Simulation of the Last 21 000 Years Using Accelerated Transient Boundary Conditions*

Abstract

Abstract The earth system model of intermediate complexity ECBilt-CLIO has been used for transient simulations of the last deglaciation and the Holocene. The forcing effects of the ice sheets, greenhouse gas concentrations, and orbital configurations are prescribed as time-varying boundary conditions. In this study two key aspects of the transient simulations are investigated, which are of broader relevance for long-term transient paleoclimate modeling: the effect of using accelerated boundary conditions and of uncertainties in the initial state. Simulations with nonaccelerated boundary conditions and an acceleration factor 10 were integrated. These simulations show that the acceleration can have a significant impact on the local climate history. In the outcropping regions of the high southern latitudes and the convective regions in the North Atlantic, the acceleration leads to damped and delayed temperature response to the boundary conditions. Furthermore, uncertainties in the initial state can strongly bias the climate trajectories in these areas over 500–700 model years. The affected oceanic regions are connected to the large heat capacities of the interior ocean, which cause a strong delay in the response to the forcing. Despite the shown difficulties with the acceleration technique, the accelerated simulations still reproduce the large-scale trend pattern of air temperatures during the Holocene from previous simulations with different models. The accelerated transient model simulation is compared with existing proxy time series at specific sites. The simulation results are in good agreement with those paleoproxies. It is shown that the transient simulations provide valuable insight into whether seasonal or annual signals are recorded in paleoproxies.