Springtime warming and reduced snow cover from carbonaceous particles

M G Flanner,C S Zender,P J Rasch,P G Hess,N M Mahowald,V Ramanathan,T H Painter

doi:10.5194/acp-9-2481-2009

Abstract

Abstract. 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−1) 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 m−2) than North America (1.2 W m−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.

Highlights

Understanding controls of snow cover evolution is important because 1) cryospheric response to climate forcing largely determines climate sensitivity (e.g., Lemke et al, 2007; Levis et al, 2007), and 2) most of the interannual variability in mid- and high-latitude planetary albedo is caused by changes in snow and sea-ice cover (Qu and Hall, 2005)
The interval between vertical bars “black carbon” and “organic matter” can be considered a reasonable range of effect for carbonaceous aerosol mixtures over snow, where fossil fuel sources are skewed towards BC and biomass burning sources towards OM (e.g., Andreae and Merlet, 2001)
Radiative transfer studies show that any mixture of carbonaceous particles induces positive top-of-atmosphere forcing over snow, and darkening caused by minute concentrations of particles within snowpack outweighs dimming forcing from atmospheric constituents, causing net warming at the surface

Summary

Introduction

Understanding controls of snow cover evolution is important because 1) cryospheric response to climate forcing largely determines climate sensitivity (e.g., Lemke et al, 2007; Levis et al, 2007), and 2) most of the interannual variability in mid- and high-latitude planetary albedo is caused by changes in snow and sea-ice cover (Qu and Hall, 2005). Atmospheric particles have short lifetimes, but influence the snowpack energy budget via several mechanisms. They reduce downwelling surface insolation (dimming) (e.g., Ogren and Charlson, 1983), decreasing absorbed solar energy by snowpack. Deposited particles reduce snow reflectance (surface darkening) (e.g., Warren and Wiscombe, 1980), counteracting the dimming effect. Absorbing particles warm the troposphere via solar heating. This warming may transfer thermal energy into snow and drive earlier melt, but may stabilize the atmosphere and reduce surface-air energy exchange. We strive to understand the relative and combined effects of these processes on continental snow cover

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Conclusion