These data represent monthly averaged values of selected variables for the Data Distribution Centre (DDC) of the Intergovernmental Panel on Climate Change (IPCC). (see also https://www.ipcc-data.org/) The model output prepared for IPCC Fourth Assessment climate of the 20th Century experiment (20C3M). These datasets are available in netCDF and GRIB format. The initial condition is taken from the state of the pre-industrial control (PIcntrl) run at 1 years. The forcing of the experiment includes greenhouse gases (CO2, CH4, N2O and CFCs), sulfate aerosol direct effects, volcanoes and solar forcing. For this experiment there are 5 ensemble runs available. Model output data for CMIP3 in higher temporal resolutions and further variables are available in ESGF at https://esgf-node.llnl.gov/search/cmip3/.