias in rate of Eurasian spring snow cover decline exhibited by all models, likely because BC deposition trends are negative or near-neutral over much of Eurasia. Improved Eurasian warming may therefore relate more to darkening induced reduction in mean snow cover.

Our study finds that black carbon is especially effective at warming climate during springtime, when the Northern Hemisphere is highly reflective and transitioning into snow-free summer. By inducing early retreat of snow cover, black carbon causes (Eurasian) land areas to absorb more sunlight and warm disproportionately.

—Marc Flanner, National Center for Atmospheric Research, lead author

The short atmospheric lifetime of black carbon offers the opportunity for fast mitigation. Flanner explained, “Our model studies suggest that eliminating black carbon emissions from fossil and biofuel sources would cause Eurasian springtime snow cover to recover at least a quarter of its estimated loss from pre-industrial times to the present.”

This is yet another major study revealing the major role of black carbon in the retreat of snow packs and glaciers around the world. Fortunately, we can do something about black carbon, for we know how to reduce emissions of black carbon from combustion of fossil fuels and biomass fuels.

—V. Ramanathan, Scripps Institute of Oceanography at the University of California, San Diego, co-author

Resources

• M. G. Flanner, C. S. Zender, P. G. Hess1, N. M. Mahowald1, T. H. Painter, V. Ramanathan, and P. J. Rasch (2009) Springtime warming and reduced snow cover from carbonaceous particles. Atmos. Chem. Phys., 9, 2481–2497