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.

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.

If a large volume of fresh water is rapidly dumped into the salty North Atlantic, for example, the surface water layer loses density and can’t mix downward.

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