The darkening of the Greenland ice sheet: trends, drivers and projections (1981–2100)

Abstract

Abstract. 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 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 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 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 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.

The sensitivity of the Greenland Ice Sheet to glacial–interglacial oceanic forcing

“…Recent observations show that the Greenland Ice Sheet (GrIS) has lost mass at an accelerated rate over the past decades (Rignot et al, 2011;Zwally et al, 2011;Sasgen et al, 2012;Shepherd et al, 2012;van den Broeke et al, 2016). On average, the GrIS contributed to 0.47 ± 0.23 mm a −1 of sea-level rise from 1991 to 2015 (van den Broeke et al, 2016), with an accelerated rate of 0.89 ± 0.09 mm a −1 from 2010 to 2014 (Yi et al, 2015).…”

“…On average, the GrIS contributed to 0.47 ± 0.23 mm a −1 of sea-level rise from 1991 to 2015 (van den Broeke et al, 2016), with an accelerated rate of 0.89 ± 0.09 mm a −1 from 2010 to 2014 (Yi et al, 2015). In the future, the GrIS is expected to continue losing mass, contributing to a sea-level rise relative to the 20th century between 90 and 280 mm by 2100 in the worst-case scenario (RCP8.5) IPCC, 2013;Clark et al, 2015;Fürst et al, 2015).…”

“…This accelerated ice loss is due to a combination of increased surface melting and enhanced ice discharge from marine-terminating glaciers to the ocean (van den . High surface melting has been attributed to rising atmospheric Greenland temperatures (Box et al, 2009;Hall et al, 2008;Tedesco et al, 2016), which may also increase crevassing and calving at the ice front. Conversely, the recently enhanced discharge of ice into the ocean is thought to be directly connected to warmer Atlantic waters entering Greenland's fjords (Holland et al, 2008a;Rignot et al, 2010;Straneo et al, 2010;Straneo and Heimbach, 2013).…”

Cross-Comparison of Albedo Products for Glacier Surfaces Derived from Airborne and Satellite (Sentinel-2 and Landsat 8) Optical Data

“…Furthermore, a widespread darkening of glacier surfaces during recent decades justifies the increasing importance and need of actual glacier albedo data. Glacier darkening was, for example, reported for the European Alps [10], the Himalayas [11], and on the Greenland ice sheet and surrounding glaciers [1,12], and related processes are studied in great detail [13]. Compared to snow albedo, ice albedo has received relatively little interest thus far.…”

“…Especially in summer, when large parts of the glaciers are snow-free, the albedo of heterogeneous bare-ice surfaces has a substantial impact on glacier melt rates and is thus highly relevant regarding the climate change response of mountain glaciers and ice caps [1][2][3][4]. Albedo of ice and snow is influenced by surface properties such as grain size, presence of water, impurity content, surface roughness, crystal orientation and structure (e.g., [5]).…”