Eleven of the world's most influential science academies warned world leaders that the threat of global climate change "is clear and increasing" and that they must act immediately to begin addressing its causes and consequences.

The stark warning came on Tuesday in an unprecedented joint statement from the heads of the science academies of Brazil, Canada, China, France, Germany, India, Italy, Japan, Russia, the UK and the US.

The statement, issued ahead of the G8 economic summit in Gleneagles, UK, in July, outlines the strong evidence for global warming.

"The evidence comes from direct measurements of rising surface air temperatures and subsurface ocean temperatures and from phenomena such as increases in average global sea levels, retreating glaciers, and changes to many physical and biological systems," the statement says, adding that most recent warming is likely to have been caused by human activity.

Major put-down

That the US National Academy of Sciences, the key scientific adviser to Congress, has signed the document is seen as a major put-down to the global warming naysayers in the Bush administration, which has refused to sign up to the Kyoto protocol to curb greenhouse gas emissions.

Following a meeting in Washington DC between President George Bush and UK prime minister Tony Blair on Tuesday, Bush conceded that climate change is a "major long-term problem" and cited US research into hydrogen-powered cars and low-pollution coal as moves already under way to help combat it. But there was no change to his opposition to the Kyoto protocol.

Nevertheless, Blair intends to make tackling climate change a central plank of the G8 summit he will be hosting. "We need to be thinking about how we move beyond a situation where there are huge emissions of greenhouse gases from present energy consumption," he said.

Environmental group Friends of the Earth welcomed the joint statement but complained that it lacks targets and a timetable for action. "G8 countries must accept their historic responsibility in creating the problem, and show genuine leadership through annual reductions in emissions," says Catherine Pearce, climate expert at Friends of the Earth.

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