Now, Greenpeace wants to know whether the petrochemical billionaire at the helm of Koch Industries has changed his mind, too.
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A letter, written by Greenpeace Executive Director Phillip Radford and shared with Whispers, will be sent to Charles Koch on Monday asking him to "tell us... do you now consider anthropogenic global warming to be on a 'solid, firm foundation' as Dr. Muller does?"
Richard A. Muller, the University of California-Berkeley physics professor who led the study, last week told told Whispers he believed the Charles Koch and his brother David were truly "interested in the science and the proof" of climate change, and do "want to get the science clarified."
Greenpeace isn't buying it.
The environmental group points out that the Koch brothers, who made their money from oil and gas, have backed numerous groups that produced studies denying the existence of climate change. By Greenpeace's estimates, the Koch brothers have given over $61 million to these groups.
For the recent Berkeley study, Charles Koch's foundation donated $150,000—a smaller sum in Koch terms, but the largest public donation the study received for its initial phase. The foundation has not said whether it will fund the study's second phase, which is due to begin soon and will look further into humankind's role in climate change.
Koch Foundation spokesperson Tonya Mullins told Whispers the research the foundation has supported—both then and now—is "nonpartisan" and "sound."
But Greenpeace feels that if the new Berkeley study is sound, Charles Koch should say whether his position on global warming has changed.
Charles Koch has long stayed mum on climate change, though David mostly denied its existence in a 2011 Think Progress interview, saying, "Climate does fluctuate. It goes from hot to cold. We have ice ages."
"Your own home state of Kansas is at the center of the summer's extreme drought," Greenpeace's Radford writes in the Monday letter. "Will groups that deny climate science continue to receive support from Koch Industries and its associated foundations?"
Neither Koch Industries nor Charles Koch's foundation responded immediately to requests for comment.