The story was originally published Nov. 30:

WASHINGTON - Fueled by global warming, polar ice sheets in Greenland and Antarctica are melting three times faster than they did in the 1990s, a study says.

So far, that's added about half an inch 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 have not 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 more pressing threat. Its melt rate has grown from about 55 billion tons a year in the 1990s to almost 290 billion tons a year recently, the study says.

"Greenland is really taking off," said National Snow and Ice Data Center scientist Ted Scambos, a coauthor of the paper released Thursday by the journal Science.

Study lead author Andrew Shepherd of the University of Leeds in England said their results provided 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 human-made global warming for the melting. Burning fossil fuels 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. Since 1992, ice sheets at the poles have lost nearly five trillion tons of ice, the study says, raising sea levels about half an inch.

That seemingly tiny extra bit probably worsened the flooding from Hurricane Sandy last month, said NASA ice scientist Erik Ivins, another coauthor. "The more energy there is in a wave, the further the water can get inland," he said.