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Thin Ice

Unlocking the Secrets of Climate in the World's Highest Mountains

Henry Holt and Company

THE MOUNTAIN GOD

The Aymara are descended from the Tiwanaku, a mysterious pre-Incan people who once ruled an empire encompassing most of present-day Chile, Peru, and Bolivia from a city on the shores of Lake Titicaca, an inland sea on the high Altiplano. Like their ancestors, many of today's Aymara revere the snow-covered mountains as gods, for they produce the rare trickles of water that make life possible in this inhospitable land. The Aymara believe water is to the land as blood is to the body — the sacred essence of life and fecundity — and that mountain spirits show their wrath through ferocious storms or by holding the water back, in the sky. In the eerie, looming presence of Sajama, one can understand and even share in this belief.

Modern society also treats Sajama with respect. Commercial jets are forbidden from flying overhead, for the mountain's core contains an enormous magnet that may twist the needle of a compass more than twenty degrees away from true magnetic north.

When Lonnie Thompson arrived in small Sajama village, a few miles west of the mountain, with twenty-odd scientists, a fleet of outfitters from La Paz, the five Peruvian mountaineers who have supported his high-altitude work for decades, and six tons of equipment, the villagers decided they did not want him disturbing the local deity. Similar visitors from the University of Massachusetts, Lonnie's collaborators, had set up a satellite-linked weather station on the summit the previous year, and the subsequent growing season had been poor. The villagers were afraid a larger expedition might induce more severe retribution. Impassioned negotiations ensued.

Bernard Francou remembers the bargaining as the most engaging cultural experience of his six years in Bolivia. It fell to Lonnie to convince the people of Sajama — more baleful an assemblage than any grant review committee back in the States — that his work was relevant to their own lives.

He told them that the ice on their mountain could give him information not only about Bolivia's changing climate but also about El Niño, the mysterious change in the weather that visits the Altiplano every two to seven years. Their mountain and their dry vegetable plots, which provide a marginal existence even in the best of years, feel El Niño's effects quite directly.

Since it generally appears at around Christmastime, the phenomenon was named for the Christ child by anchovy fishermen living less than a thousand miles northwest of here, in the coastal villages of Peru and Ecuador. They noticed that the cold waters that they fished, normally one of the most productive fisheries on earth, rose in temperature with El Niño, and the anchovy disappeared. This disrupts the entire aquatic food chain and the economy based upon it. Seabirds, which feed off the anchovy and produce the mountains of guano that the local farmers use as fertilizer and export all over the world, disappear as well. And while the sea withholds its bounty, it drives huge thunderheads into the skies above the equatorial coastal plain, normally one of the driest places on earth, producing storms, flash floods, and the occasional deadly landslide.

Simultaneously, more than nine thousand miles to the west, New Guinea, one of the wettest places on earth, experiences drought, as do Australia and the Indonesian archipelago. The Asian monsoon generally fails, while, farther west, the Horn of Africa receives torrential rain.

El Niño's sister, La Nina, who usually visits on his heels, makes Peru even drier than usual and New Guinea even wetter. This climatic seesaw, which scientists have named the "El Niño/Southern Oscillation," or ENSO, spans the Pacific and has immediate collateral effects nearly everywhere on earth.

And ENSO is just one — though perhaps the most notorious — of the myriad oscillations and seesaws known to climatologists. Seesaws and oscillations, either in time or in space — some real, some perhaps imagined and the subject of intense debate — are the basic stuff of climatology. A cycle may span years, as with El Niño, decades, several centuries, or hundreds of millennia. Only a few have El Niño's global reach.

To find a particular cycle, a climatologist must generally tease it out from among dozens of others. Consider the motion of the sun across the sky, which can change temperature by as much as sixty degrees in a single day; the hourly or weekly changes we call weather; the seasons; subtle changes in our orbit around the sun, having periods of tens or hundreds of thousands of years; sunspots and other cycles in solar output — and all of these have different strengths at different latitudes and longitudes. It's like a kid playing with a yo-yo, bouncing on a pogo stick, in a car on springs, zooming along on a roller coaster — worse, actually.

The devastation wrought by the last few El Niños has made the phenomenon a household name in recent years. Its east-west, transpacific seesaw has become relatively well known; but Sajama, in Bolivia, stands at the southern end of a second, lesser-known, north-south seesaw with equatorial Peru. When El Niño brings storms to Peru, Sajama experiences drought; when La Niña brings drought, Sajama receives rain.

AT THE NEGOTIATING TABLE, LONNIE EXPLAINED THAT IF HE COULD FIND BETTER WAYS to predict the onset and strength of these climatic siblings he might help the people of Sajama choose appropriate crops for the corresponding wet and dry years. For some inscrutable reason, however — perhaps out of caution, for they are not yet entirely confident in their predictions — he and Bernard did not reveal that the Southern Oscillation Index was dropping and an El Niño was on the way.

