With Speed and Violence: Why Scientists Fear Tipping Points in Climate Change (23 page)

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 3o 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."

25

THE PULSE

How the sun makes climate change

The Arctic pack ice extended so far south that Eskimo fishing boats landed on the northern coast of Scotland. They didn't meet much opposition, because the hungry Highlanders had abandoned their crofts after grain harvests had failed for seven straight years, and had gone raiding for food in the lowlands to the south. In the 169os temperatures in Scotland were more than 3°F below normal; snow lay on the ground long into the summer. Those who stayed behind were reduced to eating nettles and making bread from tree bark. The political repercussions of this Scottish turmoil are still with us today. The king became so worried by fears of insurrection that he shipped off angry clansmen and their starving families to set up Presbyterian colonies in Catholic Northern Ireland. And eventually, after widespread famine in the 169os brought despair about the future for the Scots as a nation, the clan chiefs forged a union with England.

This was the little ice age: a climatic affair that began early in the fourteenth century and flickered on and off before peaking in the late seventeenth century and finally releasing its grip some 150 years ago. Like a mild echo of the ice ages, it spread its icy fingers from the north across Europe, pushing Alpine glaciers down valleys, creating spectacular skating scenes for the Dutch painters Breugel and Van der Neer, and allowing Londoners to enjoy the frolics of regular frost fairs on the frozen River Thames. On one occasion, Henry VIII traveled by sleigh down the river to Greenwich, and on another an elephant was led across the ice near Blackfriars Bridge.

There were some warm periods amid the cold. In the 1420s, an armada of Chinese explorers is reputed to have sailed around the north coast of Greenland, a journey that would be impossible even in today's reduced Arctic ice. Between about 144o and 1540, England was mild enough for cherries to be cultivated in the northeastern Durham hills. Much of Europe was exceptionally warm in the 173os. But at the height of the little ice age, the Baltic Sea froze over, and there was widespread famine across northern Europe. Some suggest that half the populations of Norway and Sweden perished. Iceland was cut off by sea ice for years on end, and its shoals of cod abandoned the seas nearby for warmer climes. Some say the cold was the hidden hand behind the famine, rising grain prices, and bread riots that triggered the French Revolution in 1789.

In North America, tribes banded together into the League of the Iroquois to share scarce food supplies. The Cree gave up farming corn and went back to hunting bison. But the era was symbolized most poignantly by the collapse of a Viking settlement founded in the balmy days of the eleventh century by Leif Erikson. The Viking king had a real-estate broker's flair for coining a good name: he called the place Greenland to attract settlers. The settlement on the southern tip of the Arctic island thrived for 400 years, but by the mid-fifteenth century, crops were failing and sea ice cut off any chance of food aid from Europe.

If the Viking settlers had followed the ways of their Eskimo neighbors and turned to hunting seals and polar bears, they might have survived. But instead, they stuck to their hens and sheep and grain crops, and built everbigger churches in the hope that God would save them. He did not. When relief finally arrived, nobody was left alive in the settlement. Creeping starvation had cut the average height of a Greenland Viking from a sturdy five feet nine inches to a stunted five feet. The last women were so deformed that they were probably incapable of bearing a new generation. We know all this because their buried corpses were preserved in the spreading permafrost.

The little ice age, first documented in the 196os by the British climate historian Hubert Lamb, is now an established part of Europe's history. It has often been seen as just a historical curiosity-a nasty but local blip in a balmy world of European climatic certainty. But it is increasingly clear that what Europe termed the little ice age was close to a global climatic convulsion, which took different forms in different places.

Because it came and went over several centuries, the task of attributing different climate events around the world to the influence of the little ice age is fraught with difficulties. But reasonable cases have been made that it blanketed parts of Ethiopia with snow, destroyed crops and precipitated the collapse of the Ming dynasty in seventeenth-century China, and spread ice across Lake Superior in North America. In the tropics, temperatures were probably largely unchanged, but rainfall patterns altered substantially. In the Amazon basin, the centuries of Europe's little ice age were so dry that fires ravaged the tinderbox rainforests. In the Sahara, which often seems to experience climate trends opposite to those in the Amazon, repeated floods in the early seventeenth century washed away the great desert city of Timbuktu.

The little ice age is not the only climate anomaly in recorded history. Another, known because of its influence on European climate as the medieval warm period, ran from perhaps Boo to 1300, ending just as the little ice age began. Because it is rather more distant than the little ice age, its history and nature are rather less clear. Certainly, at various times grains grew farther north in Norway than they do today, and vineyards flourished on the Pennines, in England. Warmth brought Europe wealth. There was an orgy of construction of magnificent Gothic cathedrals. The Vikings, as we have seen, set up in Greenland at a time when parts of it could certainly be described as green. Some claim that the medieval warm period may have been warmer even than the early twenty-first century. But most researchers are much more cautious.

Reconstructions of past temperatures come mainly from looking at the growth rings of old trees. There are exceptions, but generally, the wider the rings, the stronger the annual growth and the warmer the summer. Keith Briffa, a British specialist in extracting climate information from tree rings, says: "The seventeenth century was undoubtedly cold. The evidence that the period 1570 to 1850 was also cold seems pretty robust. But the medieval warm period is still massively uncertain. There is not much data, and so much spatial bias in the data. We think there was a warm period around AD goo, certainly at high northern latitudes in summer, where we have the tree-ring evidence. But we have virtually nothing else." It looks likely that much of Europe was between 1.8 and 3.6'F warmer in the medieval warm period than it was in the early twentieth century, while the little ice age was a similar amount cooler in Europe. But any global trends were almost certainly much smaller.

In any case, to talk about a medieval warm period at all is, in the view of many, a very Eurocentric view. Tree rings from the Southern Hemisphere show no sign of anything similar there. Indeed, away from the North Atlantic, those centuries were, if anything, characterized by long superdroughts that caused the collapse of several major civilizations. In Central America, the Mayans had thrived for 2,000 years and built one of the world's most advanced and long-lasting civilizations. Theirs was a sophisticated, urbanized, and scientific and technologically advanced society of around io million people, with prolific artistic activities and strong trade links with its neighbors, and seemingly every resource necessary to carry on thriving-strikingly like our own in many respects. Yet faced with three decades-long droughts between the years Boo and 950, which may have been the worst in the region since the end of the ice age, the entire society crumbled, leaving its remains in the jungle. A few hundred miles north, a number of advanced native North American societies collapsed under the impact of sustained droughts through the American West. Best documented are the Anasazi people, ancestors of the modern Pueblo Indians. They had built elaborate apartment complexes in the canyons of New Mexico, and had developed sophisticated irrigation systems for growing crops, but were forced to flee into the wilderness after a long drought that peaked in the 128os.

The little ice age and the medieval warm period appear to have been recent natural examples of climate change. Though the warming and cooling implied in their names may have been restricted largely to the North Atlantic region, they seem to have left a signature in glaciers and megadroughts across the planet. So what caused them? And does it have anything to tell us about our own future climate? Many theories have been advanced.