# Modeling a modern-like pCO 2 warm period (Marine Isotope Stage KM5c) with two versions of an Institut Pierre Simon Laplace atmosphere–ocean coupled general circulation model

The mid-Piacenzian warm period (3.264 to 3.025 Ma) is the most recent geological period with present-like atmospheric inline-formulapCO2 and is thus expected to have exhibited a warm climate similar to or warmer than the present day. On the basis of understanding that has been gathered on the climate variability of this interval, a specific interglacial (Marine Isotope Stage KM5c, MIS KM5c; 3.205 Ma) has been selected for the Pliocene Model Intercomparison Project phase 2 (PlioMIP 2). We carried out a series of experiments according to the design of PlioMIP2 with two versions of the Institut Pierre Simon Laplace (IPSL) atmosphere–ocean coupled general circulation model (AOGCM): IPSL-CM5A and IPSL-CM5A2. Compared to the PlioMIP 1 experiment, run with IPSL-CM5A, our results show that the simulated MIS KM5c climate presents enhanced warming in mid- to high latitudes, especially over oceanic regions. This warming can be largely attributed to the enhanced Atlantic Meridional Overturning Circulation caused by the high-latitude seaway changes. The sensitivity experiments, conducted with IPSL-CM5A2, show that besides the increased inline-formulapCO2, both modified orography and reduced ice sheets contribute substantially to mid- to high latitude warming in MIS KM5c. When considering the inline-formulapCO2 uncertainties (inline-formula $M5inlinescrollmathml+/-$ 25pt14ptsvg-formulamathimgaaf64dc90f991cd4fcb7f25788bfe4fa cp-16-1-2020-ie00001.svg25pt14ptcp-16-1-2020-ie00001.png 50 ppmv) during the Pliocene, the response of the modeled mean annual surface air temperature to changes to inline-formulapCO2 (inline-formula $M7inlinescrollmathml+/-normal 50$ 39pt14ptsvg-formulamathimg5afaeba59c8bc52b10020ca531f7e775 cp-16-1-2020-ie00002.svg39pt14ptcp-16-1-2020-ie00002.png  ppmv) is not symmetric, which is likely due to the nonlinear response of the cryosphere (snow cover and sea ice extent). By analyzing the Greenland Ice Sheet surface mass balance, we also demonstrate its vulnerability under both MIS KM5c and modern warm climate.

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Tan, Ning / Contoux, Camille / Ramstein, Gilles / et al: Modeling a modern-like pCO2 warm period (Marine Isotope Stage KM5c) with two versions of an Institut Pierre Simon Laplace atmosphere–ocean coupled general circulation model. 2020. Copernicus Publications.

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