Who is James Hansen?

I was watching Jeffrey Sach's give a lecture on global poverty, and I was amused when he noted the foremost scientist on global warming is...James Hansen [note: not my martial arts instructor Jim Hanson, who is 6 foot 3, weighs 250, can kill you with his left pinky, and also generally carries a knife and has a concealed carry permit--talk about a belt and suspenders approach]. This guy rocketed to fame because he alleged the Bush administration censored him, a strange claim from someone on 60 Minutes (could you get a bigger platform?). But anyway, he's a trained as an astrophysicist. He's clearly smart, and understands the second law of thermodynamics, but I don't think this makes him the foremost expert on earth's climate. I would think oceanographers, or people more directly studying these things, would be. I guess its no different than the fact that Krugman and Stiglitz are experts at very stylized models of information asymmetries, and all the sudden their views on immigration and bank regulation are considered expert.