Well, doesn't Earth go through warm and cold periods just as Mars does? In fact not so long ago we had a mini ice age, and that was not caused by human activities. We are destroying our environment, and I'm not climate change denier, but I do think this is little more complex issue than we're being told.
In fact the Little Ice Age was probably caused in part by human activity, or rather the lack of human activity due to reduced populations in Europe and especially in the Americas.
