Abstract/Summary

The Hadley Centre Global Environment Model version 3 (HadGEM3) is the first coupled climate model to simulate an ice-free Arctic during the Last Interglacial (LIG), 127 000 years ago. This simulation appears to yield accu- rate Arctic surface temperatures during the summer season. Here, we investigate the causes and impacts of this extreme simulated ice loss. We find that the summer ice melt was predominantly driven by thermodynamic processes: atmo- spheric and ocean circulation changes did not significantly contribute to the ice loss. We demonstrate these thermody- namic processes were significantly impacted by melt ponds, which formed on average 8 d earlier during the LIG than dur- ing the pre-industrial control (PI) simulation. This relatively small difference significantly changed the LIG surface en- ergy balance and impacted the albedo feedback. Compared to the PI simulation: in mid-June, of the absorbed flux at the surface over ice-covered cells (sea-ice concentration > 0.15), ponds accounted for 45 %–50 %, open water 35 %–45 %, and bare ice and snow 5 %–10 %. We show that the simulated ice loss led to large Arctic sea surface salinity and temperature changes. The sea surface temperature and salinity signals we identify here provide a means to verify, in marine observa- tions, if and when an ice-free Arctic occurred during the LIG. Strong LIG correlations between spring melt pond and sum- mer ice area indicate that, as Arctic ice continues to thin in future, the spring melt pond area will likely become an in- creasingly reliable predictor of the September sea-ice area. Finally, we note that models with explicitly modelled melt ponds seem to simulate particularly low LIG sea-ice area. These results show that models with explicit (as opposed to parameterised) melt ponds can simulate very different sea- ice behaviour under forcings other than the present day. This is of concern for future projections of sea-ice loss.