The alleged weakening of the Atlantic Meridional Overturning Circulation appears to be triggering a growing amount of speculation about abrupt cooling, like the plot of the movie “The Day After Tomorrow”.
Crippled Atlantic currents triggered ice age climate change
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.”
Another question is whether the AMOC—currently known to be in decline—could drop off suddenly today, as depicted in the 2004 movie The Day After Tomorrow, causing temperatures to plummet across northwestern Europe. Schmittner says the past provides an eye-opener. “It’s evidence that this really did happen in the past, on short time scales.” But McManus says that studies looking deeper into the ice ages have found that the 1500-year climate oscillations tend not to be nearly as strong during interglacial periods. “It would suggest that this kind of thing isn’t so likely to happen today,” he says. On the other hand, he adds, “In most interglacials, Greenland didn’t melt … and Greenland is currently melting.”