But they did point out that a global change of another sort was taking place, that Sajama's glacier was retreating (as the locals had already observed), and that every glacier in the tropical zone will probably disappear within fifty years. Sajama lies just inside the southern edge of the tropics — the main reason Lonnie wanted to drill there. Its high glacier should outlast the others, which are generally lower and nearer the equator, but the odds are that it, too, will disappear while the children, playing as he spoke in the streets of that village, are still alive. And since Sajama's snow is their primary source of water, its disappearance stands to have a dramatic effect upon their lives.

Lonnie happened to have some idea just how dramatic that effect might be. Two ice cores he had recovered fifteen years earlier, on the summit of the Quelccaya massif on the northern Altiplano, in Peru, had shown that climate change had played a major role in the births and deaths of numerous Andean civilizations five hundred and more years ago — the ancestors of these very people among them.

Archeologists have long known that the Tiwanaku abandoned their city by the lake shortly after A.D. 1000. They had no written language, and there was no explanation for their demise until the Quelccaya ice cores revealed that a severe drought had set in on the Altiplano just before the Tiwanaku disappeared. The drought persisted for about four hundred years, and when it withdrew, the first Aymara city-states emerged. Furthermore, the drought commenced about a century after a warm spell began on the Altiplano, and its span coincides with an event that is sometimes called the Medieval Warm Period, in which temperatures in many parts of the world, including northern Europe, rose for about four centuries. Based on this evidence, one might expect the present warming to cause another devastating drought in this arid land.

You can tell just by looking that the people of the Altiplano are tough — not threatening, strong in the face of adversity. The Aymara of today grow the same crops, tuned to the Altiplano's cold, dry climate, that the Tiwanaku farmed more than a thousand years ago: tubers like potato, oca, olluco, and mashwa, and the unique, cold-adapted grains quinoa and caniwa. Their way of life and animist belief system have withstood the Incas, the Spanish conquistadors, and — at least until now — the lure of the city. But a few bad years might change that. Already El Alto, the slum by the airport on the Altiplano above La Paz, is swelling from a steady trickle of migrants across the high, windswept plain.

WHEN THE SCIENTISTS FINALLY WORKED OUT A DEAL WITH THE PEOPLE OF SAJAMA, A delegation from the village joined them at base camp to conduct a ceremony similar to those that had taken place ten and more centuries earlier in the city of Tiwanaku. They gathered together in a circle around a bonfire and passed around an endless supply of coca and legía, chasing them with shots of grain alcohol. (One scientist testifies that his head went completely numb.) Two yatiri, or holy men, offered songs and prayers to Sajama and Pacha Mama ("Lady Earth"), seeking fertility for their families and fields and forgiveness for the transgressions they planned. Lastly, the yatiri laid a white llama on a smooth stone, used before for this purpose, and slit its throat. They spilled blood, the magic elixir, into a ritual ceramic cup. Singing songs and scattering figurines of humans and animals in the fire, they sprinkled the animal's blood on the sand.

Lonnie reached the summit two days later.

AN ISLAND IN THE SKY

About three weeks after the llama sacrifice, I met a geochemist from Penn State named Todd Sowers as he passed through base camp on his descent from the summit. He had left some gear at the intermediate, nineteen-thousand-foot High Camp, so he invited me to join him as he climbed back up to retrieve it. This gave me a chance to take another step toward acclimatization and get an introduction to climatology at the same time.

High, healthy Andean glaciers receive 80 to 90 percent of their snow in the austral summer, from November to April. During the dry season in between, the wind lays a thin layer of dust and pollen on the snow. Viewed edgewise — that is, down the walls of a crevasse or along a recently exposed margin — an Andean glacier resembles layer cake. Puffy layers of clean, white snow alternate with dark, paper-thin dust bands. Annual layers can be counted by eye, and years of low snowfall are obvious. Thus, a high, healthy glacier archives climate in the manner of tree rings.

There is some art to the estimation of annual snowfall, however, because compression and the lateral flow of the glacier will cause layers to thin as they recede into the depths, buried by layers from subsequent years. At a certain depth, the compressed snow, called firn, packs to ice. The depth of this firn — ice transition varies from a few inches to hundreds of yards.

An ice core is harvested in cylindrical segments up to eight inches in diameter and one meter (or three feet) long. If there are no discontinuities caused by flow (easily identified), and the drill doesn't break down or stick in the hole (not an uncommon occurrence), a core will sample progressively older layers of ice from the surface of a glacier all the way to the bedrock or sand at its base. Detailed information about climatic conditions at the time each layer fell may be inferred by looking closely at the chemistry and structure of the ice and at the impurities it contains, either chemical or particulate — dust and pollen, for instance, sticks or leaves, even frozen bugs. Todd Sowers's specialty is analyzing the air bubbles that are trapped as the airy firn packs to ice.

