US scientists have increased the pressure on George Bush and other world leaders to tackle climate change by signing a joint statement calling on G8 nations to reduce greenhouse gas emissions.

Lord May, president of the Royal Society, said current US policy on climate change was "misguided".

The statement, from the science academies of the G8 countries, says the scientific evidence on climate change is now clear enough to compel their leaders to take action.

It says: "There is now strong evidence that significant global warming is occurring. It is likely that most of the warming in recent decades can be attributed to human activities...

"The scientific understanding of climate change is now sufficiently clear to justify nations taking prompt action. It is vital that all nations identify cost-effective steps that they can take now, to contribute to substantial and long-term reduction in net global greenhouse gas emissions."

The statement has been issued ahead of the G8 summit in Gleneagles in July. It follows months of negotiations between the UK's Royal Society, which published it yesterday, and the other academies.

One source close to the negotiations called the support of the US National Academy of Sciences "unprecedented".

In 2001 the US academy declined to sign a similar joint statement because it was preparing its own report on the issue for the Bush administration.

In a separate 1992 report it concluded: "Despite the great uncertainties, greenhouse warming is a potential threat sufficient to justify action now," but until now it has stopped short of making specific policy recommendations.

President Bush has consistently stressed the uncertainties of climate science but the new statement makes it more difficult for him to dispute the scientific consensus.

The statement calls on G8 nations to "recognize that delayed action will increase the risk of adverse environmental effects and will likely incur a greater cost."

It was released as Tony Blair was meeting Mr Bush in Washington. Mr Blair has made action on climate change and aid to Africa his priorities for the G8 summit.

Lord May, president of the Royal Society, said current US policy on climate change was "misguided".

He said: "Getting the US on board is critical because of the sheer amount of greenhouse gas emissions they are responsible for. President Bush has an opportunity at Gleneagles to signal that his administration will no longer ignore the scientific evidence and act to cut emissions."

Vicki Arroyo, director of policy analysis at the Pew Center on Global Climate Change, a US thinktank in Virginia, said the statement "makes it harder for the [Bush] administration to do what it generally does, which is to focus on the uncertainty."

Along with the Royal Society and the US National Academy of Sciences, the statement is signed by the G8 science academies of France, Russia, Germany, Japan, Italy and Canada, along with those of Brazil, China and India - among the largest emitters of greenhouse gases in the developing world.

Lord May said: "It is clear that developed countries must lead the way in cutting emissions but developing countries must also contribute. The scientific evidence forcefully points to a need for a truly international effort. Make no mistake, we have to act now."

Levels of carbon dioxide - the most common greenhouse gas in the atmosphere produced by burning fossil fuels - have increased from 280 parts per million in 1750 to over 375ppm today. Scientists say this warmed the Earth's surface by about 0.6C during the 20th century. The statement says this warming has already led to climate changes.

The Intergovernmental Panel on Climate Change predicts that average temperatures will rise further by 2100, to between 1.4C and 5.8C above 1990 levels.

Catherine Pearce, climate campaigner with Friends of the Earth, said: "The national science academies are right to call for prompt action on climate change. But this document lacks targets or a timetable for urgent action.

"G8 countries must accept their historic responsibility in creating the problem, and show genuine leadership through annual reductions in emissions."

© Copyright Guardian Newspapers Limited 2005

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