Richard A. Muller is a physicist who's been going around for years now saying that Global Warming is a hoax and that the teensy amount of climate change that may be occurring is not caused by human activity. He's the kind of guy global warming deniers cite when they want to say, "Look, look! We've got some scientists on our side!"
Or at least they did. Last year, he changed his mind and said Global Warming was real and now he's saying it is being caused by what we do, mainly in the area of carbon dioxide emissions. This is kinda like Eric Cantor suddenly deciding "Obamacare" makes good financial sense. So you might want to read this article that Muller just wrote entitled "The Conversion of a Climate-Change Skeptic."