These bubbles serve as samples of the overlying atmosphere from a time soon after the enclosing ice fell as snow. The reason for the time lag is that air from above the surface will diffuse into cavities in the firn until it is compressed enough to seal those cavities. They are usually sealed within one or two hundred years, but in areas of low accumulation and extreme cold, such as Antarctica, the process may take a few thousand years. Estimates of the close-off time can be made, but now that climatology is beginning to focus on fine details, such as where past changes began and how they spread geographically, this subject can lead to strenuous (and sometimes quite personal) disagreement. In the subtler points of climatology, as in other aspects of life, timing is turning out to be everything. As Ellen Thompson puts it, "Without a timescale, it's like trying to build a body without a skeleton."

One, perhaps obvious, use of the bubbles is to estimate the atmospheric levels of greenhouse gases in climates of the past. And, as Todd tossed out matter-of-factly on our hike up Sajama, ice cores from the polar regions, particularly those from Russia's Vostok Station in Antarctica, show that atmospheric temperature has risen and fallen in concert with atmospheric carbon dioxide for the past half-million years. To Todd and his colleagues, the greenhouse effect is not a theory, it is an experimental fact. And to the overwhelming majority of them, the debate as to whether fossil fuel burning stands to make the earth noticeably warmer in the near future ended years ago. This is not one of those fine details.

More interesting to Todd is the fact that he can use the air bubbles to estimate the age of the ice. If the average amount of atmospheric methane, a powerful greenhouse gas, were to change, for instance, it would take a year or two for the new level to mix completely in the air all over the earth — the blink of an eye in paleoclimatic terms. In the mid-nineties, Todd was involved in measuring methane levels in the two-mile-long GISP2 (Greenland Ice Sheet Project 2) core from the summit of the Greenland ice cap, in which visible annual layers date back more than forty thousand years (and a fleet of dedicated graduate students has been on hand to count them). Subject to the uncertainty in bubble close-off time and layer counting — and since layers do sometimes merge with one another or sublime or blow away, the count can be off by a few percentage points — the curve of methane concentration against time in the air above Greenland can be used as a standard to construct timescales for ice cores from other regions. One simply stretches or squeezes the timescale for the methane curve from the unknown core until its peaks and valleys match up with GISP2.

I didn't catch everything Todd said as we ventured briefly above the eighteen-thousand-foot mark and into the death zone that day. He grabbed my notebook once or twice and made a few graphs and scrawls, which I studied later. In the methane graph, he drew a large dip at about twelve thousand years ago and labeled it "Younger Dryas," the first time I'd seen or heard that term. The Younger Dryas was a cold snap that occurred a few thousand years after the most recent ice age began to loosen its grip. The atmosphere began to heat up about eighteen thousand years ago and then, during the Younger Dryas, dropped again for almost two thousand years. I see the name "Lake Agassiz" also in my notes and recall that Todd was the first to tell me a now-familiar story about the huge lake that lay on the vast, receding ice sheet that once covered North America.

As this climatological tale goes, Lake Agassiz surged through a depression in the wall of the melting continental ice sheet near what is now the St. Lawrence River Valley, on Canada's eastern coast; when the resulting pulse of cold, freshwater entered the northern Atlantic Ocean, it supposedly shut off a massive ocean current, called the great conveyor (another phrase scrawled in my notebook), which carries warm equatorial water north along the surface of the Atlantic and was thought until very recently to be the source of the excess heat that makes northern Europe and Scandinavia so warm relative to similar latitudes in North America. The shutting off of the conveyor, so this line of conjecture goes, sent the whole earth into a cold spell. When Lake Agassiz had sufficiently drained, the conveyor picked up again, and temperatures gradually rose to the levels they had reached before the Younger Dryas. This marked the beginning of the present climatic epoch, which is known as the Holocene.

This story is probably the most popular in the folklore of modern climatology and has seen quite a bit of play in the popular press. It has even generated a feature-length movie, The Day After Tomorrow. Unfortunately, however, the work that Lonnie was doing just then on Sajama would show that the story probably isn't true.

The story of Lake Agassiz and much else in the field of ice core climatology, including the techniques Todd has helped develop to understand the gas bubbles, have arisen mainly from the huge drilling efforts, involving small armies of scientists and specialized support crews, that have been undertaken in Greenland and Antarctica. The polar ice sheets contain 99 percent of the permanent ice on the earth's surface and 95 percent of all the freshwater.

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Excerpted from Thin Ice by Mark Bowen. Copyright © 2005 Mark Bowen. Excerpted by permission of Henry Holt and Company.
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