The Heat Is Online

Scientists Refocus on Conveyor Belt Shutdown

Could global warming produce a big chill?

The Boston Globe, Oct. 1, 2002

WOODS HOLE - In what would be a surprising byproduct of global warming, average temperatures in North America and Europe could drop by 5 to 10 degrees Fahrenheit in the coming decades as melting polar ice and increased water evaporation profoundly alter the ocean currents that keep both regions warm, say researchers at the Woods Hole Oceanographic Institution.

Terrance Joyce, chairman of the Woods Hole Department of Physical Oceanography, said he believes that key parts of the Gulf Stream ocean current system could virtually shut down, perhaps in our lifetime, reducing or cutting off the flow of warmer water from the equator to the North Atlantic. If these key features, called thermohaline pumps, shut down, it would signficantly cool northern latitudes, including New England.

The big-chill theory is controversial, and viewed as unlikely by some top climate forecasters. But Joyce said a four decade decline in salt levels in the North Atlantic, a trend that accelerated in the 1990s, suggests ocean currents are about to shift.

''There's no doubt in my mind that we're moving toward a shutdown'' of the thermohaline pumps, said Joyce, who is unwilling to predict when or for how long the oceanic pumps will go idle. Joyce said he was ''fairly certain'' a big chill would follow.

The idea that North America, Europe and even parts of Asia may get colder while the overall average temperature of the planet rises has been seen by researchers as a possibility for years; the debate centers on its likelihood. The Arctic icecap melted this summer at a near-record pace, flooding the North Atlantic with cold fresh water, which tends to float on top of the warmer, saltier water that has come north on the Gulf Stream. Carried to an extreme, the fresh water could block the thermohaline (a term that refers to the joint effects of salt and temperature) pumps, located east of Labrador and north of Iceland, from delivering warm water to the northern latitudes.

But some climatologists who design the complex computer models that try to predict the long-term interplay between atmosphere and oceans say they see no indication the deep-water pumps are going to quit anytime soon. Indeed, an international panel of climatologists almost two years ago said such a shutdown was ''viewed as unlikely,'' according Thomas Delworth, a research meteorologist with the federal Geophysical Fluid Dynamics Laboratory in Princeton, N.J.

Even the skeptics, however, say something unusual is happening to ocean currents.

''I'll go this far,'' Delworth said. ''We're getting a very big signal from the North Atlantic. But the implications are murkier.''

Stefan Rahmstorf, a German climatologist, took a step closer to the Woods Hole predictions in a recent issue of Nature, but stops short of a forecast: ''Oceans cover more than two-thirds of our planet. ... Increasingly clear evidence implicates ocean circulation in abrupt and dramatic climate shifts [during the past 120,000 years].'' And he added that, basically, when the system is forced, it responds with a wallop.

Joyce acknowledges the doubts, but he said that reflects the lack of research into a possible cooling trend.

''The problem is that there's so little data. We've got 10,000 measurements over time, but it's never enough,'' he said. This appears to be particularly true when it comes to charting the future of climate, which one scientist likened to predicting the path that curls of smoke will take as they rise through crisscrossing winds.

No one questions that at the heart of the matter is salt - or the lack of it. Far less clear is the effect the fresh water may have on a pair of oceanic mixing sites that work like enormous underwater ''downspouts'' driving a slow-moving ''conveyor belt'' of water around the globe.

The first stage of the Great Conveyor is the Gulf Stream, which transports warm Caribbean water - and the heat it releases en route - northeast to Iceland and beyond. Climatologists credit the Gulf Stream for winter temperatures in Europe that can be 35 degrees warmer than locations on similiar latitudes in North America.

Normally, the system works like this: At two locations, the Gulf Stream smacks into frigid, opposing surface currents, one in the Labrador Sea, the other in the Norwegian Sea. The cold, salty water drives the warm water deep, sucking more Gulf Stream water down the chute like water rushing off a gutter's downspout. Miles below the surface, the water from the Caribbean meanders on the conveyor belt, where it can surface across a broad region of the Northern Hemisphere, delivering heat energy along the way.

But rivers of relatively fresh water some 10 feet deep now cover areas of the pumps, and water samples indicate fresh water has penetrated all the way to the bottom. With a dearth of heavier saltwater to keep the system circulating downward, Woods Hole researchers say it is only a matter of time before the pumps close down. If that happens, the course of the Gulf Stream veers south, leaving ''large portions of Europe, Scandinavia, Greenland, northern Asia and Eastern North America'' colder, according to Ruth Curry, a research specialist at Woods Hole.

Where is the fresh water coming from? Warmer air temperatures at the equator has increased evaporation, making the warmer, southern waters relatively more salty - and more likely to sink.

Another source of fresh water may be melting ice. This summer, the Arctic ice cap could break the previous record, set in 1990, for the largest shrinkage documented in almost 50 years of observations, according to James Maslanik, a polar climatologist and a professor at the University of Colorado in Boulder. The polar fresh-water ice mass that used to average 2.05 million square miles in the summer is now about 1.7 million square miles (the 1990 area) ''and it may be smaller because I have not factored in the August melt,'' Maslanik said.

Climatologists such as Delworth and Ronald Stouffer, an associate who also models climate change for the Geophyiscal Fluid Dynamics Laboratory, consider predictions of a cold spell alarmist. Stouffer said that the only way his computer models can produce a cold future is if he assumes the pumps shut down and that the planet's overall temperature does not rise. ''Otherwise, I just cannot make it get colder anywhere,'' Stouffer said.

Peter Rhines, an oceanographer at the University of Washington in Seattle, stressed that, whether it's computer models or theorizing from data, the science of predicting a global-warming-induced cold snap is going to be very imprecise ''and I don't want to be Chicken Little, shouting that the sky is falling in.''

He said the best indicator of how sensitive the North Atlantic pump system is - and how quickly it can change - is an event in the 1920s. For reasons that are not understood, the pumps increased the movement of warm water to the north until the 1950s. He said that by 1930, parts of Greenland warmed 9 degrees and a herring and cod industry, previously unknown to the area, flourished in the warmer water. When the pumps returned to normal, the industries disappeared.

''One thing is fair to say,'' Rhines said. ''We're living on the edge.''

This story ran on page C1 of the Boston Globe on 10/1/2002.
Copyright 2002 Globe Newspaper Company.