Greenland Ice Sheet

The Greenland Ice Sheet contains about 10 percent of the world’s freshwater, and is vulnerable to mass loss by climate warming; if it were to melt, it would cause a 24 ft. (7.4 m) rise in global sea level. The ice sheet covers 82 percent of the total area of Greenland. It is a huge ice dome with two main peaks: one at 10,564 ft. (3,220 m) at the summit (about 72 degrees N, 29 degrees W), and the other at 9,350 ft. (2,850 m) in the south at about 64 degrees N, 44 degrees W. The ice sheet is up to about 1.8 mi. (3 km) thick in the middle, and its great weight depresses the underlying crust, which assumes the concave shape of a saucer. The ice sheet has waxed and waned in response to natural changes in solar radiation and other climatic factors over millennia: during the last ice age, which peaked about 20,000 years ago, it expanded to form the Laurentide Ice Sheet covering much of North America.

Various processes contribute to the accumulation (growth) and ablation (decay), of the net mass balance of the ice sheet. Accumulation comes mainly as precipitation (snowfall), although there is also condensation of water vapor (such as hoar frost) on the surface. Ablation includes surface meltwater runoff, iceberg calving, evaporation, and sublimation (direct conversion of solid ice to vapor). Under present climatic conditions, annual accumulation and ablation roughly balance, with about half the ablation from runoff and the other half from iceberg calving. However, iceberg calving occurs only at the margins of the ice sheet, where ice streams flow outward to reach fjords and/or the sea, and can be ignored for the rest of the ice sheet further inland (which encompasses nearly all of the sheet). The term surface mass balance is used to denote all accumulation and ablation processes, except iceberg calving. Overall (net) mass balance includes ice flow (dynamics) as well as surface mass balance processes:

The ice sheet naturally deforms and flows outward under the force of gravity. When Greenland Ice Sheet behavior is studied over periods longer than about a decade, dynamical readjustment (effectively a long-term memory) of the ice sheet to changes since the last ice age also have to be considered.

The ice sheet is divided into an accumulation zone and an ablation zone, separated by an equilibrium line. Accumulation exceeds ablation at higher elevations (accumulation zone) and vice versa at lower elevations (ablation zone), which can be explained by the much higher summer temperatures at lower elevations near the ice sheet margin. The exact height of the equilibrium line varies with location according to climatic and geographic factors, ranging from about 984 ft. (300 m) above sea level in the north to about 5,900 ft. (1,800 m) above sea level in the south.

Summer melting occurs over about half the area of the ice sheet. However, just above the equilibrium line, over more than two-thirds of the total area, the limited summer snowmelt mostly percolates into the local snow pack and refreezes, so net runoff there is negligible. The surface mass balance of the ice-sheet margin is most sensitive to slight (several degree) changes in summer temperatures, which substantially affect surface-melt runoff. This part of the ice sheet is therefore particularly sensitive to climatic changes such as global warming. Marginal melt rates are locally up to around 16 to 32 ft. (5 to 10 m) per year. These would be enhanced with future warming, which would radically affect the mass balance of the whole ice sheet, even though the area of significant melt covers only a relatively small fraction by area. Summer cooling in coastal southern Greenland between the late 1950s and early 1990s may explain the mid- to late-20th century re-advance of the ice sheet margin after retreats of a few kilometers in some places since the 19th century. This evaluation of changes in the ice margin is based on analysis of old maps, photos, and expedition reports, and cannot be expressed accurately as volume change, which can only be determined by modern altimeter surveys from aircraft and satellite.