| Summary: | Prior to conducting simulations, climate models are often "spun-up" through an indefinitely long simulation using a constant atmospheric composition taken to characterise preindustrial conditions, until rates of change of key variables are determined to be below some threshold defined for equilibrium.This procedure provides a baseline climate for the model and a range of samples of internal variability that are considered equally valid for use as initial conditions for Historical simulations. A number of Earth System models that contributed to CMIP6 can include interactive ice sheet components, and several more are developing this capability on the timeline of the next phase of CMIP. However, we have limited knowledge of the states of the Greenland and Antarctic ice sheets that could be used to create or validate ice sheet states required for a Preindustrial or Historical simulation in the CMIP paradigm. Ice sheets also cannot be assumed to maintain a constant, realistic state if coupled inside an indefinite preindustrial climate simulation, yet the adjustment timescales of ice dynamics means that accurate century-scale projections are significantly affected by the realism of the initial condition of the ice sheet model. We describe our approach to initialising interactive ice sheets for Preindustrial and Historical climate simulations with the UK Earth System Model. Methods of "spinning down" a ensemble of preindustrial ice sheets starting from modern states are described, as well as criteria for judging the suitability of the resulting preindustrial states and an experimental protocol to account for ice sheet drift in simulations.
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