The pulse

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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 1690s 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 1690s 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 1440 and 1540, England was mild enough for cherries to be cultivated in the northeastern Durham hills. Much of Europe was exceptionally warm in the 1730s. 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 ever-bigger 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 1960s 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 800 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 900, 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 10 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 800 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 1280s.

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.

The pendulum moves too fast for any orbital cycles. Some theorists have suggested a role for volcanic eruptions, which shroud the planet with aerosols that can cool it. It is true that at certain times during the little ice age, there were major eruptions. The year after the eruption of Tambora, in Indonesia, in 1815, crops failed from India to Europe and North America. It became known as "the year without a summer." But volcanic dust clouds cool temperatures for only a few years at most. They may from time to time have exacerbated the cooling, but they were not sufficiently frequent or unusual to explain a cold era that lasted on and off for almost half a millennium.

Most climatologists believe that the sun should get the blame. The coldest part of the little ice age, in the mid-to-late seventeenth century, is known as the Maunder Minimum. The popularizing of the telescope by Galileo a few decades before meant that astronomers of the day were able to note the virtual disappearance between 1645 and 1715 of the by-then-familiar spots on the surface of the sun. This is now recognized as a good indicator of a reduced output of solar energy. The best guess is that solar radiation reaching Earth's surface during the Maunder Minimum fell by perhaps half a watt per 10.8 square feet, or around 0.2 percent. But climatologists find it perplexing that such a widespread effect could result from such a modest change.

Enter an idiosyncratic, larger-than-life researcher working at the Lamont-Doherty Earth Observatory, just down the corridor from Wally Broecker. His name was Bond, Gerard Bond. Like Broecker, he hated getting bogged down in detail, and liked seeing the big picture. Like Broecker, he was willing to fly a kite, trusted his intuition, and had the confidence to propose an idea in public just to see if anyone could shoot it down. And, again like his compatriot, he had the intellectual reputation to get his kite-flying published in the often conservative scientific literature.

Bond argued forcefully until his death, in 2005, that the little ice age and the medieval warm period were the most recent signs of a pervasive pulse in the world's climatic system. This pulse, he said, had a cycle that recurred once every 1,500 years or so. It was a pulse, moreover, that seemed largely unaffected by other, apparently bigger influences on global climate, like the Milankovitch orbital cycles that triggered the major glaciations. Ice age or no ice age, he argued, the pulse just kept on going. Bond didn't invent the pulse out of thin air. Other researchers had unwittingly been on its trail for years. But, like his friend down the corridor, Bond was the man who had the confidence to compose a big picture out of the scattered fragments of evidence.

In the early 1980s, a graduate student in Germany made the first breakthrough. While at the University of Gottingen, Hartmut Heinrich was examining cores of sediment drilled from the bed of the North Atlantic. He found a number of curious layers of rock fragments that showed up in cores drilled as far apart as the east coast of Canada, the waters west of the British Isles, and around Bermuda. Radiocarbon dating revealed that these rock fragments were laid down in at least six bands over the 60,000 years before the end of the last glaciation, at intervals of roughly 8,000 years.

I looked at some of these rock fragments in the marine sediment store at Bond's old laboratory in New York. They are enormously distinctive. A browse among the trays of sediment revealed fairly subtle differences among the different cores: a change of color here, a slightly different consistency of dust there. Almost everything in these sediments has gone through the mill of being eroded from Earth's surface, discharged down rivers, and dumped in tiny bits on the seabed. But then there are Heinrich's layers. These are a mass of stones the size of gravel or pebbles, but sharp-edged and clearly untouched by the normal processes of erosion and deposition. Researchers soon gave the events that produced them their own name: Heinrich events. There was nothing like them in the sediment record.

