WASHINGTON — A man stands on a railroad track as a train rumbles closer.

"Global warming?" he says. "Some say irreversible consequences are 30 years away. Thirty years. That won't affect me."

He steps off the tracks -- just in time. But behind him is a little blonde-haired girl left in front of the roaring train.

The screen goes black. A message appears: "There's still time."

It's just an ad, part of a campaign from the advocacy group Environmental Defense, which hopes to convince Americans they can do something about global warming, that there's still time.

But many scientists are not so sure that the oncoming train of global warming can be avoided. Temperatures are going to 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.

There are limits, experts say, to how much individuals can do. The best we can hope for is to prevent the worst -- world-altering disasters like catastrophic climate change and a drastic rise in sea levels, say 10 leading climate scientists interviewed by The Associated Press. They pull out ominous phrases like "point of no return."

The big disasters are believed to be just decades away. Stopping or delaying them would require bold changes by both individuals and government.

"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."

But he and other scientists say it's too late to stop people from feeling the heat. Nearly two dozen computer models now agree that by 2100, the average yearly global temperature will be 3 to 6 degrees Fahrenheit higher than now, according to 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, according to NCAR.

A British conference on "avoiding dangerous climate change" last year concluded that a rise of just 3 degrees would likely lead to some catastrophic events, especially the melting of the Greenland's polar ice. A study in the journal Science last month said 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 Environmental Defense chief scientist Bill Chameides is more hopeful: "There's a certain amount of warming that's inevitable, but that doesn't mean that we can't avoid the really dangerous things that are happening."

Those dangerous things include: multi-century 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 believe it's possible to reduce the effects of global warming and 59 percent think their individual actions can help, according to a poll commissioned by Environmental Defense as part of its public service campaign.

Climate scientists find themselves in the delicate position of trying to balance calculations that lead to scientific despair with an optimistic public's hope.

"You don't give up," said Schneider, co-director of Stanford's Center for Environmental Science Policy. "If you have high blood pressure, do you sit there till you die or do you take Lasix (blood pressure medicine)?"

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," said Barnett.

"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 drive fuel-miserly cars, be more efficient about heating their homes.

But those efforts "are not going to change us from an irreversible course to a reversible one," said Walsh. "What you really want to say is: 'You can't go on like this. We can't go on like this.'"

Robert Correll, a top scientist in charge of an eight-country research program into Arctic problems caused by global warming, recognizes the contradictions, especially since developing nations such as China, India and those in Africa will play bigger roles in greenhouse gas pollution in the future.

The individual effort, Correll said, "is damn important, but you're not going to make much difference." That requires group or governmental action, he said.

Individual action, while crucial, "gets you 10, 20, 50 percent of the way," Schneider said.

Many of the scientists who have long been vocal skeptics of global warming now acknowledge that the Earth is getting hotter and that some of it is caused by people. Even so, this minority of scientists, such as John Christy of the University of Alabama at Huntsville, contend that the warming is "not on this dangerous trajectory."

But Environmental Defense is spending about $1.5 million over three years on the public service ads, including the child in front of the train, 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 and arranged with the Ad Council, which produced iconic anti-littering and anti-drunk driving campaigns, 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."

Krupp said scientists don't take into account the American will: "Don't underestimate the willpower of Americans when they take on a problem."

But computer model runs at the atmospheric center's Boulder, Colo., campus show Environmental Defense's train image might be too close to the truth.

"It's a train that's going downhill; that is something that people don't understand," Meehl said. "For anything to happen, it's going to have to take the public really being concerned about this problem."

Source: Associated Press