The ocean conveyor

The real day after tomorrow

Broecker is a maverick—a prodigious and fearless generator of ideas, and one of the most influential figures in climate science for half a century. Sometimes he can be more. Amid the admiration for his science, you hear some harsh words about him in the science community. A bully, some say, especially to young scientists; a man who will use his influence to suppress ideas with which he disagrees. For a man in his seventies, he certainly comes on strong and relishes conflict. Here are his unprompted, on-the-record remarks to me about one of the U.S.'s leading climate modelers, who incurred the wrath of some Republican senators: "I think the senators were well out of line, but if anyone deserves to get hit, it was him. The goddamn guy is a slick talker and superconfident. He won't listen to anyone else. I don't trust people like that. A lot of the data sets he uses are shitty, you know. They are just not up to what he is trying to do."

Broecker is not a man to cross lightly. And to be honest, I thought a bit before writing the above. Much as I like his vigor, I'd hate to be caught in his crosshairs. Some believe he has earned the right to sound off about young colleagues he thinks don't pass muster. Some worry that Broecker seems to save his invective for people who resemble him in his younger years. But he is a man in a hurry. When I met him late in 2005, at Columbia's Lamont-Doherty Earth Observatory, his distinguished friend and collaborator Gerard Bond, a man a decade younger than Broecker, had recently died.

Broecker is a geochemist with an unimpeachable track record in pioneering the use of isotopic analysis to plot ocean circulation. He has been writing and thinking for more than three decades about what he calls the ocean conveyor, which more traditional scientists call the meridional overturning circulation or the thermohaline circulation. Whatever you call it, it is the granddaddy of all ocean currents, a thousand-year circulation with "a flow equal to that of a hundred Amazon rivers," as he puts it.

The conveyor begins with the strong northward flow of the Gulf Stream pouring warm, salty water from the South Atlantic across the tropics and into the far North Atlantic. In the North Atlantic, the water is cooled, particularly in winter, by the bitter winds blowing off Canada and Greenland. This cooling increases the density of the water, a process amplified by the formation of ice, which takes only the freshwater and leaves behind increasingly saline and dense water. Eventually the dense water sinks to the bottom of the ocean, generally in two spots: one to the west of Greenland, in the Labrador Sea, and the other to the east, down Wadhams's vertical chimneys. From there the water begins a journey south along the bed of the far South Atlantic, where a tributary, formed from cold, saline water plunging to the ocean bed around Antarctica, joins up. The conveyor then heads east through the Indian and Pacific Oceans before resurfacing roughly a thousand years later in the South Atlantic and flowing north again as the Gulf Stream to the far North Atlantic—where it goes to the bottom once more.

The circulation has many roles: distributing warm water from the tropics to the polar regions, mixing the oceans, and aiding the exchange of carbon dioxide between the atmosphere and the oceans. Along the way, it keeps Europe anomalously warm in winter. In Richard Alley's words, it "allows Europeans to grow roses farther north than Canadians meet polar bears." On the face of it, the circulation is self-sustaining. The operation of the chimneys draws Gulf Stream water north, which provides more water for the chimneys. But it is also temperamental, prone to switching on and off abruptly. That switch, says Broecker, is a vital component of the entire global climate system. Not everyone agrees on the nature of the switch and how much it matters, but he makes a persuasive case.

Broecker's picture of the ocean conveyor is disarmingly simple. Too simple, some say. He admits it had its origins in a cartoon. Asked by Natural History magazine to produce a diagram to illustrate a complicated argument about ocean-water movement, he drew a map with a few arrows suggesting likely "rivers" of intense flow within the circulation. "They sent it to an artist; he drew something, and I made a couple of corrections. I didn't realize it was going to be that important, but it was a popular magazine, and suddenly the diagram became a kind of logo for climate change."

Broecker is quite candid about the crudeness of the cartoon. But while some scientists might have disowned it, he recognizes its power and has embraced it. Its origins lie in Broecker's pioneering work using chemical tracers to identify movements of water in the oceans. He noticed that water in the Pacific and Indian Oceans appeared to be a mixture of water that had plunged to the depths in the North Atlantic and lesser amounts of water that had done the same thing around Antarctica. He could also see that water that had reached the ocean floor in the North Atlantic was largely made up of water that, prior to that, had made its way north as the Gulf Stream. To some extent, he filled in the rest. "The conveyor is clearly real," he insists. "But of course it's not as highly organized as it appears in the diagram." It is more a trend than a current—"a combination of random motions." And yet his cartoon has proved to be one of the most important concepts to emerge from climate science in the past quarter century.