Apart from their size and shape, something else was odd about these rock fragments. Though they had been found way out in the middle of the Atlantic Ocean, geologists swiftly established that they came from the Hudson Bay area of northern Canada. How could they have got so far offshore and so far south? What took them there? The only logical explanation, given that all the Heinrich events took place during the last glaciation, was that they had been ripped from the bedrock by great glaciers and carried south on the underside of icebergs. They traveled a long way because the North Atlantic was extremely cold, and were eventually dumped onto the ocean floor as the icebergs melted. That raised other questions.

What climatic events would send vast armadas of icebergs sailing south into the tropics? And why the apparent 8,000-year cycle?

The next clue came a few years later, in the early 1990s, when a distinguished Danish glaciologist, Willi Dansgaard, of the University of Copenhagen, discovered in the Greenland ice-core record a series of large and sudden temperature changes that again punctuated the last glaciation. Several times, temperatures leaped up by 3.6 to i8°F within a decade or so, before recovering after a few hundred years. So far, more than twenty of these warm phases have been identified in the ice-core record. During many of them, temperatures in Europe at least may have been as warm as today.

These warming events, too, seemed to have some kind of periodicity or pulse. Temperatures moved from cold to warm and back again repeatedly, with a cycle ranging between 1,300 and 1,800 years. It was a recognizable pulse, just as a human pulse that races and then slows is recognizable, and averaged a full cycle roughly every 1,500 years. This pulse also swiftly got a name, the rather cumbersome Dansgaard-Oeschger cycle, after Dans- gaard and his Swiss colleague, Hans Oeschger. Some interpret the data as showing a continuous background temperature cycle that on most but not all occasions triggered a more substantial warming episode during its warm phase, and on rather fewer occasions triggered a Heinrich event during its cold phase.

The connection between Heinrich events and the Dansgaard-Oeschger cycle wasn't recognized immediately — understandably enough. They had different time signatures, and one was revealed in the sediments of the mid-Atlantic, while the other emerged from the Greenland ice cores. Both, in any case, seemed at first to be minor local curiosities confined to the last glaciation, and therefore of no relevance to modern climate. But Bond had a hunch that the two were linked in some way, and that they had a global significance. Both, he noted, appeared to coincide with other climate changes, such as the advances and retreat of glaciers in Europe and North America. Like the Younger Dryas event and the climate flip 8,200 years ago, they seemed either to push the world into a different climate mode or to be part of such a process. Down the corridor, Bond's buddy Broecker was on hand to suggest a possible link to the ocean conveyor. The story began to take on a life of its own. But first the pair needed evidence to back up their hunch.

Bond began to re-examine trays of sediment cores from the bed of the North Atlantic that were assembled in his New York archive. Some were old cores, taken years before by the Lamont-Doherty research vessel Verna from beneath the waters off Ireland and the channel between Greenland and Iceland. Others were new, drilled off Newfoundland under Bond's supervision.

As expected, Bond found further evidence of Heinrich's rock fragments roughly every 8,000 years or so through the last glaciation. But the marine sediment cores also revealed lesser layers of materials normally alien to the seabed of the North Atlantic. Most exciting of all, these lesser layers occurred roughly every 1,500 years, and appeared to coincide with the cold phase of the Dansgaard-Oeschger cycle in the Greenland ice cores. This was pay dirt. Doubly so when it became clear that the iceberg armadas of the Heinrich events occurred during unusually cold phases of the Dansgaard-Oeschger cycle. The pattern seemed to involve a large Heinrich event, followed by five less and less severe 1,500-year Dansgaard-Oeschger cycles, and then another big Heinrich event. Sometimes this stately progression is influenced by other cycles, such as a solar precession, but otherwise it seems to hold.

Most remarkable of all, perhaps, Bond found that although there have been no Heinrich events during the 10,000 years since the end of the last ice age—the last was 15,000 years ago—the marine imprint of the underlying 1,500-year pulse has not missed a beat. "The oscillations carry on no matter what the state of the climate," he said.

