A prominent climate change skeptic’s about-face on the subject is causing a stir in the world of environmental science.

In a self-proclaimed “total turnaround,” Richard A. Muller, a physics professor at the University of California, Berkeley, now says human greenhouse gas emissions are almost entirely to blame for global warming.

“Call me a converted skeptic,” Muller wrote in a July 28 New York Times op-ed. Three years ago, he said, he doubted whether global warming even existed. “Last year, following an intensive research effort involving a dozen scientists, I concluded that global warming was real and that the prior estimates of the rate of warming were correct.

“I’m now going a step further: Humans are almost entirely the cause.”

While many scientific organizations reached that conclusion years ago, it’s one Muller wasn’t comfortable reaching until now, he told the Star.

“If that classifies me as a skeptic, I consider that proper skepticism; something that’s a duty for any scientist.”

Muller’s opinion is based on the Berkeley Earth Surface Temperature project, which he co-founded. Its results show the average temperature of the Earth’s land has risen by 1.5 C over the past 250 years.

The match between the temperature records and carbon dioxide records suggests human greenhouse gas emissions are the best explanation for the warming, the study says.

Muller says the findings are stronger than those from the latest UN Intergovernmental Panel on Climate Change report in 2007.

The project’s results are “elegantly simple,” Muller said. It rules out changes in solar activity in global warming and shows that volcanic eruptions have short-term, but not long-term, effects on global temperatures.

Some critics have dismissed Muller’s reversal as a publicity stunt. Others caution that the research hasn’t been peer-reviewed. Judith Curry, a climatologist at the Georgia Institute of Technology, called its analysis “way over simplistic and not at all convincing” on her blog.

But Muller told the Star he’s “delighted” at the amount of reaction the project has received and thinks it will remain of interest to people he calls “thoughtful skeptics.”

“Many of them have been attacked for being anti-science. I always thought that was unfair — I thought the thoughtful skeptics were raising valid points. We tried to address those points.”

Another twist to the news: one of the project’s sources of funding is the Charles G. Koch Charitable Foundation. Koch and his brother David are the billionaire owners of Koch Industries Inc., the conglomerate with annual revenues estimated at $100 billion, and are known for bankrolling conservative causes.

Penn State geoscientist Michael Mann, whose “hockey stick” graph showing a rise in global temperatures in the 20th century has faced criticism from global-warming skeptics, pointed this out in a Facebook post. “There is a certain ironic satisfaction in seeing a study funded by the Koch Brothers — the greatest funders of climate change denial and disinformation on the planet — demonstrate what scientists have known with some degree of confidence for nearly two decades,” he wrote.

In a statement, Charles Koch foundation spokeswoman Tonya Mullins said it has long supported “sound, non-partisan, scientific research.”

“Our grants are designed to promote independent research; as such, recipients hold full control over their findings. In this support, we strive to benefit society by promoting discovery and informing public policy,” she said.

Muller dismissed criticism of the Koch brothers, whom he said have always encouraged him to go wherever the science leads.

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“It was emphasized to me over and over again that the Koch foundation is funding us because they want the science worked out in a truly objective way, and they were worried that it had not happened yet, but they felt we would do it.”

The project received funding from “the entire political spectrum,” he added. “These people were not engaging in politics when they funded us. They were supporting science.”

In his op-ed, Muller adds that the new facts don’t prove causality, “but they raise the bar: to be considered seriously, an alternative explanation must match the data at least as well as carbon dioxide does.”

And Muller remains skeptical. He argues there’s no evidence that either the recent heat waves or Hurricane Katrina were related to climate change.

Richard Muller’s changing climate views

MIT Technology Review, October 2004, on the “hockey stick” model:

“If you are concerned about global warming (as I am) and think that human-created carbon dioxide may contribute (as I do), then you still should agree that we are much better off having broken the hockey stick. Misinformation can do real harm, because it distorts predictions.”

Associated Press, October 2011:

“The skeptics raised valid points and everybody should have been a skeptic two years ago. . . . Now we have confidence that the temperature rise that had previously been reported had been done without bias.”

New York Times op-ed, July 2012:

“I hope that the Berkeley Earth analysis will help settle the scientific debate regarding global warming and its human causes. Then comes the difficult part: agreeing across the political and diplomatic spectrum about what can and should be done.”

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