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 VOL. 23, NO. 11DECEMBER 5, 1997 

Lamont's Broecker Warns Gases Could Alter Climate

Oceans' Circulation Could Collapse


Thermohaline circulation links the Earth's oceans. Cold, dense, salty water from the North Atlantic sinks into the deep and drives the circulation like a giant plunger.
On the eve of the international meeting on global warming that opened Dec. 1 in Kyoto, Japan, one of the world's leading climate experts warned of an underestimated threat posed by the buildup of greenhouse gases—an abrupt collapse of the oceans' prevailing circulation system that could send temperatures across Europe plummeting in a span of 10 years.

  If that system shut down today, winter temperatures in the North Atlantic region would fall by 20 or more degrees Fahrenheit within 10 years. Dublin would acquire the climate of Spitsbergen, 600 miles north of the Arctic Circle.

  "The consequences could be devastating," said Wallace S. Broecker, Newberry Professor of Earth and Environmental Sciences at Columbia's Lamont-Doherty Earth Observatory, and author of the new research, which appeared in the Nov. 28 issue of the magazine Science.

  A complex of globally interconnected ocean currents, collectively known as the Conveyor, governs our climate by transporting heat and moisture around the planet. But the Conveyor is delicately balanced and vulnerable, and it has shut down or changed direction many times in Earth's history, Broecker reports. Each time the Conveyor has shifted gears, it has caused significant global temperature changes within decades, as well as large-scale wind shifts, dramatic fluctuations in atmospheric dust levels, glacial advances or retreats and other changes over many regions of the Earth, he said.

  The Conveyor "is the Achilles heel of the climate system," Broecker wrote in Science. "The record ... indicates that this current has not run steadily, but jumped from one mode of operation to another. The changes in climate associated with these jumps have now been shown to be large, abrupt and global."

  The ongoing accumulation of heat-trapping industrial gases blanketing the Earth threatens to raise global temperatures, he said, but such a rise would occur gradually. Far more worrisome is the buildup's potential to stress the climate system past a crucial threshold that would disrupt the Conveyor and set off a rapid reconfiguration of Earth's climate, predicted by existing computer models.

  Broecker also offered a new theory: Scientists generally agree that periodic changes in Earth's orbit and the amount of solar radiation it receives have paced fundamental climate changes on the planet over millions of years. But the global climatic flip-flops may have been set in motion by sudden switches in the operation of the Conveyor.

  Today, the driving force of the Conveyor is the cold, salty water of the North Atlantic Ocean. Such water is more dense than warm, fresh water and hence sinks to the ocean bottom, pushing water through the world's oceans like a great plunger. The volume of this deep undersea current is 16 times greater than the flow of all the world's rivers combined, Broecker said, and it runs southward all the way to the southern tip of Africa, where it joins a watery raceway that circles Antarctica. Here the Conveyor is recharged by cold, salty water created by the formation of sea ice, which leaves salt behind when it freezes. This renewed sinking shoves water back northward, where it gradually warms again and rises to the surface in the Pacific and Indian oceans.

  In the Indian Ocean, surface waters are too warm to sink. Northern Pacific waters are cold, but not salty enough to sink into the deep. This is primarily because prevailing winds that whip around the planet hit the great mountains of the western United States and Canada and drop their moisture. The resulting snow and rain runs into the Pacific, adding a dose of fresh water that dilutes the Pacific's saltiness, said Broecker

  Today, the Conveyor comes full circle, eventually propelling warm surface waters, including the Gulf Stream, back into the North Atlantic. In winter, warm water transfers its heat to the frigid overlying air masses that come off ice-covered Canada, Greenland and Iceland. The eastward-moving air masses make northern Europe warmer in winter than comparable latitudes in North America. Without the Gulf Stream, nothing would temper the Arctic air, and Europe would enter a deep freeze.