Glaciological interest is concentrated particularly on the to a proportionately higher rate of exchange of water be-
WAIS because it rests on a bed far below sea level, which tween ice sheet and ocean than in Antarctica. Summer
may make it particularly vulnerable to accelerated dis- melting occurs over about half of the ice-sheet surface,
charge into the ocean. If the entire WAIS were to disap- with much of the meltwater flowing into the sea, either
pear, sea level would rise by 5 or 6 meters, with major along channels cut into the ice surface or by draining to
consequences (see Section 6C). the bed via crevasses. The average snow accumulation
rate is more than double that of Antarctica. There are
The WAIS was significantly larger during the last glacial only a few ice shelves and, where they do exist, basal
maximum, 20 000 years ago, and retreated to near its melting rates are much higher than in Antarctica – they
present extent within the last several thousand years and it can exceed 10 m per year. This gives an indication of the
is probably still retreating today
1
. Several postglacial mech- potential effect warmer Southern Ocean temperatures
anisms, notably isostatic uplift (a slow rise in the level of would have on Antarctic ice shelves.
the land) and the penetration of ice-softening warmth into
the deeper layers, have long response times. The ice sheet The Greenland Ice Sheet is particularly important to the
is still reacting dynamically to the glacial-interglacial tran- study of sea-level change in a warming climate for two rea-
sition and to the postglacial increase in the rate of snow- sons. First, it is likely to respond rapidly to warmer tem-
fall
2
. Consequently, the present Antarctic contribution to peratures because surface melting already occurs widely.
sea-level change probably reflects a long-term dynamic This means that small increases in air temperatures re-
response of the ice sheet as well as changes in the atmos- sult in large inland migrations of summer melt zones
pheric and oceanic climate over the last century. up the gentle slopes of interior parts of the ice sheet. In-
creasing summer melt reduces ice-sheet volume directly,
Greenland by drainage into the ocean, and indirectly, by lubricating
the base of outlet glaciers and increasing their total ice
The Greenland Ice Sheet extends from 60º to 80º N, and discharge into the ocean. Second, Greenland provides a
covers an area of 1.7 million square km. With an average picture of Antarctic conditions if climate warms enough
thickness of 1600 m, it has a total volume of about 3 mil- to weaken or remove key ice shelves. Recent observations
lion cubic km (about one ninth of the volume of the Ant- in Antarctica confirm the early predictions of substantial
arctic Ice Sheet) – roughly equivalent to a sea-level rise of glacier acceleration following ice-shelf removal.
7 m. It comprises a northern dome and a southern dome,
with maximum surface elevations of approximately 3200
m and 2850 m respectively, linked by a long saddle with el- Recent mass balance analyses
evations around 2500 m. Bedrock beneath the central part
of the ice sheet is remarkably flat and close to sea level, Until recently, it was not possible to determine whether
but the ice sheet is fringed almost completely by coastal the polar ice sheets were growing or shrinking. Over the
mountains through which it is drained by many glaciers. last decade, improved remote-sensing techniques com-
bined with accurate GPS positioning have made it pos-
Greenland’s climate is strongly affected by its proximity sible to estimate ice-sheet mass balance (see box on how
to other land masses and to the North Atlantic, leading to tell if an ice sheet is growing or shrinking).
CHAPTER 6A ICE SHEETS 103