Simulation of the climatic effects of natural forcings during the pre-industrial era

The MPM-2, an Earth system model of intermediate complexity, is employed to study the climate system response to natural forcings during the pre-industrial era (1000-1800 AD), with a special focus on the surface air temperature (SAT) evolution. Solar radiation and volcanism are the primary natural forcings during this period. In the MPM-2, the solar radiation forcing determines the long-term trend of the climate system change, and the volcanic forcing intensifies (weakens) this trend. Ultimately, the combination of solar and volcanic forcings dominates the long-term changes of the climate system. These results aro in good agreement with other model data or temperature reconstructions. Natural forcings can well explain the Little Ice Age (LIA) and the Medieval Warm Period (MWP). At the large regional scale, the SAT response to natural forcings is almost coincident with that of the Northern Hemisphere. Based on MPM-2 model results, it is concluded that the global climate gradually became cold during the pre-industrial era. However, MPM-2 model results substantially correlate with reconstructed solar and volcanic forcings. Namely, to some great extent, these results strongly rely on the forcing series data we choose. Therefore, in order to accurately simulate the secular variation of the historical climate, it is very important to reconstruct well the solar radiation change and volcanic forcing data are well reconstructed for the past 10000 years, at least for the past 2000 years, in addition to the model improvements. The sensitivity study on the abrupt solar radiation change indicates that the increased solar radiation not only strengthens the nonlinear response of SAT, but intensifies the global hydrological cycle. At the same time, the biosphere is also affected obviously.