Darkening of the Greenland Ice Sheet Responsible for Melting

M. Tedesco, S. Doherty, X. Fettweis, P. Alexander, J. Jeyaratnam, E. Noble , and J. Stroeve. The darkening of the Greenland Ice Sheet: Trends, Drivers, and Projections. The Cryosphere Discuss., 9, 5595–5645, 2015

The surface energy balance and meltwater production of the Greenland ice sheet (GrIS) are modulated by snow and ice albedo through the amount of absorbed solar radiation. Here we show, using spaceborne multispectral data collected during the 5 three decades from 1981 to 2012, that summertime surface albedo over the GrIS decreased at a statistically significant (99 %) rate of 0.02 decade−1 between 1996 and 2012. The negative trend is confined to the regions of the GrIS that undergo melting in summer with the dry-snow zone showing no trend. The period 1981–1996 showed no statistically significant trend. The analysis of the outputs of a regional 10 climate model indicates that the drivers of the observed albedo decrease is imputable to a combination of increased near-surface temperatures, which enhanced melt and promoted growth in snow grain size and the expansion of bare ice areas, as well as by trends in light-absorbing impurities on the snow and ice surfaces. Neither aerosol models nor in situ observations indicate increasing trends in impurities in the 15 atmosphere over Greenland, suggesting that their apparent increase in snow and ice might be related to the exposure of a “dark band” of dirty ice and to the consolidation of impurities at the surface with melt. Albedo projections through the end of the century under different warming scenarios consistently point to continued darkening, with albedo anomalies in 2100 averaged over the whole ice sheet lower than in 2000 20 by 0.08, driven solely by a warming climate. Future darkening is likely underestimated because of known underestimates in projected melting and because the model albedo scheme does not currently include light-absorbing impurities and the effect of biological activity, which themselves have a positive feedback, leading to increased melting, grain growth and darkening.

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