Project: IPCC Data Distribution Centre : Fourth Assessment Report data sets - The Intergovernmental Panel on Climate Change (IPCC) has been established by WMO and UNEP to assess scientific, technical and socio-economic information, relevant for the understanding of climate change, its potential impacts and option for adaption and migration. Projection of future trends for a number of key variables are provided through this section of the DDC (http://ipcc-data.org/sim/gcm_clim/SRES_AR4 ). The SRES scenarios have been constructed to explore future developments in the global enviromental with special reference to the production of greenhouse gases and aerosol precursor emission. A set of four scenario families (A1, A2, B1, B2) have been developed that each of this storylines describes one possible demographic, polito-economic, societal and technological future. Model experiments, also using different forcing scenarios, were calculated at other modeling centres. The data represent a subset of data sets from the IPCC Model Output Archive at PCMDI (https://pcmdi.llnl.gov/mips/cmip3/). _ These datasets are available in netCDF and GRIB format. The dataset names are composed of - centre/model acronym (e.g. MPEH5: Max-Planck-Institute/Echam5) - scenario acronym (e.g. SRB1: SRES B1) - run number (e.g. 1: run 1) - format identifier (e.g. N: netCDF, G: GRIB) - variable acronym [with level value] (e.g. hur850: relative humidity, 850 hPa) --> example: MPEH5_SRB1_1_G_hur850 Summary: 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. 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/.