With Speed and Violence: Why Scientists Fear Tipping Points in Climate Change (27 page)

Many would argue that all the natural variability in climate that Thompson is uncovering offers a soothing reminder that the planet and human society are no strangers to climate change. Not Lonnie. His analysis is uncovering invisible thresholds in the climate system, he says. Cross them, and the whole system goes into a spin, with dramatic cooling or warming, great droughts and the El Nino flip, turned full on or full off for centuries at a time. Should we not be just as concerned that carbon dioxide might send us above a threshold? If that happens, he says, "we won't get gradual climate change, as projected; we will instead get abrupt change."

And, of course, Thompson is tracking with concern the role of modern climate change in melting his glaciers. Back in 1976, he took a core of the ice at the summit of Quelccaya. It showed layers of ice laid down annually for 1,500 years. In 1991, when he returned to update the record, he found that the annual accumulation had stopped and the top 20 yards of ice had melted away-dramatic evidence of a recent and sudden shift in an ancient ice cap's fortunes. In the valley below, Quelccaya's largest glacier, the Qori Kalis, is retreating by 500 feet a year and has lost a fifth of its area since 1963. Across Peru, a quarter of the ice surface has disappeared in thirty years. Elsewhere in the Andes, Bolivia's Chacaltaya lost two thirds of its ice in the 199os, and Venezuela has lost four of its six glaciers since 1975•

In Africa, where 8o percent of the ice on Mount Kilimanjaro has melted away in ninety years, Mount Kenya has lost seven of its eighteen glaciers since 19oo; and most of the ice on the Rwenzori Mountains between Uganda and Congo has gone, too. Across the Indian Ocean, on New Guinea, the West Meren glacier vanished altogether in the late 199os, and its neighbor Carstensz has shrunk by 8o percent in sixty years. Thompson has seen the same trends in the Himalayas and Tibet. Glacial retreat, he says, "is happening at virtually all the tropical glaciers." In some places, there may be local factors. Occasionally, declining snowfall will do the damage. But he insists that while snowfall in high altitudes may be critical to getting a glacier started, it is rarely critical to the glacier's demise, which starts lower down the slopes. Globally, he says, there can be no explanation for the universal disappearance of glaciers other than global warming.

Thompson believes that he has only begun to explore the potential of his ice cores to answer questions about the tropics. He wants to take cores from ice still attached to the Nevado del Ruiz volcano, in northern Colombia. The mountain exploded in 1985, engulfing 20,000 people in a landslide of ash. "I think we could get a record of how often that volcano erupts," he says, apparently oblivious of the risk for researchers in such an expedition. He believes that the ice of Quelccaya can offer a history of fires and drought in the nearby Amazon. And he is looking at dust from China that has collected in ice in Alaska. It is already providing a history of pesticide use in China, and may eventually reveal whether dust out of Asia, as well as that from the Sahara, could have fertilized the soils of the Americas.

Thompson believes that by uncovering the secret climate history of the tropics, he is helping to strip climatology of an unhealthy fixation with what happens close to the homes of the researchers-in the North Atlantic: "An important reason why we think that Greenland and those places are so important is because so much research has been done thereand that is mainly because it is more convenient than going to Tibet or Patagonia." He believes that that fixation is diverting researchers from where the real climatic action is-in the tropics, in the world of El Nino and the Asian monsoon and megadroughts and the dramatic feedbacks that dried up the Sahara, which he sees as "at least as important as anything Wally Broecker has cooked up on the North Atlantic."

To Thompson, it has always seemed obvious that "the global climate is driven from the tropics." Most of the surface of Earth is in the tropics, he says. "It is where the majority of the heat reaches Earth, and from where it is distributed around the globe. It is where the great climate systems like the monsoon and El Nino are based." He argues that truly global climatic events can start only where heat and moisture can be delivered both north and south around the globe. There may be feedbacks operating in the North Atlantic or around Antarctica. But the big drivers must be in the tropics.

Thompson has his own heroes. Mercer is one. Another is James Croll, the lowly Victorian Scot who worked his way through life as a waiter, a school caretaker, and a carpenter so that he could research the astronomical forces behind the ice ages. And Thompson has simple advice for young scientists: plow your own furrow. "Go somewhere or do something that nobody else has even thought about working on." Some academics from the wrong side of the tracks would have settled quietly into faculty life, thankful for their social advance. Not Lonnie. He does research the hard way. "On one trip we were up on Quelccaya for three months. We had to cut the ice cores by hand into 6,ooo samples, take them downhill on our backs, and then melt them and put the water in bottles sealed with wax." On another occasion, he found himself in New Zealand dangling on a rope above 2,000 feet of empty space.

