Scientists believe world is at its hottest for 2,000 years

In a report to the US Congress, the National Academy of Sciences reported that the "recent warmth is unprecedented for at least the last 400 years and potentially the last several millennia".

A panel of top climate scientists told politicians that the Earth is heating up and that "human activities are responsible for much of the recent warming".

Their 155-page report said average global surface temperatures in the Northern Hemisphere rose about 1F during the 20th century.

This is shown in boreholes, retreating glaciers and other evidence found in nature, said Gerald North, a geosciences professor at Texas A&M University who chaired the academy's panel. The report was requested in November by the chairman of the House science committee, Republican Sherwood Boehlert, to address those who question whether global warming is a major threat.

Mr Boehlert said the report shows the value of having scientists advise Congress: "There is nothing in this report that should raise any doubts about the broad scientific consensus on global climate change."

The Bush administration has maintained that the threat is not severe enough to warrant new pollution controls that the White House says would have cost five million Americans their jobs.

Climate scientists Michael Mann, Raymond Bradley and Malcolm Hughes had concluded the Northern Hemisphere was the warmest it has been in 2,000 years.

The National Academy scientists concluded that the Mann-Bradley-Hughes research from the late 1990s was "likely" to be true, said John 'Mike' Wallace, an atmospheric sciences professor at the University of Washington and a panel member. The conclusions from the Nineties research "are very close to being right", and are supported by even more recent data, Mr Wallace said.

Overall, the panel agreed that the warming in the last few decades of the 20th century was unprecedented over the past 1,000 years.