Governments around the world have called for urgent action to tackle climate change, after a major report said mankind was very likely the cause.
The IPCC examined a link between warming and stronger storms
The EU said it was the starkest warning yet, while the UK said climate change threatened world peace and prosperity.
The US administration said the report was "valuable", but rejected mandatory controls to reduce greenhouse gases.
The report, by more than 2,000 top scientists, says world temperatures could increase by 3C by 2100.
The Intergovernmental Panel on Climate Change (IPCC) also projected that sea levels were most likely to rise by 28-43cm, and said global warming was likely to influence the intensity of tropical storms.
The findings are the first of four IPCC reports to be published this year.
EU Environment Commissioner Stavros Dimas said he was now deeply concerned by the accelerating pace of climate change.
Probable temperature rise between 1.8C and 4C
Possible temperature rise between 1.1C and 6.4C
Sea level most likely to rise by 28-43cm
Arctic summer sea ice disappears in second half of century
Increase in heatwaves very likely
Increase in tropical storm intensity likely
"It is now more urgent than ever that the international community gets down to serious negotiations on a comprehensive new worldwide agreement to stop global warming," he said.
UK Environment Secretary David Milliband said that international political commitment, which had so far been lacking, was now needed.
South Africa's Environmental Affairs Minister Marthinus van Schalkwyk said failure to act would be "indefensible".
And officials from Indonesia and the Maldives said they feared for the future of their countries, both threatened by rising sea levels.
US Energy Secretary Sam Bodman said he accepted the conclusions of the scientists.
"We're very pleased with it. We're embracing it. We agree with it," he said.
"Human activity is contributing to changes in our Earth's climate and that issue is no longer up for debate."
But he said that a unilateral US programme to cut emissions might damage the US economy and send business overseas.
Meanwhile the head of the Senate Environment Committee, Barbara Boxer, called on Mr Bush to show "real leadership" on the issue of climate change.
"I'm calling on the president to convene a summit at the White House of the 12 largest global warming emitters," she said, adding that she would bring the IPCC scientists to brief the Senate in the next few weeks.
The US contributes around one-quarter of the world's greenhouse gas emissions.
The report, produced by a team tasked with assessing the science of climate change, was intended to be the definitive summary of climatic shifts facing the world in the coming years.
The agency said that it would use stronger language to assess humanity's influence on climatic change than it had previously done.
In 2001, it said that it was "likely" that human activities lay behind the trends observed at various parts of the planet; "likely" in IPCC terminology means between 66% and 90% probability.
Now, the panel concluded that it was at least 90% certain that human emissions of greenhouse gases rather than natural variations are warming the planet's surface.