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

So far, that’s added only 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 haven’t agreed on how fast the mile-thick sheets are thawing – and whether Antarctica was even losing ice.

The new research concludes that Antarctica is melting, but it 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 the 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.