Scientific evidence that human activity is heating the Earth has become "far more robust" in the last five years, the head of the United Nations climate change panel said.
Rajendra Pachauri, chairman of the Intergovernmental Panel on Climate Change (IPCC), said an increase of research on global warming had added weight to the group's upcoming report, which is considered a mainstay for environmental policy-making.
"Some of the uncertainties that we had in the scientific evidence will be reduced. Our evidence will be far more robust," Pachauri told Reuters in a telephone interview.
In its last assessment in 2001, the IPCC said there was "new and stronger evidence" that gases linked to human activities, mainly from burning fossil fuels in power plants, factories and cars, were the main cause of global warming.
Its next report, the first chapter of which will be launched in Paris on February 2, will group research by about 2,000 scientists on the drivers of climate change and its impacts on weather, disease, ecology and water supply.
Marked strengthening in the IPCC conclusions might pressure the United States - which pulled out of the emissions-cutting UN Kyoto Protocol in 2001 - to toughen its policies.
President George W. Bush aims to brake the growth of emissions, which were at about 16 percent above 1990 levels in 2004. The White House says it will seek to halt or reverse the rise "as the science justifies".
James Connaughton, chairman of the White House Council on Environmental Quality, said he did not expect the new IPCC report to make a big splash.
Half the findings come from US-funded science programmes and have been factored into policy-making, Connaughton told Reuters during a US-European Union meeting in Helsinki.
Still, Pachauri said the report could add momentum to what he called "very encouraging" policy shifts in California and other states, and amongst the US corporate community.
Public interest in climate change rose after Hurricane Katrina devastated New Orleans last year, and after the release of Hollywood films such as "The Day After Tomorrow" and Al Gore's "An Inconvenient Truth".
"There is an unprecedented level of awareness of climate change and interest in the subject, and when the report comes out I expect there to be a lot of attention paid to it," Pachauri said. "Presumably that will impact the political behaviour of people across the world."