A Geologist's Obsession with the Past

December 19, 2009|


Photo courtesy Dorthe Dahl-Jensen.

The Rockefeller complex of Copenhagen's Neils Bohr Institute—a golden-hued brick building—would fit well in a Hans Christian Andersen fairy tale. Geology professor Dorthe Dahl-Jensen works there in a small office by a gable, under the building's steeply sloped tile roof. Here in Copenhagen she's dressed fashionably, in matching black skirt, blouse and jacket. She has a pageboy haircut and wears a string of pearls and a tiny, round COP15 lapel pin. I last saw Dahl-Jensen in July 2003, on the Greenland Ice Sheet. Back then, standing on a mountain of ice nearly two miles thick, she wore utilitarian heavyweight fleece and a windproof shell. It was snowing lightly. Tendrils of windblown hair brushed her face. At that time she was chief scientist of the pan-European ice-drilling team that had just obtained a sample of the most ancient ice ever cut from a glacier in the Northern Hemisphere—about 120,000 years old. There had been a water pocket at the very bottom of the hole. When the drill bit had reached the liquid—pressurized by the massive weight of the ice sheet—a muddy geyser had shot about 100 into the shaft and had frozen to the cold drilling tools, coating some the equipment with a brown deposit. Dahl-Jensen still stores ice from that hole in a walk-in freezer near her office. Today she is chief scientist of a research team seeking samples that are more ancient still. They see ice that formed from snow that fell in Greenland thousands of years before the last ice age began—during Earth's previous warm period prior the current warm spell during which civilization formed—the past epoch geologists call the Eemian.

It is no exaggeration to say that some scientists are obsessed with the Eemian period. Dahl-Jensen is an example. She has camped in sub-freezing weather for weeks at a time almost every summer for years, coveting a cylindrical hunk of sparkling Eemian ice the diameter of a loaf of bologna. A team of Americans tried twice without luck twice in the 1990s to obtain Eemian ice. Dahl-Jensen came achingly close to completing her quest in 2003, after a marathon 7-year drilling project, but the oldest sample she got was not quite ancient enough. By an unfortunate twist of fate, the drill hole had been located right above a hot spot in the granite foundation on which the ice sheet sits. Eemian age ice had melted into a puddle at the bottom.

For more than one million years, Earth has descended into an ice age every 100,000 years with the precision of a beating heart. During ice ages, global temperatures plummet and massive ice sheets cover much of Europe, Asia and North America. In warm periods, the planet heats up and, apart from the Greenland Ice sheet, the continent-size glaciers in the northern hemisphere melt. This much has been known for about a century. In the last several decades, scientists have become increasingly interested in learning details about previous warm periods—especially the Eemian—as evidence has grown more certain that greenhouse gasses are making our current warm period ever warmer. Not long ago a scientist I met said grimly that humans are conducting an uncontrolled, irreversible experiment. Sadly, he added, the experiment can't be halted or started over if the trial goes awry. But similar experiments were conducted before, naturally, in the warm periods between every ice age, the Eemian being the most recent example. By studying what happened during this earlier period, Dahl-Jensen hopes to shed new light on Earth's future climate. Researchers already know that Earth's average temperature was about 2 degrees warmer than today during the Eemian. The planet is predicted to reach this level during the 21st century. Recently a group that included Dahl-Jensen published a paper in the journal Science that showed that large parts of the Greenland Ice Sheet that are ice covered today had melted then, and had released enough water to raise the sea by about 6 feet. If sea level rose by six feet today, cities would be swamped and millions of people would be displaced; the humanitarian and economic effects would be huge. "We will reach or exceed the Eemian temperature rise in the next 100 years," says Dahl-Jensen. She adds though, that scientists don't know how long it would take the ice to melt once Eemian-level temperatures are reached. She says it could take thousands of years, though it could also happen much faster.

In the last ten to twenty years, scientists studying ice cores from Greenland have discovered past changes in the climate they have labeled abrupt. In many instances during and right after the most recent ice, Greenland's average temperature jumped almost 30 degrees Fahrenheit in a few decades, then, almost as suddenly, it plummeted. Dahl-Jensen says the ancient ice she seeks will show whether abrupt events can also occur during warm periods as well as during ice ages. If so, she says, we might experience an abrupt change in our future, perhaps one precipitated by global warming. If such an incident occurred today, it would disrupt global weather patterns and threaten the physical safety and food supplies of people around the world. Dahl-Jensen and her team return to Greenland next summer. If drilling goes as planned, she will hold piece of Eemian ice in her hands before she returns home to Copenhagen.

This story was reported for the Pulitzer Center on Crisis Reporting as part of the Copenhagen News Collaborative, a cooperative project of several independent news organizations. Check out the feed here from Mother Jones.