Climate scientists have been explaining for years that the problem with global warming isn’t just warming. It’s also about the other changes warming can bring, including heat waves, droughts, rising seas, intense storms and much more. One of the scariest possibilities is that major ocean currents could abruptly stop entirely, plunging areas like Western Europe into an abrupt deep-freeze. It’s happened before, tens of thousands of years ago, and while climate experts doubt that it will happen again anytime soon, they haven’t had especially powerful evidence to back their optimism.
But now they do, thanks to a new paper just published in Proceedings of the National Academy of Sciences. What will save Europe from disaster, say the authors, is the Bering Strait, the 50-mile-wide gap that separates Siberia from Alaska. “As long as the Bering Strait remains open,” said lead author Aixue Hu, a climate modeler at the National Center for Atmospheric Research (NCAR), in a telephone interview, “we will not see an abrupt climate event.”
The focus of Hu’s study was the globe-spanning, endlessly looping current known as the ocean conveyor belt, or, more properly, the Atlantic Meridional Overturning Circulation. To oversimplify a bit, the current brings warm surface water from the South Pacific, Indian and South Atlantic northward along the east coast of the Americas; the part that runs along the U.S. coastline is known as the Gulf Stream.
At about Massachusetts, the balmy water peels off for Europe where it gives up its remaining heat, making that continent a lot warmer than it would otherwise be (Madrid is at about the same latitude as Chicago, for example but it’s nowhere near as cold in winter). The water cools off, becomes denser, and sinks, turning into a cold subsurface current that returns south, warms and rises, and begins the loop again.
If a burst of fresh water enters the North Atlantic, however, say, from melting ice caps, the ocean’s saltiness is diluted, making it harder for surface water to sink even when it’s cold. Melt enough ice and you stop the conveyor belt completely, removing Europe’s source of heat. (This was part of the premise behind the movie “The Day After Tomorrow,” which mangles the science so badly it makes scientists cringe.)
The basic scenario, however, is sound. “This has happened several times,” Hu said, “but we weren’t sure why.” So the scientists looked for evidence of what else might have been happening at times when the current shut off. They noticed that it tended to happen when sea level was especially low — specifically when it dropped low enough to expose the sea floor at the Bering Strait, creating a “land bridge” that connected the two continents. (Anthropologists think prehistoric Asians crossed this bridge when they first populated the Americas.) “So we thought maybe,” Hu said, “this played a role.”
Screenshot from NASA's High-Def animation of surface currents. Credit: NASA.
And so it did, they discovered, when they used a powerful climate model to test their hypothesis. “When the strait is open,” Hu said, “water flows into the Arctic Ocean and eventually out into the North Atlantic.” Pacific Ocean water, it turns out, is somewhat less salty than the Atlantic, so the current is already in a constant state of mild dilution. If more freshwater gets dumped in during these times — by, say, melting a significant amount of ice on Greenland — it only adds to the dilution, slowing the conveyor belt gradually.
When the Bering Strait is closed, however, the conveyor belt is undiluted. Dump a lot of freshwater in and it’s such shock to the system that it can cause a total shutdown.
The good news is that the Bering Strait is currently open for business, and as sea level continues to rise with global warming, it’s not closing shop any time soon. So an abrupt shutdown of the conveyor belt, leading to sudden cooling in the north along more heat staying bottled in the south — the series of events that may have brought us out of the last Ice Age, a new study in Nature argued last week — is unlikely. Considering all of the disruptions we’re already seeing from climate change, and those that are likely to come over the next century, it’s slightly reassuring to realize that things could be far worse.