Global sea levels would rise on average about 3.3 metres — about half as much as previously thought — if the Western Antarctic ice sheet melts, researchers said Thursday.

The revised prediction may provide little comfort to residents of coastal cities like Vancouver and Saint John in Canada, however, as coastal regions in North America are still expected to have increases in sea levels of as much as 25 per cent more than other regions.

Researchers led by Jonathan L. Bamber of the University of Bristol in Britain report the recalculation in advance of publication of their findings in the Friday issue of the journal Science.

The Western Antarctic ice sheet is enormous, with a volume of water about 100 times that of the Great Lakes combined and a height that reaches 1,800 metres above sea level.

Its collapse is one of the more dire potential consequences of rising global temperatures, though scientists look at it as a long-term problem, one that might take centuries or even millenniums to occur.

It's important enough, however, that the Intergovernmental Panel on Climate Change included it in their most recent assessment, predicting the collapse of the ice sheet would cause sea levels to rise about five metres.

Since the IPCC included its collapse as a potential hazard of climate change, scientists have been taking a closer look at how such an event might happen.

Bamber's group calculated that the entire sheet would not collapse, with parts of it remaining grounded on the continent, leading to a less-than-expected sea level rise.

Their study echoes the findings of University of Toronto geophysicists Jerry Mitrovica and Natalya Gomez and U.S. scientist Peter Clark, who in February published a paper in Science demonstrating that coastal regions in North America and the Indian Ocean would be harder hit.

The model by the two Canadian scientists and their American colleague factored the impact of the gravitational pull the ice sheet exerts today on the surrounding water, as well as the shift that would occur in the Earth's rotation axis should the ice sheet melt.