Decreasing cloud cover driving Greenland Ice Sheet Loss

Abstract: The Greenland Ice Sheet (GrIS) has been losing mass at an accelerating rate since the mid-1990s. This has been due to both increased ice discharge into the ocean and melting at the surface, with the latter being the dominant contribution.

This change in state has been attributed to rising temperatures and a decrease in surface albedo. We show, using satellite data and climate model output, that the abrupt reduction in surface mass balance since about 1995 can be attributed largely to a coincident trend of decreasing summer cloud cover enhancing the melt-albedo feedback. Satellite observations show that, from 1995 to 2009, summer cloud cover decreased by 0.9 ± 0.3{154653b9ea5f83bbbf00f55de12e21cba2da5b4b158a426ee0e27ae0c1b44117} per year. Model output indicates that the GrIS summer melt increases by 27 ± 13 gigatons (Gt) per percent reduction in summer cloud cover, principally because of the impact of increased shortwave radiation over the low albedo ablation zone. The observed reduction in cloud cover is strongly correlated with a state shift in the North Atlantic Oscillation promoting anticyclonic conditions in summer and suggests that the enhanced surface mass loss from the GrIS is driven by synoptic-scale changes in Arctic-wide atmospheric circulation.

INTRODUCTION

The mass balance of the Greenland Ice Sheet (GrIS) has changed significantly over the last two decades. Until the mid-1990s, losses from surface meltwater runoff and ice discharge into the ocean (D) were roughly balanced by snow accumulation (12). However, since then, mass loss has accelerated (3) as the surface mass balance (SMB) has declined and D has increased (1), with a possible link between meltwater production and ice dynamics (45). As a consequence, the GrIS has become the dominant source of barystatic sea level rise, with an average (1991–2015) contribution of 0.47 ± 0.23 mm/year [equivalent to 171 gigatons (Gt) of ice] (2).

About 60{154653b9ea5f83bbbf00f55de12e21cba2da5b4b158a426ee0e27ae0c1b44117} of this recent mass imbalance has been associated with a declining SMB predominantly due to enhanced surface melt (12). Studies based on in situ observations suggest that surface melt rates are controlled by variations in summertime shortwave (SW) radiation (67). However, to date, only the impact of a declining albedo (α) on the SW radiation budget (Eq. 1) has been considered (89).

Paper: ‘Decreasing cloud cover drives the recent mass loss on the Greenland Ice Sheet’  Stefan Hofer1,*, Andrew J. Tedstone1, Xavier Fettweis2 and Jonathan L. Bamber1  Science Advances  28 Jun 2017: Vol. 3, no. 6, e1700584 DOI: 10.1126/sciadv.1700584

Read full paper at  http://advances.sciencemag.org/content/3/6/e1700584

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