Has this happened before?

One morning last week, I was listening to the radio while carpooling into work in San Francisco. The morning folks were speaking with callers about global warming, and I have to say that the message really seems to be getting out there. One of the callers, though, raised the point that the Earth has warmed before in the past because of greenhouse gases, and in fact, has been much warmer. Surprisingly, a friend mentioned the same thing to me later that day. I think that people are interested in this point because (1) it means that sometimes global warming is natural, and (2) it seems that the Earth can survive it. Absolutely! The Earth has been extremely warm in the past, and greenhouse gases have definitely contributed. And Life has survived! Life survives because some living organisms, given enough time, can evolve and adapt. But not all do, and many went extinct during times of warming. Understanding which organisms can adapt and survive is a major challenge for scientists today.

Learning about warming in the past is important because it helps us to understand some of the consequences of climate change, but it is of only limited relevance for several reasons. First, the level of warming that we expect in this century will be extremely fast compared to the past. It is questionable whether organisms will be able to adapt successfully. Second, human civilization was not present in those past episodes. How do we begin to understand what the consequences will be for us? Life will certainly carry on in some form or another. Will it be a world in which we wish to live?