On the reversibility of Arctic sea ice loss

It is now expected carbon emissions will follow an overshoot trajectory. A realistic emission-driven overshoot scenario esm-SSP534-over is available from the CMIP6 archive. We analysed the simulations to examine reversibility of the Arctic sea ice cover. Reversibility here means that at the end of the 21^st century the sea ice extent is the same as that at the earlier point in the century with the same atmospheric CO₂ concentration. Firstly, in an emission driven simulation the system behaves differently on the upward and downward branches of CO₂ concentration. We show it is better to use atmospheric CO2 concentration rather than Arctic surface air temperature, as the relation between the two is not linear. Total Arctic sea ice extent shows consistent behaviour in 3 out of 4 models (CNRM-ESM2, MIROC, UKESM1) with a CO₂ concentration threshold above which sea ice becomes irreversible. This can be explained by the continued ocean heat transport into the Arctic even though the Atlantic Meridional Overturing Circulation (AMOC) declines. The NorESM model has very different behaviour, sea ice extent is reversible and even overshoots beyond the present-day extent. We suggest this is caused by the known strong AMOC decline in this model. The analysis indicates Arctic air temperature is a result of the changes in sea ice extent rather than the driving factor, as is often assumed, both ultimately controlled by ocean heat transport. From the available simulations we conclude there is large uncertainty in the future Arctic climate state. This uncertainty extends to the future global air temperatures as different models show different inertia on CO2 concentrations, which only materialises in the downward emission branch. This affects many other climate variables with their own time lag. Climate inertia and time delays in the earth system should be investigated further to improve fidelity of future projection. This necessitates the use of emission-driven scenarios instead of concentration-driven ones which do not allow for the full inclusion of internal earth system feedbacks. 

We acknowledge funding from the projects COMFORT (grant agreement no. 820989) and OceanNETs (grant agreement no. 869357) under the European Union’s Horizon 2020 research and innovation programme, and from the EC Horizon Europe project OptimESM “Optimal High Resolution Earth System Models for Exploring Future Climate Changes”, grant 101081193 and UKRI grant 10039429, from the project EPOC, EU grant 101059547 and UKRI grant 10038003. For the EU projects the work reflects only the authors’ view; the European Commission and their executive agency are not responsible for any use that may be made of the information the work contains.