The National Academy of Sciences, reaching that conclusion in a broad review of scientific work requested by Congress, reported Thursday that the "recent warmth is unprecedented for at least the last 400 years and potentially the last several millennia."
A panel of top climate scientists told lawmakers that the Earth is heating up and that "human activities are responsible for much of the recent warming."
Their conclusion: The globe grew hotter by 1 full degree in the 20th century, reports CBS News correspondent Bill Whitaker.
That may not sound like much, Whitaker reports, but a second scientific report released today says that was enough to account for half the ocean warmth that fueled last year's deadly hurricanes, and that natural warming cycles have only a minor effect. That study is being published by the American Geophysical Union.
"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," said Gerald R. North, a professor of meteorology and oceanography at Texas A&M University, who headed the panel.
"The committee finds it plausible that the Northern Hemisphere was warmer during the last few decades of the 20th Century than during any comparable period over the preceding millennium," North said at a news conference.
The report was requested in November by the chairman of the House Science Committee, Rep. Sherwood Boehlert, R-N.Y., to address naysayers who question whether global warming is a major threat.
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.
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.
The Bush administration has maintained that the threat is not severe enough to warrant new pollution controls that the White House says would have cost 5 million Americans their jobs.
Climate scientists Michael Mann, Raymond Bradley and Malcolm Hughes had concluded the Northern Hemisphere was the warmest it has been in 2,000 years. Their research was known as the "hockey-stick" graphic because it compared the sharp curve of the hockey blade to the recent uptick in temperatures and the stick's long shaft to centuries of previous climate stability.