Optimizing sea ice parameters mitigates the underestimation of Arctic marine access in CMIP6 climate models

Abstract Maritime activity in the Arctic is increasing, and climate change projections suggest that commercial vessels may soon have seasonal access to high-latitude waters. Accurate assessments of Arctic marine access are essential, yet the ability of climate models to reproduce historical navigability remains underassessed. Here, we show that the multi-model mean of the Coupled Model Intercomparison Project Phase 6 (CMIP6) underestimates overall Arctic navigability, despite capturing historical changes in summer navigable areas. Since the early 21st century, the multi-model mean underestimates the length of the shipping season in many regions of the Arctic Ocean, following a dipole-like pattern for moderately ice-strengthened vessels, with underestimates in the East Siberian and Laptev Seas, and overestimates in the Kara and Barents Seas. Moreover, inaccurate ice thickness simulations hinder the identification of trans-Arctic routes through the Northwest Passage. Improved simulation of ice concentration has a greater impact on assessing the length of the shipping season for ordinary ships, while improvements in ice thickness substantially improve the assessments for ice-strengthened ships. Better simulation of ice thickness is critical for accurately identifying trans-Arctic routes for both types of vessels.