When the Intergovernmental Panel on Climate Change (IPCC) publishes its Fourth Assessment Report of the Scientific Basis of Climate Change (AR4) in 2007, a significant portion of the report will analyze coupled general circulation model (GCM) simulations of the climate of the past century as well as scenarios of future climates under prescribed emission scenarios. Modeling groups worldwide have contributed to the report. Three U.S. contributors arec the Community Climate System Model (CCSM) project, the NASA Goddard Institute for Space Sciences, and the U.S. National Oceanic and Atmospheric Administration (NOAA) Geophysical Fluid Dynamics Laboratory (GFDL). This collection of model results is providing a wealth of new information that will be used to examine the state of climate science, the potential impacts from climate changes, and the policy consequences that they imply.