Even as the amount of sunlight hitting the region diminished, parts of Greenland and North America warmed by nearly 3 degrees F. Glaciers then shrank and sea levels rose by up to 100 feet, only to reverse. This cycle repeated every few thousand years. Why?
The authors argue that the Bering Strait, a choke point, is the critical factor. When sea levels dropped sufficiently, dry land emerged between North America and Asia. This dam halted the flow of water from the North Pacific into the Arctic.
At present, about 800,000 cubic meters (211 million gallons) of water per second flow into the Arctic from the North Pacific. That's about 3.6 times the discharge of the Amazon, the world's largest river.
This water from the north Pacific eventually flows to the north Atlantic. Water in the north Pacific is much fresher, and therefore much lighter, than the saltier water of the North Atlantic. And the influx of freshwater into the North Atlantic impedes a process that's critical to heat distribution around the globe.
Scientists call this conveyor-beltlike flow the "meridional overturning circulation." And it's responsible for keeping Europe balmy compared to regions at similar latitudes elsewhere.
When the overturning is impeded, however, the transport of tropical heat to high northern latitudes slows, and the north Atlantic grows colder.
In other words, freshwater flowing into the north Atlantic can bring temperatures down in the region. Conversely, lessening the flow of freshwater into the north Atlantic can cause temperatures to rise. That's what the authors of this paper say happened repeatedly during the past 100,000 years.