WASHINGTON -- Now that most scientists agree that human activity is causing earth to warm, the central debate has shifted to whether climate change is progressing so rapidly that, within decades, humans may be helpless to slow or reverse the trend.
This ''tipping point" question has begun to consume many prominent researchers in the United States and abroad, because the answer could determine how drastically countries need to reduce their greenhouse gas emissions in the coming years.
While scientists remain uncertain when such a point might occur, many say it is urgent that policy makers cut global carbon dioxide emissions in half over the next 50 years or risk the triggering of changes that would be irreversible.
There are three specific events that these scientists describe as especially worrisome and potentially imminent, although the time frames are a matter of dispute: widespread coral bleaching that could damage the world's fisheries within three decades; dramatic sea level rise by the end of the century that would take tens of thousands of years to reverse; and, within 200 years, a shutdown of the ocean current that moderates temperatures in northern Europe.
The debate has been intensifying because earth is warming much faster than some researchers had predicted.
James Hansen, who directs NASA's Goddard Institute of Space Studies, last week confirmed that 2005 was the warmest year on record, surpassing 1998.
Earth's average temperature has risen nearly 1 degree Fahrenheit over the past 30 years, Hansen noted, and another increase of about 4 degrees over the next century would ''imply changes that constitute practically a different planet."
''It's not something you can adapt to," Hansen said in an interview. ''We can't let it go on another 10 years like this. We've got to do something."
Princeton University geosciences and international affairs professor Michael Oppenheimer, who also advises the advocacy group Environmental Defense, said one of the greatest dangers lies in the disintegration of the Greenland or West Antarctic ice sheets, which together hold about 20 percent of the fresh water on the planet.
If either of the two sheets disintegrates, sea level could rise nearly 20 feet in the course of a couple of centuries, swamping the southern third of Florida and Manhattan up to the middle of Greenwich Village.
While both the Greenland and the Antarctic ice sheets as a whole are gaining some mass in their cold interiors because of increasing snowfall, they are losing ice along their peripheries.
That indicates that scientists may have underestimated the rate of disintegration they face in the future, Oppenheimer said. Greenland's current net ice loss is equivalent to an annual 0.008 inch sea level rise.
The effects of the collapse of either ice sheet would be ''huge," Oppenheimer said. ''Once you lost one of these ice sheets, there's really no putting it back for thousands of years, if ever."
Last year the British government sponsored a scientific symposium on ''Avoiding Dangerous Climate Change," which examined a number of possible tipping points.
A book based on that conference, due to be published tomorrow, suggests that disintegration of the two ice sheets becomes more likely if average temperatures rise by more than 5 degrees Fahrenheit, a prospect ''well within the range of climate change projections for this century."
Some scientists, including President Bush's chief science adviser, John Marburger III, emphasize that much uncertainty remains about when abrupt global warming might occur.
''There's no agreement on what it is that constitutes a dangerous climate change," said Marburger, adding that the US government spends $2 billion a year on researching this and other climate change questions. ''We know things like this are possible, but we don't have enough information to quantify the level of risk."
John Christy, the director of the Earth Science System Center at the University of Alabama in Huntsville, said that increased warming could possibly be offset by other factors, such as increased cloudiness that would reflect more sunlight. ''Whatever happens, we will adapt to it," Christy said.
Scientists who read the history of earth's climate in ancient sediments, ice cores, and fossils find clear signs that it has shifted abruptly in the past on a scale that could prove disastrous for modern society.
Peter deMenocal, an associate professor at the Lamont-Doherty Earth Observatory of Columbia University, said that about 8,200 years ago, a very sudden cooling shut down the Atlantic Ocean conveyor belt.
As a result, the land temperature in Greenland dropped more than 9 degrees Fahrenheit within one or two decades.
''It's not this abstract notion that happens over millions of years," deMenocal said. ''The magnitude of what we're talking about greatly, greatly exceeds anything we've withstood in human history."