Feb. 2 (Bloomberg) -- Global warming is ``very likely'' caused by humans, and world temperatures and sea-levels will increase by the end of the century, the United Nations said in its most comprehensive report yet on climate change.

Temperatures are likely to rise by 1.1 to 6.4 degrees Celsius by the end of this century relative to the last, with a probable 2 to 4.5 degree range if carbon dioxide doubles from pre-industrial levels, the UN Intergovernmental Panel on Climate Change said in the report. Sea-level gain over the same period may range from 18 to 59 centimeters (7 to 23 inches).

The Bush administration said the human role in climate change is no longer debatable following the report. ``Human activity is contributing to changes in the Earth's climate,'' Energy Secretary Samuel Bodman said at a press conference in Washington. ``That issue is no longer up for debate.''

A key change in the report's language compared with the panel's 2001 document showed there is more certainty that human activity is causing the warming. The report, released to reporters in Paris, puts the probability of the link at more than 90 percent, against the 66 to 90 percent likelihood signaled in 2001.

``Clearly we are endangering all species on earth, we are endangering the future of the human race,'' IPCC chairman Rajendra Pachauri said in an interview. ``We are probably beyond the stage where we could have called it urgent. I would say it is immediate,'' he said, referring to the need for governments to reduce emissions.

Extreme Weather

The global atmospheric concentration of carbon dioxide (CO2) stood at 379 parts per million in 2005, up from about 280 ppm in 1750, before the industrial revolution, the report said. Amounts of CO2 and methane, another greenhouse gas, exceed ``by far'' the highest in a 650,000-year Antarctic ice-core record.

The report ``puts a full stop behind the questioning of the science underlying the issue of whether humans are causing global warming,'' Achim Steiner, director of the UN Environment Program, said in an interview in Paris. ``This is critical because it allows us to now to shift the attention to what kind of policy responses and international initiatives we need to achieve emissions reductions.''

Scientists have said global warming caused by man-made emissions is responsible for melting glaciers and ice sheets, and increased instances of storms, droughts and floods. Over this century, those effects may be magnified, according to today's report.

`Smoking Gun'

``Continued greenhouse gas emissions at or above current rates would cause further warming and induce many changes in the global system during the 21st century that would very likely be larger than those observed during the 20th century,'' the report said.

The increases in greenhouse gases are primarily attributable to fossil fuel use and land-use change, Susan Solomon, who chaired talks this week, said at a Paris news conference. The man-made ``forcing'' of temperature rise in 2005 was more than ten times greater than natural warming caused by changes in solar patterns, the IPCC said.

``We now have the smoking gun on global warming,'' said Philip Clapp, president of the U.S. National Environmental Trust in a Jan. 31 phone interview after seeing portions of the draft.

Today's report, ``Climate Change 2007: the Physical Science Basis,'' is the first in a four-volume survey that involves more than 2,500 scientists from over 130 countries. Governmental delegations and scientists went over the wording of today's report line-by-line, in talks that ended past midnight in Paris.

Rising Sea Levels

On the rise in sea-levels, language was added to the statement to reflect concerns that the new forecast doesn't bear in mind recent discoveries and so underestimates the potential, said Sharon Hays, associate director of the U.S. Office of Science and Technology Policy, in a telephone interview today.

The wording of the document reads: ``dynamical processes related to ice flow not included in current models, but suggested by recent observations could increase the vulnerability of the ice sheets to warming, increasing future sea-level rise.''

Scientists at the University of Texas at Austin on Aug. 10 said in the journal Science that melting of Greenland's ice sheet ``increased dramatically'' in the past few years. The breakup in 2002 of the Larsen B ice shelf opened the way for the west Antarctic ice sheet's glaciers to flow faster, accelerating melting.

Melting Glaciers

``We've seen a significant increase of the glaciers melting, the ice caps have been retreating, the area of snow cover has declined, and the arctic sea ice extent has dropped considerably,'' said Tingjun Zhang, a scientist at the U.S. National Snow and Ice Data Center who was lead author of one of the report's chapters. ``There's a big cost for construction and maintenance of infrastructure,'' because of the loss of stability when permafrost thaws, Zhang said in a telephone interview from Boulder, Colorado, where the NSIDC is based.

Infrastructure affected includes roads, railways, buildings and oil installations based on permafrost in places like the Tibetan plateau and the Russian arctic, he said.

The full temperature range across all scenarios examined by the IPCC was for a rise of 1.1 to 6.4 degrees Celsius, and ``best estimates,'' which weren't given in 2001, for each scenario ranged from 1.8 to 4 degrees, showing a greater degree of certainty about climate change.

Greater Certainty

``We have greater certainty, and that should worry us more,'' UNEP's Steiner said. Steiner and environmental groups including WWF International, Greenpeace and Friends of the Earth said that now the causes of global warming have been established with near certainty, governments need to reduce greenhouse gas emissions.

``The report, if anything tells politicians to act,'' said Hans Verolme, director of WWF's Climate Change Programme, in an interview in Paris. ``The costs of acting are minute with respect to the costs of inaction,'' he said of the risks of droughts, floods, storms and disease posed by global warming.

Many industrialized nations have already begun taking action by setting caps on emissions. Under the UN's Kyoto Protocol treaty, 35 countries and the European Union agreed to cut emissions of greenhouse gases by a combined 5 percent from 1990 levels by the 2008-2012 period. The U.S., the largest emitter of greenhouse gases, and Australia didn't ratify the treaty, and developing nations such as India and China aren't subject to emissions reductions.

Comprehensive Policies

U.S. President George W. Bush ``has put in place a comprehensive set of policies aimed at addressing what he himself has called a very serious problem,'' Hays, the U.S. negotiator, said. ``The Kyoto treaty was flawed and we believe that our policies are the right ones.''

In 2002, Bush set out plans to reduce greenhouse gas intensity, or the emissions per unit of gross domestic product, by 18 percent by 2012 -- preventing the release of more than 500 million metric tons of CO2. The top U.S. climate change negotiator, Harlan Watson, told Bloomberg in November that the U.S. is a ``little ahead of schedule'' on achieving that goal, which would still see total emissions 30 percent above 1990 levels, compared with the seven percent reduction Kyoto would have set for the country.

The second section of the IPCC report focuses on the impacts of and ways of adapting to climate change and will be released April 6. The third part, entitled ``mitigation of climate change'' is slated for release May 4, and the final part, a review for policy-makers, is due to be published on Nov. 16.

In December, politicians will meet in Indonesia for talks on reducing emissions held by the UN's Framework Convention on Climate Change. ``It is politically significant that all the governments have agreed to the conclusions of the scientists, making this assessment a solid foundation for sound decision making'' UNFCCC Executive Secretary Yvo de Boer told reporters in Paris. ``The world urgently needs new international agreement on stronger emission caps for industrialized countries, incentives for developing countries to limit their emissions, and support for robust adaptation measures.''

To contact the reporter on this story: Alex Morales in Paris at amorales2@bloomberg.net.

Last Updated: February 2, 2007 11:09 EST