This is an example of a positive feedback.Another thing that happened during past climate changes was that when the warming occurred, more ice melted, as I talked about in the last topic.And of course, when the ice melts, more dark surfaces are exposed, leading to more warming.This is yet another example of a positive feedback.The really important thing to remember about these past climate change is that they occurred very, very slowly.The Earth warmed or cooled just a few degrees over thousands or perhaps millions of years.And that's very different to what's been going on recently.One of the previous warmest period on Earth was called the mediaeval warm period, because it occurred between A.D. 800 and A.D. 1300.But it didn't occur everywhere, it occurred in just some parts of the northern hemisphere.But during that time the Vikings took advantage of the fact that Greenland was warm enough to farm, and moved there.