The Bering Strait, the 80-kilometer-wide stretch of ocean between Russia and Alaska, can strongly influence the climate of the entire Northern Hemisphere, researchers have calculated. The findings answer a question that has dogged scientists for the past decade, and they demonstrate how seemingly slight changes in certain factors can impact global climate.
For the past several million years, Earth has been locked in a glacial cycle. Ice ages come and go with remarkable regularity, lasting 100,000 years or so and then letting up for 10,000 to 15,000 years. The last such ice age ended about 11,000 years ago. Within each ice age, the kilometer-thick ice sheets that cover much of the top of the world also go through cycles, thickening and thinning, extending and receding, over periods lasting a few thousand years. But until now, no one has been able to pinpoint the cause of these oscillations.
This pressing question has now been tackled by Aixue Hu, an oceanographer at the U.S. National Center for Atmospheric Research in Boulder, Colorado, and colleagues. They studied the Bering Strait's potential impact on glacial oscillations. They knew that every so often during past ice ages, drops in sea level created a land bridge across the strait. By analyzing data from ocean sediments, Hu's team found that the timing of the bridge's appearance seemed to be linked to the oscillations of the ice sheets. So the researchers combined the ocean sediment data with calculations of Earth's orbital and rotational variations--which are known to trigger or end the ice ages--into a new supercomputer model. As they report online this week in Nature Geoscience, the model shows that the Bering Strait's status was directly responsible for the ice-sheet oscillations.
Based on the model, this is what happens. At the beginning of an ice age, massive sheets of ice build up and advance across North America, Greenland, Europe, and northern Asia. These sheets sequester a great deal of water, causing sea levels to plunge and exposing the land bridge across the Bering Strait. When the strait closes, water cannot flow from the Pacific Ocean into the Arctic Ocean. The loss of that inflow means saltier--and warmer--water from the Atlantic Ocean has an easier path into the Arctic. The warmer water melts the ice sheets around their edges, which in turn floods the Arctic and Atlantic oceans with fresh water, raising sea levels again and moderating the northern climate. Once the seas are high enough, the Bering Strait reopens, cold Pacific water reenters the Arctic, the climate cools enough for the ice sheets to advance, and the whole process repeats.
Still undetermined, Hu says, is exactly where and how the warming waters interact with the ice sheets. Finding that out is going to require a new generation of models, he adds.
Fluid dynamicist Ronald Stouffer of the National Oceanic and Atmospheric Administration in Princeton, New Jersey, agrees. A full understanding of the process described in the paper will have to wait until a comparable ice-sheet model can be developed, he says. Nevertheless, the research improves our understanding of past climate variations "and therefore gives us more confidence in future projections."