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The Darkening of the Greenland Ice Sheet: Trends, Drivers and Projections (1981-2100)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 space-borne multispectral data collected during the 3 decades from 1981 to 2012, that summertime surface albedo over the GrIS decreased at a statistically significant (99 %) rate of 0.02 decade(sup -1) between 1996 and 2012. Over the same period, albedo modelled by the Modele Atmospherique Regionale (MAR) also shows a decrease, though at a lower rate (approximately -0.01 decade(sup -1)) than that obtained from space-borne data. We suggest that the discrepancy between modelled and measured albedo trends can be explained by the absence in the model of processes associated with the presence of light-absorbing impurities. The negative trend in observed albedo 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 also showed no statistically significant trend over the whole GrIS. Analysis of MAR outputs indicates that the observed albedo decrease is attributable to the combined effects of increased near-surface air temperatures, which enhanced melt and promoted growth in snow grain size and the expansion of bare ice areas, and to trends in light-absorbing impurities (LAI) on the snow and ice surfaces. Neither aerosol models nor in situ and remote sensing observations indicate increasing trends in LAI in the atmosphere over Greenland. Similarly, an analysis of the number of fires and BC emissions from fires points to the absence of trends for such quantities. This suggests that the apparent increase of LAI in snow and ice might be related to the exposure of a "dark band" of dirty ice and to increased consolidation of LAI at the surface with melt, not to increased aerosol deposition. Albedo projections through to the end of the century under different warming scenarios consistently point to continued darkening, with albedo anomalies averaged over the whole ice sheet lower by 0.08 in 2100 than in 2000, driven solely by a warming climate. Future darkening is likely underestimated because of known underestimates in modelled melting (as seen in hindcasts) and because the model albedo scheme does not currently include the effects of LAI, which have a positive feedback on albedo decline through increased melting, grain growth, and darkening.
Document ID
20160003692
Acquisition Source
Goddard Space Flight Center
Document Type
Reprint (Version printed in journal)
External Source(s)
Authors
Tedesco, Marco
(Lamont-Doherty Earth Observatory Palisades, NY, United States)
Doherty, Sarah
(City Univ. of New York New York, NY, United States)
Fettweis, Xavier
(Liege Univ. Belgium)
Alexander, Patrick
(City Univ. of New York New York, NY, United States)
Jeyaratnam, Jeyavinoth
(NASA Goddard Inst. for Space Studies New York, NY, United States)
Stroeve, Julienne
(Colorado Univ. Boulder, CO, United States)
Date Acquired
March 23, 2016
Publication Date
March 3, 2016
Publication Information
Publication: The Cryosphere
Publisher: Copernicus Publications
Volume: 10
Issue: 2
e-ISSN: 1994-0424
Subject Category
Meteorology And Climatology
Report/Patent Number
GSFC-E-DAA-TN30574
Funding Number(s)
CONTRACT_GRANT: NSF-PLR-1304807
CONTRACT_GRANT: NSF-ANS-0909388
CONTRACT_GRANT: NNH06CC03B
CONTRACT_GRANT: NNX1498G
Distribution Limits
Public
Copyright
Other
Keywords
aerosols
Greenland
surface energy
darkening
impurities
fires
trends
albedo
ice
summer

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