The science academies of the world's leading nations have urged their governments to take prompt action to combat possible climate change.
The statement will increase pressure to cut CO2 emissions
They have agreed that all countries could and should take cost-effective action to cut carbon dioxide emissions.
The statement was released on Wednesday by the academies of the G8 nations, including the UK's Royal Society and the US National Academy of Sciences.
It was signed by scientists from 11 countries, including China and India.
The academies are making their voices heard ahead of July's G8 meeting in Scotland, where the British Prime Minister has promised to put climate change high on the agenda.
Their statement read: "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.
"Action taken now to reduce significantly the build-up of greenhouse gases in the atmosphere will lessen the magnitude and rate of climate change."
Lord May, the current President of the UK's Royal Society, added: "It is clear that world leaders, including the G8, can no longer use uncertainty about aspects of climate change as an excuse for not taking urgent action to cut greenhouse gas emissions."
He called US policy "misguided" and noted that crucial to the international acceptance of the statement was the fact that leading scientists from three of the world's biggest developing world emitters China, India and Brazil had also signed it.
White House position
But the statement was immediately rejected by the prominent American global warming sceptic Professor Fred Singer, president of the Science and Environmental Policy Project based in Arlington, Virginia.
He told the BBC News there was no firm evidence of global warming. He claimed the data was contradictory and there was no consensus within the scientific community.
He said: "There is simply no consensus. That's a myth. Even if there were a warming, it's a question of how much. Obviously the greenhouse effect is real; the problem is the data do not show a significant warming."
In turn, Stephen Cox, executive secretary of the Royal Society, said contrarians such as Professor Singer were increasingly becoming isolated, and pointed to Tuesday's comments by George Bush as evidence that the White House, too, was shifting its position.
"If one listened very carefully to what President Bush said last night at his press conference, it appears to me there has been a change and that President Bush has accepted implicitly that there is a problem and has accepted therefore the advice of his own academy, the National Academy of Sciences in Washington, who themselves signed this statement, that there is a really serious problem as far as climate change is concerned."