Warming Could Slow Ocean Currents

March 1, 2007 — There is both good and bad news about the conveyor belt of heat-moving currents in the North Atlantic Ocean.

A new study of 17 different climate models concludes that the thermohaline circulation, which drives the Gulf Stream and helps warm Europe, won't stop dead and abruptly change regional climates anytime this century. That's the good news.

"Abrupt climate change in this century is unlikely," said climate researcher Andrew Weaver of Canada's University of Victoria.

The thermohaline circulation is seen as one of the few climate switches that could be flipped quickly and morph climates worldwide — at least in theory.

In the 2004 movie The Day After Tomorrow, the large-scale circulation came to a speedy halt, hurling Europe into another ice age in a matter of days — a wildly exagerated scenario. But despite Hollywood's over-the-top version, abrupt climate change over years or decades is a genuine concern.

And now for the bad news.

Many experts have thought it would take something dramatic, like the complete meltdown of Greenland's ice sheet (and subsequent dumping of buoyant freshwater into the ocean) to throw a wrench into the thermohaline circulation. The models show that may not be the case.

The warming of ocean surface temperatures alone may be enough to slow the circulation.

All of the models showed a reduction in the conveyor belt-like thermohaline circulation over time, said Weaver. "But it turns out it's not the freshwater that's driving it," he added.

Weaver and his colleagues published their findings in the latest issue of Geophysical Research Letters.

The models show that the main force slowing the thermohaline circulation is the heating of surface waters caused by global warming. Warmer water is more buoyant, and therefore less likely to sink. If less water sinks, the overturning circulation pattern of the North Atlantic slows down, regardless of what happens to Greenland's ice.

"In most of the models the heating causes a 20 percent reduction all by itself," confirmed climate researcher Ron Stouffer, of the National Oceanic and Atmospheric Administration's Geophysical Fluid Dynamics Laboratory. He cautions, however, that the models are limited in their ability to reproduce the real oceans.

So the jury is still out on what is likely to happen to ocean currents as the world warms.

The main weakness of the models, said Stouffer, is that they simulate the currents as broad and gentle, whereas in the real world they are narrow and intense. The main obstacle to more accurate modeling has been the cost of bigger, more powerful computers, he said.

As for the role of Greenland's melting glaciers, "it's something that might potentially matter," he said, but it's not at the top of the worry list yet.