US Scientists Back Manmade Warming Claim

US scientists have endorsed the contentious findings of a global warming study that showed humans are drastically altering the climate.

The US National Academies of Science said the last few decades of the 20th century were the warmest for 400 years. It adds that many parts of the globe were warmer during the past 25 years than during any comparable period over the last 1,000 years. But it had less confidence in more specific claims that the 1990s were the warmest decade, and 1998 the warmest year.

The new report, published yesterday, focused on the "hockey stick" graph - a reconstruction of average global temperatures over the last 1,000 years that shows a sharp upswing in the late 20th century.

The image was produced in 1999 by a team led by climate expert Michael Mann who estimated past temperatures using centuries-old sediments and tree rings. It appeared to show the rise in global temperatures since the industrial revolution. It quickly became a main factor in the climate change debate but critics said Dr Mann's techniques were flawed.

Yesterday's report, prepared by the National Academies' national research council, said: "The committee found the Mann team's conclusion that warming in the last few decades of the 20th century was unprecedented over the last 1,000 years to be plausible."

It added: "There is sufficient evidence from the tree rings, boreholes, retreating glaciers and other 'proxies' of past surface temperatures to say with a high level of confidence that the last few decades of the 20th century were warmer than any comparable period for the last 400 years."

Before 1600, it said there was not enough reliable data to be sure, but that it was more likely that the most recent decades were warmer. But it said that "even less confidence" could be placed in the Mann team's conclusions about the 1990s, and 1998 in particular.

One British expert who helped to write the report believed some climate scientists had gone too far in an effort to force people to take action. Neil Roberts, a climate scientist at Plymouth University, said: "We have to recognise that some of the climate supporters have tended to use data that are not by any means proven because they believe it's in a good cause."