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Chronicling the Ice

Long before global warming became a cause célà¨bre, Lonnie Thompson was extracting climate secrets from ancient glaciers. He finds the problem is even more profound than you might have thought

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Thompson is starting the arduous climb to the drilling camp, located high on an ice-filled corridor between two glaciers. He moves steadily but slowly, drawing his breath in ragged gasps. Every now and then he pauses to bend at the waist, as if taking a bow. It's a trick, he says, for easing the burden that high altitude places on the heart.

He stops at the top of a tower of rocks deposited by a past advance of ice. Directly below is the glacier he plans to climb. "It'll be a walk in the park," Thompson says, panting. Shortly, he moves off, clambering across the ice-mauled debris that limns the glacier's course. "That's what you said last time," I yell after him.

My husband and I decide to head back to Lhasa.

Thompson's team would spend two weeks on the glacier and pull out three cores, one more than 500 feet long (contained in about 140 tubes), representing thousands of years of the glacier's and the atmosphere's history. Then, because their permits had expired, they returned to Lhasa, entrusting their Chinese colleagues with getting the ice off the glacier. It was not an easy task. The first truck hired to haul the cores the 900 miles back to Lhasa never showed up. Porters and yak herders threatened to quit. A two-day snowstorm struck. A second truck choked on the thin air; to keep it running, its drivers had to inject oxygen into the engine from a bottle fetched from Lhasa.

About two months after leaving Tibet, I step into the cavernous freezer at OSU's Byrd Polar Research Center. The temperature stands at minus 30 degrees Fahrenheit. Here, stashed on steel shelves, are thousands of shiny aluminum tubes that hold Thompson's collection of ice cores. Organized by expedition, the tubes measure a meter in length; their caps bear an identifying set of letters and numbers.

My companion, graduate student Natalie Kehrwald, is making a first pass through the Naimona'nyi cores, and even though she's dressed in a wool hat and down jacket, she doesn't linger long in the freezer. Pulling out the tube she wants, she dashes from the freezer to a small anteroom that, mercifully, is some 50 degrees warmer. There she pulls out a cylinder of ice and places it on a light table. This section of the core contains subtly alternating bands of clear and cloudy ice. The transparent bands mark intervals of high precipitation, while the more opaque bands signify drier, dustier times. The pattern is strangely beautiful.

Kehrwald examines other lengths of ice. One, from a depth of about 365 feet, is filled with fine air bubbles, which often form under extremely cold conditions. Another, from an even greater depth, contains ice so clear it looks like glass. But it's the ice from closer to the surface that causes the most excitement, for some of it contains intriguing dark flecks that may be fragments of insects or plants—remains that can provide solid rungs in the ladder of time.

Thompson's Andean ice, for example, contains ash from known volcanic eruptions, like Huaynaputina's in southern Peru in a.d. 1600. It also incorporates organic detritus that can be radioactively dated. In 1998, Thompson found the remnants of a 6,000-year-old insect in the ice he wrested from a dormant Bolivian volcano. In 2004 and 2005, he recovered 5,200-year-old marshland plants from the Quelccaya ice cap's shrinking edges. Insects and plants near the top of an ice cap or glacier are not so important, since the upper layers bear stripes that reveal the years like tree rings. But establishing dates becomes critical deep in the core, where the weight of overlying ice squeezes annual layers of snow so close together they seem to merge. Just a smattering of independently derived dates from organic material would help nail the Tibetan timelines to the wall.

As Thompson looks at his cores across a long fetch of space and time, he sees what appears to be a wavelike sweep of ice growth proceeding south to north across the Equator. This pattern, Thompson says, bears a striking correspondence to a 21,500-year astronomical cycle. Known as the precessional cycle, it derives from the fact that the earth, like a child's top, wobbles as it spins, altering the time of year in which the Northern and Southern hemispheres come closest to the sun. That, in turn, affects precipitation patterns, including the strength of monsoons.

The precessional pattern is still at work, says Thompson, but its influence is becoming harder to detect. "To me this is what makes our world today seem so different from the past," he muses. "If nature alone were in charge, then glaciers should be growing in the lower latitudes of one hemisphere and retreating in the lower latitudes of another. But that's not what's happening." As he sees it, the fact that glaciers and ice fields are dwindling virtually everywhere constitutes the clearest sign yet that rising concentrations of greenhouse gases are profoundly damaging the natural system.

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