GLOBAL WARMING CONFIRMED BY TOP U.S. STUDY

by J.R. Pegg

(ENS) WASHINGTON -- The Earth is hotter today than it has been in four centuries and likely warmer than it has been in the past 1,000 years, according to a review of surface temperature research released Thursday by the U.S. National Academies of Science.

The 155 page report provides additional evidence that "human activities are responsible for much of the warming," the authors said.

The study, written by a panel of 12 climate experts, assesses the state of scientific efforts to reconstruct surface temperature records for the Earth over approximately the last 2,000 years.

Widespread reliable instrument records of global temperatures are available only for the last 150 years, leaving scientists to estimate past climatic conditions by analyzing proxy evidence from sources such as tree rings, corals, ocean and lake sediments, cave deposits, ice cores, boreholes, and glaciers.

Committee chair Gerald North said the panel's review of instrument and proxy data affords "a high level of confidence that global mean surface temperature was higher during the last few decades of the 20th century than during any comparable period during the preceding four centuries."

"This statement is justified by the consistency of the evidence from a wide variety of geographically diverse proxies," said North, a geosciences professor at the University of Texas A&M.

The committee said average surface temperatures in the Northern Hemisphere rose about one degree Fahrenheit (0.6 degrees Celsius) during the 20th century.

The report was requested last year by House Science Committee Chairman Sherwood Boehlert, a New York Republican, in a bid to silence global warming skeptics critical of a surface temperature reconstruction widely known as the "hockey stick" study.

Published in 1998 by climate scientists Michael Mann, Raymond Bradley and Malcolm Hughes, the study suggested average surface temperatures in the Northern Hemisphere during the late 20th century were higher than at any time in the past 1,000 years.

The upward curve of a graph illustrating the research indicates that temperatures were relatively flat from A.D. 1000 to 1900 before sharply rising after 1900 -- the chart resembles a hockey stick.

The graph appeared in the 2001 report by the United Nations Intergovernmental Panel on Climate Change and emerged as oft-cited evidence that human greenhouse gas emissions are driving an increase in global temperatures.

Some scientists questioned the statistical methods used by the researchers and the findings of the study. Climate skeptics have latched onto this criticism and repeatedly attacked the study as flawed in a bid to cast doubt on humanity's contribution to climate change.

Last June, House Energy and Commerce Committee Chairman Joe Barton, a Texas Republican, attempted to launch a probe into the background of the authors -- a move that drew sharp criticism from Boehlert and several scientific organizations worldwide.

The controversy prompted Congressman Bohlert to ask the National Academies, an independent organization chartered by Congress to advise the government on scientific matters, to review the study as well as other research on surface temperatures.

North said the panel largely endorsed the findings of the hockey stick study.

The conclusions of the study have "subsequently been supported by an array of evidence," including large-scale surface temperature reconstructions and pronounced changes in proxy indicators, North said.

Some of these changes, such as melting on icecaps and the retreat of glaciers around the world, "appear to be unprecedented during at least the last 2,000 years," North told reporters at a briefing in Washington, DC.

The panel said it had less confidence that warming was unprecedented over the last thousand years because of a lack of precisely dated proxy evidence for temperatures prior to 1600, in particular in the Southern Hemisphere.

"The picture is much murkier before 1600 and considerably murkier when you go further back in time," said committee member Kurt Cuffey, a geography professor at the University of California, Berkeley. "We start losing sources of information ... and start relying more and more on data from fewer and fewer geographic locations."

The study by Mann and his colleagues also concluded that the 1990s were the warmest decade, and 1998 the warmest year in 1,000 years -- the panel said larger uncertainties in temperature reconstructions for decades and individual years precluded it from outright support of those conclusions.

Questions about the statistical methods used by the authors of the hockey stick study should not cast doubt on its primary conclusions, said committee member Peter Bloomfield, a statistics professor at North Carolina State University.

"All of the statistical choices made were reasonable at the time," Bloomfield said. "I saw nothing that spoke to me of any manipulation ... it was an honest attempt to construct a data analysis."

The panel said its review of other research gave it "very little confidence" in statements about average global surface temperatures prior to A.D. 900 because the proxy data for that time frame are sparse.

The panel stressed that global temperature records are only one piece of the global climate picture and not the primary evidence of climate change.

"Human activities are increasing the concentrations of certain greenhouse gases and these gases inevitably cause warming," Cuffey said. "That is not up for debate."

Boehlert praised the work of the committee and said it "shows the value of Congress handling scientific disputes by asking scientists to give us guidance."

Scientists need to do more work to develop a better sense of what global temperatures were prior to 1600, Boehlert said, and "Congress ought to let them go about that work without political interference."