A new international scientific study says polar ice sheets in Greenland and Antarctica are now melting three times faster than they did in the 1990s. Researchers blame global warming.

So far, that's only added about half an inch to rising sea levels, not as bad as some earlier worst case scenarios. But scientists worry that the melting's pace has quickened, especially in Greenland.

Dozens of climate scientists, including researchers at the University of Washington, compared their measurements of ice sheet changes in Antarctica and Greenland over the past two decades. The results, published in the journal Science, provided a more accurate view of the state of continental ice sheets and eliminated some conflicting observations.

We are just beginning an observational record for ice, said co-author Ian Joughin, a glaciologist in the University of Washington's Applied Physics Laboratory who is lead author on an accompanying review article. This creates a new long-term data set that will increase in importance as new measurements are made.

Until now, researchers haven't agreed on how fast the mile-thick sheets are thawing -- and if Antarctica was even losing ice. Study lead author Andrew Shepherd at the University of Leeds in the UK says it is clear that Greenland's melting is a problem.

We don't fully understand why it's accelerating, Joughin said. But the longer-term observations we have, the more solid predictions we will be able to make.

The UW portions of the research were funded by the National Science Foundation and the National Aeronautics and Space Administration.

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