Tropical high

Why an ice man is rewriting climate history

There are two special things about Lonnie Thompson. First, doctors reckon that he has spent more time on mountains above 20,000 feet than any other lowlander on the planet. And second, in his freezer back home in Columbus, Ohio, he has probably the most detailed physical record anywhere of the climate of planet Earth over the past 20,000 years. Not bad for the sixty-year-old son of a hick from Gassaway, a tiny railroad town in West Virginia.

Make that three things. Because Thompson is, in a mild-mannered but determined way, a revolutionary in the world of glaciology. For four decades now, climate scientists have been drilling ice cores in the polar regions to find the secrets of climates past. They have found a lot, and they have developed some impressive theories about how the world's climate system is driven from these cold wastelands. But thirty years ago, Thompson, then still a graduate student in the geology of coal with a temporary post drilling ice cores in Antarctica, set out to prove them wrong about the origins of climate.

With his early mentor, the legendary British glaciologist John Mercer, Thompson ignored the poles and began drilling ice cores in glaciers high in the Andes, the Himalayas, and other mountain regions of the tropics. This was unheard of at the time. Finding funding was hard, because nobody had a budget for such work. But in the years since, he has uncovered a new, entirely unexpected world of tropical climate change. And now, after fifty expeditions to five continents, and with 20,000 feet of ice cores stored in his freezer, he believes he is on the path to proving that the true triggers and drivers and Achilles heels and thresholds and tipping points for the world's climate lie in the tropics.

For men like Broecker, this is sacrilege. But although Thompson's case is not yet proven, he has found some unexpected fans. Richard Alley, a career member of the "polar school," is an admirer of the senior from Gassaway. He told me with a smile: "Lonnie is a legend, and he may well turn out to be right." Whether he is right or not, Thompson's ice cores and the data he has painstakingly extracted from them are the lifeblood of an emerging debate between the polar and tropical schools—a debate that might not be happening at all without him.

Thompson is a loner. He has always avoided the big organizations and funding bodies that dominate so much climate science. Sometimes that has been out of necessity; now he sees it as a virtue. It has given him the freedom to do and think things his way. With his researcher wife, Ellen Mosley-Thompson, he set up a small team at the Byrd Polar Research Institute, part of Ohio State University. "We started small and we try to be self-contained," he says. "That makes us flexible. We don't have to stand in line for analysis of cores, or for supplies. And we have our own workshops to make everything."

The Thompsons build their own lightweight drills and photovoltaic generators, because these are the only means of getting the right gear by horseback onto the high slopes of the world's tallest mountains. And they have their own four automatic mass spectrometers, working 24 hours a day 365 days a year to analyze the samples brought back from around the world. Thompson doesn't even trust the big science institutions to look after his ice cores when he's gone. With the prize money that has come his way in recent years, he has created a trust fund to keep the freezers going in perpetuity.

Being independent means he can pack his bags and head around the world on a whim if he thinks there is an ice core to be had. Back in 1997, he took advantage of a brief thaw in diplomatic relations between Moscow and Washington to fly to Franz Josef Land, in the Siberian Arctic. There he extracted a thousand feet of ice from near an old Russian nuclear bomber base, and persuaded the bomber pilots to fly it back to Moscow for him. More recently, after years of stonewalling by the Tanzanian authorities, he took his drilling kit on a tourist flight to Dar es Salaam and smooth-talked his way up Kilimanjaro to extract vital evidence of the demise of its ice cap. Thompson has spent half a lifetime taking his ice pick, crampons, and drilling gear to the Andes and the Himalayas, Tibet and the Russian Arctic, Alaska and East Africa. Back in Columbus, he has interrogated the ice and the bubbles of air trapped inside for signs of dust, metals, salts, and isotopes of oxygen and carbon to discover not just temperatures and rainfall but the comings and goings of El Nino events, forest fires, droughts, and monsoons.

His first love, he says, is Quelccaya, the first ice cap he scaled in Peru with John Mercer. It is the one he keeps going back to. He can see the whole world evolve there, he says, from the revival of El Ninos in the Pacific around 5,500 years ago to the decades of drought that finished off the pre-Columbian Moche empire; from the first record in the tropics of the little ice age to the recent isotopic signature of global warming. Here and elsewhere across the tropics, he has also found a dust "spike" in the ice that shows that dust storms were sweeping across the tropics 4,200 years ago—evidence, it seems, of a sudden near-global megadrought.

Most intriguing for glaciologists, Thompson's collection of worldwide ice cores has revealed a previously unknown pattern in the formation of glaciers across the tropics. The pattern seems to be independent of the great glaciations that waxed and waned in the polar regions. It seems instead to follow latitude, starting in the Southern Hemisphere close to the Tropic of Capricorn, where he has found evidence that glaciers began to form in Bolivia 25,000 years ago. Then, as if by clockwork, other glaciers began to form and grow farther north. One by one, they started through Peru and Ecuador. Then, 12,000 years ago, a continent to the east but following the same northward trajectory, an ice cap began to form at the summit of Kilimanjaro, on the equator. Skipping north again to the Himalayas, around 8,000 years ago, glaciers started to grow near the Tropic of Cancer. Across three continents, glacier formation was oblivious of longitude or the equator or anything else. Latitude ruled.

Why? Thompson has tied this extraordinary progression to the precession, the wobble in Earth's orbit that gradually alters the line of latitude where the most intense solar heating occurs. This is the same wobble that sustained the African monsoon over the Sahara when the sun was overhead there in the early Holocene, but snuffed out the rains as the sun moved on. In the mountains of the tropics, glaciers generally started where the sun was fiercest. The sun was most intense over the Tropic of Capricorn 25,000 years ago and then moved north, becoming most intense over the Tropic of Cancer. It appears to have triggered the formation of glaciers all the way.

On the face of it, this seems odd. Why would the harshest sun and hottest temperatures create glaciers? Thompson has a simple explanation. The zone of maximum sun in the tropics is also the zone of maximum rainfall, which in the highest mountains means the zone of maximum snow. Up there, he says, it has always been cold enough for glaciers to form. So temperature is not an issue. What the high valleys have often lacked is moisture to feed the growth of glaciers. The sun brought the moisture, and with it the snow and the glaciers.

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 1990s, and Venezuela has lost four of its six glaciers since x975-

In Africa, where 80 percent of the ice on Mount Kilimanjaro has melted away in ninety years, Mount Kenya has lost seven of its eighteen glaciers since 1900; 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 1990s, and its neighbor Carstensz has shrunk by 80 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 col- lected 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 there—and 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 6,000 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 chat his eyes lit up when I suggested he might be on the first flight to the red planet.

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 300-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 300-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 12,000-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 17,000-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?

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