A key problem in climate research is understanding how humans have and will influence Earth's climate system. Climate system models are the most comprehensive tools available to predict climate change. Prediction of climate change depends on our ability to define various factors that force the system (e.g. greenhouse gases, aerosols) and our ability to model feedback processes in the system (e.g. water vapor, clouds). Forcing factors of the climate system from 1870 to 2100 will be presented. In particular, the role of changes in sulfate aerosols and atmospheric ozone will be discussed. A brief evaluation of water vapor feedback will be provided as an example of how model feedback processes can be evaluated. Simulations of the 20th and 21st century climate will be presented. These simulations employ the NCAR Climate System Model, a coupled ocean-atmosphere-land-sea ice model. Finally, major uncertainties in our ability to predict the forcing of past and future climate will be summarized.