Monitoring Of The Greenland Ice Sheet

Several means have been used to assess the current state of balance of the ice sheet to effectively model its response to natural- and human-induced climatic change. Observational methods include direct meteorological and glaciological measurements at the icesheet surface with weather stations, ablation stakes and ice cores, and remote sensing by satellite radar, gravity surveys, and airborne laser. It is essential to establish the ice sheet's current state of mass balance and climatic sensitivity, and detect any early-warning signs that might signify its future response.

Despite much effort, the Greenland Ice Sheet is still poorly-sampled by direct glaciological measurements, although it appears that there have been rela tively large fluctuations in surface mass balance over the early 21st century. However, a Greenland Climate Network of automatic weather stations and an extensive ice-core network have been established across the ice sheet in the 1990s, providing much-needed meteorological and glaciological measurements across the interior regions.

Airborne and satellite laser-altimetry data analyses indicate substantial volume losses from the ice sheet since around 1993. Various recent analyses of gravimetric (GRACE) satellite data suggest particularly high mass losses 2002-06; the largest mass losses are generally indicated from low elevations (less than 2,000 m.) and especially in southeast Greenland, with partly compensating mass gains at higher elevations (over 2,000 m.). Other altimetry data suggest that there has been substantial growth of the ice-sheet interior in the last 15 years, which may be partly attributable to increased atmospheric moisture and precipitation and/or shifting storm tracks forced by enhanced greenhouse gases. There are considerable discrepancy among these pioneering observational estimates. Moreover, most of the observational studies have data spanning less than a decade, so they have yet to provide a convincing timeframe of how the Greenland Ice Sheet might be responding to long-term climatic change, most notably the evident Arctic and global warming since the 1970s.

Outlet glaciers have typically thinned by several meters per year since the 1990s. Satellite radar inter-ferometry reveals widespread acceleration of Greenland margin glaciers, a pattern progressing northward since 1996, with an accompanying doubling of the ice sheet's volume deficit, although it is too early to say if these changes are exceptional. Some of these margin changes are the likely response to recent climatic warming through infiltration of surface melt water and lubrication of the glacier bed, speeding up glacier flow and mass loss. Notably, several key glaciers in southern Greenland (such as Jakobshavn Isbrae, Helheim, and Kangerdlussuaq) have shown dramatic accelerations, retreats and/or breakup of their floating tongues. This thinning of the Greenland Ice Sheet margin could account for at least 10 percent of the total increase in global sea level during this period.

Studies have shown that the Greenland Ice Sheet experiences rapid changes over periods of just a few years, and some of these could be in response to cli mate change. Scholars are currently focusing on surface ablation and glacial dynamics in the marginal (edge) regions of the ice sheet, and their sensitivity to summer warming.

Recent relatively high summer temperatures (1995-2005) are associated with increased net ice loss over Greenland. However, recent warm summers are not unprecedented as they are similar to those experienced an early-20th-century warm period (1918-47). Nevertheless, most computer-model predictions of future climate suggest that sustained warming around the margins will lead to a negative mass balance and give a positive contribution to sea level. The latest Intergovernmental Panel on Climate Change (IPCC) prediction is for a Greenland Ice Sheet contribution to sea-level rise of 1-13 cm. during the current century. The ice sheet could respond more rapidly to continued warm temperatures via hydraulic acceleration, but at the moment, the physical response mechanisms are too poorly understood to be included in the IPCC projections.

SEE ALSo: Glaciers, Retreating; Glaciology; Greenland Cores; Ice Component of Models; Iceland.

BIBLIoGRAPHY. J.L. Bamber, and A.J. Payne, eds., Mass Balance of the Cryosphere: Observations and Modelling of Contemporary and Future Changes (Cambridge University Press, 2004).

Edward Hanna University of Sheffield

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