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. Experiment was initiated from model year 85 (nominal year 1850) of the control simulation. The time varying forcing agents were varied based upon observations and reconstructions for the late 19th and 20th centuries. Varying forcing agents: CO2,CH4,N2O,CFC11(including other CFCs and HFCs),CFC12 and sulfate aerosols (Boucher data, only direct effect). Non-varying forcing agents: Background aerosols: black carbon,sea salt,desert dust, stratospheric and tropospheric ozone, solar irradiance (1368 W/m2) and the distribution of land cover types. No volcanic aerosols. No heat or fresh water adjustments. For this experiment there is only one run 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/.