Science News: For the past several decades, scientists have been in agreement that a 1200-year-long cold spell called the Younger Dryas was probably triggered by the massive melting of North American ice sheets some 13 000 years ago. But what path the floodwaters took has been the source of some debate. New computer simulations of ocean currents show that the most likely pathway was the Mackenzie Valley in Canada’s Northwest Territories; meltwater rushing through the valley would have been directed into the Arctic Ocean, where coastal boundary currents would have carried it toward the North Atlantic near Greenland. Because the lighter freshwater would have tended to stay on top of the ocean’s denser salt water, the massive inflow could have disrupted normal ocean circulation, resulting in cooler air temperatures. Understanding the conditions that led to Earth’s last Big Freeze could be important for understanding modern climate change. The researchers’ results were published online yesterday in the Proceedings of the National Academy of Sciences.