The Climateprediction.com program is a state-of-the-art simulation of the climate system which runs on one's PC. By downloading the program one will be contributing to the study of climate change and, in particular, helping to resolve some of the uncertainties in the science of climate change. The program uses the idle time on one's PC. Climateprediction.com is the biggest ever study of climate change and builds upon hundreds of millions of dollars of international research effort over the past 20-30 years.