(Adds Chirac's call for new environmental body, edits)

By Gerard Wynn and Alister Doyle

PARIS, Feb 2 (Reuters) - Mankind is to blame for global warming, the world's top climate scientists said on Friday, sending governments a "crystal clear" warning they must take urgent action to avert damage that could last centuries.

The United Nations panel, which groups 2,500 scientists from more than 130 nations, predicted more droughts, heatwaves and a slow gain in sea levels that could continue for more than 1,000 years even if greenhouse gas emissions were capped.

The panel's report predicts a "best estimate" that temperatures would rise by between 1.8 and 4.0 Celsius (3.2 and 7.8 Fahrenheit) in the 21st century.

"Faced with this emergency, now is not the time for half measures. It is the time for a revolution, in the true sense of the term," French President Jacques Chirac said. "We are in truth on the historical doorstep of the irreversible."

The scientists said it was "very likely" -- or more than 90 percent probable -- that human activities led by burning fossil fuels explained most of the warming in the past 50 years.

That is a toughening of the position taken by the Intergovernmental Panel on Climate Change's (IPCC) last report in 2001, which judged a link as "likely", or 66 percent probable.

Many governments, U.N. agencies and environmental groups urged a widening of the U.N.'s Kyoto Protocol, which binds 35 industrial nations to cut emissions by 2012 but excludes top emitters led by the United States, China and India.

"The signal we've received from the scientists today is crystal clear and it's important that the political response is also crystal clear," said Yvo de Boer, head of the U.N. Climate Secretariat.

GREEN SUMMIT

He wants an emergency environment summit of world leaders this year to push for wider action. Kyoto has been weakened since the United States pulled out in 2001 and emissions by many backers of Kyoto are far above target.

The Bush administration played down the U.S. contribution to climate change although the country is the biggest single source of greenhouse gases, with a quarter of the world total.

"We are a small contributor when you look at the rest of the world," U.S. Energy Secretary Sam Bodman said of greenhouse gas emissions after the IPCC report. "It's really got to be a global discussion."

A 21-page summary of IPCC findings for policy makers outlines wrenching change such as a possible melting of Arctic sea ice in summers by 2100 and says it is "more likely than not" that greenhouse gases have made tropical cyclones more intense.

The report projects a rise in sea levels of between 18 and 59 centimetres (7 and 23 inches) in the 21st century -- and said bigger gains cannot be ruled out if ice sheets in Antarctica and Greenland thaw.

Rising seas threaten low-lying islands, coasts of countries such as Bangladesh and cities from Shanghai to Buenos Aires.

Temperatures rose 0.7 degrees in the 20th century and the 10 hottest years since records began in the 1850s have been since 1994. Greenhouse gases are released mainly by burning fossil fuels in power plants, factories and cars.

U.S. President George W. Bush says Kyoto-style caps would harm the economy and that Kyoto should include developing nations. His policies will brake the growth of emissions but stop short of caps favoured by most of his industrial allies.

Democrats who control both houses of Congress want tougher action.

The president of Kiribati, a group of 33 Pacific coral atolls threatened by rising seas, said time was running out.

"The question is, what can we do now? There's very little we can do about arresting the process," President Anote Tong said.

"There is no single solution," the International Energy Agency said. It wants more energy savings, more renewable energy, nuclear power and efforts to make fossil fuels cleaner.

Speaking to a meeting of scientists, environmental campaigners, foreign officials and business leaders, Chirac said the United Nations Environment Programme (UNEP) had insufficient power and should be overhauled and upgraded.

"Our aim must be to transform (the UNEP) into a fully fledged United Nations organisation. This United Nations Environment Organisation will carry the global ecological conscience," he said, suggesting the new body's name.