Enlarge / The Mackenzie River empties into the Beaufort Sea along Canada's northwest coast. (credit: NASA Earth Observatory) 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.