During the last glacial period, climatic variation in the Northern and Southern Hemispheres was evidently linked. Modelling work points to freshwater discharge into the North Atlantic as a driving factor. Climate during the period from 60,000 to 25,000 years ago, referred to as Marine Isotope Stage 3, was exceptionally variable. Ice-core records from Greenland suggest that the Northern Hemisphere underwent a series of rapid warming episodes, each followed by gradual cooling. The Southern Hemisphere had a similar climate pattern, but with relatively slow warming and less extreme temperatures. These so-called Dansgaard-Oeschger (DO) events occurred several times during this period, but the physical processes behind the timing and amplitude of the recorded temperature changes are unclear. On page 851 of this issue, Knutti and colleagues present a conceptual model suggesting that discharges of fresh water into the North Atlantic had a larger part to play than was previously thought.
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