An ice free Arctic during the Last Interglacial: CMIP6-PMIP4 progress on Arctic sea ice

The Arctic was around 4-5C warmer in summer during peak Last Interglacial (LIG), compared to the preindustrial. However this summer warming was not accurately captured by CMIP models until 2020. Before then the lack of LIG Arctic warmth in CMIP models was most commonly postulated to be due to a lack of dynamic vegetation feedbacks. However in 2020 a UK CMIP6 model accurately captured the summer warming (Guarino et al., 2020). The warming in this model is due to a complete summertime loss of Arctic sea ice, rather than dynamic vegetation feedbacks. Whilst marine data, until 2023,  were not adequate for assessing the accuracy of this modelled LIG Arctic sea ice loss (Kagayama et al., 2021), this has now been rectified by valuable new marine core evidence from the Arctic (Vermassen et al., 2023). Here, we show firstly why we are confident that melt pond physics (albedo feedbacks) are sufficient to melt LIG sea ice, raise the Arctic temperature, and also why they are important for the accurate projection of Arctic sea ice loss during warm climate – including the future (Diamond et al., 2021; 2024). Secondly, we quantify the Arctic warmth, and discuss the nature of polar amplification in CMIP models, during the LIG (Sime et al., 2023). We find an Arctic-wide warming of 3.7±1.5 K at the LIG, alongside a climatological minimum sea ice area of 1.3 to 1.5 million km2, i.e that the peak LIG Arctic likely experienced a mixture of ice-free and near-ice-free summers.