Such dramatic climatic reversals occurring in such a short time cannot be explained by
Milankovitch cycles (that is, cyclical changes to the shape of
Earth’s orbit, the tilt of Earth’s axis, and the wobblelike movement of Earth on its axis with respect to the Sun), which play out over tens of thousands of years. A number of
hypotheses have been proposed to explain the Younger Dryas, but so far there is no
consensus on its cause.
The leading
hypothesis, first proposed by Finnish scientist Claes Rooth in 1982 and later expanded by American climatologist Wallace Broecker and others, postulated that large amounts of fresh
water were discharged into the North Atlantic about 12,800 years ago. More specifically, the retreat of the
Laurentide Ice Sheet allowed
Lake Agassiz, a glacial meltwater lake that covered a large part of north-central
North America, to drain eastward into the
Atlantic Ocean rather than southward into the
Mississippi River. Broecker and American geologist George Denton proposed that this large influx of fresh water may have stopped higher-density
seawater in the North Atlantic from descending to lower depths, thereby interrupting
thermohaline circulation (a system of surface and deepwater
currents that distributes large amounts of
heat around the globe) and initiating a short-term return to glacial conditions.
