An arctic flower

Clues to a climate switchback t must have felt like the springtime of the world. Anybody living on Earth 13,000 years ago could only have felt elation. An ice age of some 80,000 years was coming to an end. Temperatures were rising; ice was melting; rivers were in flood; and permafrost was giving way to trees and meadows across Europe and North America. In the Atlantic Ocean, the Gulf Stream was pushing north again, bringing warm tropical water and reestablishing an ocean circulation system that had shut down entirely in the depths of the ice age. Westerly winds blowing across the ocean were picking up the heat and distributing it across Europe and deep into Asia.

Meanwhile, in the tropics, the deserts were in retreat, the rainforests were expanding again from their ice-age refuges, and the Asian monsoon was kicking back in. Most spectacularly, the Sahara was bursting with life, covered in vegetation and huge lakes. This was the dawn of the age of Homo sapiens, who had supplanted the last of the Neanderthals during the long glaciation. If there had been a Charles Keeling around, he would have measured rising atmospheric levels of carbon dioxide and methane that were amplifying the thaw. He might even have invented the term "global warming" to describe it.

Then the unthinkable happened: the whole thing went into reverse again. Almost overnight, the thaw halted and temperatures plunged. Temperatures became as cold as they had been in the depths of the ice age. The forests returning to northern climes were wiped out; the permafrost extended; and ice sheets and glaciers started to regain their former terrain.

The springtime seemed to be over almost before it began. But this reversal was not the first. The previous 5,000 years had been full of them. Some 18,000 years before the present, there was still a full-on ice age. By 16,000 years ago, the world was warming strongly. But by 15,000 years ago, it was cold again, with ice sheets reforming. At 14,500 years ago, it became so warm that within 400 years the ice caps melted sufficiently to raise sea levels worldwide by 65 feet. The cold gained the upper hand once more, only to give way to the pronounced warming of 13,000 years ago, which crashed again 12,800 years ago.

Today we can see this extraordinary climatic history recorded in ice cores extracted from the ice of Greenland and Antarctica. Graphs of the temperatures back then look like seismic readings during a big earthquake—or cardiac readouts during a heart attack. They show a climate system in a protracted series of spasms. Looking back, we recognize the death throes of the ice age. But that is with hindsight. At the time, there was little evidence that the climate system had any sense of direction at all. It lurched between its glacial and interglacial modes. The one thing it didn't do was settle for a happy medium.

The last great cold snap of the ice age, 12,800 years ago, is known today as the Younger Dryas era. The dryas is a white Arctic rose with a yellow center that suddenly reappeared in European sedimentary remains, indicating that the old cold reasserted itself. The era is called the Younger Dryas to distinguish it from the Older Dryas, the climate reversal of a thousand years earlier, and the Oldest Dryas, which came before that. The Younger Dryas, like the others, was swift and dramatic. Within about a generation, temperatures fell worldwide—perhaps by as little as 3 to 5°F in the tropics, but by an average of as much as 28 degrees farther north, and, according to ice cores analyzed by George Denton, of the University of Maine, by 54 degrees in winter at Scoresby Sound, in eastern Greenland.

Not only temperatures crashed. Records of Chinese dust and African lakes and tropical trade winds and South American river flows and New Zealand glaciers all reveal dramatic changes happening in step 12,800 years ago. The world was much drier, windier and dustier. But in the Southern Hemisphere, temperatures may have gone in the opposite direction. Marine sediment cores show dramatic warming in the South Atlantic and the Indian Ocean—as do temperature records in most Antarctic ice cores.

The Younger Dryas freeze lasted for fifty or so generations: 1,300 years. One can imagine tribes of Homo sapiens desperately relearning the crafts that got their ancestors through the ice ages. But it may also have triggered innovation. Some believe that dry conditions in the Middle East at the time may have encouraged the first experiments with crop cultivation and the domestication of animals. And then the freeze ended, and temperatures returned to their former levels even faster than they had fallen. Analysts of the Greenland ice-core chronology say publicly that the warming must have happened within a decade. But that is the minimum time frame for the change of which they can be certain, given the resolution of the ice cores.

