What role does the Bering Strait play in moderating global temperatures?
Alaska’s Bering Strait may play a critical role in the regulation of the global climate — including a knack for maintaining the Atlantic Ocean “conveyor belt”that bathes northern Europe in eon-long kisses of sultry currents and warm wet weather.
But squeeze shut the 53-mile-wide narrows between the Pacific and Arctic oceans off the western tip of Alaska — something that occurred during the last ice age when continental ice sheets locked up much of the world’s fresh water — and the oceanic engine that stabilizes the home planet's climate becomes much more likely to go on the fritz and stay that way for a long time.
These resulting shutdowns have previously stalled the Gulf Stream and triggered abrupt swings between warmer and frigid climates, what scientists call Dansgaard-Oeschger and Heinrich events. These jarring shifts struck the North Atlantic as many as 25 times between 80,000 and about 11,000 years ago, all during moments when the Bering Land Bridge blocked all flow between Pacific and Arctic oceans.
The possibility that a modern version of this process might cause an overnight return to ice age conditions was dramatized (and exaggerated to a fantastic and preposterous degree) by Hollywood in the 2004 climate-disaster flick "The Day After Tomorrow."
But a new study using highly sophisticated computer models predicts there will be no real-life sequel any time soon — not as long as the Pacific and Arctic keep swapping spit through the gap dividing Alaska from Siberia.
“A diverse group of climate researchers has found after running computer simulations that the strait that separates North America and Russia might be serving as a global temperature stabilizer,” explained this Physics.org story about the findings. “When the strait is blocked, melting glacial freshwater in the Arctic Ocean can’t make its way to the Pacific, causing it to back up and eventually flow into the Atlantic.”
The study — The role of the Bering Strait on the hysteresis of the ocean conveyor belt circulation and glacial climate stability — is part of a broader research effort into how ocean circulation impacts the Earth’s climate. Climate scientist Aixue Hu at the National Center for Atmospheric Research and many of 11 co-authors on the new study have previously found that the Bering Strait may be a key factor in global climate dynamics.
"The global climate is sensitive to impacts that may seem minor," Hu explained in this 2010 story. "Even small processes, if they are in the right location, can amplify changes in climate around the world."
In the newest study, Hu and her team ran two sets of computer simulations — one where plunging sea levels had closed the Bering Strait again, and one where the Bering Strait remained open and allowed salty Pacific water into the Arctic and fresh Arctic water to escape into the Pacific.
In each scenario, the researchers gradually added more and more fresh water to the North Atlantic in latitudes spanning from Cuba to England. The goal was to trigger the shutdown of the Atlantic circulation and cause one of those abrupt cooling events inside the computer simulation.
They succeeded. But first, how does all this colossal mixing cause such jarring climate shocks in the real world?
The flow of the Gulf Stream and other elements of the global ocean circulation system deliver warm salty water to the North Atlantic, where it cools, grows denser, and sinks. At depth, this dense salty water starts flowing south. It then keeps rolling, eventually crawling into other hemispheres along a network of deep ocean currents that meander the globe over hundreds of years while equalizing the climate.
But introduce massive amounts of less dense fresh water into the mix, and the North Atlantic sinking starts to sputter, slowing the deep currents to the point where they temporarily die. (Here’s a discussion of a sudden cooling event about 8,000 years ago possibly caused by the draining of massive glacier lakes in North America.)
"One thing that can slow this circulation down, is, if you add freshwater to that area of the North Atlantic, it lowers the density," explained Woods Hole researcher Bruce Peterson, in this 2002 National Geographic story about the process. "It counteracts the process that is increasing density. … If you stop the process, you stop the conveyor that brings warm water north.”
The paradoxical result, Peterson added, can be a chilldown of northern Europe and perhaps the entire northern hemisphere.
Thus, smooth functioning of the Atlantic ocean “conveyor” becomes a “critical link” in keeping the world’s climate from making these wild swings.
“If waters of the far North Atlantic don't sink,” Hu told Science Now here, “much of the large-scale ocean circulation worldwide temporarily collapses. One result: the Gulf Stream, which brings climate-warming waters from the equator to the North Atlantic, comes to a halt.”
If this sounds familiar, you might be remembering the iron-jawed turn by Dennis Quaid in the 2004 climate disaster movie, when he skied to the rescue through an instantaneous ice age caused by shutdown of the Atlantic’s internal circulation. At the time, scientists called the plot of the movie absurd, but acknowledged there was a kernel of scientific truth in the premise.
Back to the computer modeling study. Hu and her team found that the Bering Strait’s status appears to play a key and curious role in the timing of the whole process.
“In both sets of simulations, surface waters became so fresh that they never got denser than the underlying salty water, and therefore never sank, shutting down ocean circulation and plunging areas around the North Atlantic, including Greenland, into a cold spell,” wrote Sid Perkins in this Science Now story about the study.
“However, the researchers noted a critical difference between the sets of simulations: When the Bering Strait was closed, it took as many as 1,400 years for ocean circulation to recover; when the strait was open, the circulation rarely took more than 400 years to recuperate.”
Here’s how hydro-geologist Scott Johnson explains the new findings in this story on the technology news blog Ars Technica:
“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.”
The bottom line?
“Even for greenhouse warming, abrupt climate transitions similar to those in the last glacial time are unlikely to occur as (long as) the Bering Strait remains open,” Hu and her team wrote in the paper.
“And that’s just one more reason why the day after tomorrow probably won’t resemble The Day After Tomorrow," Johnson added here.
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