The report was requested to help prepare Bush for his trip to Europe next week, but the academy was not asked for policy recommendations and it made none.

However, White House press secretary Ari Fleischer said the report does not definitely conclude that human activity is the cause of rising temperatures.

``Yes, temperatures (are) rising. It is uncertain what has caused it and what the solutions might be,'' he said.

Bush wanted the study to help the administration decide what steps to take to combat climate change.

In Europe, Bush has meetings on global warming scheduled with various officials. Many Europeans protested vigorously after Bush, citing looming energy shortages, in March reversed a campaign promise to limit CO2 emissions from power plants.

Bush's Cabinet-level task force plans to keep studying the issue after the president goes to Europe, where he is expected to outline a set of mostly voluntary steps that countries could take to reduce emissions.

The 24-page National Academy of Sciences report, an assessment based on previous studies of the phenomenon, says, ``The primary source, fossil fuel burning, has released roughly twice as much carbon dioxide as would be required to account for the observed increase'' in temperature.

The report also blames global warming on other greenhouse gases directly affected by human activity: methane, ozone, nitrous oxide and chlorofluorocarbons.

``Despite the uncertainties, there is general agreement that the observed warming is real and particularly strong within the past 20 years,'' it says. ``Global warming could well have serious adverse societal and ecological impacts by the end of this century.''

One U.S. area likely to be hard hit by climate change is the United States' breadbasket, the Great Plains.

Two senior Bush advisers, John Bridgeland, who oversees domestic policy, and Gary Edson, an economist, wrote to the academy May 11 asking for help with ``identifying the areas in the science of climate change where there are the greatest certainties and uncertainties.''

In preparation for his round of meetings with European allies, Bush held a lengthy meeting with Cabinet members Tuesday to come up with a strategy on how to sell his almost-finished proposal for a global-warming agreement, according to senior administration officials. In March, he rejected an international pact former Vice President Al Gore signed in Kyoto, Japan, that would have set tight limits on emissions of many greenhouse gases.

Sen. John Kerry, D-Mass., said the academy report was unnecessary and ``underscores the lack of leadership'' by Bush on global warming. ``The science on this has been strong enough that presidents and foreign ministers of other countries have moved on this for years,'' Kerry said.

But now that the report is in hand, he said, ``It increases the imperative for them to take action.''

Sen. Chuck Hagel, R-Neb., a major participant in the debate on global warming, said the report ``provides us with a basis to move forward with an alternative'' global warming strategy.

Though the report is neutral on that, scientists ``really do know that CO2 is the main driver'' behind global warming, said the report's lead author, Ralph Cicerone, chancellor of the University of California, Irvine.

Prepared in less than a month by 11 scientists, the report finds agreement with the assessment of human-caused climate change by the Intergovernmental Panel on Climate Change, an agency of the United Nations.

``The IPCC's conclusion that most of the observed warming of the last 50 years is likely to have been due to the increase in greenhouse gas concentrations accurately reflects the current thinking of the scientific community on this issue,'' the report says.

It says, however, the increase of global fossil fuel carbon dioxide emissions in the past decade has averaged about 0.6 percent per year, less than the range of IPCC scenarios.

Other findings are:

By 2100, temperatures are expected to increase between 2.5 degrees and 10.4 degrees Fahrenheit above those of 1990.

The predicted warming is larger over higher latitudes than over low latitudes, especially during winter and spring, and larger over land than over the oceans.

Rainfall rates and the frequency of heavy precipitation events are predicted to increase, particularly over the higher latitudes.

``The likelihood that this effect could prove important is greatest in semiarid regions, such as the U.S. Great Plains,'' the report says.

On the Net:

National Academy: http://www.nationalacademies.org

United Nations: http://www.ipcc.ch

EPA global warming: http://www.epa.gov/globalwarming

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