Making sense of millennial-scale climate change

Recent results, and especially those presented at the AGU Chapman Conference on Mechanisms of Millennial-Scale Global Climate Change, allow formulation of a consistent hypothesis for millennial-scale climate change. The observed large, abrupt, widespread millennial-scale climate changes of the last glaciation are hypothesized to have been forced by North Atlantic atmosphere-ocean-ice interactions. Shifts in ocean circulation (oceanic jumps) between modem and glacial modes caused the Dansgaard/Oeschger oscillations, in which the cold, dry and windy signal of the glacial mode was transmitted through the atmosphere to hemispheric or broader regions. These jumps were triggered by changes in freshwater delivery to the North Atlantic, and possibly by other causes extending beyond the North Atlantic. The additional jumps to the Heinrich mode caused atmospheric anomalies similar to but somewhat stronger and more-widespread than during the cold phases of the Dansgaard/Oeschger oscillation. Heinrichmode effects also were transmitted through the ocean, with antiphase behavior between much of the world and some southern regions centered on and downwind of the South Atlantic. Surging of the Laurentide Ice Sheet supplied extra freshwater to force jumps to the Heinrich mode; surging probably was triggered by Dansgaard/Oeschger cooling after Laurentide growth exceeded a MacAyeal threshold for thawing the ice-sheet bed.