Rapid ice-age warming events amplified by strong vegetation-albedo feedback

Abstract The last glacial period was punctuated by >20 Dansgaard-Oeschger (D-O) cycles each manifested as a large, rapid (decadal-scale) warming in Greenland followed by slower cooling. D-O warming events have been identified as instances of a global climate-tipping point being crossed repeatedly. The mechanisms are not fully resolved. A role for the interaction of ice-sheet dynamics with the ocean circulation is widely accepted. Less attention has been paid to land-surface changes, although both decreasing snow and increasing tree cover in high latitudes could amplify warming. Here we use a global dataset of pollen records spanning multiple D-O cycles during Marine Isotope Stage 3 to reconstruct changes in land temperature, vegetation cover and vegetation height, and combine these with snow cover changes generated by the LOVECLIM climate model, using a 3D-variational data assimilation technique to estimate global patterns of change in land-surface albedo and temperature. We find a large (1.94 ± 0.65 W m–2 K–1) positive feedback from the combined effect of vegetation and snow, which is about 40 % larger than the pure snow-albedo feedback. This may be large enough to have destabilized the climate system, helping to explain the abruptness of D-O warming events.