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World History on Ice

From the outside, the storage shed on the University of New Hampshire (UNH) campus in Durham looks inconspicuous enough-a standard white 48-by-12-foot box. It doesn’t look too remarkable from the inside, either, housing a few electric jigsaws and racks holding thousands of cylindrical canisters filled with ice. This is not your average ice locker, however. It contains all the pieces of a two-mile strip of ice drilled from a massive ice sheet in Greenland. Moreover, this ice holds vital data about the earth’s climate over the past 250,000 years and offers the most detailed record yet of the last 110,000 years of our planet’s history.

“In some ways, the ice sheets tell us more about what the environment was like in northern latitudes 100,000 years ago than we can learn about the 1700s and 1800s from human records,” says Paul Mayewski, director of glacial research at UNH and chief scientist for the Greenland Ice Sheet Project Two (GISP2). “Those written records consist mainly of temperature readings, but we can use the ice to analyze 45 different variables.”

Mayewski views the ice sheets as a “time machine” that not only tells us about the earth’s history, including the effects of hundreds of volcanic eruptions, but also about human history. This frozen repository is providing a bounty of information to both earth scientists and archaeologists.

How can they extract so much information from ordinary chunks of ice? The Greenland ice sheets are composed of snow that falls to earth carrying compounds from the air, including chemicals, metals, dust, even radioactive fallout. The snow piles up layer by layer, year after year, trapping these substances. Pressure from the accumulating snow eventually creates ice, and bubbles that form in the ice seal off small samples of the atmosphere. In laboratories at UNH and elsewhere, scientists can precisely identify the yearly layers in the ice-like the rings in a tree trunk-to determine the composition of the atmosphere at that time.

Greenland’s frozen archives contain remarkable remnants of industrial enterprise over the ages. For instance, the record shows that the earliest large-scale pollution started about 2,500 years ago and continued for the next 800 years-the result of mining and smelting lead and silver during the Greek and Roman eras. In fact, lead pollution in that period rose to four times natural background levels, according to Claude Boutron, a French scientist whose team studied ice chunks from a parallel sampling effort, the European Greenland Ice-Core Project.

Other findings indicate that the decline of the Roman Empire was followed by a steady drop in lead pollution: lead concentrations in the ice cores fell during the Middle Ages and did not surpass the Roman levels until the start of the Industrial Revolution. An even sharper rise occurred in the twentieth century when lead concentrations rose to some 200 times natural (pre-Greek and Roman) levels, presumably owing largely to the introduction of lead additives to gasoline.

Other chemicals have also shown a dramatic upsurge. According to the ice core data, atmospheric concentrations of carbon dioxide climbed almost 30 percent, methane concentrations more than doubled, and concentrations of sulfate (a byproduct of coal combustion) have roughly tripled since the onset of the Industrial Revolution.

New pollutants began showing up in Greenland in the late-1950s-radioactive strontium-90 and cesium-137, fallout primarily from U.S., Soviet, and British nuclear testing programs. “This fallout reached a peak in 1963 and then dropped off with the signing of the atmospheric Test Ban Treaty later that year,” says Jack Dibb, a UNH scientist in the Glacier Research Group. “We still see little bumps in the 1970s and ’80s from tests by the Chinese, French, and perhaps some others we don’t know about.” More radioactive debris in the form of cesium-134 and 137 drifted to Greenland in May 1986 courtesy of the Chernobyl nuclear accident in the Ukraine. This radioactive cloud deposited isotopes in Antarctic ice, suggesting that the entire planet was contaminated by the core meltdown.

But the story the ice tells is not all bad. Concentrations of key pollutants (including lead) reaching Greenland have actually declined since the passage of the U.S. Clean Air Act in 1970 and the subsequent clamp-down on emissions. Still, over the 100,000-plus years these ice cores span, levels of carbon dioxide and methane, both greenhouse gases, have never been higher than they are today, says Martin Wahlen, a physicist at the Scripps Institute of Oceanography, and the magnitude of this human-induced change is truly remarkable. With respect to carbon dioxide and methane concentrations, he says, “humanity has brought about a change of roughly the same magnitude as that which naturally occurs between glacial and interglacial periods.” Whereas this natural shift took place over the course of tens of thousands of years, however, the human-induced change occurred within only the past few centuries.

One of the biggest surprises to emerge from the GISP2 project is the discovery of rapid climate shifts that occur within a time frame of decades or less. “We’ve shown, on at least eight separate occasions, that climate change has occurred abruptly as civilizations were developing in the last several thousand years,” Mayewski says. These changes can put people living in extreme environments-either very cold or arid-at risk. “If you live in a marginal area like that, a slight change in temperature or moisture can put you out of business.”

For example, Mayewski and Yale archaeologist Harvey Weiss have found a surprising correlation between a climatic “event” in 2,200 B.C., which resulted in extreme drought from Europe to India, and the collapse of the Mesopotamian Empire, which was based near a desert region in what is now Iraq. “That doesn’t mean climate change was the only factor, but it probably played some role,” Mayewski says.

Mayewski teamed up with archaeologist Tom McGovern of Hunter College and others to investigate a similar longstanding mystery regarding the disappearance of Norse settlers in Western Greenland beginning in the mid-1300s. “The core records indicate a really cold winter around the year 1350 and a series of progressively colder summers,” McGovern says. “The worst news for these people would have been a series of cold summers, which would have reduced an already short growing season, and that’s exactly what happened.” The climate, he adds, had always been suspected of playing a role in wiping out the settlement, but “we needed the new ice core data, which has a resolution on the scale of individual years and seasons, to really pin it down.”

McGovern next hopes to find out whether the widespread die-offs of mastodons, woolly mammoths, and other animals 10,000 years ago at the end of the Pleistocene era were due mainly to climate change or to human predation. “There’s been a tremendous debate in archaeology for years, and the Greenland data can finally help us resolve it.”

Mayewski expects that future studies will turn up many other associations between the climate events revealed in the ice sheets and major turning points in human history. The next step, he says, is to produce ice cores from other parts of the world-hence a deep-drilling program that began last year in Antarctica. The GISP2 collaborators are also beginning to compare the ice core data with corresponding climate records obtained from tree rings, lake sediments, and coral.

The key is not just to pool the data, McGovern says-“You really need to bring people together to form diverse teams,” and collaborations of this sort between climatologists, archaeologists, paleontologists, and historians are “opening up a whole new area” with tremendous potential. In terms of exploiting the body of information locked deep in the world’s ice sheets, Mayewski adds, “we’ve only begun to scratch the surface.

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