Modelling Miocene climate with the IPSL earth system model. Uncertainties and challenges.

Modelling the different climate states of the long Miocene period is challenging, as it requires a precise knowledge of the different parameters that serve as boundary conditions for the earth system models (ESM), specifically paleogeography, ice sheet extent and atmospheric CO2 concentration. Through years, pCO2 reconstructions have been refined and icesheet volumes estimated, allowing modellers to test scenarios consistent with data. Still, several paleogeography reconstructions have been published for the Miocene in the last ten years, sometimes varying strongly in terms of paleoelevation and extent of mountain ranges, seaway depth, and global paleobathymetry. Another source of uncertainty is the ability of ESMs tuned to the present climate to reproduce correctly the climate forced with the peculiar boundary conditions of the deep past. Here we present numerical simulations carried out with the Institut Pierre Simon Laplace ESM (IPSL-CM5A2) forced by different Miocene boundary conditions (paleogeography and CO2) representative of the Miocene Climatic Optimum, the Miocene Climate Transition and the Late Miocene. We further carry out high-resolution atmospheric and vegetation modelling with two generations of atmospheric models (LMDZ5 and LMDZ6, respectively for CMIP5 and CMIP6) and provide a quantitative model comparison with a paleovegetation database. Whereas the large trends of climate variation through Miocene are captured, our results raise the question of the value of present-day model-observation agreement for studying past periods and the limits of quantitative data-model comparison for deep-time periods.