Quantifying Effects of Earth Orbital Parameters and Greenhouse Gases on Mid-Holocene Climate

Abstract. The mid-Holocene (MH) is the most recent typical climate period and a hot topic for global paleocultural research. Following the latest Paleoclimate Modelling Intercomparison Project (PMIP) protocol and using a fully coupled climate model, we simulated the climate difference between the MH and the pre-industrial (PI) periods, and quantified the effects of Earth orbital parameters (ORB) and greenhouse gases (GHG) on climate difference. More attention was paid to the simulated differences in the Atlantic meridional overturning circulation (AMOC) between these two periods. Compared to the PI conditions, the ORB effect in the MH simulation led to the seasonal enhancement of temperature, consistent with previous findings. For the MH simulation, the ORB effect led to a remarkably warmer climate in the mid-high latitudes and increased precipitation in the Northern Hemisphere, which were partially offset by the cooling effect of the lower GHG. The AMOC in the MH simulation was about 4 % stronger than that in the PI conditions. The ORB effect led to 6 % enhancement of the AMOC in the MH simulation, which was, however, partly neutralized by the GHG effect. The simulated stronger AMOC in the MH was mainly due to the thinner sea ice in the polar oceans caused by the ORB effect, which reduced the freshwater flux export to the subpolar Atlantic and resulted in a more saline North Atlantic. This study may help us quantitatively understand the role of different external forcing factors in the Earth’s climate evolution since the MH.