Threat of Localized Cooling Flows
By Curt Suplee
From Global Warming
Washington Post Staff Writer
Monday, December 1, 1997; Page A03
As the mass palaver begins today at the international climate summit in Kyoto, Japan, the main focus will be on rising worldwide temperatures.
Yet paradoxically, one of the most horrendous potential consequences of global warming is catastrophic local cooling, specifically across Europe.
A glance at the map puts the threat in chilling perspective. London is farther north than Winnipeg; Denmark has the same latitude as the Aleutians. Yet European winters are comparatively mild. The reason is that the North Atlantic is warmed by a mighty ocean "conveyor belt" that transports stupendous amounts of heat in a mile-deep layer of warm water that flows northward from the equator.
If that beneficent system were to stop -- as it apparently has many times in the past during glacial periods -- northern Europe's average winter temperatures would be 10 or 20 degrees Fahrenheit below what they are now.
"If there were a full shutdown," said Wallace S. Broecker of Columbia University's Lamont-Doherty Earth Observatory, "Ireland would become like Spitsbergen, regardless of global warming. Iceland would have glaciation right down to sea level. People would have to abandon it. Northern Europe would become, not exactly uninhabitable, but more like Siberia than New York."
As a result, determining whether global warming could hinder or stop the Atlantic "thermohaline circulation" (or THC, as the conveyor belt is formally known) and how rapidly it might do so are among the most urgent questions in climate research.
In the Nov. 28 issue of the journal Science, Broecker offers a cautionary analysis, arguing that evidence from ice cores and ocean sediments shows that past changes in THC-related climate have been "large, abrupt and global," occurring "on a time scale of a few decades to as little as a few years." Other experts think the intervals are considerably longer, perhaps entailing centuries of gradual or fluctuating change.
Either way, because "the consequences could be devastating," Broecker warns, "it behooves us to get a better grasp than we now have of this phenomenon."
Similar sentiments have been expressed repeatedly since the early 1990s, when researchers at the National Oceanic and Atmospheric Administration's climate modeling center in Princeton, the Geophysical Fluid Dynamics Laboratory (GFDL), calculated that appreciable increases in atmospheric carbon dioxide could possibly turn the THC off altogether.
Earlier this year, two Swiss researchers reported in the Aug. 28 Nature that their computer-model calculations showed "a permanent shutdown" within 100 years if carbon dioxide levels continue to increase at the present rate. That poses a risk "that no nation bordering the North Atlantic would willingly take," Stefan Rahmstorf of the Potsdam Institute for Climate Impact Research in Germany wrote in an accompanying commentary.
Unfortunately, no one knows exactly what it takes to disrupt the critical relationships between heat (the "thermo-" of THC) and salt content ("-haline") in the conveyor belt. But the general principles are well understood, and involve interconnected events thousands of miles apart.
Around the equator, the ocean is heated by strong sunlight. The expanding mass of water is less dense, and thus rises. As this mass drifts north, constant surface evaporation makes the upper water progressively saltier. When it reaches as far north as Iceland and Greenland, it cools dramatically -- having shed much of its heat into the atmosphere, where winds from the west blow it toward Europe.
This incoming current has a much higher salt content than the surrounding local waters, which are constantly being diluted by precipitation and runoff from land masses. So when the saltier immigrant flow cools, it sinks to the ocean bottom and begins to run south, forming the lower half of the THC conveyor belt: North Atlantic Deep Water. As it plunges toward the abyss, it draws in water behind it, providing much of the driving force for the conveyor. (A corresponding heavy-water sink exists near Antarctica, but it has far less effect on our weather.)
But if the North Atlantic surface were to become too warm for the incoming current to cool and sink, or the surrounding waters have an abnormally low salt content, then the THC system could grow chaotic or simply slam to a halt. That's what many scientists think happened during a bizarre cold snap called the Younger Dryas about 11,400 years ago when the planet was just coming out of the last ice age. Things were warming up nicely when suddenly "there was a dramatic return to ice age-like conditions in less than a hundred years," Stanford University climate researcher Stephen H. Schneider writes in his new book, "Laboratory Earth."
The leading theory for the big chill is that a large volume of fresh water from melting glaciers in North America suddenly spilled out into the North Atlantic, diluting the incoming THC flow so much that it shut the whole conveyor down.
The effects of prolonged global warming, many experts believe, could achieve something like the same conditions. If warming puts more water vapor in the air, it would increase precipitation (especially at higher latitudes) and contribute to melting of ice masses, thus reducing the salt content of northern waters. At the same time, greenhouse warming would presumably increase ocean surface temperatures, making it harder for the denser incoming water to cool enough to sink.
The combined effect could switch off the THC, though probably only for a few decades. The sudden cooling brought on by conveyor shutdown would result in more ice formation at high latitudes; and as ice forms from sea water, it expels salt, making the surrounding surface water denser. At the same time, precipitation would decrease, reducing ocean dilution. Pretty soon, in theory, the THC would kick itself back into action just as it did after the Younger Dryas episode. Broecker suspects that this naturally recurring "salt oscillator" takes about 1,000 years to reverse itself.
Jerry Mahlman, director of GFDL, doubts large alterations in THC are likely within the next few decades but agreed that "it's a very legitimate issue." In the Nov. 21 issue of Science, he cites a "greater than two out of three chance" that if there are substantial precipitation increases at high latitudes, they will reduce ocean salinity, "suppressing the overturning circulation."
If that were to happen, Mahlman noted, there could be other grave outcomes. One is a probable decline in waterborne nutrients drawn up from the ocean bottom if upwelling at the equator slowed or stopped. Projecting what that might do to marine ecosystems is so difficult, he said, that "it makes climate seem simple by comparison."
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