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 1%year CO2 increase experiment (to doubling). 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. In the CM2.1U-D4_1PctTo2X_I1 experiment atmospheric CO2 levels were prescribed to increase from their initial mixing ratio level of 286.05 ppmv at a compounded rate of +1 percent per year until year 70 (the point of doubling). CO2 levels were held constant at 572.11 ppmv from year 71 through the end of the 220 year long experiment. For the entire 220 year duration of the experiment, all non-CO2 forcing agents (CH4, N2O, halons, tropospheric and stratospheric O3, tropospheric sulfates, black and organic carbon, dust, sea salt, solar irradiance, and the distribution of land cover types) were held constant at values representative of year 1860. 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/.