Ocean temperature map

A panel of scientists says evidence of global warming is clear. They say the last few decades have been warmer than any comparable period in the past several hundred years. Scientists believe the warming trend is also to blame for the harsh hurricane season in the North Atlantic last year.

Experts convened by the U.S. National Academy of Sciences say the 20th century advance of global warming is shown by consistent evidence from a wide variety of geographically diverse sources.

Their new report comes at the request of U.S. lawmakers seeking to answer critics of earlier studies that said the warming of the past few decades was unprecedented in the last one-thousand years.

The earlier studies were based on indirect natural climate evidence going back a millennium. Such indirect evidence included tree rings and deep boreholes in ice. Researchers have collected such data in recent years to reconstruct temperatures that existed before scientific instruments began giving readings about 150 years ago.

The National Academy of Sciences' panel concludes the studies' findings are plausible.

Gerald North

"We do roughly agree with the substance of their findings," said Gerald North.

Gerald North of Texas A&M University chaired the panel.

But he qualified the group's support for the studies, indicating that they are limited in their ability to gauge temperatures during the first several hundred years of the millennium accurately. He cautions about the reliability of natural data older than 400 years because it is scarce.

"There is a disagreement, a small disagreement, over exactly how sure we are, particularly about the earlier 600 years or so in their curve," he said.

The panel also takes exception to the studies' assertions that the 1990s were likely the warmest decade in a millennium and 1998 the warmest year. It says climate reconstructions for individual years or decades are more uncertain than for longer periods because not all indirect natural evidence records temperatures on such short timescales.

But panel member Kurt Cuffey of the University of California at Berkeley says the part of the studies addressing the last 400 years is valid.

"As far as the implications for policy go, this information is certainly consistent with the prevailing view that climate is warming due to human influences and will continue to do so in the future," said Kurt Cuffey.

Another study from the National Center for Atmospheric Research in Colorado finds that global warming accounted for half of the extra ocean warmth that caused the severe 2005 hurricane season in the tropical waters of the North Atlantic ocean.

The study contradicts recent claims that only natural cycles are responsible for the increase in Atlantic hurricane activity since 1995. It supports the premise that hurricane seasons will become more active as global temperatures rise.

Last year produced a record 28 tropical storms and hurricanes in the Atlantic. Four reached top strength, including Katrina, which devastated New Orleans and much of the U.S. Gulf of Mexico coast.