Land albedo feedback may have shaped the ‘fast-slow’ pattern of Dansgaard-Oeschger cycles

A global set of pollen records extending through Dansgaard-Oeschger (D-O) cycles during Marine Isotope Stage 3 was combined with LOVECLIM model simulations to infer patterns of land temperature and albedo dynamics, using a 3D variational data assimilation technique. The calculated global land albedo feedback was unexpectedly large and positive, suggesting a potential role (alongside oceanic mechanisms) in the causation of these cycles. A simple zero-dimensional climate model, equipped with a capacitor standing for ocean heat storage, and an inductor standing for the non-linear feedback, illustrates a mechanism that can generate ‘fast-slow’ oscillations similar to those observed, when the equilibrium point lies in a certain range ­– while tending to converge to the equilibrium point when it lies outside that range. This critical range corresponds to a state of the Earth system where the gain of the non-linear feedback is ≥ 1. The equilibrium point is sensitive to orbital forcing, suggesting a possible mechanism by which the Earth system could enter and exit the oscillatory state.