Implications of Reduced Antarctic Ice Sheets for the Global Climate System

Abstract

As anthropogenic CO2 emissions warm the global climate and cause retreat of polar ice sheets, there is an urgent need to understand the consequences of ice sheet mass loss. The paleoclimate record is a valuable resource for illuminating how the Earth system may have behaved under warmer conditions of the past. However, proxy records are spatially and temporally constrained, and crucially, the drivers of past warming periods - changes in insolation due to orbital forcing - fundamentally differ from the present-day greenhouse gas emission driven warming. As such, Earth system modelling can be a useful tool to evaluate the impact of different interacting forcings on Earth system elements and feedbacks. Despite evidence that Antarctic ice sheets were reduced in extent during the last interglacial period (LIG; c. 130 - 115 ka), many Earth system model simulations employ present day Antarctic ice sheet configurations in model simulations of the LIG (Barnett et al. 2023; Dutton et al. 2015; Golledge et al. 2021; Otto-Bliesner et al. 2021). This neglects a key feature of the LIG and may contribute to poor model skill in representing the LIG Southern Ocean, constraining our ability to reliably project future change. Furthermore, contemporary mass loss from the Antarctic ice sheets is accelerating under global warming due to ever-rising atmospheric CO2 emissions. Therefore, it is increasingly urgent to understand the climate response to ice sheet retreat under relevant past interglacial conditions, and across a range of possible future warming pathways. This work focuses on the impact of reduced Antarctic ice sheet extents, and their interaction with orbital forcing and atmospheric CO2 concentrations. Simulations are conducted using UVic ESCM, an Earth system model of intermediate complexity, with three different Antarctic ice sheet configurations in order to explore the sensitivity of the Earth system to reduced ice sheet extent. The ice sheet configurations include modifications to the land/ocean mask which reflect plausible past interglacial ice sheet extents. A suite of steady-state experiments are presented in three research chapters. First, the effect of the ice sheet configuration alone is explored, to establish a baseline for the response of key Earth system elements to changes in ice sheet extent under preindustrial orbital conditions and atmospheric CO2 concentrations. This allows for the identification of key processes affected by the ice sheet extent, revealing that Southern Ocean heat content and deep water formation is sensitive to the extent and location of ice sheet retreat. The changes in the Southern Ocean also have wide-reaching consequences, such as modification of the Atlantic Meridional Overturning Circulation (AMOC), highlighting the significance of polar processes on the wider Earth system. In the second research chapter, the modified ice sheets are applied in a paleoclimate context. Experiments with orbital and CO2 forcing representative of the LIG “peak warmth" are employed and evaluated against proxy data from this period as well as the Paleoclimate Model Intercomparison Project (PMIP4) lig127k ensemble of experiments. While the modified ice sheet configurations result in only small improvements to UVic ESCM's ability to reproduce features of the LIG reconstructed from proxy data, these experiments reveal the interacting effects of orbital forcing and ice sheet configuration. The third research chapter explores the sensitivity of the modelled climate and ocean to both Antarctic ice sheet extent and a range of higher-than-preindustrial atmospheric CO2 concentrations. This reveals a non-linear relationship of the Earth system to CO2, as the ocean behaves as a bistable system that undergoes abrupt state transitions in response to CO2 forcing. Below and above a CO2 window the ocean is in a qualitatively different state. At intermediate CO2 concentrations the ocean oscillates between two quasi-stable states reminiscent of the millennial-scale oscillations observed in the paleoclimate record of past glacial periods (Menviel et al., 2019; Cheng et al., 2009}). The exact response of the system depends on Antarctic ice sheet extent. These experiments demonstrate that the modelled ocean can exhibit self-sustaining oscillations driven by deep convection in the Southern Ocean, without changes in the boundary conditions.