Richard Muller, Former Climate Change Skeptic, Warns Congress That Global Warming Is Real (VIDEO)

WATCH: Koch-Funded Climate Skeptic Warns Congress That Global Warming Is Real

WASHINGTON -- Richard Muller, a prominent scientist long skeptical of the science behind climate change, declared at a congressional briefing on Monday that the world is getting warmer and he has the evidence to prove it.

The well-known physicist from the University of California, Berkeley, recently spearheaded the Berkeley Earth Surface Temperature study, a two-year study underwritten in part by the Charles G. Koch Foundation, whose results confirmed the scientific underpinnings of global warming.

At a briefing organized by ranking member of the House Energy and Commerce Committee Ed Markey (D-Mass.), Muller explained how his team reached the conclusion that in the last half-century the earth's temperature has risen roughly 1 degree Celsius, a number that exceeds the conservative 0.64 degree estimate put forth by the United Nations' Intergovernmental Panel on Climate Change.

Analyzing a tremendous amount of data from temperature monitoring stations around the world, the BEST study examined four criticisms -- in Muller's mind, legitimate ones -- often made of existing climate change research: that so-called "urban heat islands," which retain inordinate amounts of heat, have led to exaggerated greenhouse warming estimates; that researchers have cherry-picked station data; that certain stations offer unreliable data that has been unduly influenced by the proximity of other heat sources; and that researchers have engaged in data adjustment bias, inappropriately altering data when a change in measuring instruments or other local variables were in play.

Muller's team found none of these concerns to be significant. "This means that the list of potential biases had not unduly influenced the results that had been published by prior groups," Muller said in a statement on Monday.

The "human component is somewhat uncertain," Muller cautioned of climate change science, and it is an area "worth some additional scientific addressing."

He encouraged climate skeptics to evaluate his research critically and acknowledge the earth is getting warmer, adding, "I think we need to listen to the skeptics and see when they are raising good issues."

Rep. Henry Waxman (D-Calif.), ranking member of the House Natural Resources Committee, suggested he had his doubts about how many climate deniers would cross the aisle on the politically polarized debate, even in the face of the overwhelming scientific evidence. "To be a denier is not a scientific position," Waxman said.

Those who would cast doubt on climate science can be divided into two groups, Muller replied: those who are legitimately skeptical of the science and seek more evidence, and those who start with the conclusion that climate change is a hoax and seek out any data that seems to justify the notion. Muller is one of the former.

"It is ironic if some people treat me as a traitor, since I was never a skeptic -- only a scientific skeptic," he told The Huffington Post's Tom Zeller Jr. in a recent email exchange. "Some people called me a skeptic because in my best-seller 'Physics for Future Presidents' I had drawn attention to the numerous scientific errors in the movie 'An Inconvenient Truth.' But I never felt that pointing out mistakes qualified me to be called a climate skeptic."

But the fact that a foundation created by the conservative billionaire Koch brothers, which famously underwrites climate deniers, contributed $150,000 to help fund his study would seem to suggest otherwise. The foundation has since distanced itself from the findings, saying they're still in need of peer review.

Muller was joined on Monday by Ben Santer, a research scientist with the Lawrence Livermore National Laboratory, and William Chameides, dean of Duke University's Nicholas School of the Environment and vice-chair of the National Academies' Committee on America's Climate Choices.

Santer argued that scientists frequently examine alternative hypotheses to try to reconcile them with observable data, but climate change, he says, is the only explanation that makes any sense. "I don't think Climategate casts any doubt on climate science," he added, referring to the 2009 controversy in which emails between researchers at the Climatic Research Unit associated with the University of East Anglia appeared to suggest scientists had inappropriately altered data to exaggerate the effects of climate change in at least three sets of oft-cited data. It was later shown there was no evidence of deliberate scientific malpractice.

"It's not rocket science," said Chameides, who discussed findings from the National Academies' America's Climate Choices report that he helped to author. "It's looking at the possible explanations and using rational explanations to eliminate them."

WATCH highlights from the briefing below:

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