Broecker chose the term "conveyor" because, he says, "I think names are very powerful, and that was much better than the proper scientific term. Some scientists say it is stupid, but laypeople can imagine a conveyor belt much more easily." He certainly has a way with words. Broecker was the first scientist to use the term "global warming," in a paper in the 1970s.

I first discovered the conveyor back in the late 1980s, while researching a book on environmental change. I was fascinated by the simplicity of the idea; by the fact that the conveyor might have two natural states, on and off; and by the scary possibility that climate change might shut the conveyor down if the ocean off Greenland became so flooded with freshwater that the dynamics of dense saline water formation around the chimneys broke down. For me, that idea was the first real inkling that climate change might not be as it was in the mainstream models—that the greenhouse effect might unleash something altogether nastier.

Early on, Broecker was often ambivalent about the potential for truly disastrous events. But by 1995, he felt confident enough to title a lecture on the conveyor to a big science conference "Abrupt Climate Change: Is One Hiding in the Greenhouse?" In it he outlined how evidence from seafloor and lake sediments, ice cores, coral, and glacier records "demonstrates unequivocally" that an on-off switch on the global conveyor operated at the beginning and the end of the last ice age. The suggestion was that the conveyor had shut down and single-handedly started the ice ages, lowering temperatures by "4 degrees C [7.2°F] or more. . . often within the lifespan of a generation"—a claim he inflated soon afterward, in the pages of Scientific A?nerican, to "10 degrees C [18°F] over the course of as little as a decade."

Broecker's picture, then, is of a powerful but fickle ocean conveyor with an on-off switch functioning in the far North Atlantic. Switched on, it warms the world, especially the Northern Hemisphere, and is typical of periods between ice ages. Switched off, it cools the Northern Hemisphere, and is typical of glaciations. But the system flickers at other times, too, he says. It triggered warm episodes that punctuated the depths of the last ice age, and perhaps drove more recent events such as Europe's medieval warm period and the little ice age. Broecker accepts that the ultimate forcing for these dramatic changes may lie in a celestial event like the slow movements of the Milankovitch cycles. But when a threshold is crossed and sudden climate change occurs, it is the conveyor that throws the switch.

These claims remain extremely controversial. Most would accept that Broecker is right that the conveyor slowed during the ice ages and probably shut down at various points. But most researchers believe that it was a consequence, and not a cause, of the glaciation. The big forces behind the cooling were the shift of carbon dioxide into the oceans and the spread of ice. And how important the ocean conveyor was in those processes has yet to be demonstrated. While the conveyor may have intensified cooling in the North Atlantic region, where the Gulf Stream is an acknowledged important feature in keeping the region warm, it is far less clear whether its global effects are anything like as big as Broecker claims.

But Broecker has rarely been bogged down in detail. Two years after making his claims for the ocean conveyor and the ice ages—and just a week before the world met in Japan to agree to the Kyoto Protocol—he was warning that climate change could trigger a future shutdown of the conveyor. "There is surely a possibility that the ongoing buildup of greenhouse gases might trigger yet another of those ocean reorganizations," he said. If it did, "Dublin would acquire the climate of Spitzbergen in ten years or less ... the consequences would be devastating." He called the conveyor the "Achilles heel of the climate system."

Broecker was also, I think, making a wider point. He wants to generate a change in the way we think about the planet. Climate systems work, he suggests, rather as Stephen Jay Gould said evolution worked: not gradually, through constant incremental change, but in sudden bursts. Gould's phrase "punctuated equilibrium" sounds right for Wally's world of climate, too. And his new paradigm also fits the science of chaos theory, in which his ocean conveyor is an "emergent property" in the wider Earth system.

But the crux of the public debate on Broecker's ocean conveyor remains a very simple question: Could global warming shut the conveyor down? Broecker seems rarely to have doubted it. And the claim has in recent years seemed almost to have a life of its own. This struck me most strongly at a conference on "dangerous" climate change held at the Hadley Centre for Climate Prediction, in Exeter in 2005. There I met Michael Schlesinger, of the University of Illinois at Urbana-Champaign. He is a sharp-suited guy sporting a pastiche of 1950s clothes and hairstyle. But if there were serious doubts in Exeter about whether his style sense would ever come back into fashion, there was no doubt that his ideas about climate change had found their moment.

