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Report backs global warming claims

A signature piece of evidence for global warming -- a chart showing that a sharp rise in temperatures made the late 20th century the warmest period in at least 1,000 years -- is most likely correct, a national panel of scientific experts concluded today.

The graph is often referred to as “the hockey stick” because it is shaped like one: A shaft that shows a long period of gradual change in Northern Hemisphere surface temperatures, and then a sharp spike upward during the last 100 years or so that represents the blade. Since it was first published in a scientific journal in 1998, environmentalists have seized on it as proof of human-induced climate change while scientific critics have attacked its methodologies and data.

Last year, the graph catapulted into the national political arena after Joe Barton, a Texas Republican who is chair of the House energy committee, asked its three authors – including a University of Massachusetts professor -- for a detailed accounting of their funding and data. The request was blasted as an intimidation tactic by a range of scientists and other congressmen, and the National Academy of Sciences, which advises Congress and the government, was asked to conduct an independent review.

“Our conclusion is that this recent period of warming is likely the warmest in a (millennium),’’ said John Wallace, one of the 12 members on the panel and professor of atmospheric science at the University of Washington.

Scientists widely believe that emissions of carbon dioxide from power plants and cars that get trapped in the atmosphere are helping drive global warming. The report says that there are multiple lines of evidence supporting this conclusion. "This doesn't change the scientific landscape in terms of the greenhouse warming debate," Wallace said.

Because reliable temperature records stretch back only about 150 years, scientists must extrapolate past temperature and climate data indirectly, from natural archives such as tree growth rings, corals, ice cores, and cave deposits. For example, trees grow faster in warmer temperatures, so scientists can determine how warm it was in the past from the size of growth rings. And sediments at the bottom of some lakes can provide clues, because more sediment is deposited in warm periods when there is more snowmelt to carry the soil.

The National Academy of Sciences study looked at the validity of\ the initial report and concluded its main point about warming in the late 20th century is likely correct, basing that finding on a review of the evidence in the initial report and subsequent studies.

However, the authors found little confidence in the initial report's statement that the 1990s was the warmest decade, and 1998 the warmest year, in at least a millennium. While it may be true, they said temperature data for the last 1,000 years are not reliable enough to trust those statements.

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