Late Pliocene ice sheets as an analogue for future climate: a sensitivity study of the polar Southern Hemisphere

Abstract. The Earth's ice sheets, including the Antarctic Ice Sheet (AIS), are critical tipping points in the climate system. In recent years, the potential future collapse has garnered increased attention due to its cascading effects, which could significantly alter global climate patterns and cause large-scale, long-lasting, and potentially irreversible changes within human timescales. This study investigates the large-scale response of the polar Southern Hemisphere (pSH; comprising the Southern Ocean and Antarctica (60–90° S)) to the geometric reduction in ice sheets to a reconstructed Late Pliocene (LP) extent and imposing increased greenhouse gas (GHG) forcing in the Earth System. Using the PRISM4D reconstruction, where ice sheets such as the West Antarctic Ice Sheet (WAIS) were significantly diminished, we conducted multi-centennial simulations with the EC-Earth3 model at atmospheric CO2 concentrations of 280 and 400 ppmv. The simulation performed with LP ice sheet extent leads to a 9.5 °C rise in surface air temperature, approximately a 16 % reduction in sea ice concentration (SIC) over Antarctica and the Southern Ocean. These changes far exceed those driven by CO2 increase alone, which result in a 2.5 °C warming and a 9.3 % sea ice decline. Additionally, both experiments deduce there is a reversal in sea level pressure (SLP) polarity with respect to pre-industrial (PI) patterns. Higher-than-normal SLP is present over Antarctica, and lower-than-normal SLP is present in the mid-latitudes, indicative of a negative phase of the Southern Annular Mode (SAM). This is supported by a weakening of the westerly jet, which in turn contributes to the formation of a fresh cap in the upper ocean, induced by the imposed climatic impacts of our sensitivity experiments. This overall freshening of the upper ocean increases stratification in the water column and prevents deep convection in the Southern Ocean, thus leading to the formation of the Antarctic Bottom Water (AABW), which is paramount for the ventilation of the global ocean. Overall, our findings suggest that, by increasing the atmospheric concentration of CO2, the AABW is suppressed at a multi-centennial timescale; however, by reducing the ice sheet extent, compensatory mechanisms, involving an extensive salinisation of the ocean interior, trigger partial recovery of this water mass. This emphasises the non-linearity of the climate system, since consequences of reducing the ice sheets induce an amplified warming and freshening in the near-surface, whereas they induce opposing mechanisms in the deep ocean that significantly alter the dynamics of water masses that feed the AABW. By isolating the climatic response to ice sheet extent reduction, whilst holding other parameters fixed, this study offers critical insights into the mechanisms driving atmospheric and oceanic variability around Antarctica and their broader implications for global climate dynamics. Here we provide a unique, targeted approach, specifically focusing on the direct impact of ice sheet retreat on regional climate.