The deglaciation of the northern hemisphere: A global perspective

Orbitally induced increase in northern summer insolation after growth of a large ice sheet triggered deglaciation and associated global warming. Ice-a lbedo, sealevel, and greenhouse-gas feedbacks, together with tropical warmi ng from weakening winds in response to polar amplification of warming, caus ed regional-to-global (near-) synchronization of deglaciation. Effects were larger at orbital rather than millennial frequencies because ice sheets an d carbon dioxide vary slowly. Ice-sheet-linked changes in freshwater delive ry to the North Atlantic, and possibly free oscillations in the climate sys tem, forced millennial climate oscillations associated with changes in Nort h Atlantic deep water (NADW) flow. The North Atlantic typically operates in one of three modes: modern, glacial, and Heinrich. Deglaciation occurred f rom a glacial-mode ocean that, in comparison to modern, had shallower depth of penetration of NADW formed further south, causing strong northern cooli ng and the widespread cold, dry, and windy conditions associated with the g lacial maximum and the cold phases of the millennial Dansgaard-Oeschger osc illations. The glacial mode was punctuated by meltwater-forced Heinrich con ditions that caused only small additional cooling at high northern latitude s, but greatly reduced the formation of NADW and triggered an oceanic "sees aw" that warmed some high-latitude southern regions centered in the South A tlantic.