With Speed and Violence: Why Scientists Fear Tipping Points in Climate Change (21 page)

There may have been other feedbacks at work to push the planet into ice ages and drag it back out again. Methane may have been important. Its atmospheric concentration is in lockstep with temperature apparently as fixedly as that of carbon dioxide. One likely explanation is that the and ice ages dried up wetlands and reduced their emissions of methane. Likewise, a colder atmosphere would have contained less water vapor-which would also have amplified the cooling.

A final amplifier may have been the ocean circulation system, with its huge ability to move heat around the planet. There is good evidence that the circulation system slows down during ice ages, and may have shut down entirely at the coldest point in the last glaciation. This is the province of a legend in the climate debate, Wally Broecker, and we will return to it in the next chapter.

The study of the ice ages suggests that over the past couple of million years at least, the natural climate system has constantly returned to one of two conditions. One is glaciated; the other is interglacial. The former has an atmosphere containing around 44o billion tons of carbon dioxide; the latter has an atmosphere containing about 66o tons. The planet oscillates between the two states regularly, repeatedly, and rapidly. But it doesn't hang around in any in-between states.

The evidence, says Berrien Moore III, the director of the Institute for the Study of Earth, Oceans, and Space, at the University of New Hampshire, "suggests a tightly governed control system with firm stops." There must be negative feedbacks that push any small perturbation back to the previous position. But there must also be strong positive feedbacks. Once things go too far, and the system seems to cross a hidden threshold, those positive feedbacks kick it to the other stable state. Each time, the guiding feedback seems to have rapidly moved about 220 billion tons of carbon between the atmosphere and the ocean.

That appears to have been the story for about the past two million years -until now. For the first time in a very long time, the system is being pushed outside this range. In the past century or so, human activity has moved another 22o billion tons of carbon into the atmosphere, in addition to the high concentrations of the interglacial state. The atmosphere now contains twice as much carbon as it did during the last ice age, and a third more than in recent interglacial eras, including the most recent. And we are adding several billion tons more each year. This extra carbon in the atmosphere has not been part of recent natural cycles. It comes mainly from fossilized carbon, the remains of swamps and forests that grew tens of millions of years ago.

This addition of carbon to the atmosphere is perhaps the biggest reason why Earth-system scientists feel the need to talk about the Anthropocene era. We are in uncharted territory. And the big question is: How will the system respond to this vast injection? Where will the carbon end up? There seem to be three possibilities. First, as some optimists hope, the system may deploy negative feedbacks to suppress change. Perhaps an accelerating biological pump in the ocean might remove the carbon from the atmosphere. It is possible. But the oceans generally like it cold. And there is no sign of such negative feedbacks kicking in yet, nor any obvious reason why they might. If anything, the biological pump has slowed in recent years.

The second possibility is the one broadly embraced by most climate models and the scientific consensus of the IPCC. It is that the system will carry on operating normally, gradually accumulating the carbon and gradually raising temperatures. There will be no abrupt thresholds that launch the climate system into a new state. This is moderately comforting, and fits the standard computer models, but it is contrary to experience over the past two million years.

And that raises a third possibility. Many Earth-system scientists think that their climate-modeling colleagues have not yet got the measure of the system. They fear that we may be close to a threshold beyond which strong positive feedbacks take hold, as they do when Earth begins to move between glacial and interglacial eras. The feedbacks may flip the system into a new, as-yet-unknown state. Most likely it would be one with much higher atmospheric concentrations of carbon dioxide and methane-more like the early days on planet Earth. That state might mean an era of huge carbon releases from the soil, or massive methane farts from the ocean floor, or wholesale changes to the ocean circulation system, or the runaway melting of the ice caps. That is conjecture. We simply don't know. But hold on to your hat: we could be in for a bumpy ride.

23

THE OCEAN CONVEYOR

The real day after tomorrow

Wally 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-therecord 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 argu ment 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 r98os, 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 American, to "1o 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.