So far, that's only added about half an inch (1.3 centimeters) to rising sea levels, not as bad as some earlier worst case scenarios. But the melting's quicker pace, especially in Greenland, has ice scientists worried.
One of the biggest wild cards in climate change has been figuring out how much the melting of the massive sheets of ice at the two poles would add to the seas. Until now, researchers haven't agreed on how fast the mile-thick sheets are thawing — and if Antarctica was even losing ice.
The new research concludes that Antarctica is melting, but points to the smaller ice sheet in Greenland, which covers most of the island, as the bigger and more pressing issue. Its melt rate has grown from about 55 billion tons a year in the 1990s to almost 290 billion tons a year recently, according to the study.
"Greenland is really taking off," said National Snow and Ice Data Center scientist Ted Scambos, a co-author of the paper released Thursday by the journal Science.
Study lead author Andrew Shepherd of the University of Leeds in England, said their results provide a message for negotiators in Doha, Qatar, who are working on an international agreement to fight global warming: "It's very clear now that Greenland is a problem."
Scientists blame man-made global warming for the melting. Burning fossil fuels, such as coal and oil, emits carbon dioxide and other greenhouse gases that trap heat, warming the atmosphere and oceans. Bit-by-bit, that erodes the ice sheets from above and below. Snowfall replenishes the ice sheets, but hasn't kept pace with the rate of melting.
Because the world's oceans are so big, it takes a lot of ice melting — about 10 trillion tons — to raise sea levels 1 inch (2.5 centimeters). Since 1992, ice sheets at the poles have lost nearly 5 trillion tons of ice, the study says, raising sea levels by about a half inch.
That seemingly tiny extra bit probably worsened the flooding from an already devastating Superstorm Sandy last month, said NASA ice scientist Erik Ivins, another co-author of the study. He said the extra weight gives each wave a little more energy.
"The more energy there is in a wave, the further the water can get inland," Ivins said.
Globally, the world's oceans rose about half a foot (15 centimeters) on average in the 20th Century. Melting ice sheets accounts for about one-fifth of sea level rise. Warmer water expands, contributing to the rise along with water from melting glaciers outside the polar regions.
Just how much ice is melting at the two poles has been difficult for scientists to answer. In 2007, the Intergovernmental Panel on Climate Change did not include ice sheet melt in its calculations of future sea level rise because numbers were so uncertain.
It's an important factor because if all the polar ice sheets somehow melted — something that would take centuries — global sea levels would jump by more than 200 feet, said Pennsylvania State University ice scientist Richard Alley, who wasn't part of the research.
Some past studies showed melting on the polar ice sheets, while others said that the Antarctic ice sheet was growing and offsetting melting in Greenland. The new work by 47 scientists around the world combines three methods and measurements from 10 satellites to come to a scientific consensus on what's happening to the polar ice sheets.
In the 1990s, the two ice sheets combined on average lost 110 billion tons of ice each year to melting, the researchers reported. That increased and by 2005 to 2010, they were losing three times as much — 379 billion tons yearly. The numbers don't include the summer of 2012 when Greenland experienced a melt that hadn't been seen in more than a century, researchers said.
The consensus says that as a whole the Antarctic ice sheet is melting. Part of the issue is that the southern continent is not reacting to climate change uniformly, with some areas growing and others shrinking. The entire Antarctic ice sheet is about the size of the U.S. and Mexico combined.
NASA chief scientist Waleed Abdalati, one of the few top ice researchers who wasn't part of the study, praised the work.
"Understanding how and why the ice sheets are changing today better equips us for understanding and predicting how much and in what ways they will change in the future," he said.