Most Climate Prediction models that watch the greenhouse effect on Earth are run on huge supercomputer in a research lab. However, most off the shelf PC and Mac are capable of running the same model prediction if given enough time. Your computer's CPU spends alot of time being idle. Why not use that idle time to do some real science. Climateprediction.net is a site that gives you a screensaver that works on climate change models. Check it out other useful at Massive Parallel Computng Projects or Distributed Computing.