The non-linear response of the AABW to the removal of the West Antarctic Ice Sheet and implications to future climate

The West Antarctic Ice Sheet (WAIS) is considered one of the tipping points of the Earth System. Its retreat due to climate change progressively results in sea level rise, affecting large numbers of the world's population. We aim to understand the potential consequences of a future WAIS collapse by implementing a mid-Pliocene Warm Period (MP) Antarctic Ice Sheet configuration, based on reconstructions, where the WAIS is severely reduced. We perform simulations with the EC-EARTH3.3 model at low resolution spanning 1400 years under 280, 400, and 560 ppmv of CO2 and derive the mean state of the last 200 years of simulation and the variability of climate patterns for the entire runtime. With the near removal of the WAIS, we find a non-linear response in Antarctic Bottom Water (AABW) formation to increasing CO2 levels. The AABW formation is highly sensitive to increased stratification, which results from sea surface temperature increase driven by current climate change. With the presence of a modern WAIS, Antarctic surface air temperature and Southern Ocean sea surface temperature are positively correlated to atmospheric CO2, and we see a strengthening of the positive phase of the Southern Annular Mode (SAM). This affects mid-latitude westerlies and reinforces the negative feedback between surface warming and AABW formation. However, with the near removal of WAIS, we observe a dampening in the otherwise doubling of atmospheric warming observed with increasing CO2 and the same pattern occurs for the SAM. This results in a non-linear behaviour of AABW formation, where the AABW is suppressed up to 4 Sv during a longer period compared to the control experiments (modern WAIS), followed by a recovery to pre-industrial strength levels that is not sustained under 560 ppmv. This response also induces a further weakening of the Atlantic Meridional Overturning Circulation (AMOC) and a reduced reach of the AABW transport into the Atlantic and Pacific Oceans, with potential cascading effects on the global climate. Our longer simulations reveal that the AABW formation thresholds are highly dependent on atmospheric CO2concentrations and the freshwater input into the surrounding basins of the Antarctic region. These results suggest that WAIS retreat already deeply impacts societal development, but a collapse would induce a new climate regime that needs further investigation to allow for climate adaptation.