The CMIP7-PMIP FastTrack abrupt-127k simulation

A rapidly warming climate with substantial polar amplification will lead the Arctic becoming ice free during summer. An Arctic that „turns blue“, i.e. that changes from a current Arctic Ocean covered by high-albedo sea ice to a future low-albedo ice free water surface, may occur as early as the 2050s even under low emissions scenarios (Kim et al., 2023). Absence of summer sea ice will further exacerbate Arctic warming and will have ramnifications from regional to global scale (Bruhwiler et al., 2021).

The study of past warm climate states with significantly reduced prevalence of Arctic sea ice enables an integrated proxy-data and climate modelling approach. This provides a valuable out-of-sample test for climate models from which future projections are derived and may help us to better understand processes and climate patterns related to a blue Arctic.

Based on the Last Interglacial (~127,000 years ago), a time when orbital parameters caused much increased boreal high-latitude insolation forcing in particular from boreal spring to boreal autumn, the fourth iteration of the Paleoclimate Modelling Intercomparison Project (PMIP4) identified in their simulation lig127k 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). 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.

With this presentation we will outline 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).