Abrupt climate changes during the last ice age: a study of millennial-scale variability in climate simulations

Simulating the climate is more important than it has ever been if we want to foresee the disruptions of global climate change. Climate simulations are, however, limited by our lack of understanding of the tipping points of the climate systems. Such tipping points have been crossed the last deglaciation, between 19 and 11.7 thousand years before present, whose chain of events recalls the demise of the modern ice sheets. Deglacial abrupt climate changes are linked to complete reorganisations of Atlantic Meridional Overturning Circulation (AMOC) by the effect of the freshwater released by the melting ice sheets. The AMOC can shift between several modes, but the precise drivers and consequences of these shifts are still to be established.

To investigate this problem, I produced a new set of simulations from a general circulation model displaying millennial-scale variability that resembles the abrupt climate change sequence of the last glacial period and the last deglaciation. Under the right balance of magnitude and location of the freshwater forcing field derived from the ice sheet melting history, the North Atlantic climate experiences a pseudo-oscillating behaviour with a periodicity of around 1500 years and Greenland temperature changes of about 10°C. I proposed a mechanism to explain the millennial-scale variability in these simulations based on a slow global salt oscillator and abrupt North Atlantic convection component coupled by modifications in the AMOC regime. This mechanism is key to understanding what combination of boundary conditions and forcings can lead to millennial-scale variability in glacial simulations, and this theory was applied to new last glacial maximum simulations forced with deglacial meltwater history using two different ice sheet reconstructions.

The choice of the ice sheet reconstruction can modify the processes at stake in North Atlantic abrupt climate shifts. In particular, this mechanism relies on an abrupt recovery of deep water formation in the Nordic Sea that is obtained only when Eurasian ice sheet katabatic winds are strong enough. The uncertainties around the ice sheet reconstructions and their associated meltwater discharge are still preventing the exact reproduction of the chain of events of the last deglaciation, but this study provides a new way to understand the window of parameters where millennial-scale variability in a more systematic way to trigger them in climate simulations.