WASHINGTON (AP) -- Global warming's "slow-motion time bomb" of trapped greenhouse gases in the Arctic's thawing tundra may not go off quite as fast as once feared, a new study found.

Even so, it remains a problem that in the long run is still likely to worsen global warming in an uncontrollable way, researchers reported.

The study, published in Thursday's journal Nature, looked at thawing parts of Alaska and found that greenhouse gas releases initially are sucked up by new plants as the Arctic gets warmer and greener. But that helpful effect doesn't last.

Eventually, between 15 and 50 years, those plants "can't keep up" and get overwhelmed, said study lead author Ted Schuur, a University of Florida ecologist.

At that point, a billion tons of carbon a year can be released into an atmosphere that is already warming because of carbon dioxide emissions from power plants, cars, and other industrial activities, Schuur said. That would contribute the same amount to global warming as the deforestation of the tropics, he said.

"I call it a slow-motion time bomb," Schuur said.

Making matters worse is that much of the gas trapped in permafrost is methane, which is more than 20 times more potent than carbon dioxide.

It's a vicious cycle. As the world warms, more permafrost thaws. As more permafrost thaws, more greenhouse gases are released and the world warms even more, scientists say.

For the long-term, the new paper "heightens rather than lessens the concern" about the effects of trapped greenhouse gases in permafrost, said Stanford University ecologist Chris Field, who wasn't part of the study.