Bond died in 2005, at the age of sixty-five. His longtime colleague Peter deMenocal has continued his work, looking for more signs of the pulse. Examining seabed sediments off Africa's west coast, he has found that every 1,500 years or so there were huge increases in dust particles in the sediments, suggesting big dust storms on land. The sediments also revealed dramatic increases in the remains of temperature-sensitive marine plankton, suggesting a temperature switchback in tropical Africa of as much as 9°F. "The transitions were sharp," deMenocal says. "Climate changes that we thought should take thousands of years to happen occurred within a generation or two."

Bond's final claim, that the pulse can be seen in recurrent climatic events right through to the present, seems to be vindicated, especially by temperatures in Europe and North America. There was an especially strong cooling event in the Northern Hemisphere that ended around 2,000 years ago; it was replaced by the medieval warm period that reached its height perhaps 1,100 years ago, and then by another cold era that bottomed out around 350 years ago, during the Maunder Minimum—when temperatures fell by up to 3-6°F in northern Europe, and the Eskimos reached Scotland in their kayaks.

Bond's study was an extraordinary piece of detective work. But it raises more questions than it answers. Two stand out. What, if any, is the relationship between these cycles and other parts of the climate system, such as Broecker's ocean conveyor? And, of course, what causes the mysterious pulse?

Heinrich originally argued that his ice armadas must be the result of some instability in the North American ice sheet that caused periodic collapses into the North Atlantic. There might thus be some link to big freshwater breakouts like that which triggered the Younger Dryas event. Certainly they involved huge amounts of ice. But the timing is fuzzy. Bond argued that while instabilities in the ice sheet could explain Heinrich events, only some of his pulses produced Heinrich events. So instability in ice sheets is unlikely to explain the pulses themselves, which in any case seem to have been unaffected by glaciations. By 2001, Bond believed he had confirmed the answer that many suspected all along.

He went back to the Greenland ice cores to look for evidence of solar cycles. There is no known direct marker for solar cycles in the cores. But other researchers had discovered that isotopic traces of cosmic rays bombarding the atmosphere were left in the ice cores—and that when solar radiation is at its most intense, cosmic rays are literally blown away from the solar system. Thus fewer "cosmogenic" isotopes, like carbon-14 and beryllium-10, are left in the ice cores during periods of strong solar radiation.

Bond came up trumps again. The evidence tallied. Over the past 12,000 years, fluctuations in detritus from the iceberg armadas in the Atlantic coincided with changes in cosmogenic isotopes in the ice cores. Thus there was a solar pulse that translated into a pulse in icebergs, global temperatures, and recurrent climatic events found through both the glacial and the postglacial eras.

Bond was convinced before his death that most climate change over the past 10,000 years had been driven by his solar pulse, amplified through feedbacks such as ice formation and the changing intensity of the ocean conveyor. He worried that people might interpret this as showing that global warming was natural. "But that would be a misuse of the data," he told me in an interview shortly before his death. Rather, he said, the most important lesson from his research is what it shows about the sensitivity of the system itself: "Earth's climate system is highly sensitive to extremely weak perturbations in the sun's energy output." And if it is sensitive to weak changes in solar forcing, it is likely to be sensitive also "to other forcings, such as those caused by human additions of greenhouse gases to the atmosphere."

What, exactly, drives the amplifications is another matter, however. For years, as Bond worked on his ideas, Broecker had declared that the Dansgaard-Oeschger temperature cycle in Greenland was linked to fluctuations in his ocean conveyor. Certainly the geography seemed right. Both appeared to originate in the far North Atlantic. It seemed clear, too, that during the periods when ice armadas were floating south in the Atlantic, temperatures in the North Atlantic were cold, and the amount of deep water being formed around Greenland declined. In extreme cases—perhaps during full-scale Heinrich events—the conveyor probably shut down. Perhaps a reduction in solar radiation triggered the entire sequence. But the evidence of what caused what was largely circumstantial. And as we will see later, there is another explanation, producing a large amplification from another quarter entirely.

But whatever the amplifier, the pulse is real and extremely pervasive. In the postglacial era, perhaps only in the past fifty years has something come along with greater power to disrupt climate.

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