Photo: Konrad Steffen
Ice on the Land Ice Sheets
Annual total loss of mass from the Greenland Ice Sheet ciers that feed the ice shelves are observed to accelerate,
more than doubled in the last decade of the 20th centu- as much as eight-fold, following ice-shelf break-up.
ry and may have doubled again by 2005. This is related Observations made over the past five years make it
to more melting and also to increased discharge of ice clear that existing ice-sheet models cannot simulate
from outlet glaciers into the ocean. Warmer Green- the widespread rapid glacier thinning that is occur-
land summers are extending the zone and intensity of ring, and ocean models cannot simulate the changes
summer melting to higher elevations. This increases in the ocean that are probably causing some of the ice
both meltwater runoff into the ocean and meltwater thinning. This means that it is not possible now to pre-
drainage that lubricates glacier sliding and potentially dict the future of the ice sheets, in either the short or
increases ice discharge into the ocean. long term, with any confidence.
There is uncertainty concerning recent overall changes The Greenland and Antarctic ice sheets hold about 99
in ice mass in the Antarctic Ice Sheet but there is prob- per cent of the world’s freshwater ice (the equivalent of
ably an overall decline in mass with shrinking in the 64 m of sea level rise) and changes to them will have
west and addition in the east due to increased snowfall. dramatic and world-wide impacts, particularly on sea
Ice shelves are thinning and some are breaking up. Gla- level but also on ocean circulation.
12 GLOBAL OUTLOOK FOR ICE AND SNOW