In the strongest language it has ever used, a United Nations panel says global warming is "very likely" caused by human activities and has become a runaway train that cannot be stopped.
The warming of Earth and increases in sea levels "would continue for centuries ... even if greenhouse gas concentrations were to be stabilized," according to a 20-page summary of the report that was leaked to wire services.
The summary of the fourth report by the U.N.'s Intergovernmental Panel on Climate Change, or IPCC, was scheduled for release this morning in Paris. But scientists involved in the final editing process have been leaking bits and pieces from it all week, culminating in the leaking of the full report eight hours before its release.
The phrase "very likely" indicates a 90% certainty. The last IPCC report, issued five years ago, said it was "likely" that human activity was at fault, indicating a certainty of 66%.
Many scientists had argued during the editing process that the report should say it is "virtually certain" that human activities are causing global warming. That would indicate a 99% certainty.
But the change was strongly resisted by China, among other nations, because of its reliance on fossil fuels to help build its economy.
The report also says scientists' "best estimate" is that temperatures will rise 3.2 to 7.8 degrees by 2100. In contrast, the increase from 1901 to 2005 was 1.2 degrees.
The report also projects that sea levels could rise by 7 to 23 inches by the end of the century, and perhaps an additional 4 to 8 inches if the recent melting of the Greenland ice sheet and the Larsen B ice shelf in western Antarctica continues at current rates.
That is a decrease from the maximum of 35 inches predicted in the earlier study.
Nonetheless, such an increase would inundate many low-lying areas around the world, including islands such as Kiribati in the western Pacific Ocean and marsh areas near New Orleans. Such flooding would affect more than 10 million people.
The report also predicts a melting of Arctic ice during summers and a slowing of the Gulf Stream.
In addition, the report says, for the first time, that it is "more likely than not" that the strong hurricanes and cyclones observed since 1970 have been produced by global warming. The 2002 report said scientists did not yet have enough evidence to make such a link.
The summary is a purely scientific document and does not offer any recommendations on ways to control the problem. Those are expected in a chapter to be released this year.
The obvious solution would be to cut emissions of carbon dioxide, the most important greenhouse gas, by reducing the use of fossil fuels in automobiles, factories and power plants.
The 1997 Kyoto Protocol was designed to reduce such emissions, but some major countries, including the United States, China and India, have no defined targets. President Bush withdrew the U.S. from the protocol in 2001, arguing that it was an "economic straitjacket" and that it failed to set standards for developing nations.
The earlier IPCC report was heavily criticized by conservative critics and a variety of online bloggers who said it exaggerated the effects of global warming. But a new study reported Thursday in the online version of the journal Science said that the IPCC report actually significantly underestimated both the extent of warming and the extent of the rise in sea levels.
An international team of climate experts said in the Science report that data showed global temperatures had increased by 0.6 degree, at the upper limit of the U.N.'s predictions, and that sea levels had risen 0.13 inch per year, compared with the U.N. report's estimate of less than 0.08 inch per year.
The data show that "IPCC is presenting a consensus view that has been OKd by a very large number of interests, so it tends to err on the side of making cautious statements and not exaggerating," said geochemist Ralph Keeling of the Scripps Institution of Oceanography in La Jolla, one of the authors of the Science study.
The Science study "looks quite solid to me, indicating ... that the climate is changing in a very significant way -- and model projections are not overestimates, as some charge," said atmospheric scientist Michael MacCracken of the Climate Institute, an independent think tank in Washington.
The unexpectedly large rise in sea levels may be at least partially due to the recently observed melting of the ice sheets, the authors of the Science study said.
The increase also may be due in part to a natural variability in sea levels superimposed onto rises produced by global warming, they said. It would be "premature," they concluded, to assume that sea levels will continue to increase at the current rate.
The Associated Press and Reuters were used in compiling this report.