Insolation-driven 100,000-year glacial cycles and hysteresis of ice-sheet volume

Comprehensive climate and ice-sheet models show that insolation and internal feedbacks between the climate, the ice sheets and the lithosphere–asthenosphere system explain the 100,000-year period on which the Northern Hemisphere ice sheets grow and shrink. Driving force behind the 100,000-year glaciation cycle For the past million years Earth's climate has been dominated by a 100,000-year cycle of glaciation involving the advance and retreat of the Northern Hemisphere ice sheets. Variations in insolation — the solar radiation reaching Earth's surface — caused by constant changes in Earth's orbit and orientation to the Sun are closely associated with glaciations, but the physical mechanisms driving the cycle have remained unclear. Now Ayako Abe-Ouchi and colleagues demonstrate that insolation variations interact with the size of the ice sheets and the solid Earth to control the 100,000-year cycles. Their model shows that internal feedbacks between insolation, climate and the elevation of the land beneath the ice sheet allows ice sheets to grow gradually until a maximum size is reached. At that point the lowered ice sheet elevation caused by the delayed rebound of the solid Earth means that a small increase in insolation can destroy the ice sheet in just a few thousand years. The growth and reduction of Northern Hemisphere ice sheets over the past million years is dominated by an approximately 100,000-year periodicity and a sawtooth pattern 1 , 2 (gradual growth and fast termination). Milankovitch theory proposes that summer insolation at high northern latitudes drives the glacial cycles 3 , and statistical tests have demonstrated that the glacial cycles are indeed linked to eccentricity, obliquity and precession cycles 4 , 5 . Yet insolation alone cannot explain the strong 100,000-year cycle, suggesting that internal climatic feedbacks may also be at work 4 , 5 , 6 , 7 . Earlier conceptual models, for example, showed that glacial terminations are associated with the build-up of Northern Hemisphere ‘excess ice’ 5 , 8 , 9 , 10 , but the physical mechanisms underpinning the 100,000-year cycle remain unclear. Here we show, using comprehensive climate and ice-sheet models, that insolation and internal feedbacks between the climate, the ice sheets and the lithosphere–asthenosphere system explain the 100,000-year periodicity. The responses of equilibrium states of ice sheets to summer insolation show hysteresis 11 , 12 , 13 , with the shape and position of the hys