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 http://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. Initial conditions for this experiment were taken from 1 January of year 1 of the 1860 control model experiment named CM2.1U_Control-1860_D4. Several forcing agents varied during the 140 year duration of the CM2.1U-D4_1860-2000-AllForc_H1 experiment in a manner based upon observations and reconstructions for the late 19th and 20th centuries. The time varying forcing agents were atmospheric CO2, CH4, N2O, halons, tropospheric and stratospheric O3, anthropogenic tropospheric sulfates, black and organic carbon, volcanic aerosols, solar irradiance, and the distribution of land cover types. The direct effect of tropospheric aerosols is calculated by the model, but not the indirect effects. For this experiment there are 3 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/.