A Bush administration report suggests that evidence of global warming has begun to affect animal and plant populations in visible ways, and that rising temperatures in North America are due in part to human activity.

The report to Congress, issued Wednesday, goes further than previous statements by President Bush. He has said more scientific research is needed before he imposes new restrictions on greenhouse gases such as carbon dioxide.

In 2001, after the release of a National Academy of Sciences report on global warming, Bush said the concentration of greenhouse gases has increased, in large part, because of human activity, but he emphasized that other factors could have influenced warming. Referring to the NAS report, he said, "We do not know how much effect natural fluctuations may have had on warming."

Several administration officials characterized the study as a routine annual summary of scientific research on global warming. John H. Marburger, the president's science adviser, said the report has "no implications for policy."

"There is no discordance between this report and the president's position on climate," Marburger said.

But environmentalists and conservatives said the report reveals contradictions within the administration's stance on global warming.

Jeremy Symons, who heads the National Wildlife Federation's global warming program, characterized the study as "nothing new in terms of the science of global warming, but this is definitely new in terms of the administration's position."

Myron Ebell, director of global warming policy for the Competitive Enterprise Institute, also said it signals a change. "We're frustrated and disappointed the administration seems to have an incoherent global warming policy," Ebell said.

Although the government issues this report each year, in this instance the overview covers two years of research on climate change. In one key finding, it describes carbon dioxide as "the largest single forcing agent of climate change."

Administration officials, including James R. Mahoney, director of the administration's climate change science program, said that although carbon dioxide is the primary human-generated influence on climate change, the largest influence on climate conditions is water vapor. He added that the report "was not a significant science finding or policy finding."

But Annie Petsonk, international counsel for Environmental Defense, said the report underscores the need for immediate curbs on greenhouse gas emissions.

"The administration is finally admitting what the National Academy of Sciences and virtually every other scientific body has concluded: Climate change is happening now," Petsonk said. "It's time for the United States, the world's biggest greenhouse gas polluter, to step up to the plate and start cutting emissions."

The overview also concludes that based on studies of more than 1,700 species, "the balance of evidence" shows global warming effects are evident in plants and animals.

In response, Marburger said: "Climate change can have broad implications for our environment, and that is why the president has put forward an aggressive strategy to develop the best science and technology to address the issue."