H11 meltwater and standard 127 ka Last Interglacial simulations suggest more modest peak temperatures for both Greenland and Antarctica: a multi-model study of water isotopes

The Last Interglacial (LIG) period, approximately 130 000 to 115 000 years ago, represents one of the warmest intervals of the past 800 000 years. Here, we simulate water isotopes in precipitation over Antarctica and the Arctic during the LIG, using three isotope-enabled atmosphere–ocean coupled climate models: HadCM3, MPI-ESM-wiso, and GISS-E2.1. These models were run following the Paleoclimate Modelling Intercomparison Project phase 4 (PMIP4) protocol for the LIG at 127 ka (kiloyears ago), supplemented by a 3000-year Heinrich Stadial 11 (H11) experiment using HadCM3. The long H11 simulation applies Northern Hemisphere meltwater to the North Atlantic, causing large-scale changes in ocean circulation – including cooling in the North Atlantic and Arctic and warming in the Southern Ocean and Global Ocean. While the standard 127 ka simulations do not capture the observed Antarctic warming and sea ice reduction in the Southern Ocean and Antarctic regions, they do capture around half of the warming in the Arctic. The H11 simulations align more closely with observations than the 127 ka simulations. H11 captures more than 80 % of the warming, sea ice loss, and δ¹⁸O changes for both Greenland and Antarctica. Decomposition of seasonal δ¹⁸O drivers highlights the dominant role of sea ice retreat and associated changes in precipitation seasonality in influencing isotopic values across all simulations, alongside a smaller common response to orbital forcing. We use the H11 and multi-model 127 ka simulations together to infer LIG surface air temperature (SAT) changes based on ice core measurements. The peak inferred LIG Greenland SAT increase is $+\mathrm{2.89}\pm \mathrm{1.32}$ K at the NEEM ice core site – less than half the previously inferred warming. Peak inferred LIG Antarctic SAT increases are $+\mathrm{4.39}\pm \mathrm{1.45}$ K at EDC, dropping to $+\mathrm{1.67}\pm \mathrm{3.67}$ K at TALDICE. These calculated warming values reflect climate effects alone and do not account for any ice-flow- or site-elevation-related impacts. Coastal sites in Greenland and Antarctica appear to have experienced less warming compared with higher central regions.