Global Warming Could Trigger Ice Age

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 University's Lamont-Doherty Earth Observatory, and author of the new research, which appears in the Nov. 28 issue of the magazine Science.

The Conveyor "is the Achilles heel of the climate system," Professor 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.

Professor 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, he said.

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, Professor 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 equatorial 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 Professor Broecker, a geochemist at Lamont-Doherty, Columbia's earth science research institute in Palisades, N.Y.

Northern Atlantic surface waters have only about 7 percent more salt than their counterparts in the northern Pacific, but that is just enough to reach the threshold that allows them to sink. But if the North Atlantic waters warmed by only a few degrees, or if they were diluted by just a bit more fresh water from melting glaciers and sea ice or more rainfall, for example, the threshold would not be achieved, and the waters would not sink. Computer models that simulate the Earth's climate system show that the ocean's so-called thermohaline circulation (from the Latin for 'heat' and 'salt') is sensitive to such small changes, Professor Broecker said. The entire Conveyor might shut down or rearrange in a different pattern, with serious effects on global climate, he said.

Today, the Conveyor comes full circle, eventually propelling warm surface waters, including the Gulf Stream, back into the North Atlantic. In winter months, this warm water transfers its heat to the frigid overlying air masses that come off ice-covered Canada, Greenland and Iceland. Thus tempered, the eastward-moving air masses make northern Europe noticeably 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.

There is also strong evidence, from tropical latitude glaciers, that the water vapor content of Earth's atmosphere can shift, too. Water vapor is the most abundant greenhouse gas in Earth's atmosphere and a marked reduction would lower air and ocean temperatures significantly.

"Although the exact linkages that promote such climate changes have yet to be discovered, a case can be made that their roots must lie in the ocean's large- scale thermohaline circulation," Professor Broecker said. The most telling clue is that the boundaries that mark climate changes in continuous sediment or ice core records are sharp, not gradual. That is true even in climate change records spanning millions of years, whose rhythms are governed by Earth's orbit. Professor Broecker suggests that sudden switches in thermohaline circulation may act as a trigger that sets off ice ages and other large-scale climate cycles. Professor Broecker noted that abrupt climate changes have occurred not only during ice ages but during warmer eras such as today's. The Eemian Period -- the last major warm period before the last ice age began about 115,000 years ago -- ended with a brief but intense cold period. A brief, intense cold spell also occurred about 8,000 years ago -- about 2,500 years after the last ice age ended -- when conditions were similar or warmer than they are now.

"Through the record kept in Greenland ice, a disturbing characteristic of the Earth's climate system has been revealed, that is, its capability to undergo abrupt switches to very different states of operation. I say 'disturbing' because there is surely a possibility that the ongoing buildup of greenhouse gases might trigger yet another of those ocean reorganizations and thereby the associated large atmospheric changes," Professor Broecker said. "Were this to happen a century from now, at a time when we struggle to produce enough food to nourish the projected population of 12 to 18 billion, the consequences could be devastating."

Professor Broecker is one of the world's leading authorities on global climate change. He has won nearly every major geological award, including the Vetlesen Prize, considered by many to be the equivalent of the Nobel Prize in earth sciences. Last year he was awarded the National Medal of Science and the Blue Planet Prize, for achievements in global environmental research.