Causes
The prevailing theory holds that the Younger Dryas was caused by a significant reduction or shutdown of the North Atlantic "Conveyor", which circulates warm tropical waters northward, in response to a sudden influx of fresh water from Lake Agassiz and deglaciation in North America; however, geological evidence for such an event is thus far lacking. The global climate would then have become locked into the new state until freezing removed the fresh water "lid" from the north Atlantic Ocean. A recent alternative theory suggests instead that the jet stream shifted northward in response to the changing topographic forcing of the melting North American ice sheet, bringing more rain to the North Atlantic which freshened the ocean surface enough to slow the thermohaline circulation. There is also some evidence that a solar flare may have been responsible for the megafaunal extinction, though it cannot explain the apparent variability in the extinction across all continents.
Previous glacial terminations probably did not have Younger Dryas-like events, suggesting that its cause has a random component. Nevertheless, there is evidence that some previous glacial terminations had post glacial cooling periods somewhat similar to the Younger Dryas.
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