MARTHA MACCALLUM: I mean the climate has changed over the course of the thousands and thousands of years that the Earth has been in existence, it has changed, you know, by several degrees up and down over the course of it. I just don't think that there is convincing evidence that the presence of man has altered that more dramatically than say the earth being covered with volcanoes emitting, you know, naturally noxious gases.

ALISYN CAMEROTA: So you're saying that it's cyclical? Brian wouldn't even go that far, by the way.

MACCALLUM: Yeah. I'm saying that it's nature.