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  • Flanner, M. G., C. S. Zender, P. G. Hess, N. M. Mahowald, T. H. Painter, V. Ramanathan, P. J. Rasch, 2009: Springtime warming and reduced snow cover from carbonaceous particles. Atmos. Chem. Phys., 9, 2481-2497.

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"). We apply a range of models and
observations to explore impacts of these processes on springtime
climate, 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. 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, we estimate 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)
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). 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.

Full Article: http://www.atmos-chem-phys.org/9/2481/2009/acp-9-2481-2009.html

Last Updated: 2009-05-10

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