These 21 Last Interglacial (LIG) summer surface air temperature (SSAT) observations were compiled to assess LIG Arctic sea ice (Guarino et al 2020). Twenty of the observations were also previously used in the IPCC-AR5 report. Each observation is thought to be of summer LIG air temperature anomaly relative to present day and is located in the circum-Arctic region. All sites are from north of 51N. There are 7 terrestrial based temperature records; 8 lacustrine records; 2 marine pollen-based records; and 3 ice core records included in the original compilation. This compilation includes 1 additional ice core record. This work was funded by NERC standard research grant nos. NE/P013279/1 and NE/P009271/1.

The HadGEM3 (HadGEM3-GC3.1 or HadGEM3-GC3.1-N96ORCA1) PI simulation was initialized using the standard CMIP6 protocol using constant 1850 GHGs, ozone, solar, tropospheric aerosol, stratospheric volcanic aerosol and land-use forcing. The PI spin-up was 700 model-years, which allowed the land and oceanic masses to attain approximate steady state. The HadGEM3 LIG (Last Interglacial) simulation was initialized from the end of the spin-up phase of the equivalent pre-industrial (PI) simulation. After initialization, the LIG was run for 350 model-years. This 350 LIG spin-up permits the model to reach atmospheric equilibrium and to achieve an upper-ocean equilibrium. The model was then run for a further 200 model-years of LIG production run. This has been demonstrated to be an adequate run length to appropriately capture the model internal variability. This dataset contains outputs from the 200 years of production run of the period. The HadCM3 PI simulation was run for a period of over 600 years. The HadCM3 LIG simulation was initialized from the end of a previous CMIP5 LIG simulation, which was of length 400 years and initiated from the end of the corresponding PI, and run for further 250 years. The total spin-up phase for the HadCM3 LIG simulation used in this study was thus 600 model-years, and the length of the production (at atmospheric and upper-oceanic equilibrium) LIG HadCM3 simulation is 50 model-years. This work was funded by NERC standard research grant nos. NE/P013279/1 and NE/P009271/1.