The planet's polar ice sheets are melting three times faster than they were in the the 1990s, a 20-year study of satellite records says.

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The definitive analysis of the mass of ice sheets covering Greenland and Antarctica should end decades of speculation about how much their melting may contribute to sea-level rise. The study finds their combined melting has contributed 11.1 milimetres to global sea levels since 1992.

The results come as nations meet in Doha, Qatar, for this year's global climate change negotiations. Previous estimates of sea-level rise from melting ice date from 2004 and are highly disputed.

We can now say for sure that both Antarctica and Greenland are shrinking - Andrew Shepherd, University of Leeds

"The measurements we had to hand then couldn't tell us whether Antarctica in particular was growing or shrinking," said Andrew Shepherd from the University of Leeds who led the research.

"We can now say for sure that both Antarctica and Greenland are shrinking because of the changes in climate that they've been exposed to."

The researchers combined data from 10 satellites designed to measure ice thickness in different ways. Some use lasers or radar to measure the height of ice, others detect changes in gravity to "weigh" the ice lying over Greenland and Antarctica.

9,000 Lake Windermeres

They concluded Antarctica has lost around 1,320 gigatonnes of ice since 1992. Greenland, which has experienced much greater warming has lost around 2,940 gigatonnes over the same period - equivalent to more than 9,000 Lake Windemeres.

Sea level is rising for a number of different reasons - mainly as the oceans expand as they warm up due to a gradually warming planet. But the contribution of the melting ice caps now accounts for about a third of the rise, the study reveals.

Read more on global warming from Tom Clarke's recent visit to the Arctic.

The analysis doesn't sigificantly alter previous, rather uncertain predictions, of how fast ice sheets may be melting. But to have more concrete data will now improve other scientists' ability to forecast future melt.

"This project is a spectacular achievement," said Richard Alley, a climate scientist at Penn State University. "The data will support essential testing of predictive models and will lead to a better understanding of how sea-level change may depend on the human decisions that influence global temperature."