A scientific panel on Wednesday warned President Bush that greenhouse gas emissions will cause global warming to continue, a finding that could complicate Bush's efforts to sell his environmental policies to wary allies.
A report issued by a committee of the National Academy of Sciences' National Research Council concluded that more research is needed to determine exactly how much global climate change is attributable to human activities.
Nevertheless, it said, "global warming could well have serious adverse societal and ecological impacts by the end of this century, especially if globally averaged temperature increases approach the upper end of . . . projections."
The report, which was requested by the White House, comes as Bush is preparing to defend his approach to global warming during meetings with European leaders next week.
Bush has been widely criticized by U.S. allies for his decision to withdraw from the Kyoto accord, a global agreement to reduce greenhouse gas emissions, and reverse a campaign pledge to limit carbon dioxide emissions from power plants.
Environmentalists and congressional allies who favor aggressive action to combat climate change said the report will make it difficult for the administration to question the reality of global warming or propose toothless efforts to deal with it.
"I think it's a very important message for the White House to hear; it underscores the need for them to come up with a constructive policy before they go to Europe," said Sen. John F. Kerry (D-Mass.).
The panel said the large body of international research linking climate change to human activity is reliable. That conclusion appears to rebuff assertions by some conservative lawmakers that previous warnings about global warming have been tainted by political bias.
Among the main findings in the 28-page report:
* Climate change is occurring as a result of human activity, and will continue to cause air and ocean temperatures to rise.
* Previous international research is scientifically valid, but may have underemphasized some of the uncertainties surrounding global warming.
* By 2100, the planet's average surface temperature will increase by at least 2.5 degrees and possibly as much as 10.4 degrees Fahrenheit.
* Anticipated climate change will have devastating effects: increased drought in semiarid regions; higher sea levels in coastal areas; an increase in flooding and polluted storm-water runoff in some areas.
Although Bush has said he intends to propose an alternative to the Kyoto agreement, a top aide said Wednesday he will not have a detailed proposal to present to European leaders during next week's six-day, five-nation tour.
National Security Advisor Condoleezza Rice said a Cabinet-level advisory group is still studying the issue and is not ready to formulate specific policy options. The working group includes Vice President Dick Cheney, Secretary of State Colin L. Powell, Agriculture Secretary Ann M. Veneman, Interior Secretary Gale A. Norton, EPA administrator Christie Whitman and others.
But environmentalists were heartened by the potential effect of the study on the administration's policy development. "No one in the White House has any basis left to question the science of global warming or to question whether serious immediate action is necessary," said Phil Clapp, president of the National Environmental Trust.
"It completely undercuts any basis the administration had for walking away from a strong international agreement on climate change or for not acting to reduce further greenhouse emissions," he said.
Shogren reported from Washington and Mehta from Orange County. Times staff writer Robin Wright in Washington contributed to this story.