Melting point: Grey Glacier in Chile's Torres del Paine National Park, part of the southern Patagonian ice field, as seen from the International Space Station in July 2007.

Credit: NASA

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WASHINGTON: Contrary to common belief, glaciers melting with global warming are contributing more to the rising sea level than the Antarctic and Greenland ice sheets.

Scientists have found that the ebb and flow of glaciers, where they meet the water, causes them to speed up and deliver more ice into the world's oceans than previously estimated.

Accelerating contribution

Glaciers and ice caps account for 60 per cent of the meltwater that flows into the oceans, which has been speeding up over the past 10 years from global warming, said Mark Meier of the University of Colorado in Boulder's Institute of Arctic and Alpine Research.

His team report today in the U.S. journal Science that, together, glaciers and ice caps drop into the oceans 417 km³ of ice each year, equal to the entire volume of Lake Erie, one of the five U.S. Great Lakes. And the volume of ice is growing by 12.5 km³ annually.

By comparison, the study says, ice breaking off and melting from Greenland's ice sheet contributes just 28 per cent of the world's ice to the oceans, and the Antarctic ice sheet another 12 per cent.

The accelerating contribution of glaciers and ice caps is due in part to rapid changes in the flow of tidewater glaciers that discharge icebergs directly into the ocean, the researchers said. When a glacier with its "toe in the water" thins, a larger fraction of its weight is supported by water and it slides faster and sends more ice into the ocean.

Rising sea levels

Alaska's Columbia Glacier, which drops three cubic kilometres into Prince William Sound, is a good example, said geologist and study co-author Robert Anderson.

The glacier has thinned up to 400 m in a few places, has shrunk about 14.5 km in length since 1980 and is expected to shrink the same amount in the next two decades, he said.

The melting of the ice sheets of Greenland and the Antarctic is not expected to catch up with that of glaciers and ice caps until the end of the century, the scientists said. They estimated the accelerated melting of glaciers and ice caps will add from 10.2 to 24.1 cm to the sea level rise globally by 2100.

The figures do not include the expansion of the oceans as they get warmer which could potentially double the levels, the researchers said.

A 30-centimeter rise in the sea level causes a shoreline retreat of 30 metres or more, they said, adding that currently some 100 million people live within about one metre of sea level.