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.
But the pathway of the suspected Lake Agassiz flood has remained a mystery. It didn't go down the Mississippi River into the Gulf of Mexico, evidently, and there was no physical evidence for such a flood down the St. Lawrence River into the Atlantic. But this week researchers report in the journal Nature that they have found evidence that Lake Agassiz suddenly emptied to the North, down the Mackenzie River into the Arctic Ocean.
On the extraterrestrial front, as reported by Michael Reilly in Discovery News, Cardiff University astrobiologist Bill Napier refines the idea expressed earlier that an asteroid or comet crashing into Earth was responsible for the sudden climate change 13,000 years ago. Napier proposes that the sudden freeze came not from a single meteorite but rather a long, intensive shower of sunlight-dimming ash and dust as Earth's orbit passed through a trail of rubble left by the passage of a comet.
Often overlooked in the debate over the cause of the Younger Dryas is the common elements of the competing scenarios. What has brought these different ideas forward is a new understanding about the natural behavior of climate. The concept of a system with a static, slumbering, smooth profile of gradual change is a thing of the past. The ice cores and ocean sediments and pollen layers all point to a natural system that frequently has shown itself inclined to suddenly plunge from one mode into another. Whatever the cause, a changing climate can be sudden, and lethal.
(Image: I have been waiting for a good excuse to share this goofy cartoon by an anonymous artist for a long time and I'm delighted to have finally found one!)