Years ago, a student in the field with Thompson died of the aftereffects of altitude sickness. His father sued. That still hurts. Thompson would be the last professor on Earth to send his students somewhere he wasn't prepared to go himself. He is still prepared to live for months under canvas in freezing cold and lung-achingly thin air. Just turning sixty when we met, he was recently back from his biannual trip to the Andes, and his calendar included upcoming trips to Kilimanjaro and central Africa's "mountains of the moon." He had tentative plans for expeditions to the last glaciers in New Guinea and a Siberian island near where the last mastodon froze to death 5,000 years ago.

He told me he reckoned that his techniques could one day help uncover the remains of life in the ice caps of Mars. And I swear that his eyes lit up when I suggested he might be on the first flight to the red planet.

29

THE CURSE OF AKKAD

The strange revival of environmental determinism

Around 4,200 years ago, the world's top empire was run by Sargon, the despotic but otherwise unexceptional ruler of the Akkadian empire. Some have called this the first true empire in the world. Certainly it seemed to be a new form of society, created out of a number of previously autonomous city-states on the floodplains of the Tigris and Euphrates Rivers in Mesopotamia. Its rule extended all the way from the headwaters of the two rivers, in Turkey, across much of Syria and as far south as the Persian Gulf. But Sargon's empire had been in business for only a century or so when it suddenly collapsed. Archaeologists initially put this down to an invasion of barbarian hordes from the surrounding mountains. But an energetic field archaeologist called Harvey Weiss, of Yale, changed that rather lazy assumption-and with it changed much else about our perceptions of the rise and fall of past civilizations.

In the late 1970s, while working in Syria, Weiss discovered a "lost city" beneath the desert sands, close to the Iraqi border. Over more than a decade he excavated the remains of the settlement, named Tell Leilan. He pieced together the story of a highly organized city that had grown over several thousand years from a small village to a prosperous outpost of the Akkadian empire. But there was a mystery. It appeared that for some 300 years, the city had been abandoned and its streets had filled with wind-blown dust.

Weiss tied the events at Tell Leilan to a contemporary cuneiform text titled "The Curse of Akkad," which recorded a great drought in which the fields of most of northern Mesopotamia were abruptly abandoned. The granaries emptied, the fruit trees died in the orchards, and even the fish departed as the great rivers dried up. Refugees flooded south. The people of southern Mesopotamia built a hundred-mile wall to keep them out. Archaeologists had previously dismissed "The Curse of Akkad" as mythology. The idea that climatic and other environmental change determined the progress of societies had been hugely out of fashion. The prevailing view was that politics, economics, wars, and dynasties made and broke empires, and that climate was just a more or less benign backdrop.

But Weiss was convinced that only a massive shift in climate could explain a Soo-year collapse, after which the climate apparently recovered enough for the northern plains to be settled once more. When he published his findings, they provoked consternation in the archaeological community but huge interest among climate scientists-not least Peter deMenocal, of Lamont-Doherty. `After Weiss's publication, environmental determinism had a huge revival," deMenocal says. Especially after it emerged that the dust storms of Mesopotamia were part of a wider process of aridification right across the Middle East and beyond, which had seen off other societies, too.

In New York, deMenocal was working with a student, Heidi Cullen, on analyzing a core of marine sediment drilled from beneath the Gulf of Oman, 1,500 miles south of Tell Leilan. They decided to look for evidence of dust storms in the core. "We thought the dust might be visible there, and Heidi started to go through it," he told me. "It was very painstaking work, and to be honest, she was about to give up. Then boom. One day she found it. The Soo-year layer of dust, dated at 4,200 years ago, and much of it clearly derived from Mesopotamia. We sent it to Harvey, and he was ecstatic."

The news spread. Lonnie Thompson and his team went back to their tropical ice cores and found similar layers of black dust. "It was a huge global dust spike," he said. In the ice on the summit of Kilimanjaro, in East Africa, there is only one dust "spike" in the 12,ooo-year record. And it occurs right at 4,200 years ago, he said. On the other side of the planet from Syria, at Quelccaya, in Peru, the same period produced "the biggest dust event in the ice core in a 17,ooo-year record." Fallout of dust onto the glacier was a hundred times as much as normal levels. "And it shows up in the Asian monsoon region of the Himalayas, too," says Thompson's dust analyst, Mary Davis.

