WASHINGTON -- Many scientists say the world is moving closer to the point at which it will not be able to avert global warming disasters such as drastic climate upheavals and severe rises in sea levels.
There is still time, but stopping or delaying them will require bold changes by both individuals and the government, according to several climate scientists.
Either way, temperatures will rise for decades to come because the chief gas that causes global warming lingers in the atmosphere for about a century.
''We certainly aren't going to stop that 18-wheeler that's rolling down the hill. In the short term, I'm not sure that anyone can stop it," said John Walsh, director of the Center for Global Change and Arctic System Research at the University of Alaska in Fairbanks.
''The big payoff is going to be for our children," said Tim Barnett, a senior scientist at the Scripps Institution of Oceanography in California.
''Together, if we take a concentrated action as a people, we might be able to slow it down enough to avoid these surprises," Barnett added.
Nearly two dozen computer models agree that by 2100, the average yearly global temperature will be 3 to 6 degrees higher than now, said Gerald Meehl, a senior scientist at the National Center for Atmospheric Research.
Even if today the world suddenly stops producing greenhouse gases, temperatures will rise 1 degree by 2050, the organization said.
A British conference with an aim, it said, of ''avoiding dangerous climate change" concluded last year that a rise of just 3 degrees would probably lead to some catastrophic events, especially the melting of Greenland's polar ice.
A study in the journal Science last month said that the melting, which is happening faster than originally thought, could trigger a 1- to 3-foot rise in global ocean levels.
Stephen Schneider of Stanford University put the odds of a massive Greenland melt at 50-50.
But the chief scientist for the nonprofit ecological group Environmental Defense, Bill Chameides, expressed more hope. ''There's a certain amount of warming that's inevitable, but that doesn't mean that we can't avoid the really dangerous things," he said.
Those dangerous things include multicentury melts of polar ice sheets and an accompanying major sea-level rise, abrupt climate change from a dramatic slowing of the ocean current systems, and the permanent loss of glacier-fed ancient water supplies for China, India, and parts of South America.
Despite what scientists say, 70 percent of Americans think it is possible to reduce the effects of global warming, and 59 percent say their individual actions can help, according to a poll commissioned by Environmental Defense as part of its public service campaign.
Climate scientists find themselves trying to balance calculations that lead to scientific despair with public hope.
It takes decades to stabilize emissions of greenhouse gases -- which are spewed by power plants, cars, and factories -- and another half-century after that to slow revved-up ocean warming, so ''you're stuck with say 100 years of warming," Barnett said.
''I believe we are past the point of no return," he said. ''What does the point of no return mean? To me, it means we've reached a point where we are seeing the impacts of global warming . . . The question is: How much worse is it going to get? That is a case in which we can control our destiny -- if we act now."
Both Barnett and Walsh said the question they get most from the public is: What can I do personally about global warming? They tell people to drive less and to drive fuel-efficient cars, and to be more vigilant about heating their homes.
But those efforts alone ''are not going to change us from an irreversible course to a reversible one," Walsh said.
Individual action ''gets you 10, 20, 50 percent of the way," Schneider said.
Many scientists who have long been skeptics of global warming now acknowledge that Earth is getting hotter and that some of the heat is caused by people. Even so, this minority of scientists, such as John Christy of the University of Alabama at Huntsville, say that the warming is ''not on this dangerous trajectory."
But Environmental Defense is spending about $1.5 million over three years on public service ads to drive home to the public that warming is on a dangerous track and that individuals can and should do something about it.
The ads, released in late March, are being run for free nationwide, said Fred Krupp, Environmental Defense's executive director.
''We expect at least $100 million worth of time and space over the next two years, so it is a big deal," Krupp said. ''When we are successful in making an issue that every American feels responsible to act on, that in itself can reduce emissions."