"historical" is an experiment of the CMIP5 - Coupled Model Intercomparison Project Phase 5 (https://pcmdi.llnl.gov/mips/cmip5). CMIP5 is meant to provide a framework for coordinated climate change experiments for the next five years and thus includes simulations for assessment in the AR5 as well as others that extend beyond the AR5.

                3.2 historical (3.2 Historical): Simulation of recent past (1850 to 2005). Impose changing conditions (consistent with observations).

                Experiment design: https://pcmdi.llnl.gov/mips/cmip5/experiment_design.html
                List of output variables: https://pcmdi.llnl.gov/mips/cmip5/datadescription.html
                Output: time series per variable in model grid spatial resolution in netCDF format
                Earth System model and the simulation information: CIM repository

                Entry name/title of data are specified according to the Data Reference Syntax
                (https://pcmdi.llnl.gov/mips/cmip5/docs/cmip5_data_reference_syntax.pdf)
                as activity/product/institute/model/experiment/frequency/modeling realm/MIP table/ensemble
                member/version number/variable name/CMOR filename.nc.