Tuesday, 10 April 2012

Land Bridge Caused Wild Temperature Swings

Much of the last ice age was characterized by violent climate swings. At seemingly random times beginning about 80,000 years ago, average temperatures in and around the North Atlantic rose or fell by 10°C or more in the course of a decade or two—a pattern that lasted for 70,000 years. Researchers have debated whether the climate swings were driven by sharp variations in solar activity or simply by unstable climatic processes, but a new study also points to a more earthbound culprit: the presence of a land bridge connecting Asia to North America.

Earth's climate has been relatively stable since the end of the last ice age, says Aixue Hu, a climate scientist at the National Center for Atmospheric Research in Boulder, Colorado. And temperatures were fairly stable, too, after the ice age began in earnest about 100,000 years ago. But some 20,000 years later, things became unhinged. Around this time, Hu notes, something else happened: As the ice sheets that covered North America and northern Eurasia snatched up more and more of Earth's water, global sea level dropped to about 50 meters lower than it is today. That exposed a broad strip of land that connected what is today Alaska and Siberia. Ancient animals used the land bridge, which measured as much as 1500 kilometers wide in spots, to roam back and forth between Asia and North America, and many researchers have proposed that early humans used the dry land as a route to the New World. But according to a new study by Hu and colleagues, there were also huge consequences for Earth's climate.

Hu's team ran two sets of climate simulations: one in which the Bering Strait was open, as it is today, and one in which it was blocked, separating the North Pacific from the Arctic Ocean. In each set of simulations, the researchers gradually added large amounts of fresh water to the North Atlantic between the latitudes of 20° and 50°. At the time, the researchers propose, this swath, which spans the latitudes from southern Cuba to southern England, would have received large amounts of meltwater from Northern Hemisphere ice sheets during warm spells that occasionally punctuated the ice age. Today, the surface waters in this swath affect the temperature and salinity of water even farther north in the Atlantic, a region where surface waters cool, sink to the seafloor, and then flow southward—a critical link of the worldwide conveyor belt of ocean circulation. If waters of the far North Atlantic don't sink, says Hu, 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.

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. However, the researchers noted a critical difference between the sets of simulations: When the Bering Strait was closed, it took as many as 1400 years for ocean circulation to recover; when the strait was open, the circulation rarely took more than 400 years to recuperate—a sign that ocean circulation is stable when the strait is open, the team reports online today in the Proceedings of the National Academy of Sciences.

Whenever the ocean circulation shut down in the simulations, temperatures in Greenland suddenly dropped by 12°C—a decrease similar in magnitude to many abrupt cold snaps chronicled in the Greenland ice core records. But in the future, such shifts are unlikely to occur, because now—and in the future, as sea levels are predicted to rise from current levels—the Bering Strait will remain open.
"This is a very nice study, and their results make a lot of sense," says Ronald Stouffer, a climate modeler with the National Oceanic and Atmospheric Administration's Geophysical Fluid Dynamics Laboratory in Princeton, New Jersey. The team's results "are a really important contribution," adds Lloyd Keigwin, a paleoceanographer at Woods Hole Oceanographic Institution (WHOI) in Massachusetts. In 2007, Keigwin and Mea Cook, a geoscientist then at WHOI, proposed that ocean circulation through the Bering Strait helps stabilize climate in the North Atlantic. "This is good supporting evidence for the model we proposed a few years ago," he says.

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