Into the cold?
Slowing ocean circulation could presage dramatic and chilly climate change
Call it global warming's dirty little secret. Those much-publicized scenarios of how carbon-dioxide (CO2) pollution may gradually heat up the earth don't tell you another key fact: that climate has sometimes changed without warning. It can go from warm to cold or cold to warm in less than decade, and stay that way for centuries.Skip to next paragraph
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Water-circulation data from the North Atlantic now suggest the climate system may be approaching that kind of threshold. If man-made warming or natural causes push it over the edge, the system will chill down many temperate parts of North America and Europe, even while the planet as a whole continues to warm.
Terrence Joyce, chairman of the physical-oceanography department at Woods Hole Oceanographic Institution in Massachusetts, is one of a handful of scientists trying to raise awareness about this possibility. He says he is "not predicting an imminent climate change only that once it started (and it is getting more likely) it could occur within 10 years."
Mr. Joyce explains that many of the computer simulations of climate change "never predict any abrupt transition." But, he says, such an event could occur. "Abrupt climate change has been a part of our history," he says.
That's what happened when the so-called Little Ice Age cut in about 500 years ago. Take a look at Bruegel's famous paintings of skaters on frozen Dutch canals to get an idea of what would be in store for regions that haven't known such harsh winters since we emerged from the Little Ice Age during the last century.
There is as yet no conclusive evidence that the Dutch should stock pile ice skates. But Woods Hole director Robert Gagosian feels an urgency to settle the question. He sees enough disturbing information in the North Atlantic data, which oceanographers from Woods Hole and other institutions have gathered, to call it "strong evidence that we may be approaching a dangerous threshold." He says we need to know whether we are blindly walking toward the edge of a cliff.
North Atlantic water circulation raises this level of concern because it is a key factor in the climate system. Broadly speaking, that system redistributes solar heat from the tropics around the planet. The atmosphere carries heat north and south in the form of warm air and water vapor. The latter releases its heat when it condenses into droplets. That's about half the distribution; ocean currents carry the rest.
Winds move heat around quickly. Ocean currents can take centuries. Oceanographers call their stately flow the Great Ocean Conveyor. Warm surface currents distribute tropical heat. Deep currents carry cold water back toward the equator. Together, these currents form an interconnected system that circulates through the North and South Atlantic into the Indian Ocean and the Pacific.
The "pump" that drives the conveyor is in the northern part of the North Atlantic. There, the Gulf Stream brings in warm, relatively salty water. This cools as it gives up heat to the winds that warm Britain and Europe. Cold, salty water is relatively heavy. Mingling with Arctic outflows, the Gulf Stream water sinks to great depths and flows southward. More Gulf Stream water flows in to replace it.