Sometimes the plucky investigator in a mystery story isn’t baffled by a “whodunnit.” Sometimes they are pretty sure about the who, but the evidence ain’t where it ought to be. Studying past climate events can be like that, with likely explanations waiting in limbo for years until good evidence turns up—or points to another explanation.

About 13,000 years ago, the warming out of the last ice age temporarily reversed course around the North Atlantic. This cold “Younger Dryas” period lasted almost 2,000 years. Like most climate events that primarily affect the North Atlantic region, ocean circulation is the prime suspect.

Jamming the conveyor

Global ocean circulation is a bit like a branching conveyor belt, with currents pushing water one way at the surface and allowing it to return along the bottom. In the Atlantic, surface currents move north until they grow salty and cold, at which point they stop being less dense than the underlying deep water. In several areas around Greenland, surface and deep waters can mix while a deepwater current heads off to the south.