Joking about death is part of human nature and doesn't mean that people don't care! Generally the jokes are made because there is nothing that you can do to stop it. Lets face it, we are all going to die at some point and we cannot stop that.
I agree that wen there is something that can be done about it then we should. In the intance of Global Warming we can all do our bit to try to help and I'd like to think we would all do that (well....apart from those that hope it will shut down the NAD and give us areal winter!!!!).
However, there is not a lot I or the average person in the street can do about the possibility of an Asteroid hitting us, hence the jokes. On a lighter note, just before it hits it would make great viewing!!!!