For more than 30 years, climate scientists have debated whether flood waters from melting of the enormous Laurentide Ice Sheet, which ushered in the last major cold episode on Earth about 12,900 years ago, flowed northwest into the Arctic first, or east via the Gulf of St. Lawrence, to weaken ocean thermohaline circulation and have a frigid effect on global climate. Now, using new, high-resolution global ocean circulation models, researchers report the first conclusive evidence that this flood must have flowed north into the Arctic first down the Mackenzie River valley....