Summer surface air temperature proxies point to near sea-ice-free conditions in the Arctic at 127 ka

Abstract. The Last Interglacial (LIG) period, which had higher summer solar insolation than today, has been suggested as the last time that Arctic summers were ice-free. However, the latest suite of Coupled Modelling Intercomparison Project 6 Paleoclimate (CMIP6-PMIP4) simulations of the LIG produce a wide range of Arctic summer minimum sea ice area (SIA) results, ranging from a 30 % to 96 % reduction from the pre-industrial (PI). Sea ice proxies are also currently neither abundant nor consistent enough to determine the most realistic state. Here we estimate LIG minimum SIA indirectly through the use of 21 proxy records for LIG Summer Surface Air Temperature (SSAT) and 11 CMIP6-PMIP4 models for the LIG. We use two approaches. First, we use two tests to determine how skilful models are at simulating observed proxies for ΔSSAT (where Δ refers to LIG-PI). This identifies a positive correlation between model skill and the magnitude of ΔSIA: the most reliable models simulate a larger sea ice reduction. Averaging the most skilful two models yields an average SIA of 1.3 mill. km² for the LIG. This equates to a 4.5 mill. km², or a 79 %, SIA reduction from the PI to the LIG. Second, across the 11 models, the averaged ΔSSAT at the 21 proxy locations is inversely correlated with ΔSIA (r = -0.86). In other words, the models show that a larger Arctic warming is associated with a greater sea ice reduction. Using the proxy record-averaged ΔSSAT of 4.5 ± 1.7 K and the relationship between ΔSSAT and ΔSIA, suggests an estimated ΔSIA of 4.4 mill. km² or 77 % less than the PI. The mean proxy-location ΔSSAT is well-correlated with the Arctic-wide ΔSSAT north of 60° N (r=0.97) and this relationship is used to show that the mean proxy record ΔSSAT is equivalent to an Arctic-wide warming of 3.7±0.1 K at the LIG compared to the PI. Applying this Arctic-wide ΔSSAT and its modelled relationship to ΔSIA, results in a similar estimate of LIG sea ice reduction of 4.5 mill. km². The LIG climatological minimum SIA of 1.3 mill. km² is close to the definition of a summer ice-free Arctic, which is a maximum sea ice extent less than 1 mill. km². The results of this study thus suggest that the Arctic likely experienced a mixture of ice-free and near ice-free summers during the LIG.