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Is ocean circulation slowing down?

The ocean “conveyor belt” may be showing signs of slowing down, according to provocative research published in December. New measurements of temperature and salinity in the North Atlantic indicate that changes are occurring in this segment of the ocean’s circulation that could eventually affect Earth’s climate.

One way to throw the planet into a cooler state is to flip a switch on the ocean conveyor belt, which carries warm Atlantic water to northern latitudes, where it loses its heat, sinks and eventually sends cold deepwater south toward the equator, keeping Europe’s climate milder than it would be otherwise (see Geotimes, November 2005). Changes in global climate and salinity have the potential to affect this transport system of heat in the oceans, with subsequent consequences for the planet’s climate system, particularly in the northern latitudes.

The current situation “is not The Day After Tomorrow.”
Andreas Schmittner, Oregon State University

One example that illustrates some of the potential feedbacks are so-called Heinrich events that occurred during the last ice age, when the rapid addition of thousands of icebergs from the North American ice sheet changed salinity and slowed ocean circulation. As recorded in the Greenland ice cores, local temperatures dropped as much as 10 degrees Celsius over timescales of a century. The most recent such event occurred 16,000 years ago (though the Younger Dryas event 12,000 years ago may also have been a Heinrich event). Any signs of slowing today could indicate that some relatively abrupt changes could be in store for the planet’s climate system.

Recent measurements and analyses of the North Atlantic made by Harry Bryden and co-workers at the National Oceanography Center in Southampton, United Kingdom, show some distinct changes over the past five decades that could indicate such a slowdown could be taking place right now. The researchers collected temperature and salinity data on cruises following transects across the North Atlantic in 2004, compared with transects made in 1957, 1981, 1992 and 1998 (two of which were made along a different sightline, from Africa to the Bahamas).

From their data, they calculated that several distinct segments of cooler water at varying depths showed dramatic decreases and increases in their movement south, while the volume of warm water moving north remained about the same. Taken altogether and compared to the past cross-Atlantic transects, circulation weakened by 30 percent over the half-century covered by the data, according to Bryden’s team’s new modeling results, published Dec. 1 in Nature.

“The general buzz is that they did a very good job given what they had,” says Gavin Schmidt, a climate researcher at NASA Goddard Institute for Space Studies in New York. But with only five transects, separated across decades and also short in duration, these “snapshots” of ocean slices may not have covered seasonal and annual variability, “which is, theoretically speaking, very large,” Schmidt says. Still, “lots of people are excited because [the result] is quite dramatic.”

The week after its publication, the paper had an excited yet mixed response at the American Geophysical Union meeting in San Francisco, where researchers presented new models and data on ocean circulation, as well as on freshening of North Atlantic waters. The Greenland ice cap and sea ice in the Arctic are both melting rapidly, with mixed temperature and salinity changes in both the Atlantic and Pacific, according to data presented by several speakers. The potential impacts of releasing this freshwater into the North Atlantic remain to be seen.

Andreas Schmittner, an ocean modeler at Oregon State University in Corvallis, said that the various circulation models do not yet fit the data accumulated from a variety of sources. The current situation “is not The Day After Tomorrow,” Schmittner said in a press conference, referring to the movie in which an abrupt short circuit of the North Atlantic circulation led to a frozen northern hemisphere in a matter of days.

Researchers hope that long-term monitoring stations deployed on the same cruise where Bryden and colleagues got their latest measurements eventually will provide enough information to confirm whether or not the real ocean conveyor belt is indeed changing.

Naomi Lubick

"Salting a stagnant ocean," Geotimes, November 2005

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