That is actually not known. The historical temperature record only goes back a couple of hundred odd years, and is constanly being revised and argued over, and generally shown to be fairly inaccurate. Any further back than that and we are talking proxies - and that is such a wild can of worms that its not even worth arguing about. So we don't really know a) what the current rate of temperature change is, and b) if it has in the past been as high or not.

The whole concept of 'tipping points' (no return) is a an unproven theory that does not have a lot of supporters even in the AGW proponent camp. So let's not have all this 'we are all doomed' talk just yet, eh?