You may remember 2004’s disaster movie and CGI delivery vehicle, The Day After Tomorrow. The premise of the film (which like any self-respecting disaster film, is excessively absurd) is that global warming suddenly plunges the world into the depths of an ice age. New York City drowns under the largest storm surge in history, and then flash freezes. As is the case with many disaster movies, there’s a small kernel of truth at the eye of this hurricane of exaggeration.

That kernel relates to ocean circulation and the Gulf Stream. The Gulf Stream carries warm water toward Western Europe, helping to keep it more temperate than its latitude would otherwise dictate. It depends on the downward flow of dense, salty water in the North Atlantic that drives a "conveyor belt" of ocean circulation in the Atlantic. Large amounts of fresh water discharged to the North Atlantic (from melting ice sheets, for example) can clog up that overturn by decreasing the density of the surface water. Slowing down Atlantic circulation drives down temperatures in Europe and affects climate around the globe.

During the most recent ice age, changes to the Atlantic conveyor system appear to have triggered bursts of extremely rapid climate change. A new study pins these changes on an event that took place elsewhere in the globe: the closing of the Bering Strait between Alaska and Russia.

Although it's not able to generate cinematic quality climatic chaos, researchers think that the shutdown in the Atlantic conveyor is behind some of the most rapid climate changes visible in ice core records from Greenland—the Dansgaard-Oeschger oscillations. These events occurred in cycles roughly 1,500 years long throughout much of the last glacial period.

Although North Atlantic overturning seems to be involved in these events, it’s unclear what alters the currents. It could be an external trigger (though no orbital or solar cycles really fit the bill), or it could be a sort of ice sheet heartbeat. It may be that the events can only occur when ice sheets reach a critical size, meaning that the rhythm of the cycles could be determined by the growth rate of ice sheets.

Whatever the trigger is, it appears to have been absent or ineffective at the start of the most recent ice age. The last glacial period began around 115,000 years ago, but Dansgaard-Oeschger oscillations were only prevalent between 11,000 and 80,000 years ago. They didn’t appear for the first 35,000 years of the glacial period, and they haven’t been seen since it ended. A paper published this week in Proceedings of the National Academy of Sciences pins this difference on a feature that's an ocean away.

It had been proposed that the Bering Strait between Alaska and Eastern Russia—which is replaced by a land bridge when sea level drops during glacial periods—could have something to do with these rapid climate shifts. So, a group of researchers set out to test the idea using the latest Community Climate System Model (CCSM3).

The model was run under two scenarios—one with modern sea level and an open Bering Strait, and one with a lower sea level and a closed Bering Strait. In each, freshwater was added to the North Atlantic at a slowly increasing rate until the overturning circulation slows down, after which the freshwater input is ramped back down to zero.

During the Dansgaard-Oeschger oscillations, the overturning circulation seems to show a sort of double equilibrium. One state is the normal mode, like it behaves today. That seems to collapse to a low-circulation state that can hang around for quite a while before flipping back to full strength.

The simulation with an open Bering Strait couldn’t replicate this behavior. The overturning circulation would slow down, but as soon as the freshwater addition started to drop, the circulation would smoothly recover right along with it. With the Bering Strait closed, however, the circulation would collapse more quickly, hold steady there for a while, and then abruptly kick back into gear. Much like the real thing is thought to have done.

The Bering Strait exerts its influence by controlling flow between the Arctic and the North Pacific. Normally, fresher water flows into the Arctic, but when freshwater is being added to the North Atlantic some of it leaks into the Arctic and out to the Pacific. That helps keep the overturning circulation in the North Atlantic from clogging up so easily. In contrast, when the Bering Strait is closed, the freshwater in the North Atlantic piles up and lingers.

Beyond offering an explanation of why the Dansgaard-Oeschger oscillations happened when they did (during the period when sea level was low enough that the Bering Strait was closed off), this work also has something to say about the future. Since the Bering Strait is open today, an abrupt collapse of overturning circulation in the North Atlantic due to melting Greenland ice could be much less likely. And that’s just one more reason why the day after tomorrow probably won’t resemble The Day After Tomorrow.

PNAS, 2012. DOI: 10.1073/pnas.1116014109 (About DOIs).