Earth's average temperature has been hotter over the last quarter century than during the previous four centuries and possibly much longer, the National Academy of Sciences said in a report Thursday that substantially supports the findings of a controversial 1998 climate study.

The report by a scientific panel appointed by the academy backed the most vivid feature of the so-called hockey stick graphic, a chart showing a long-term rise in temperature between A.D. 900 and today.

The panelists agreed that "the last few decades of the 20th century were warmer than any comparable period in the last 400 years." But they showed less confidence in the researchers' conclusion that the climate is warmer now than it has been in 1,000 years, a conjecture the scientists said was only scientifically plausible. More research is needed to investigate that possibility, the panelists said.

Even so, the 141-page report by a dozen prominent scientists who reviewed the latest scientific reports on global warming and heard testimony from climate experts could offer ammunition to those who are calling for tightened controls on greenhouse gases that many scientists believe may be causing global warming. The study was sparked by a 2005 congressional dispute over the reliability of the 1998 study and chart.

The chart got its name for its vague resemblance to a hockey stick, the shaft depicting temperature fluctuations over a long stretch of time until about the mid-19th century, when warming suddenly spiked into the form of a blade.

First published in Nature magazine, the study thrilled global warming activists and angered some congressional Republicans and oil-industry executives.

The work by climate scientist Michael Mann, then at the University of Virginia, and two colleagues depicted a soaring rise in the Earth's average temperature since about the beginning of the Industrial Age, compared to generally cooler global temperatures through most of the Middle Ages.

Reliable, widespread thermometer records weren't available before the mid-19th century. To infer temperature averages in different eras, the scientists based most of their chart on natural or proxy records such as the width of rings inside trees that grew during the Middle Ages, the ratios of oxygen isotopes in polar ice cores and centuries-old paintings of glaciers in locales that are now ice-free.

However, the methodology was not sufficient to persuade the panel to throw its full weight behind the most sweeping claim by Mann and his colleagues: that temperatures through the Middle Ages were generally much cooler than they are today.

The panel's use of the word "plausible" in judging the claim puzzled Raymond Bradley of the University of Massachusetts, who published the original study with Mann and Malcolm Hughes of the University of Arizona. In an e-mail, Bradley called the use of the word strange and speculated the panel chose it to strengthen consensus. "I guess it was selected so that everybody on the committee could agree," he said.

A more sardonic view was taken by prominent Bay Area physicist Richard Muller of Lawrence Berkeley National Laboratory, who served as a peer reviewer for the academy's report. In 2004, he publicly criticized the Mann team's work, calling it "an artifact of poor mathematics ... when applied to the (temperature records of the) last millennium," he recalled in an e-mail Thursday.

Although Muller estimates 2 in 3 odds that humans are causing global warming, "the fact that the original conclusion of Mann et al. is 'plausible' is damning with faint praise," he said. "Theories are plausible; discoveries are supposed to be proven."

Despite these caveats, the scientists involved in Thursday's report agreed that there's no doubt that the planet is getting unusually hot -- and fast.

"The last 400 years has experienced a warming. The last 25 to 30 years have been warmer than any comparable period (in) that span," the panel's chairman, physicist-climatologist Gerald North of Texas A&M University said Thursday in Washington.

The academy panelists also dismissed critics' earlier insinuations that the Mann team played fast and loose with data, a point that pleased Mann, who is now at Pennsylvania State University.

"The report ... provides absolutely no support for the oft-heard claims that the original hockey stick was the result of 'programming errors,' or was 'not reproducible,' or there was some scientific misconduct involved," he said in an e-mail. "These claims were always spurious and should now finally be laid to rest ... The (academy) report is very good, and I'm pretty happy with it, especially given the short time interval over which the committee had to familiarize themselves with a complex and often quite technical debate."

The academy report was born from a political dispute. In 2005, the hockey-stick chart became the target for Sen. James Inhofe, R-Oklahoma, who regards global warming theory in general as a hoax. Last year, another doubter, Rep. Joe Barton, R-Texas, asked Mann to give the House Energy Committee his team's scientific data, plus details on their funding.

Barton's request sparked protests from committee member Rep. Henry Waxman, D-Los Angeles, and from scientific organizations who feared it was an attempt to intimidate researchers.

To cool tempers, Rep. Sherwood Boehlert, R-N.Y. and chair of the House Committee on Science, asked the academy to assess the Mann team's work.

The result is the 141-page report unveiled Thursday. It is available online at national-academies.org.

©2006 San Francisco Chronicle

###