There's quite a lot of evidence that the earth is going through natural temperature changes. It's also pretty definite that those changes may be exacerbated by global warming and pollution, but there's not really anything to say that this wouldn't be happening anyway.
The earth goes through climate cycles. It's natural, and it happened quite a bit before we were even here.
Anyone who wants more information, I'd suggest they read "A Short History of Nearly Everything" by Bill Bryson. Excellent read.