What caused the last great stab of cold 13,000 years ago? Almost overnight, it seems, something drove the gradually warming Northern Hemisphere back into the ice age for 1,000 years or more until warming resumed.
People researching the behavior of ancient climate have been ruminating over this question for 20 years now, ever since they detected unexpectedly sharp changes in temperatures in a variety of sources - ice cores, ocean sediments, pollen layers in old dirt.
You might think they would have it settled by now, but Earth does not so easily give up its big secrets.
The episode is called the Younger Dryas, named for a particular layer of pollen left by the pretty little, white-petaled Dryas flower - a cousin of the rose - that tolerates cold so well it is one of few plants found around the edges of glaciers.
The question has provoked two lines of thought. One calls up a terrestrial explanation, the other an extraterrestrial origin for the abrupt climate change.
Most climate scientists favor the idea conceived by geophysicist Wally Broecker in 1989 that a large meltwater lake at the edge of the Laurentide Ice Sheet burst through an ice dam, flooding the North Atlantic with relatively light freshwater that upset the balance of the Ocean Conveyor, stopping the northerly transport of Tropical warmth in its tracks.