The fossil record drop off and the evidence of huge fires and sudden, massive climate cooling are others. (blogs.discovermagazine.com)
There were huge fires in the Arctic this year, including some fires reported burning in the Tundra, where there are not even any trees. (ecoshock.org)
There were also huge fires you could light, which not only looked fantastic in game (I remember being blown away by them the first time I saw them) but could also work as a distraction to sneak around your foes. (useapotion.com)