On a high-profile and bi-partisan fact-finding tour in Alaska and Canada's Yukon territory, Senators John McCain, a Republican, and Hillary Clinton, the Democratic senator for New York, were confronted by melting permafrost and shrinking glaciers and heard from native Inuit that rising sea levels were altering their lives.

  "The question is how much damage will be done before we start taking concrete action," Mr McCain said at a press conference in Anchorage. "Go up to places like we just came from. It's a little scary." Mrs Clinton added: "I don't think there's any doubt left for anybody who actually looks at the science. There are still some holdouts, but they're fighting a losing battle. The science is overwhelming."

Their findings directly challenge President George Bush's reluctance to legislate to reduce America's carbon emissions. Although both senators have talked before of the need to tackle global warming, this week's clarion call was perhaps the clearest and most urgent. It also raises the prospect that climate change and other environmental issues could be a factor in the presidential contest in 2008 if Mrs Clinton and Mr McCain enter it. Mrs Clinton and Mr McCain, who represents Arizona, are among the leading, and the most popular, likely contenders.

That they chose Alaska as the stage from which to force global warming on to the American political agenda was not a matter of chance. In many ways, this separated US state is the frontline in the global warming debate. Environmentalists say the signs of climate change are more obvious there than perhaps anywhere else in the US.

Dan Lashof, a scientist with the Natural Resources Defense Council, a respected Washington-based group, told The Independent: "People in Alaska are starting to freak out. The retreat of the sea ice allows the oceans to pound the coast more, and villages there are suffering from the effects of that erosion. There is permafrost melting, roads are buckling, there are forests that have been infested with beetles because of a rise in temperatures. I think residents there feel it's visible more and more, more than any other place in the country."

President Bush's administration has repeatedly questioned the evidence of global warming and the contribution of human activity to any shift. Mr Bush, who in 2001 refused to ratify the Kyoto treaty on global warming weeks after he took office, has repeatedly been accused of doing nothing to enforce tighter controls on emissions of carbon dioxide and other "greenhouse gases". But this summer, the US National Academy of Sciences - and the scientific academies of the other G8 nations as well as Brazil, China and India - issued a statement saying there was strong evidence that significant global warming was happening and that "it is likely that most of the warming in recent decades can be attributed to human activities". They called on world leaders to recognize"that delayed action will increase the risk of adverse environmental effects and will likely incur a greater cost". Mrs Clinton, who must first win her re-election to the US senate next year if she is to enter the 2008 White House race, said at the press conference that she had spoken to scientists as well as native Alaskans during the trip.

She said that, flying over the Yukon, she saw forests devastated by spruce bark beetles, believed to be increasing at an unprecedented rate because of warmer weather. She also talked of what a 93-year-old woman at a fish camp at Whitehorse told her. The woman said she had been fishing there all her life but now fish have strange bumps on them.

"It's heartbreaking to see the devastation," Mrs Clinton said. Mr McCain, Mrs Clinton and Senators Lindsey Graham of South Carolina and Susan Collins of Maine, also went to Barrow, the northernmost city in the US. There, they spoke to scientists and Inupiaq Inuit. They also saw shrinking glaciers in Kenai Fjords National Park.

Mr McCain - with Senator Joe Lieberman - is behind proposed legislation that would require power-generating companies to reduce carbon emissions to their 2000 levels. Mr Graham, a Republican, said he had been moved by what he had seen. "Climate change is different when you come here, because you see the faces of people experiencing it. If you go to the people and listen to their stories and walk away with any doubt that something's going on, you're not listening."

Mrs Collins, a Democrat, was even more convinced. She said the evidence in Alaska represented the "canary in the mine shaft of global warming crying out to us to pay attention"