The Greenland and Antarctic ice sheets: how fast do they melt?

By Prof.dr. M. van den Broeke
IMAU, University of Utrecht

The ice sheets of Antarctica and Greenland (AIS and GrIS) together contain more than 23 million km3 of water, sufficient to raise global sea level by 65 m (1 km3 of water equals 1 Gt or 1012 kg). With this volume, they are larger by two orders of magnitude than all other (200,000+) glaciers and ice caps combined. Each year, the combined mass turnover of the AIS and GrIS (the amount of snow that falls on the grounded parts of their surface and leaves the ice sheets through iceberg calving and runoff) exceeds 2500 Gt of water, equivalent to 7 mm of global sea level change. Even relatively small fluctuations in the mass balance of the AIS and GrIS will therefore have considerable impact on global sea level rise (SLR), which currently is about 3.4 mm/yr. Until quite recently, ice sheet models predicted that the AIS and GrIS, owing to their sheer size, would react only slowly to changes in ambient conditions. This view has changed profoundly during the last decade, when remotely sensed data revealed fast and dramatic changes in the marginal areas of the AIS and GrIS. An iconic example is the catastrophic disintegration of numerous Antarctic Peninsula (AP) ice shelves, the floating extensions of the ice sheet. In just several decades, more than 28,000 km2 of AP ice shelves was lost, a total of 18% of the original area of floating ice, culminating in the breakup of the northern part of Wilkins Ice Shelf in 2008 and 2009 . With the buttressing effect removed, velocities of glaciers flowing into the former Larsen B Ice Shelf increased two to eightfold, thereby thinning rapidly. Out of 244 analyzed marine glacier fronts on the AP, 87% have retreated over the past 61 years. The boundary between mean advance and retreat has migrated progressively southward.
Further to the south and west, glaciers in coastal West Antarctica draining the West Antarctic Ice Sheet (WAIS) also experience dramatic acceleration and thinning. Between 1996 and 2007, Pine Island Glacier accelerated by 42% and neighboring Smith Glacier by 83%. Total ice discharge from glaciers draining this part of the WAIS increased by 30% in 12 years time, and the net mass loss increased from 39 Gt/yr to 105 Gt/yr. Pine Island Glacier presently undergoes thinning at rates of up to 15 m per year. This may be the onset of WAIS collapse, but model studies suggest that this is unlikely to occur on short timescales. Reports that the huge East Antarctic Ice Sheet also shows the first signs of dynamic mass loss are debated. Runoff in Antarctica is small, and melt occurs mainly in the Antarctic Peninsula and the northward protruding ice shelves; here, melt extent and duration seems to have remained steady.
During the last decade, the GrIS has also been losing mass at an accelerating rate. Between 2000 and 2005, the GrIS annual ice discharge increased by about 30%, mainly a result of widespread acceleration of outlet glaciers in the southeast and in the west. In 2006, the two largest outlet glaciers in the southeast, Helheim and Kangerdlugssuaq, slowed down simultaneously, ceased thinning and readvanced, followed by other glaciers in the region. Despite this, Jacobshavn Isbrae, the largest glacier in the Northern Hemisphere, has continued to accelerate; from an average of 15 m per day in 2000 it presently flows at 26 m per day. During the same period, the glacier thinned by 170 m and discharge increased by 50%, resulting in a 262 Gt mass loss over the last decade, two-thirds of which occurred since June of 2005. The rate of discharge of Jacobshavn Isbrae is now so large that the glacier is losing mass throughout the year, even in winter.
As opposed to the AIS, the GrIS has a well-defined ablation area. Changes here may be less visible and less widely reported, but they do strongly impact the ice sheet mass balance. Changes in brightness temperature derived from passive microwave sensors onboard satellites show that the melt index (melt area times number of melt days) of the GrIS has steadily increased since 1990, to reach a maximum in the record melt year 2010. Model results show that recent mass loss from the GrIS is about equally partitioned between increased discharge through marine-terminating outlet glaciers and enhanced melting and subsequent runoff. Meltwater runoff from the GrIS has increased from ~220 Gt/yr in 1990 to almost 400 Gt/yr in 2010, a staggering 80% increase in just two decades. Combining radar-inferred ice discharge and SMB from atmospheric models, it is estimated that during the last decade (2000-2009), the GrIS and AIS experienced a combined loss of 330 Gt/yr, equivalent to 0.9 mm/yr SLR. Importantly, the mass loss from the AIS and GrIS accelerated by 37 Gt/yr2 over the last 19 years, a rate three times larger than that of mountain glaciers and ice caps. This implies that the GrIS and AIS will soon become the dominant contributors to sea level rise and remain so during the 21st century. Moreover, these large freshwater fluxes can impact ocean currents by changing the ocean stratification. This, in turn, may effect the formation of dense bottom waters, which drive the thermohaline circulation.
Willem Barentsz Poolinstituut

Bundeling van kennis, onderzoek en onderwijs over de Noord- en de Zuidpool

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