Into the cold?

This circulation – sucking in Gulf Stream water at the top and forcing it down and out at the bottom – propels the North Atlantic branch of the conveyor. Shut down that pump, and you could have what Dr. Gagosian calls "dramatic" climate change. He explains in a posting to the Woods Hole website that "average winter temperatures could drop by 5 degrees Fahrenheit over much of the United States, and by 10 degrees in the northeastern United States and in Europe."

The way to shut down the pump is to dilute the inflow water to the point where it is no longer salty enough to sink deeply and flow southward near the bottom. That seems to be happening now. Last April, Robert Dickson of Britain's Centre for Environment, Fisheries, and Agricultural Science, together with colleagues from Canada, Germany, and Scotland reported in Nature magazine that fresh water has been diluting the North Atlantic for the past four decades. Research by other groups confirms this trend.

Joyce says the evidence "strongly suggests" the North Atlantic pump is "threatened by fresh-water dilution." The cause is unclear. It could be a subtle effect of global warming. Changes in air circulation have altered the freezing and melting patterns of Arctic ice generally. Ice in the Arctic Ocean, in particular, has thinned. Also, the Arctic has warmed to the point where melting permafrost now is a major concern. But there is no clear causal pattern to the North Atlantic fresh-water dilution.

The urgent need, Joyce says, is for "specific research to clarify what is going on." That includes more upper-ocean salinity measurements and monitoring of the North Atlantic conveyor circulation.

Last December, the National Academy of Sciences released a report urging research to understand abrupt climate change generally. Richard Alley of Pennsylvania State University at University College, chairman of the Academy committee, warned at that time that "it will be a long time, if at all, before we are really good at predicting climate change...." He added, "Any reality may be very different from the predictions, and we need to anticipate changes and surprises."

Right now, those climate simulations don't deal with the nasty surprises Gagosian anticipates if the North Atlantic circulation pump shuts down, as it has done in some past climate changes. Instead of half a century or more to adapt to global warming, the next 10 to 20 years might bring a climate change that would change the world and the world economy. In Gagosian's words, it could "freeze rivers and harbors and bind North Atlantic shipping lanes in ice ... disrupt the operation of ground and air transportation ... cause energy needs to soar exponentially ... force wholesale changes in agricultural practices and fisheries." Efforts to curb CO2 emissions to slow global warming would become a secondary issue as people tried to cope with more immediate challenges.

Dr. Alley says there's no reason yet for alarm, although there is a case to be made for more intensive research to find out what's happening to North Atlantic circulation. He also sees a larger challenge. If drastic climate change were imminent, there is little we could do to stop it. The best strategy, he says, is to work harder now to build resiliency into agriculture, housing, energy use, and into economies generally. That's essentially the conclusion a US Department of Energy climate-change study group reached 25 years ago.

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