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PARIS - Global warming is happening now and it will continue for centuries no matter what we do. It could be devastating. But don't give up hope.

Got that?

That's the complex message that scientists are trying to get across after Friday's release of a landmark climate report by the world's leading experts and government officials.

It's a balancing act. Scientists are trying to warn about dire consequences, but not scare people into throwing up their hands and doing nothing - a reaction that would only guarantee the worst scenarios would come true.

What's the worst?

Maybe more than 1 million dead and hundreds of billions of dollars in costs by 2100, in a world adapting to more extreme weather such as droughts, hurricanes and wildfires, said Kevin Trenberth. He is one of the many co-authors of the report from the Intergovernmental Panel on Climate Change (IPCC).

And if nothing is done soon to curb greenhouse gas emissions, experts say, by 2100 the melting of Greenland's ice sheet would become inevitable. Over the following centuries, the world's seas would rise by more than 20 feet.

''If you're in Florida or Louisiana, or much of western Europe or southeast Asia or Bangladesh . . . or Manhattan . . . you don't want that,'' said Andrew Weaver of the University of Victoria in Canada.

Utah is not in danger of being washed away by rising sea levels, but it likely still will feel the effects of global warming. Middle-of-the-road temperature projections for the Western U.S., including Utah, by 2100 are roughly estimated to increase an average of 8 degrees, said Linda Mearns, a study author who is with the U.S. National Center for Atmospheric Research in Boulder, Colo.

Snowfall levels are likely to also take a hit in parts of North America, which could hurt Utah's ski industry.

"The snow depth would decrease, and the snow season would certainly decrease by several weeks by the end of the century," she said.

But can the worst be headed off?

''It's not too late'' if greenhouse gas emissions are curbed quickly, said Australian scientist Nathaniel Bindoff, another co-author.

But ''it's later than we think,'' said panel co-chairwoman Susan Solomon, the U.S. National Oceanic and Atmospheric Administration scientist.

There is no easy solution to reverse the damage humans have been inflicting on the planet, said Thomas Reichler, a University of Utah climate researcher who examined a summary of the IPCC report. The report authors indicate that our past and future greenhouse gas emissions will take more than 1,000 years for the planet to remove from the atmosphere.

"Our descendants will still have to fight against this problem," he said. "That is quite a message."

John Horel, a U. meteorologist, said the report continues the trend of improving researchers' confidence in the data being gathered. This version fixed a problem with satellite temperature records that didn't correspond to the predicted warming trend, which skeptics latched onto in the last report.

"There really hasn't been much credible debate in the science community for a number of years" that global warming is occurring already. The report was the first of four to be released this year by the panel, which was created by the United Nations in 1988.

The report said no matter how much civilization slows or reduces its greenhouse gas emissions, global warming and sea level rise will continue for centuries.

''This is just not something you can stop. We're just going to have to live with it,'' said Trenberth, director of climate analysis for NCAR. ''We're creating a different planet. If you were to come back in 100 years' time, we'll have a different climate.''

The report spurred bleak reactions from world leaders.

''We are on the historic threshold of the irreversible,'' warned French President Jacques Chirac, who called for an economic and political ''revolution'' to save the planet.

In Washington, Bush administration officials praised the report but said they still oppose mandatory cuts in greenhouse gas emissions. The problem can be addressed by better technology that will cut emissions, promote energy conservation and hasten development of nonfossil fuels, said Energy Secretary Samuel Bodman.

About three-fourths of Americans say they expect global warming will get worse, according to a recent AP-AOL News poll. However, other recent polls have found they don't consider it a top priority for the U.S. government.

United Nations environmental leaders are talking about a global summit on climate change for world leaders, and they hope President Bush will attend.

''The signal that we received from the science is crystal clear,'' said Yvo de Boer, the executive secretary of the United Nations Framework Convention on Climate Change, a multinational body that tries to change policy to fight global warming.

The major player that has at times been absent is the United States, the biggest emitter of greenhouse gases.

''The world cannot solve the climate change problem without the United States,'' said Achim Steiner, who heads the U.N. Environment Program.

''The world is looking to the Bush administration and to the United States and how it has to be a key part'' of solving global warming, he said.

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* Tribune reporter GREG

LAVINE and AP science writer MALCOLM RITTER contributed to this report.