A sea ice free Arctic: CMIP7 Assessment Fast Track abrupt-127k experimental protocol and motivation

Sime, L. C., Diamond, R., Stepanek, C., Brierley, C., Schroeder, D. ORCID: https://orcid.org/0000-0003-2351-4306, Kageyama, M., Pollock, M., Malmierca-Valle, I., Blockley, E., West, A., Feltham, D. ORCID: https://orcid.org/0000-0003-2289-014X, Ridley, J., Braconnot, P., Williams, C. J. R., Shi, X., Otto-Bliesner, B. L., Macarewich, S. I., Buarque, S. R., Zhang, Q., LeGrande, A., Zheng, W., Jiang, D., Morozova, P., Guo, C., Zhang, Z., Yeung, N., Menviel, L., Narayanasetti, S., Yoshimori, M., Reeves, O. and Zhao, A. (2026) A sea ice free Arctic: CMIP7 Assessment Fast Track abrupt-127k experimental protocol and motivation. Geoscientific Model Development, 19 (13). pp. 5881-5905. ISSN 1991-9603 doi: 10.5194/gmd-19-5881-2026

Abstract/Summary

Given that the Arctic could be ice-free in summer within the next ten to 20 years, accurately predicting lowice states is of crucial importance. Paleo-evidence shows that the strong orbitally-induced high latitude insolation anomaly at 127 000 years ago (127 ky), of around +70 Wm−2 in the Arctic during spring-summer, led to warm conditions and an Arctic that was occasionally or often ice-free during summer. Building on two Coupled Model Intercomparison Projects (CMIPs): the Sea-Ice Model Intercomparison Project and the Paleoclimate Modelling Intercomparison Project, we propose an Assessment Fast Track experiment, abrupt-127k, focusing on this seasonally ice-free, or near ice-free, Arctic at 127 ky. The abrupt-127k experiment is initialised from a piControl simulation and abruptly imposes observed values for the insolation distribution and greenhouse gas forcing at 127 ky. It provides a new opportunity to evaluate models used to compute climate projections, both against paleoevidence and each other, during a known low Arctic sea ice state. As CMIP models are not usually tuned to paleo observations, abrupt-127k represents a true “out-of-sample” test. The abrupt-127k experiment has four key scientific objectives, to: ascertain the simulated Arctic sea ice state, including the presence and characteristics of last-ice areas; evaluate the simulated climates using Arctic paleo-evidence; characterise the central Arctic surface energy budget; and analyse the ice budget including ice melt, growth, and transport. We show that a large Arctic ice response will manifest within the first 30 years of the simulation, thus a single 100-year long run is sufficient for these objectives. Modelling groups are requested to follow standard CMIP output protocol for analysis, including the use of standard “fixed-length” output. Given abrupt-127k is similar in setup to abrupt-2xCO2 and abrupt-4xCO2 CMIP7 experiments, combined analysis of these abrupt-experiments will facilitate understanding of the impacts of instantaneous radiative forcing in the Arctic.

Item Type	Article
URI	https://centaur.reading.ac.uk/id/eprint/131180
Identification Number/DOI	10.5194/gmd-19-5881-2026
Refereed	Yes
Divisions	Science > School of Mathematical, Physical and Computational Sciences > Department of Meteorology
Publisher	European Geosciences Union
Download/View statistics	View download statistics for this item