From Lake Van, in eastern Turkey, to the Dead Sea, in Palestine, and in Africa from Kenya to Morocco, water levels fell by tens or even hundreds of yards 4,200 years ago. Civilizations were ending everywhere. In Egypt, those years produced a collapse of order that marked the break between the Old and Middle Kingdoms. "On the tombs of the Pharaohs, their histories talk of expansion until 4,200 years before the present, when there were droughts and mass migrations and sand dunes crossing the Nile," says Thompson. In Palestine, the situation was even worse, according to Arie Issar, an Israeli hydrologist and the author of a detailed study of climate change and civilization in the region. The level of the Dead Sea dropped a hundred yards. "All the urban centers were abandoned, and the cities, which had existed for several hundred years, remained only as large heaps of ruins. They were not resettled until nearly half a millennium later." Farther east, in the Indus Valley of modern-day Pakistan, the urban centers of the Harappan civilization collapsed at the same time.

What caused all this? Nobody is sure. Jeffrey Severinghaus, of the Scripps Institution of Oceanography, has found tantalizing evidence of a dust signal in the Greenland ice cores 4,200 years ago. But instead of more dust than before, he found less. There was also a decline in sea ice in the North Atlantic. This has been interpreted as evidence of a change in the ocean conveyor. Did Broecker's conveyor drive things once again? On the face of it, that interpretation looks unlikely. For on this occasion, rather as during the great climatic disruption of 5,500 years ago, events in the North look like mere ripples flowing out from much bigger events in the tropics.

It is more evidence, says deMenocal, that climate switches may lurk in the tropics at least as much as at the poles. Richard Alley again reaches for common ground. Perhaps, he says, the Arctic feedbacks were at their height during and immediately after the ice ages, but lost their influence once most of the ice had gone. During the height of the Holocene, at least, perhaps the tropics ruled. But if so, what is driving the feedbacks in the tropics? Where are the tropical equivalents of Broecker's conveyor, Alley's "sink or freeze" switch, and Juergen Mienert's clathrate gun?

30

A CHUNK OF CORAL

Probing the hidden life of El Nino

Some researchers have a way of combining business with pleasure. Not for Dan Schrag, the Harvard geochemist, the arduous journeys into thin, cold air on tropical glaciers. Back in 1997, he was on his fourth trip to the paradise islands of the East Indies in search of ancient coral. One day, he was sauntering along a beach on Bunaken Island, a speck of old atoll off the Indonesian island of Sulawesi. "We had had a glorious dive, during which we saw a huge school of barracuda," he remembers. "We stopped for lunch, and I took a walk down the beach, behind the mangrove swamp. It was the last day of the trip. We had failed to find anything useful, and I was preparing to go home. Then I saw this massive coral head on the beach, incredibly well preserved." He chiseled out a piece and headed for the plane.

Back in the lab at Harvard, Schrag discovered that this fossilized piece of coral was 125,000 years old and contained sixty-five years' worth of growth rings that gave a brief window on the climate of the western Pacific back before the last ice age. It was a "fantastic discovery," he says. "I guess I got really, really lucky." The coral he had found was the first piece ever located that was large enough and well enough preserved to give a good snapshot of ancient El Ninos. What's more, says Schrag, it came from a region that is in the "bull's-eye" of El Nino, in the heart of Indonesia. His preprandial discovery is helping transform our understanding of El Nino's place in the climate system.

Until recently, climatologists looked on El Nino as a minor aberration in the tropical Pacific, of only passing interest to the wider world. But in the past two decades it has become the fifth horseman of the Apocalypse, a bringer of devastating floods, fires, and famine from Ethiopia to Indone sia to Ecuador, and a sender of weird weather around the world. It has been appearing more frequently, and with effects that are much more violent and last longer. Its current level of activity is unparalleled in the historical record. Yet the historical record doesn't go back far, so nobody has been sure whether this is a perfectly normal upturn or an alarming consequence of global climate change. Schrag's coral has helped provide some answers. It makes a strong case that global warming is already having a profound effect on what climatologists are coming to regard as the flywheel of the world's climate.