PARIS - Global warming is so severe that it will "continue for centuries," leading to a far different planet in 100 years, warned a grim landmark report from the world's leading climate scientists and government officials. Yet, many of the experts are hopeful that nations will now take action to avoid the worst scenarios.

They tried to warn of dire risks without scaring people so much they'd do nothing - inaction that would lead to the worst possible scenarios.

"It's not too late," said Australian scientist Nathaniel Bindoff, a co-author of the authoritative Intergovernmental Panel on Climate Change report issued Friday. The worst can be prevented by acting quickly to curb greenhouse gas emissions, he said.

The worst could mean more than 1 million dead and hundreds of billions of dollars in costs by 2100, said Kevin Trenberth of the National Center for Atmospheric Research in Colorado, one of many study co-authors. He said that adapting will mean living with more extreme weather such as severe droughts, more hurricanes and wildfires.

"It's later than we think," said panel co-chair Susan Solomon, the U.S. National Oceanic and Atmospheric Administration scientist who helped push through the document's strong language.

Solomon, who remains optimistic about the future, said it's close to too late to alter the future for her children - but maybe it's not too late for her grandchildren.

The report was the first of four to be released this year by the panel, which was created by the United Nations in 1988. It found:

_Global warming is "very likely" caused by man, meaning more than 90 percent certain. That's the strongest expression of certainty to date from the panel.

_If nothing is done to change current emissions patterns of greenhouse gases, global temperature could increase as much as 11 degrees Fahrenheit by 2100.

_But if the world does get greenhouse gas emissions under control - something scientists say they hope can be done - the best estimate is about 3 degrees Fahrenheit.

_Sea levels are projected to rise 7 to 23 inches by the end of the century. Add another 4 to 8 inches if recent, surprising melting of polar ice sheets continues.

Sea level rise could get worse after that. By 2100, if nothing is done to curb emissions, the melting of Greenland's ice sheet would be inevitable and the world's seas would eventually rise by more than 20 feet, Bindoff said.

That amount of sea rise would take centuries, said Andrew Weaver of the University of Victoria in Canada, but "if you're in Florida or Louisiana, or much of western Europe or southeast Asia or Bangladesh ... or Manhattan ... you don't want that," he said.

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.

"While climate changes run like a rabbit, world politics move like a snail: Either we accelerate or we risk a disaster," said Italy's environment minister, Alfonso Pecoraro Scanio.

And South Africa's Environmental Affairs Minister Marthinus van Schalkwyk said failure to act would be "indefensible."

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 non-fossil 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.

But doing nothing about global warming could mean up to a 10-degree Fahrenheit temperature rise by the end of the century in the United States, said report co-author Jonathan Overpeck at the University of Arizona.

Elsewhere, the projected effects of global warming would vary on different parts of the globe.

Temperatures would spike higher near the poles, according to the report. Within 22 years - whether greenhouse gases are controlled or not - most of the Northern Hemisphere will see more high temperature extremes, the report showed. Places like Northern Africa will get even less rainfall.

This climate change "is just not something you can stop," said Trenberth. "We're just going to have to live with it. If you were to come up back in 100 years time, we'll have a different climate."

People experience the harshest effects of global warming through extreme weather - heat waves, droughts, floods, and hurricanes - said study co-author Philip Jones of Britain's University of East Anglia. Those have increased significantly in the past decade and will get even worse in the future, he said.

Given all the dire predictions, why are scientists nearly all optimistic? They think their message is finally getting through to the people in charge.

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 multi-national body that tries to change policy to fight global warming.

"That makes it imperative that the political response that comes from this crystal-clear science is as crystal-clear as well.

"I sense a growing sense of urgency to come to grips with the issue," de Boer said. "I think the major challenge is to further the negotiating agenda in a way that makes major players feel safe to step forwardly on this issue."

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," Achim Steiner, who heads the UN Environment Program, told The Associated Press.

"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.

De Boer was optimistic, there too. In an interview, he said that despite U.S. greenhouse gas emissions increasing 16 percent since 1990, change is afoot.

Citing congressional interest and carbon dioxide emission limits requested by top industry CEOs, de Boer said: "I see a very important momentum building throughout the country."