Abstract
Abstract. The Last Interglacial period (LIG) is a period with increased summer insolation at high northern latitudes, which results in strong changes in the terrestrial and marine cryosphere. Understanding the mechanisms for this response via climate modelling and comparing the models' representation of climate reconstructions is one of the objectives set up by the Paleoclimate Modelling Intercomparison Project for its contribution to the sixth phase of the Coupled Model Intercomparison Project. Here we analyse the results from 16 climate models in terms of Arctic sea ice. The multi-model mean reduction in minimum sea ice area from the pre industrial period (PI) to the LIG reaches 50 % (multi-model mean LIG area is 3.20×106 km2, compared to 6.46×106 km2 for the PI). On the other hand, there is little change for the maximum sea ice area (which is 15–16×106 km2 for both the PI and the LIG. To evaluate the model results we synthesise LIG sea ice data from marine cores collected in the Arctic Ocean, Nordic Seas and northern North Atlantic. The reconstructions for the northern North Atlantic show year-round ice-free conditions, and most models yield results in agreement with these reconstructions. Model–data disagreement appear for the sites in the Nordic Seas close to Greenland and at the edge of the Arctic Ocean. The northernmost site with good chronology, for which a sea ice concentration larger than 75 % is reconstructed even in summer, discriminates those models which simulate too little sea ice. However, the remaining models appear to simulate too much sea ice over the two sites south of the northernmost one, for which the reconstructed sea ice cover is seasonal. Hence models either underestimate or overestimate sea ice cover for the LIG, and their bias does not appear to be related to their bias for the pre-industrial period. Drivers for the inter-model differences are different phasing of the up and down short-wave anomalies over the Arctic Ocean, which are associated with differences in model albedo; possible cloud property differences, in terms of optical depth; and LIG ocean circulation changes which occur for some, but not all, LIG simulations. Finally, we note that inter-comparisons between the LIG simulations and simulations for future climate with moderate (1 % yr−1) CO2 increase show a relationship between LIG sea ice and sea ice simulated under CO2 increase around the years of doubling CO2. The LIG may therefore yield insight into likely 21st century Arctic sea ice changes using these LIG simulations.
Highlights
The Last Interglacial period (LIG) was the last time global temperature was substantially higher than the pre-industrial period (PI) at high northern latitudes
Models used during the previous Coupled Model Intercomparison Project 5 (CMIP5) disagree on the magnitude of Arctic sea ice retreat during the LIG: the diversity of sea ice behaviour across models was linked to the spread in simulated surface temperatures and in the magnitude of the polar amplification (Otto-Bliesner et al, 2013; Lunt et al, 2013; IPCC, 2013)
To help understand the role of Arctic sea ice in these changes, we present a new synthesis of LIG sea ice information using marine core data collected in the Arctic Ocean, Nordic Seas and northern North Atlantic and compare this to Paleoclimate Model Intercomparison Project 4 (PMIP4)-LIG simulations
Summary
The Last Interglacial period (LIG) was the last time global temperature was substantially higher than the pre-industrial period (PI) at high northern latitudes. It is important in helping us understand warm-climate sea ice and climate dynamics (Otto-Bliesner et al, 2013, 2017; Capron et al, 2017; Fischer et al, 2018). It was difficult to compare some of the LIG simulations because they were not all run using identical protocol
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