The kind of weather headlines we've had this summer always prompt talk of global warming, from a severe drought across much of the nation to the unusual melting on the surface of Greenland's ice sheet.
As science journalist Michael Lemonick writes, there's always debate, even in the scientific community, about the extent to which those events are part of a natural cycle.
But almost all scientists believe the theory that global warming is real, and that it's caused by human beings and the greenhouse gases we generate from power plants, cars and other sources.
Until recently, Richard Muller, professor of physics at the University of California, Berkeley, was not in that camp.
He had long doubted the existence of man-made global warming, and he received funding from a foundation supported by David and Charles Koch, prominent backers of the Tea Party.
But after undertaking his own scientific study, he changed his mind.
"I now believe that there has been significant warming for the last 260 years," he told Here & Now's Robin Young. "The clear evidence... is that essentially all of that is caused by humans."
He wrote about the evolution in the New York Times:
Call me a converted skeptic. Three years ago I identified problems in previous climate studies that, in my mind, threw doubt on the very existence of global warming. 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.
- Richard Muller, professor of physics at the University of California, Berkeley
This segment aired on August 2, 2012.