How reversible are carbonate chemistry changes triggered by future Arctic sea ice loss?

The ongoing rapid decline in Arctic sea ice is considered as a tipping element of our climate system. It is exposing a warmer and more acidified ocean directly to the atmosphere, permitting greater light penetration and enhanced exchange of heat, momentum, and gases across the air-sea interface. Earth system models project that these thermal and biogeochemical changes will dramatically perturb Arctic Ocean carbonate chemistry. As one of the consequences, the projections indicate that the seasonal maximum in surface ocean pCO₂ generally shifts from winter to summer during this century. Yet, it is unknown whether such biogeochemical changes in the Arctic would be reversible, if we managed to reduce atmospheric carbon dioxide concentrations. Here we analyse the reversibility of Arctic biogeochemistry changes using idealised 1pctCO2-cdr simulations from six earth system models. These model experiments simulate a 140-year period of 1% annual atmospheric CO₂ increase (rampup to 4x preindustrial levels), followed by a 140-year period of 1% annual CO₂ decrease (rampdown). Our results indicate that the present day pCO₂ cycle is largely recovered when atmospheric CO₂ returns to preindustrial levels. However, most models exhibit substantial hysteresis, particularly during summer, where surface ocean pCO₂ remains more elevated during the rampdown phase relative to the rampup phase (difference in Arctic average up to 60 𝜇atm pCO₂ for the same atmospheric CO₂ levels). Despite model differences, their projections consistently show pronounced regional variability in the pCO₂ hysteresis, with high hysteresis occurring for example in the Nordic Seas and the Barents Sea. Our results indicate that the pCO₂ hysteresis is particularly sensitive to sea surface temperature and net primary productivity, both of which show regionally varying hysteresis as well. These findings underscore the complex impacts of Arctic sea ice loss on biogeochemical cycles, emphasising the importance of accounting for hysteresis in CO₂ overshoot scenarios and climate mitigation strategies.