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

That has added only about half an inch to rising sea levels, not as bad as 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 Antarctica is melting, but points to the smaller ice sheet in Greenland, which covers most of the island, as the 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.

Andrew Shepherd, lead author of the paper published Thursday in the journal Science, said the research team's 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."

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.