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 albe...
Main Authors: | , , , , , |
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Format: | Article |
Language: | English |
Published: |
Copernicus Publications
2016-03-01
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Series: | The Cryosphere |
Online Access: | http://www.the-cryosphere.net/10/477/2016/tc-10-477-2016.pdf |
Summary: | 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</sup> between 1996 and 2012. Over the same
period, albedo modelled by the Modèle Atmosphérique Régionale (MAR)
also shows a decrease, though at a lower rate ( ∼ −0.01 decade<sup>−1</sup>)
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. |
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ISSN: | 1994-0416 1994-0424 |