Paleoclimate simulations and reconstructions. An intercomparison for the last millennium

A suite of paleoclimate simulations spanning the last millennium are analyzed with several high complexity general circulation models (AOGCMs), such as ECHO-G, HadCM3, CSM1.4, CCSM3, MPI-ESM, CNRM and IPSL and compared with available paleoclimatic reconstructions. The simulations are forced with different estimates of natural and anthropogenic external forcings, including solar and volcanic activity and atmospheric concentrations of greenhouse gases and anthropogenic aerosols. The various external forcing configurations applied in each model are compared, evidencing that the largest differences refer to the variability of solar forcing, the implementation of volcanic forcing, the exclusion of some anthropogenic forcing in some model runs and the fact that one of the models uses an interactive carbon cycle. . The analysis is mainly focused on the temperature response in the various model simulations, paying special attention to the internal variability and its sensitivity to the external forcing. The temporal evolution is studied at global and hemispherical scales, emphasizing relevant periods of the last millennium as the Medieval Climate Anomaly, the Little Ice Age, the industrial times or significant volcanic eruptions. For most of the simulations a linear relationship between the total equivalent external forcing applied and the temperature response is found which allows to define a paleo-transient climate sensitivity (PTCS) for the last millennium. The PTCS is compared in each model with the equilibrium climate sensitivity as well as with the transient climate response obtained for future climate change projections. In addition, the variations produced by important volcanic events is analysed in order to provide complementary estimations of climate sensibility focusing on higher frequencies.