Temperatures have been warmer over the past 25 years than any time in the past four centuries - and possibly the past 1,000 years, a prestigious national advisory panel reported yesterday.
In a study conducted to help settle a continuing political debate, the National Research Council reviewed the latest scientific reports on global warming, heard testimony from climate experts and concluded that it has been "warmer during the last few decades of the 20th century than during any comparable period over the preceding millennium."
The council released a 155-page report to Congress that validated the major findings of studies used to create one of the most controversial images in climate science: the "hockey stick" chart that illustrates the spike in global temperatures since 1900.
Temperature estimates, covering a 1,000-year pattern of climate change, were based on studies of tree rings, sea coral, ice cores, glaciers and other evidence. They were first published in the journal Nature in 1998 and in Geophysical Research Letters in 1999.
Although they initially attracted only modest press attention, they became lightning rods for debate when the Intergovernmental Panel on Climate Change included them in its 2001 report.
The group urged global policymakers to reduce greenhouse gas emissions, including carbon dioxide from cars and power plants. It also created the hockey stick chart, showing a sharp spike in temperatures beginning about 1900 that looks like a hockey stick lying on the ground with the blade pointing up at the end of the 20th century.
"That clearly attracted attention," said Malcolm Hughes, a tree ring expert at the University of Arizona and a co-author of the studies.
The original study's authors said they felt vindicated by the NRC findings. Michael E. Mann, who is now at Pennsylvania State University and was lead author of both reports, said skeptics were likely to "cherry-pick" the contents of yesterday's report to try to fuel the debate over whether the climate is changing.
"Hopefully, the scientific community can start to move forward with the science again," Mann said in an e-mail.
Critics had argued that the study's authors used flawed statistical methods and that evidence from tree rings, sediment cores and other sources was too scant to draw reliable conclusions about ancient temperatures.
"When you're doing a reconstruction, it's easy to underestimate the uncertainties," said Ross McKitrick, a researcher at the University of Guelph in Ontario, Canada and a critic of the initial studies.
The warming debate prompted U.S. Rep. Sherwood Boehlert, a New York Republican who is chairman of the House Science Committee, to request the report issued yesterday. The NRC is a branch of the independent National Academy of Sciences.
Panel members said yesterday that the evidence of a long-term warming pattern becomes weaker if you try to go back more than 400 years.
"Before then, it gets murky," said Kurt M. Cuffey, a geography professor at the University of California at Berkeley and a member of the panel appointed by the National Academy of Sciences.
They said that it's harder to determine climate conditions before 1600 because evidence used, such as tree rings and sediment cores, are harder to find and harder to decipher.
"The question becomes, are the trees responding to temperature changes the same way?" said Douglas Nychka, a senior scientist at the National Center for Atmospheric Research who served on the panel.
The study authors had estimated that the 1990s were the warmest decade and that 1998 was the warmest year in the past 1,000 years.
But the panel said ancient records for any given year or decade are insufficient to identify a single year or decade as being the hottest in the past 1,000 years. The members also said that the original findings have been substantiated by more recent studies, that the authors used sound scientific principles and that temperature patterns are just one of several signals showing how human activities are warming the planet.
"There were things they could have done different, could have done better. But after all this was the first paper," said Gerald R. North, a professor of meteorology and oceanography at Texas A&M; and the panel's chairman.