Transient climate simulations of the Holocene (version 1) – experimental design and boundary conditions

Abstract

Abstract. The Holocene, started approximately 11.5 thousand years before present (ka), is the latest interglacial period with several rapid climate changes from decades to centuries time scales superimposed on the millennium scale mean climate trend. Climate models provide useful tools to investigate the underlying dynamic mechanisms for the climate change during this well-studied time period. Thanks to the improvements of the climate model and computational power, transient simulation of the Holocene offers an opportunity to investigate the climate evolution in response to time-varying external forcings and feedbacks. Here, we present the design of a new set of transient experiments for the whole Holocene from 11.5 ka to the preindustrial period (1850 CE) (HT-11.5ka) to investigate both combined and separated effects of the main external forcing of orbital insolation, atmospheric greenhouse gas (GHG) concentrations, and ice sheets on the climate evolution over the Holocene. The HT-11.5ka simulations are performed with a relatively high-resolution version of the comprehensive Earth system model CESM1.2.1 without acceleration, both fully- and singly-forced by time-varying boundary conditions of orbital configurations, atmospheric GHGs, and ice sheets. Preliminary simulation results show a slight decrease of the global annual mean surface air temperature from 11.5 ka to 7.5 ka due to both decreases in orbital insolation and GHG concentrations, with an abrupt cooling at approximate 7.5 ka, which is followed by a continuous warming until the preindustrial period mainly due to increased GHG concentrations. The simulated cooling magnitude at 6 ka lies within the range of the 14 PMIP4 model results and is close to their median result for the mid-Holocene simulations. Further analyses on the HT-11.5ka transient simulation results will be covered by follow-up studies.