Climate has changed in the past. We have means of analyzing why, through various tools like ice core samples. This data shows that the climate is very sensitive to various forcings like solar activity, volcanic eruptions, and green house gases. Furthermore, the earth has positive feedback loops that kick in once a little bit (relative) of heat is added. From this data, we can model the effects of different forcings to determine that recent warming trends are not from natural means, but from increased human emissions. That is what the mountain of peer reviewed data from climate scientists shows.

That is all I will say about past warming/cooling for tonight, I don't have time to give the issue justice. I'll be sure to link you to a future post addressing this issue.