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a new study also points to a more earthbound culprit: the presence of a land bridge connecting Asia to North America.

debated whether the climate swings were driven by sharp variations in solar activity or simply by unstable climatic processes

Earth's climate has been relatively stable since the end of the last ice age

temperatures were fairly stable, too, after the ice age began in earnest about 100,000 years ago

20,000 years later, things became unhinged

global sea level dropped to about 50 meters lower than it is today

As the ice sheets that covered North America and northern Eurasia snatched up more and more of Earth's water

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

also huge consequences for Earth's climate

two sets of climate simulations

one in which the Bering Strait was open

one in which it was blocked

each set of simulations, the researchers gradually added large amounts of fresh water to the North Atlantic between the latitudes of 20° and 50

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 circulatio

Gulf Stream, which brings climate-warming waters from the equator to the North Atlantic, comes to a halt.

If waters of the far North Atlantic don't sink, says Hu, much of the large-scale ocean circulation worldwide temporarily collapses

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

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

sign that ocean circulation is stable when the strait is open

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

if it stopped, air temperatures could change quickly.

computer simulation actually showed the same temperature fluctuations over Greenland as researchers have found, via core ice samples, occurred the last time the Strait closed during the last Ice Age.