The melting of mountain glaciers and ice caps as a result of global warming over the next century is likely to cause bigger than expected increases in sea levels. An assessment of the volume of water running into the oceans from melting ice caps suggests that sea levels could rise by two to three times the amount previously expected from this source. The study used satellite monitoring to assess the contribution to sea levels made by all land-based ice, except for the two continental-sized ice sheets of Greenland and Antarctica.

It found that the volume of water melting into the sea each year from glaciers and ice caps was 100 cubic miles (417 cubic km), which is almost equal in size to the amount of water in Lake Erie. However, this volume of meltwater is increasing by a further three cubic miles each year because of an acceleration in the rate at which ice caps and glaciers are melting, said Professor Mark Meier, of the University of Colorado. "One reason for doing this study is the widely held view that the Greenland and Antarctic ice sheets will be the principal cause of sea-level rise," Professor Meier said. "But we show that it is the glaciers and ice caps, not the two large ice sheets, that will be the big players in the sea rise for at least the next few generations."

The Intergovernmental Panel on Climate Change (IPCC) estimated that melting ice caps and glaciers will add about three inches (7.6cm) to sea levels this century. But the latest assessment, published in the journal Science, suggests they are more likely to add between four inches and 9.5 inches to global sea levels.

This does not include the rise in sea levels caused by the thermal expansion of water, which could potentially double this figure. A 12-inch rise in sea level can typically cause a shoreline to retreat by 100ft (30m) or more. About 100 million people now live in areas within three feet of sea level. "At the very least, our projections indicate that future sea-level rise may be larger than anticipated, and that the component due to glaciers and ice caps will continue to be substantial," Professor Meier said.

Asked why the last report of the IPCC estimates a lower increase in sea-level rise, Professor Meier said that the scientists had to deal with scientific data that was out of date by the time the latest report of the IPCC was published this year. "They were restricted to the use of numbers in the peer-reviewed literature that was published before early 2006. And some of that data was gathered long before that. We used data that was newer," he said.

The study covered several hundred thousand glaciers and ice caps in polar and temperate regions. The research also included vast mountain glaciers such as the Columbia Glacier in Alaska, which is discharging about two cubic-miles of water into Prince William sound; and the Bering Glacier, also in Alaska, which measures about 5,000 square-miles.

Professor Robert Anderson, of Colorado University, who took part in the study, said polar glaciers that run into the sea are retreating because of complex "dynamic" processes. "We need to acknowledge the role of all the ice masses and understand the physical mechanism by which they deliver water to the sea," he said. Scientists at the British Antarctic Survey have found that the movement of about 300 glaciers in the Antarctic Peninsular had accelerated towards the sea. The scientists believe they were disintegrating at a faster rate than before.
(By Steve Connor, The Independent, 20/07/2007)