The hypothesis was that the Gulf Stream’s shutdown, after floods of fresh water had come off the melting ice cap over North America, had meant immediately colder temperatures in Europe and the eastern half of North America. This accounted for the almost unbelievably quick beginning of the Younger Dryas, which analysis of the Greenland ice cores revealed had happened in only three years. Three years, for a major global shift from the worldwide pattern that climatologists called warm-wet, to the worldwide pattern called cool-dry-windy. It was such a radical notion that it had forced climatologists to acknowledge that there must be nonlinear tipping points in the global climate, leading to general acceptance of what was really a new concept in climatology: abrupt climate change.

“What caused the stall again?” Diane said.

Kenzo clicked to the next slide, an image of the Earth portraying the immense ice cap that had covered much of the northern hemisphere throughout the last ice age. The end of that one had arrived slowly, in the old-fashioned linear way, melting the top of the ice cap and creating giant lakes that rested on the remaining ice. These lakes had been held in place by ice dams that were themselves melting, and when the dams had at last given way, extraordinarily large floods of fresh water had rushed down into the ocean, emptying volumes as large as the Great Lakes in a matter of weeks. Signs left on the landscape indicated this had happened down the watersheds of the St. Lawrence, the Hudson, and the Mississippi, and out west, a lake covering most of Montana had drained down the course of the Columbia River several times, leaving an area in Washington called the scablands which gave eloquent testimony to the power of these floods to tear into the bedrock. Presumably the same thing had happened on the East Coast, but the signs had mostly been submerged by rising sea levels or the great eastern forest, so that they were only now being discerned.