For more than a decade, Schlesinger has been making Broecker's case that a shutdown of the ocean conveyor could be closer than mainstream climate modelers think. Some critics feel that he just doesn't know when to give up and move on. But he has stuck with it, criticizing the IPCC and its models for systematically eliminating a range of quite possible doomsday scenarios from consideration. "The trouble with trying to reach a consensus is that all the interesting ideas get eliminated," he said at the conference. Science by committee ends up throwing away the good stuff—like the idea of the conveyor's shutting down. But in Exeter, Schlesinger was back in vogue. He had been invited to present his model findings that a global warming of just 3-6°F would melt the Greenland ice sheet fast enough to swamp the ocean with freshwater and shut down the conveyor. The risk, he said, was "unacceptably large."

Although he had been saying much the same for a decade, he was now considered mainstream enough to be invited across the Atlantic to expound his ideas at a conference organized by the British government. And he was no longer alone. Later in the day, Peter Challenor, of the British National Oceanography Centre, in Southampton, said he had shortened his own odds about the likelihood of a conveyor shutdown from one in thirty to one in three. He guessed that a 3-degree warming of Greenland would do it. Given how fast Greenland is currently warming, that seems a near certainty.

But all this is models. What evidence is there on the ground for the state of the conveyor? The truth is that dangerous change is already afoot in the North Atlantic. And, whatever the skepticism about some of Broecker's grander claims, the conveyor may already be in deep trouble. Since the mid-1960s, says Ruth Curry, of the Woods Hole Oceanographic Institution, the waters of the far North Atlantic off Greenland—where Wad hams's chimneys deliver water to the ocean floor and maintain Broecker's conveyor—have become decidedly fresher.

In fact, much of the change happened back in the 1960s, when some 8 billion acre-feet of freshwater gushed out of the Arctic through the Fram Strait. Oceanographers called the event the Great Salinity Anomaly. To this day, nobody is quite sure why it happened. It could have been ice breaking off the great Greenland ice sheet, or sea ice caught up in unusual circulation patterns, or increased flow from the great Siberian rivers like the Ob and the Yenisey. Luckily, most of the freshwater rapidly headed south into the North Atlantic proper. Only 3 billion acre-feet remained. Curry's studies of the phenomenon, published in Science in June 2005, concluded that 7 billion acre-feet would have been enough to "substantially reduce" the conveyor, and double that "could essentially shut it down." So it was a close call.

With the region's water still substantially fresher than it was at the start of the 1960s, the conveyor remains on the critical list. Another single slug of freshwater anytime soon could be disastrous. In the coming decades, some combination of increased rainfall, increased runoff from the land surrounding the Arctic, and faster rates of ice melting could turn off the conveyor. And there would be no turning back, because models suggest that it would not easily switch back on. "A shift in the ocean conveyor, once initiated, is essentially irreversible over a time period of many decades to centuries," as Broecker's colleague Peter deMenocal puts it. "It would permanently alter the climatic norms for some of the most densely populated and highly developed regions of the world."

As I prepared to submit this book to the publisher, new research dramatically underlined the risks and fears for the conveyor. Harry Bryden, of the National Oceanography Centre, had strung measuring buoys in a line across the Atlantic, from the Canary Islands to the Bahamas, and found that the flow of water north from the Gulf Stream into the North Atlantic had faltered by 30 percent since the mid- 1990s. Less warm water was going north at the surface, and less cold water was coming back south along the ocean floor. This weakening of two critical features of the conveyor was, so far as anyone knew, an unprecedented event.

Probing further, Bryden found that the "deep water" from the Labrador Sea west of Greenland still seemed to be flowing south. But the volume of deep water coming south from the Greenland Sea, the site of Wadhams's chimneys, had collapsed to half its former level. The implication was clear: the disappearing chimneys that Wadhams had watched with such despair were indeed hobbling the ocean circulation. Broecker seemed on the verge of being proved right that the ocean conveyor was at a threshold because of global warming.

None of this demonstrated that Broecker's bleaker predictions of what would happen if the conveyor shut down were about to come true—that "London would experience the winter cold that now grips Irkutsk in Siberia." Something more like the little ice ages was the worst that most climate modelers feared. But there did seem to be a real possibility that many of Broecker's ideas were about to be put rather dramatically to the test.

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