WASHINGTON — Weighing in on the highest profile debate about global warming, the nation's premier science policy body on Thursday voiced a "high level of confidence" that Earth is the hottest it has been in at least 400 years, and possibly even the last 2,000 years.
A panel convened by the National Research Council reached that conclusion in a broad review of scientific studies, reporting that the evidence indicates “recent warmth is unprecedented for at least the last 400 years.”
The panel of top climate scientists told lawmakers that the Earth is running a fever and that “human activities are responsible for much of the recent warming.” Their 155-page report said average global surface temperatures in the Northern Hemisphere rose about 1 degree F during the 20th century.
The report was requested last November by the chairman of the House Science Committee, Rep. Sherwood Boehlert, R-N.Y., to survey what science says about climate change over the last 2,000 years.
Last year, when the House Energy and Commerce Committee chairman, Rep. Joe Barton, R-Texas, launched an investigation of three climate scientists, Boehlert said Barton should try to learn from scientists, not intimidate them.
The Bush administration also has maintained that not enough is known about the threat to warrant new emission controls that the White House says would have cost 5 million Americans their jobs.
Boehlert said Thursday the report shows the value of having scientists advise Congress. “There is nothing in this report that should raise any doubts about the broad scientific consensus on global climate change,” he said. Interactive: The greenhouse effect
Many scientists tie warming temperatures to rising emissions of certain gases like carbon dioxide. While essential to survival, carbon dioxide has seen a spike as fossil fuels are burned by cars and factories, leading to concerns that it and other gases are exacerbating the greenhouse effect that keeps Earth inhabitable.
“The numerous indications that recent warmth is unprecedented for at least the last 400 years and potentially the last several millennia, in combination with estimates of external climate forcing variations over the same period, supports the conclusion that human activities are responsible for much of the recent warming,” the panel wrote.
'Hockey stick' debate
The report focused on research data criticized by warming skeptics and which was published in 2001 by the Intergovernmental Panel on Climate Change, a United Nations body that has led international warming research.
Climate scientists Michael Mann, Raymond Bradley and Malcolm Hughes had prepared the research, concluding that the Northern Hemisphere was the warmest it has been in 1,000 years. Their research included a graphic dubbed the “hockey stick” because of the way the temperature data looked when presented over time — stability followed by a sharp curve upwards over the last 140 years.
The National Research Council panel concluded that the Mann-Bradley-Hughes research was “likely” to be true, said John Wallace, an atmospheric sciences professor at the University of Washington and a panel member. The conclusions from the research “are very close to being right” and are supported by even more recent data, Wallace said.
The panel looked at how other scientists reconstructed the Earth’s temperatures going back hundreds of years, before there was data from modern scientific instruments.
For all but the most recent 150 years, the academy scientists relied on “proxy” evidence from tree rings, corals, glaciers and ice cores, cave deposits, ocean and lake sediments, boreholes and other sources. They also examined indirect records such as paintings of glaciers in the Alps.
Combining that information gave the panel “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,” the panel said in its report.
'Plausible' scenario beyond 400 years
“We do roughly agree with the substance of their findings,” panel chairman Gerald North, a geosciences professor at Texas A&M University, said of the "hockey stick" researchers.
Speaking at a news conference Thursday, North did say there was “some disagreement” among panel members on the "hockey stick" data before 1600.
In their report, the panel said it had less confidence in the evidence of temperatures before 1600, but still found it “plausible” that temperatures during the last few decades of the 20th century were warmer “than during any comparable period over the preceding millennium.”
They also considered the data 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 were the main causes of changes in greenhouse gas levels. But those temperature changes “were much less pronounced than the warming due to greenhouse gas” levels since the mid-19th century, the report said.
The National Research Council is part of the National Academies, a private organization chartered by Congress to advise the government on scientific matters.
The full report is online at www.nas.edu/morenews/20060622.html.
MSNBC.com's Miguel Llanos and The Associated Press contributed to this report.