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, with a 30g% to 96g% reduction from the pre-industrial (PI) period. 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 reconstructed ISSAT from proxy records (where I refers to LIG-PI). This identifies a positive correlation between model skill and the magnitude of ISIA: The most reliable models simulate a larger sea ice reduction. Averaging the two most skilful models yields an average SIA of 1.3×106gkm2 for the LIG. This equates to a 4.5×106gkm2 or 79g% SIA reduction from the PI to the LIG. Second, across the 11 models, the averaged ISSAT at the 21 proxy locations and the pan-Arctic average ISSAT are inversely correlated with ISIA (rCombining double low line-0.86 and-0.79, respectively). In other words, the models show that a larger Arctic warming is associated with a greater sea ice reduction. Using the proxy-record-Averaged ISSAT of 4.5±1.7gK and the relationship between ISSAT and ISIA suggests an estimated sea ice reduction of 4.2±1.4×106gkm2 or about 74g% less sea ice than the PI period. The mean proxy-location ISSAT is well correlated with the Arctic-wide ISSAT north of 60ggN (rCombining double low line0.97), and this relationship is used to show that the mean proxy record ISSAT is equivalent to an Arctic-wide warming of 3.7±1.5gK at the LIG compared to the PI period. Applying this Arctic-wide ISSAT and its modelled relationship to ISIA, results in a similar estimate of LIG sea ice reduction of 4.1±1.2×106gkm2. These LIG climatological minimum SIA of 1.3 to 1.5×106gkm2 are close to the definition of a summer ice-free Arctic, which is a maximum sea ice extent of less than 1×106gkm2. 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.