The CMIP7-PMIP FastTrack abrupt-127k simulation : an opportunity to test Arctic sea ice representation in climate models

According to CMIP6 projections, all socio-economic scenarios show a rapidly warming climate with substantial polar amplification that will lead the Arctic becoming ice free during summer. This absence of summer sea ice acts as a positive feedback for Arctic warming and impacts regional to global scale (Bruhwiler et al., 2021).

The last interglacial, ~127,000 years ago, is a past warm climate state with significantly reduced prevalence of Arctic sea ice. This period provides a valuable out-of-sample test for climate models with which future projections are computed and may help us to better understand processes and climate patterns related to a blue Arctic.

The Last Interglacial (~127,000 years ago) is a period when orbital parameters caused much increased boreal high-latitude summer insolation forcing. The fourth phase of the Paleoclimate Modelling Intercomparison Project (PMIP4) identified in the lig127k simulation substantial model-spread of simulated minimum annual Arctic sea ice conditions (Kageyama et al., 2020; Sime et al., 2023). To enable a better understanding of the origin of model-model discord the paleoclimate science community has proposed simulation abrupt-127k (Sime et al., in prep.) as part of the FastTrack portfolio of the seventh interation of the Climate Modelling Intercomparison Project (CMIP7, Dunne et al., 2024). While simulation abrupt-127k inherits orbital and greenhouse gas parameters of PMIP4 simulation lig127k, its layout follows the approach of CMIP simulation abrupt-4xCO2, where the initial scientific focus is on a comparably short period (~100 model years) after model initialisation rather than on the quasi-equilibrated climate as in PMIP4 simulation lig127.

This presentation will outline the rationale and utility of CMIP7 FastTrack simulation abrupt-127k to a) increase the model ensemble from the classical PMIP to the wider CMIP framework; b) focus on processes and feedbacks that translate modified climate forcing into Arctic climate towards refining our understanding of the apparent model-model discord found in lig127k; c) enhance analysis of simulated sea ice conditions and dynamics based on the standardized protocol for sea-ice related climate model outputs by the Sea-Ice Model Intercomparison Project (SIMIP; Notz et al., 2016).