Ice Sheets
Ice sheet: a mass of land ice, continental or
sub-continental in extent, and thick enough
to cover most of the underlying bedrock
topography. Its shape is mainly determined
by the dynamics of its outward flow. There
are only two ice sheets in the modern world,
on Greenland and Antarctica; during glacial
periods there were others.
Ice shelf: a thick, floating slab of freshwater ice extending from the coast (originating as
land ice). Nearly all ice shelves are in Antarctica.
Area Covered Ice Volume Potential Sea
(million square km) (million cubic km) Level Rise (cm)
Ice sheets (total) 14.0 27.6 6390
Greenland 1.7 2.9 730
Antarctica 12.3 24.7 5660
Ice shelves 1.5 0.7 0
Source: IPCC 2007
35
Summary Corresponding sea-level rise increased from about 0.2
mm per year in the early 1990s to perhaps 0.8 mm per
The vast polar ice sheets are shrinking as our climate be- year since 2003, contributing to the total observed rise
comes warmer. Floating ice shelves and glacier tongues during the 1990s of approximately 3 mm per year. Some
are thinning and even breaking up in both Greenland of the thinning glaciers extend many tens to hundreds
and Antarctica, probably because of the combined ef- of kilometres inland, and whether or not ice losses con-
fects of warming ocean waters and increasing summer tinue to accelerate will depend partly on whether ice
air temperatures. Much of this floating ice fills coastal shelves continue to thin, and partly on how far inland
embayments, and is pushed seawards by tributary gla- the zones of glacier acceleration can extend. These ques-
ciers, which are observed to accelerate, as much as eight- tions represent a major challenge to scientists, and their
fold, following ice-shelf break-up. At the same time, answers could have a profound impact on all of us. Re-
warmer summers are extending the zone and intensity search planned for the International Polar Year in 2007-
of summer melting to higher elevations, particularly in 2008 aims to answer them.
Greenland. This increases both meltwater runoff into
the ocean and meltwater drainage to the bed, where it
lubricates glacier sliding and potentially increases ice Introduction to the ice sheets
discharge into the ocean.
Greenland and Antarctica contain 98–99 per cent of the
Together these changes have resulted in net losses from freshwater ice on Earth’s surface. Buried layers of ice,
both ice sheets at rates that are increasing with time. formed from annual snowfall, preserve records of past
100 GLOBAL OUTLOOK FOR ICE AND SNOW
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