He finally came around to what other climate scientists have been spouting for years. Richard A. Muller, a physics professor at the University of California-Berkeley, announced over the weekend that his much-publicized investigation into climate data has found that humans' production of carbon dioxide is causing the world to slowly warm up. And this process could speed up dramatically in the coming years.

Muller's conclusions attract special attention because of his vocal self-styling as a converted climate change skeptic. Muller criticized global warming studies for sloppy and self-serving data selection and a lack of transparency that obscured errors; he then lambasted fellow scientists for circling the wagons and calling any climate change deniers wrong. Muller says he's still upset that the American Physical Society declared the evidence for warming "incontrovertible" a few years ago in an official statement.

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"We don't do things in science that are incontrovertible," Muller said in an interview with Yahoo News.

Muller took matters into his own hands and embarked on his own investigation into the data with his daughter Elizabeth and a team of scientists two years ago. His Berkeley Earth Surface Temperature project attracted funding from the Charles Koch Charitable Foundation, the nonprofit outfit of a wealthy businessman who denies that global warming is happening. Three years later, Muller ended up surprising himself when his research confirmed everything those same studies that drew his skepticism concluded, and then some. Muller says his study's results are more reliable than many previous ones because he intentionally avoided the data pitfalls he objected to, such as only using a portion of the global temperatures available. (He expounds on his methods here.)

Muller's study has not yet been published in a peer-reviewed scientific journal, but he says he plans to do so at some point. One climate scientist, Benjamin D. Santer, told the Los Angeles Times he thinks posting the study online and not in a journal is in "the spirit of publicity, not the spirit of science" and may do more to hurt the global warming cause than help it. But Muller wants to get feedback on his methods and to share his results with everyone, avoiding what he sees as a secrecy and lack of transparency that surrounded earlier climate change studies.

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Though Muller is now entirely convinced that the Earth is warming due to man-made causes, he still expresses disdain for people who try to raise passions around the issue by pointing to local weather events, such as the drought scorching up America's Midwest right now, as proof of the phenomenon. (He attributes the drought to La Niña, a temporary cooling of the ocean.) The effects of global warming on local weather patterns are unknown, and even as two-thirds of the world has heated up, another one-third has shown a gradual cooling over the past 250 years, he says. The overall effect is a troubling global warming, but Muller has no patience for simplifications that stray from the truth.

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"I'm personally very worried," he says of global warming. Muller says that so far the warming has been "tiny," but that everything points to the process speeding up. "I personally suspect that it will be bad."

Muller is now wading into another controversy, by endorsing the process of natural gas extraction called fracking for developing countries, which tend to rely more on coal. Coal production creates more carbon dioxide, but fracking has also drawn its share of environmentalist critics.

"I believe the only kind of action that is sustainable is that which is profitable, and fortunately we can do that," he says. "We can become much more energy efficient."