Impacts of freshwater fluxes on ice shelves tipping points in UKESM

There is a strong concern about how fast and how much the global sea level will rise in the next few decades due to the current global warming. However, the projection range is large due to uncertainties about the future evolution of the Antarctic ice sheet, particularly the possibility of the ice shelves' collapse.  

With the base submerged in seawater, these ice shelves are strongly influenced by the surrounding oceanic conditions, which can be split into two regimes: cold or warm cavity. When ice shelves are exposed to warm water, basal melt increases sharply, leading to a loss of buttressing of the grounded ice upstream and potentially the collapse of the shelf. Results from TIPMIP (Tipping Points Modelling Intercomparison Project) idealised experiments carried out by the UK Earth System Model (UKESM) with an interactive Antarctic ice sheet component suggest that tipping points for several ice shelves will be reached in the future at relatively high global warming levels (GWLs). However, it is questionable whether the warming thresholds for tipping that we find are realistic due to the model biases and other uncertainties. 

This study focuses on exploring the uncertainty in the climate simulated by UKESM and assessing the consequences of ice shelf tipping on the wider Earth System. To do this, we induce the cavity regime shift at a lower GWL than the reported threshold by mimicking the key climate change forcing identified from the higher GWL experiments via an artificial freshwater around the Antarctic ice sheet margin. By doing so, we obtain a pair of low GWL experiments with and without ice shelf tipping, which allows us to isolate the impact of ice shelf tipping on the Earth System. In addition, the experiment setup also allows us to explore the consequences of different scenarios of various freshwater hosing values. The preliminary results indicate that the excessive freshwater induces an expansion of Southern Ocean sea ice, leading to a cooling trend in global mean temperature.