It's nature's cycle. You need to read a very good, and well argued book: Cool It: The Skeptical Environmentalist's Guide to Global Warming [ame="http://www.amazon.com/Cool-Skeptical-Environmentalists-Global-Warming/dp/0307266923"]Amazon.com: Cool It: The Skeptical Environmentalist's Guide to Global Warming (9780307266927): Bjørn Lomborg: Books[/ame]
There have been many cooling and warming trends in our recorded history, as well as those that can be discerned from looking at geological layers and trees.
In fact, more people die from cold deaths each year than they do from heat deaths. So, it really wouldn't be that bad to raise the average temperature of the Earth a couple degrees (even if we were the cause of Global Warming). It's save more lives.
Anyway, it's an awesome book and points out a lot of flaws in many arguments.