How the Bering Strait influences Earth's climate
The Bering Strait is only 50 miles wide, but it has quite an influence on Earth's climate, say scientists.
But a new study in the journal Nature Geoscience concludes that this shallow strait between the North Pacific and the Arctic oceans has played a large role in climate fluctuations during recent ice ages. Depending on whether it's closed or open, the strait dramatically changes the distribution of heat around the planet.
When sea levels decline enough that water can no longer flow from the Pacific to the Arctic through the strait, the North Atlantic responds by growing warmer. That warmth is strong enough to melt ice sheets and temporarily reverse the glaciation of the Northern Hemisphere.
Generally, scientists think that changes in Earth's orbit around the sun have driven the repeated advance and retreat of glaciers during the Pleistocene — the period starting 2.58 million years ago and ending about 10,000 years ago.
When less sun reaches the Northern Hemisphere during summer months, winter snows don't melt. The white snow reflects more of the sun's energy back into space, further cooling the region. Glaciers form and begin creeping southward. These ice sheets, a mile or more thick in places, suck up large quantities of water.
Compared to today, sea levels dropped by as much as 400 feet during the Pleistocene.
But while glacial periods follows Earth's orbital variations quite closely — they occur very roughly on 100,000-year cycles — for the past 100,000 years or so, a shorter warming and cooling cycle has played out over the larger one.
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.
Here’s the cycle: Cooling brought on by changes in Earth's orbit caused glaciers to grow and sea levels to fall. Eventually, the seas dropped far enough that the Bering Strait was closed off. The flow of relatively fresh water from the north Pacific to the north Atlantic stopped, or was dramatically decreased. Without interference from this freshwater, the meridional overturning in the North Atlantic strengthened — by about 13 percent. Parts of Greenland, northeastern North America, and Europe warmed by 2.7 degrees F. Glaciers around thenNorth Atlantic then began melting.
Meanwhile, as the northward flow of water in the Pacific was stymied, temperatures there dropped by the same amount — 2.7 degrees F.
In the end, however, this warming was self-limiting. As increased warmth melted glaciers around the north Atlantic, sea levels began to rise.
Eventually they rose sufficiently to again engulf the Bering land bridge. The flow of water from the north Pacific into the Arctic resumed. The meridional overturning in the north Atlantic again weakened. And the glaciers of the Northern Hemisphere again began advancing southward.
None of this bears directly on the current trend of human-induced global warming. But it does indicate that scientists, enabled by ever-more powerful computers and more complicated — some might say "realistic" — climate models, are improving their understanding of Earth's climate system.
It also highlights an important lesson: In complex systems (Earth’s climate), seemingly small changes, such as closing the 50-mile-wide Bering Strait, can have large consequences, like temporarily reversing a hemisphere-wide cooling trend.
Or, as we talked about last week, a little warming might cause a lot more.