Runoff from ice caps in Antarctica and Greenland along with melting mountain glaciers have replaced expanding oceans as the main driver of rising sea levels, according to a new study.

The rate at which the global ocean water mark rises could have a devastating impact on hundreds of millions of people living in low-lying areas around the world.

Earlier research had shown that sea levels crept up and average of 3.1 millimetres (0.12 inches) per year from 1993 to 2003.

More than half of that increase came from a process called thermal expansion whereby the ocean gains in mass as climate change pushes global temperatures upward.

The other half, climate scientists calculated, was caused by land ice, especially dwindling glaciers in mountain ranges such as the Himalayas and Andes.

The new study, drawing on data from two new observational systems, shows that thermal expansion -- which is cyclical over periods measured in decades -- essentially stopped after 2003.

But sea levels continued to rise, though at the slightly diminished rate of 2.5 millimetres (0.1 inches) per year.

Which left scientists wondering: if the water had stopped expanding, what was now driving the continuing elevation of the world's oceans?

The answer, it turns out, are the only two masses of ice on Earth big enough to qualify as ice sheets: Greenland and Antarctica. Both are up to three kilometres (two miles) thick, and Greenland -- the smaller of the two -- is about the size of Mexico.

"During the last decade, Antarctica and Greenland only contributed about 0.5 mm (0.02 inches) per year to rising sea levels whereas today it is about 1.0 mm (0.04 inches) per year," said Anny Cazenave, a scientist at France's National Centre for Space Studies and lead author of the paper.

"This was surprising," she told AFP by phone.

The ice sheet that sits atop Greenland contains enough water to raise world ocean levels by seven metres (23 feet).

The most dire predictions do not foresee a complete meltdown in the foreseeable future. But even a modest increase in the rate at which these continent-sized ice-blocks are changing from solid to liquid could spell disaster.

The UN Intergovernmental Panel on Climate Change (IPCC) warned last year that thermal expansion will push sea levels up 18 to 59 centimetres (7.2 to 23.2 inches) by 2100, enough to wipe out several small island nations and severely disrupt mega deltas in Asia and Africa.

But the report failed to take into account recent studies on the observed and potential impact of the melting ice sheets, prompting the Nobel-winning body to later remove the upward bracket from its end-of-century forecast.

The new study was published in the peer-reviewed journal Global and Planetary Change.