(AP) - The Earth is running a slight fever from greenhouse gases, after enjoying relatively stable temperatures for 2,000 years. The National Academy of Sciences, after reconstructing global average surface temperatures for the past two millennia, said Thursday the data are "additional supporting evidence ... that human activities are responsible for much of the recent warming."
Other new research showed that global warming produced about half of the extra hurricane-fueled warmth in the North Atlantic in 2005, and natural cycles were a minor factor, according to Kevin Trenberth and Dennis Shea of the National Center for Atmospheric Research, a research lab sponsored by the National Science Foundation and universities, reports Trend.
The academy had been asked to report to Congress on how researchers drew conclusions about the Earth's climate going back thousands of years, before data was available from modern scientific instruments. The academy convened a panel of 12 climate experts, chaired by Gerald North, a geosciences professor at Texas A&M University, to look at the "proxy" evidence before then, such as tree rings, corals, marine and lake sediments, ice cores, boreholes and glaciers.
Combining that information gave the panel "a high level of confidence that the last few decades of the 20th century were warmer than any comparable period in the last 400 years," the panel wrote. It said the "recent warmth is unprecedented for at least the last 400 years and potentially the last several millennia," though it was relatively warm around the year 1000 followed by a "Little Ice Age" from about 1500 to 1850.
Their conclusions were meant to address, and they lent credibility to, a well-known graphic among climate researchers a "hockey-stick" chart that climate scientists Michael Mann, Raymond Bradley and Malcolm Hughes created in the late 1990s to show the Northern Hemisphere was the warmest it has been in 2,000 years.
It had compared the sharp curve of the hockey blade to the recent uptick in temperatures a 1 degree rise in global average surface temperatures in the Northern Hemisphere during the 20th century and the stick's long shaft to centuries of previous climate stability.
That research is "likely" true and is supported by more recent data, said John "Mike" Wallace, an atmospheric sciences professor at the University of Washington and a panel member.
Rep. Sherwood Boehlert (news, bio, voting record), R-N.Y., chairman of the House Science Committee, had asked the academy for the report last year after the House Energy and Commerce Committee chairman, Rep. Joe Barton (news, bio, voting record), R-Texas, launched an investigation of the three climate scientists.
The Bush administration has maintained that the threat from global warming is not severe enough to warrant new pollution controls that the White House says would have cost 5 million Americans their jobs.
"This report shows the value of Congress handling scientific disputes by asking scientists to give us guidance," Boehlert said Thursday. "There is nothing in this report that should raise any doubts about the broad scientific consensus on global climate change."
The academy panel said it had less confidence in the evidence of temperatures before 1600.
But it considered the evidence reliable enough to conclude there were sharp spikes in carbon dioxide and methane, the two major "greenhouse" gases blamed for trapping heat in the atmosphere, beginning in the 20th century, after remaining fairly level for 12,000 years.
Between 1 A.D. and 1850, volcanic eruptions and solar fluctuations had the biggest effects on climate. But those temperature changes "were much less pronounced than the warming due to greenhouse gas" levels by pollution since the mid-19th century, the panel said.
The National Academy of Sciences is a private organization chartered by Congress to advise the government of scientific matters.