Melting icebergs may have triggered or exacerbated ice age slowdowns to Atlantic Ocean currents.
By Eric Hand
The last ice age wasn’t one long big chill. Dozens of times temperatures abruptly rose or fell, causing all manner of ecological change. Mysteriously, ice cores from Greenland and Antarctica show that these sudden shifts—which occurred every 1500 years or so—were out of sync in the two hemispheres: When it got cold in the north, it grew warm in the south, and vice versa. Now, scientists have implicated the culprit behind those seesaws—changes to a conveyor belt of ocean currents known as the Atlantic Meridional Overturning Circulation (AMOC).
These currents, which today drive the Gulf Stream, bring warm surface waters north and send cold, deeper waters south. But they weakened suddenly and drastically, nearly to the point of stopping, just before several periods of abrupt climate change, researchers report today in Science. In a matter of decades, temperatures plummeted in the north, as the currents brought less warmth in that direction. Meanwhile, the backlog of warm, southern waters allowed the Southern Hemisphere to heat up.
AMOC slowdowns have long been suspected as the cause of the climate swings during the last ice age, which lasted from 110,000 to 15,000 years ago, but never definitively shown. The new study “is the best demonstration that this indeed happened,” says Jerry McManus, a paleo-oceanographer at Columbia University’s Lamont-Doherty Earth Observatory, and a study author. “It is very convincing evidence,” adds Andreas Schmittner, a climate scientist at Oregon State University, Corvallis. “We did not know that the circulation changed during these shorter intervals.”
To assess the strength of ancient ocean currents over the course of 35,000 years in the middle of the ice age, McManus and his colleagues examined a 10-meter section of a 38-meter sediment core drilled from the bottom of the Atlantic Ocean. The core came from an elevated patch of sea floor known as the Bermuda Rise, where sediments accumulate abnormally fast. The thick layer allows for a more detailed reading of chemical changes within the sediments when they were buried.
Ice Age climate swings may have come from the weakening of powerful Atlantic currents, in which shallow, warm waters move north (red), and deep, cold waters move south (blue).