East Antarctic Ice Sheet regime shifts during climate transitions

Reconstructions of the East Antarctic Ice Sheet based on geological records commonly assume that the relationship between a given proxy and changes in ice mass remains constant in time, and that this relationship is independent of climate state. This assumption, however, has yet to be comprehensively tested. To address this shortcoming, we use a coupled ice sheet--ice shelf model representing an East Antarctic-type ice sheet to determine how ice sheets respond to ocean--atmosphere states ranging from warm and wet with weak ocean forcing, to cold and arid with strong ocean forcing.
 
We find that where warm climates are accompanied by a weak sensitivity to ocean forcing, net ice volume oscillates in phase with oceanic and atmospheric forcing, whereas under cold climates with strong ocean forcing the behaviour is anti-phased. Transitions between these two regimes are characterised by ice volume fluctuations that resonate at half the frequency of the forcing. Calving, reflecting ice discharge, exhibits a highly complex relationship to imposed forcings, transitioning from smooth oscillations to abrupt pulses as the dominance of ocean forcing increases.

Focusing on the evolving balance between surface melt, basal melt, and calving, we are also able to demonstrate that the local Shannon entropy signature of our simulations maps out specific ice sheet regime types. Under both warm and cold extremes the ice sheet exists in a low entropy state of high predictability. Between these end-members, however, the ice sheet exhibits less predictable and more variable behaviour, characterised by overall higher entropy but also abrupt flickering between states. The transition from the cold to intermediate regime can occur under an atmospheric temperature change of as little as 0.5 - 1 K, whereas the transition to the warmest regime occurs over a 1 - 2 K range. 
  
Our findings are based on an ensemble of coupled ice sheet--ice shelf model simulations totalling 100 million model years, spanning climates from five degrees colder than present to fifteen degrees warmer than present. As such they provide a comprehensive framework for interpreting future East Antarctic Ice Sheet changes over multi-centennial to multi-millennial timescales. Most importantly, our results suggest that ice sheet reconstructions based on geological proxy records must take into account the background climate state and behavioural regime of the ice sheet in order to be most accurate.