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Ice loss from Antarctica and Greenland has accelerated over the last 20 years, research shows, and will soon become the biggest driver of sea level rise.
From satellite data and climate models, scientists calculate that the two polar ice sheets are losing enough ice to raise sea levels by 1.3mm each year.
Overall, sea levels are rising by about 3mm (0.12 inches) per year.
Writing in Geophysical Research Letters, the team says ice loss here is speeding up faster than models predict.
They add their voices to several other studies that have concluded sea levels will rise faster than projected by the Intergovernmental Panel on Climate Change (IPCC) in its landmark 2007 assessment.
By 2006, the Greenland and Antarctic sheets were losing a combined mass of 475Gt (gigatonnes - billion tonnes) of ice per year.
On average, loss from the Greenland sheet is increasing by nearly 22Gt per year, while the much larger and colder Antarctic sheet is shedding an additional 14.5Gt each year.
If these increases persist, water from the two polar ice sheets could have added 15cm (5.9 inches) to the average global sea level by 2050.
A rise of similar size is projected to come from a combination of melt water from mountain glaciers and thermal expansion of seawater.
"That ice sheets will dominate future sea level rise is not surprising - they hold a lot more ice mass than mountain glaciers," said lead author Eric Rignot from Nasa's Jet Propulsion Laboratory (JPL) in Pasadena, California.
"What is surprising is this increased contribution by the ice sheets is already happening."