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 Pre-industrial control experiment (picntrl). These datasets are available in netCDF and GRIB format. Initial conditions for this experiment were derived from a multi-step process. First, an atmosphere + land surface model was spun up with forcing agents at levels representative of the late 20th century. Second, the spun up atmosphere + land surface model was coupled to an ocean + sea ice model. Initial conditions for sea ice, ocean potential temperature and salinity were derived from late 20th century observations. Forcing agents consistent with year 1860 were applied to the coupled model which was integrated for a ~220 year adjustment period. Year 1 of the archived CM2.1U_Control-1860_D4 experiment data begins at the end of this adjustment period. CM2.1U_Control-1860_D4 forcing agents representative of conditions circa-1860 include the well-mixed greenhouse gases (CO2, CH4, N2O), tropospheric and stratospheric O3, tropospheric sulfates, black and organic carbon, dust, sea salt, 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 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/.