State dependent ice-sheet resonance under Cenozoic and future climates

Inferences of ice-sheet change through geological time rely on environmental proxies, yet these inferences assume an unchanging ice-sheet response to climate. Here, using 500-kyr long ice-sheet simulations, we show that the directionality of ice sheet change depends on the background state of the climate. Under cold atmospheric conditions with high-amplitude glacial–interglacial changes in sub-shelf melt, ice sheets advance during cold phases and retreat as the climate warms. However, under warmer air temperatures with reduced glacial–interglacial ice-shelf melt variability, ice sheets advance during warm phases and retreat during colder periods. Forced with a linearly changing climate, the ice sheet switches from one mode to the other, and a resonant response arises at half the forcing frequency. These findings imply that climate–ice sheet phasing is not constant over time, and suggest that ice sheet behaviour under a future, warmer, climate may be substantially different from today. Gradual cooling or warming of the atmosphere, combined with strong ocean-driven basal melt, could have led to major changes in the periodicity, phasing, and asymmetry of past ice sheet growth and decay, according to an ensemble of 500-kyr long ice sheet simulations.