Looking for a way to get involved in the IPCC Fourth Assessment Report (AR4)? Here's an opportunity to do so. You can sign up now to analyze some aspect of the twentieth-century or future climate simulations being performed by about 14 modeling groups around the world. These analyses will begin when the model simulations currently running are completed and the model data is collected and archived in September of this year. This unique opportunity has arisen because climate modeling groups around the world have been charged with performing an unprecedented set of coordinated twentieth and twenty-first century climate change experiments, in addition to commitment experiments extending to the twenty-second and twenty-third centuries, for the IPCC AR4. This will require a considerable expenditure of human and computer resources to complete these experiments, and must be matched by an equivalent effort to analyze the output from these simulations for assessment in the IPCC process. This multi-model dataset will be a unique and valuable resource that will enable international scientists to gauge model performance, model sensitivity, and model response to a variety of forcings for twentieth-, twenty-first-, twenty-second-, and twenty-third-century climate variability and climate change.