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Is the world's ice melting?

Ice sheets

Earth has two huge ice sheets: Antarctica and Greenland. Antarctica holds enough ice to raise sea levels by 70m if it melts, Greenland by 7m.

Antarctica

Climate models suggest the Antarctic ice sheet will not melt, indeed there may be some growth. Scientists predict more snowfall on the Antarctic continent making the ice sheet thicker, counteracting to some extent sea level rise caused by global warming. Even if the ice sheet grows it will not completely balance out the thermal expansion of the oceans, and the melting of glaciers outside the Antarctic region, so sea levels are still predicted to continue to rise.

NERC's British Antarctic Survey and the Centre for Polar Observation & Modelling (CPOM) are increasing knowledge of this issue.

Scientists use two methods to see whether the Antarctic ice sheet is growing or shrinking. They compare how much snow accumulates, with estimates of how much ice is lost as calving icebergs or through melting ice shelves. But this isn't very precise. Satellites can estimate how thick the ice sheet is by measuring the height of the ice surface with an altimeter. One such study showed that most of the ice sheet grows as much as it melts, but there were only a few years of satellite data and not all of the ice sheet was covered.

We may be able to predict the fate of ice sheets if we understand their past. NERC scientists at the British Antarctic Survey are investigating the 740,000 year climate record locked up in Antarctica's 4km thick ice sheet. Trapped bubbles in the ice hold an archive of atmospheric gases, and evidence for levels of global pollution by industry, agriculture and even atomic bombs. Studies on the ice sheet and its contribution to world sea level rise are vital in helping us understand global change.

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Greenland

The Arctic is one of the fastest warming regions on the planet. Climate models indicate that the local warming over Greenland is likely to be one to three times the global average. Ice sheet models project that a local warming of larger than 3°C, if sustained for thousands of years, would lead to virtually a complete melting of the Greenland ice sheet, with a resulting sea level rise of about 7m. A local warming of 5·5°C, if sustained for one thousand years, would be likely to result in a contribution from Greenland of about 3m to sea level rise.

Sea ice

Arctic sea ice is melting. Scientists monitored record losses in August and September 2005. September is the month when sea ice reaches its minimum. Satellite data suggests the Arctic has lost 20% of its September sea ice since 1978. Climate models predict that as early as the year 2070 there will be no sea ice in the Arctic during the summer months.

Melting sea ice does not directly affect sea levels, as the ice is already afloat. But the melting will affect climate indirectly by changing ocean circulation patterns and reducing the amount of the sun's heat reflected back out to space - accelerating global warming. Sea ice loss is already becoming a problem for some animals, such as polar bears.

Glaciers

The world's glaciers are receding. The melting is making a significant contribution to global sea level rise (thermal expansion of the oceans is the cause of most of the sea level rise). The World Glacier Monitoring Service in Switzerland surveyed 88 glaciers in 2002 and 2003. They found 79 were melting and only four were growing.

In the short term this could cause more flooding, in the long term it could add to the drought problems in some areas as rivers dry up.