Richard Alley, who was there handling the ice cores, says: "Most of that change looks like it happened in a single year. It could have been less, perhaps even a single season. It was a weird time indeed." Like The Day After Tomorrow, only in reverse.

All this is doubly strange, because the Younger Dryas cooling went against the grain of all the long-term trends for the planet. The orbital changes that had triggered the glaciation had faded by then; astronomical forces were pushing the planet toward the next interglacial era. Of course, the real work was being done by feedbacks like melting ice, the return of greenhouse gases like carbon dioxide and methane into the atmosphere, and the revival of the ocean conveyor. These feedbacks would have turned a smooth progression into a series of jumps. But they would not easily have altered the direction of change. So why the backward flip? What made climate plunge back into the icy abyss when all the forcings and all the feedbacks should have been kicking the world into warmer times?

Chaos theory may help here. Alley says that it is just when conditions are changing fastest that the chances for seemingly random, unexpected, and abrupt change are greatest. The system is stirred up and vulnerable. The drunk is on a rampage. And there is a reasonable chance that some of the abrupt changes will be in the opposite direction to that expected. This is what, in the clever subtitle to his 2001 report on abrupt climate change, Alley called "inevitable surprise." What is equally clear is that at the time, the entire planetary climate system had just two possible states: glacial and interglacial. It knew no third way. And so, during the several thousand years when it was on the cusp between the two, it flickered between them.

On the ground, one element was a sudden switch in Broecker's ocean conveyor. It would be going too far to say that the Younger Dryas proves that the global conveyor is the great climate switch that Broecker claims. But the event makes a compelling case that events in the far North Atlantic can, without help from astronomical or any other forces, sometimes have dramatic and long-lasting effects on global climate.

The unexpected switch of the ocean conveyor was almost certainly triggered by melting ice. In the final millennia of the ice age, as melting made fitful but sometimes dramatic progress, a very large amount of liquid water was produced. Often it did not pour directly into the oceans but formed giant lakes on the ice or on land around the edges. The largest known of these is called Lake Agassiz, after the discoverer of the ice ages. It stretched for more than 600 miles across a wide area of the American Midwest, from Saskatchewan to Ontario in Canada, and from the Dakotas to Minnesota in the U.S., generally moving with the advancing front of warming.

In the early stages of the déglaciation, the lake drained south, down the Mississippi River into the Gulf of Mexico. But about 12,800 years ago, it seems, something stopped this and forced the lake to drain east. Perhaps the route south was blocked by land gradually rising after the weight of the ice was removed. Perhaps the lake simply passed over a natural watershed as it moved north with the retreating face of the ice sheet. But at any rate, there was eventually a huge breakout of freshwater from the heart of North America into the basin now occupied by the Great Lakes, and on into the North Atlantic.

The vast inrush of cold freshwater would have drastically cooled and freshened the ocean. High salinity was critical for sustaining the newly revived, and perhaps still precarious, ocean conveyor. So a fresher ocean shut down the conveyor once more. The warm Gulf Stream was no longer drawn north. Temperatures crashed across the North Atlantic region, and probably particularly around Greenland. The entire global climate system would have been shaken, and may have lurched back from its interglacial to its glacial mode.

Little of this narrative is cut-and-dried. The evidence is patchy. Some doubt whether even a vast eruption of freshwater down the Saint Lawrence Seaway would have had much influence on ocean salinity on the other side of Greenland. And others, hard-line opponents of the Broecker hypothesis, wonder exactly how important the ocean conveyor is to global climate. Even Broecker admits that parts of the story are "a puzzle."

But new evidence is emerging all the time. One compelling rewrite of the Broecker narrative has come from John Chiang, of the University of California at Berkeley. His modeling studies of the North Atlantic suggest that the most critical event at the start of the Younger Dryas may have been not the shutdown of the ocean conveyor itself but the impact of the freshwater invasion on the formation of sea ice in the North Atlantic. He says that an invasion that diluted the flow of warm water from the Gulf Stream would have rapidly frozen the ocean surface. The freeze itself would have flipped a climate switch, preventing further deepwater formation, sealing out the Gulf Stream, and, through the ice-albedo feedback, dramatically chilling the entire region.

