Coupled climate-carbon simulations of the Penultimate Deglaciation and Last Interglacial in the PLASIM-GENIE model

Theoretical understanding of paleoclimate change such as deglaciations comes primarily from time slice simulations in state-of-the-art atmosphere-ocean general circulation models, where multimillennial transient simulations would be too computationally expensive. Such steady state runs may be missing long-timescale processes involving ocean circulation or the carbon cycle, which could be captured by long transient simulations. The PLASIM-GENIE (Planet Simulator – Grid-Enabled Integrated Earth System) model is capable of running fast, multimillennial climate-carbon cycle simulations, comprising a fully 3D spectral atmosphere and frictional geostrophic ocean with marine and terrestrial carbon cycle modules. Here, we present comparisons between steady state and pseudo-transient experiments in PLASIM-GENIE, starting from the Penultimate Glacial Maximum (140,000 years before present) through the Last Interglacial, applying the PMIP4 Penultimate Deglaciation protocol. In pseudo-transient simulations, the model is stopped at every 500 years and restarted with updated prescribed ice sheets, orbital forcings, meltwater fluxes and relaxed CO₂ (with an active carbon cycle). These are compared to steady state time-slice simulations where the model is spun-up at each 500-year interval, to test for hysteresis in atmosphere, ocean and carbon cycle processes. Particular focus is on the timing of Atlantic Meridional Overturning Circulation weakening and recovery. We supplement these baseline simulations with a series of sensitivity experiments where individual forcings are varied.