The Great Ocean Conveyor Belt
by Daniel Grossman

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Urgent Problem or Not?

Thomas Stocker, a climate scientist at the University of Bern in Switzerland - photo courtesy of Thomas Stocker

Climate scientist Andrew Weaver believes that the effect of global warming alone is not enough to stop the Great Conveyor. - photo courtesy of University of Victoria, Canada

Scientists are united in agreement that huge, abrupt climate changes have occurred, and most likely will occur again. There's also widespread agreement that global warming could bring one on. However, nobody can predict the likelihood, which leads to differences about whether the threat is urgent. Thomas Stocker at the University of Bern in Switzerland is in the "urgency" camp.

Based on a computer simulation he ran of the impact of global warming Stocker says, "A complete shutdown of the Thermohaline Circulation cannot be excluded if the warming persists long enough and strong enough."

Stocker also says a complete shutdown is unlikely until the next century. But a slowdown of the conveyor, which could also have big impacts, is possible much sooner.

Climate scientist Andrew Weaver is one of the most vocal scientists in the "not urgent" camp. A professor at the University of Victoria in British Columbia, Weaver published a paper in the journal Science questioning whether changes in precipitation and evaporation, also called the hydrological cycle, could harm the Great Conveyor.

"I cannot see a mechanism," says Weaver, "that would bring the amount of fresh water required to actually cause it to collapse. Because increased hydrological cycle because of climate change and global warming doesn't cut it as far as I'm concerned."

The huge floods that caused some of the past shutdowns were dozens of times bigger than the amount of water that global warming could add to the north Atlantic argues Weaver. He says Stocker's simulations and others like them are flawed. He accuses his colleagues of exaggeration.

"The one thing I cannot stand is fear-mongering to try to get people to change and to take action."

Columbia University's Wally Broecker agrees that computer simulations showing a shutdown are inadequate, and he agrees that global warming will not cause a repeat of the past. So is the threat urgent or not? Broecker says the stakes are too high to ignore the possibility, however slight, that humans might trigger an abrupt change.

"Unfortunately the world is going to have to make a very important decision on the basis of inadequate data. All we can say is that in the past, the earth has done incredibly crazy things all on its own."

Meanwhile, the amount of carbon dioxide in the air continues to grow. Last year it made the second biggest leap since systematic measurements began in the 1960s. So the gradual warming, and the risk of a dramatic, abrupt change, continues to grow.

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