Broecker has adopted this idea as an elaboration of his conveyor scenario. Some others see it as a replacement or even a refutation. Alley says: "It looks like this is the real switch in the North Atlantic. In the winter, does the water sink before it freezes, or freeze before it sinks? Sink or freeze. There are only two possible answers. That's the switch." Fresher, colder water will freeze; warmer, more saline water will sink. If the water sinks, the conveyor remains in place and the Northern Hemisphere stays warm. If it freezes, the circulation halts and the westerly winds crossing the ocean toward Europe and Asia stop being warmed by the Gulf Stream and instead are chilled by thousands of miles of sea ice. "The difference between the two is the difference in places between temperatures at zero degrees Celsius [32°F] and at minus 30 degrees [-22°F]," says Alley.

And that switch flipped, Alley argues, at the start and the finish of the Younger Dryas. At the start, freshwater invaded the North Atlantic; the ocean froze, and within a decade "there were ice floes in the North Sea and permafrost in the Netherlands." The westerly winds would have picked up the cold of the Atlantic ice and blown it right across Europe and into Asia. They would have cooled the heart of the Eurasian landmass, preventing it from warming enough to generate the onshore winds that bring the monsoon rains to Asia. This revised narrative also explains the concurrent warming in the Southern Hemisphere. If the Gulf Stream was not flowing north, the heat that it once took across the equator stayed in the South Atlantic. So as the North of the planet froze, the South warmed. A freshwater release in northern Canada had become a global climatic cataclysm. One, moreover, that went against all the long-term trends of the time.

It took about 1,300 years before the North Atlantic water switched back to sinking rather than freezing in winter. There is no consensus on what finally flipped the switch. But when it happened, it was at least as fast as the original freeze. The North Atlantic no longer froze; instead, the water was salty and dense enough to sink. The ocean warmed; the winds warmed; temperatures were restored in a year; nature returned to reclaim the tundra; and deglaciation got back on track.

For some, this story is encouraging. If it takes huge volumes of cold water flowing out of a lake to switch off the ocean conveyor, they say, we should be safe. There are no unstable lakes around of the kind created by the melting of the ice sheets. In any case, the world is warmer today than it was even at the start of the Younger Dryas. It may be, says Alley, that the world climate system is much more stable in warm times than in cold times. But equally it may not. For one thing, the superwarm world we are creating may contain quite different perils. For another, even the old perils may not have been neutralized as much as optimists think.

There is a cautionary tale in what happened 8,200 years ago. Despite large amounts of warming after the demise of the Younger Dryas cold event, the ice had one last hurrah. Again there was a large intrusion of cold freshwater into the North Atlantic. Again there was a big freshwater release; again the ocean was covered by ice; and again there seems to have been a disruption to the global conveyor. This was a lesser event than the Younger Dryas—probably only regional in its impact on climate, and lasting for only about 350 years. But it was nonetheless one of the biggest climate shifts of the past 10,000 years. And perhaps most significant for us today, says Alley, it happened in a world markedly more like our own than that of the Younger Dryas. Temperatures were generally rather close to those of today, and the ice sheets were quite similar. The event suggests, if nothing else, that if sufficient freshwater were to invade the North Atlantic today, it could have a similar impact.

As we have seen, in recent decades large slugs of freshwater have poured into the far North Atlantic. They may have come close to triggering a shutdown of the ocean conveyor. This trend is unlikely to end. As the climate warms and the permafrost melts in Siberia, river flows from there into the Arctic Ocean are rising strongly. And there is always the prospect of future catastrophic melting of the Greenland ice sheet, where glaciers are accelerating and lakes are forming.

Gavin Schmidt, one of Hansen's climate modelers at the Goddard Institute for Space Studies, says that the event 8,200 years ago is a critical test for today's climate models. "If we are to make credible predictions about the risks we run today of catastrophic climate change, those models need to be able to reproduce what happened 8,200 years ago," he says. "If we could do that, it would be really good. It could tell us a lot about processes highly relevant for the climate of the twenty-first century."

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