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Photo captions by
Joel K. Bourne, Jr.

TimeSigns @ National Geographic Magazine
By Virginia MorellPhotographs by Peter Essick

What causes climate change? Could a climate "flip" happen virtually overnight?

Get a taste of what awaits you in print from this compelling excerpt.

"One. Two. Three. Lift!" barks Cathy Whitlock, a fossil pollen expert and paleoclimatologist at the University of Oregon. She and the three of us—two of her students and I—tighten our grips on the cold metal tube of a lake-bed drilling rig and heave. "Again," she commands. Slowly, inch by inch and groan by groan, the coring barrel that Whitlock and her students had manhandled into the marshy shore of Little Lake, a blue jewel of water in Oregon's central Coast Range, emerges from the mud.

"Once more," orders Whitlock. We bend to the task and at last free the barrel from the muck. Whitlock has extracted a couple hundred similar cores from the deep sediments of this lake, but she beams like a kid getting her first bike as she slides her latest sample of old mud, five centimeters (two inches) thick and a meter long (three-feet long), out of the barrel.

"Oh, that's a lovely core," she says. To me it looks about as interesting as a Tootsie Roll. But to Whitlock's trained eye even the chocolate hue of the mud holds a story. "That rich brown color tells you it's full of organic matter—especially pollen," she says, slicing the core in half lengthwise with her pocketknife. "You can't see the pollen without a microscope, but it's there."

And in that pollen lie clues to one of the greatest puzzles facing researchers like Whitlock: What has caused—and will cause again—the sudden climate changes that our Earth periodically undergoes? Not the 100,000-year fluctuations between a glaciated and a warmer Earth that have occurred for the past million years or so, but the more rapid shifts that scientists have recently identified when the Earth switched suddenly from frozen ice age to picnic-warm and back again. How often and how quickly have such dramatic changes happened? Perhaps most important, what do these past abrupt reversals tell us about the direction of Earth's climate today and in the future?

To answer such questions, scientists are busy unearthing signs of ancient climate in a surprising array of sources: glacial ice and moraines, stalagmites from caverns, tree rings and corals, dust and sand dunes, and the microscopic shells of organisms buried in deep-ocean sediments. Others, hoping to piece together the climate of the more recent past, turn to human records, using archaeological inscriptions, vintner and gardening diaries, and ship captains' logs. "We need both human and natural records," explains Ohio State University glaciologist Lonnie Thompson, who specializes in retrieving ice cores from the dwindling glaciers on tropical mountains. "We want to understand how the climate worked before and after people appeared. That's the only way we'll figure out what impact people have on climate, how much we're responsible for the way it's changing now."

To print the whole National Geographic story, click here.

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Interactive Quiz

What do tree rings, pack rats, and stalagmites have in common? Put your climate IQ to the test.

More to Explore

In More to Explore the National Geographic magazine team shares some of its best sources and other information. Special thanks to the Research Division.

Did You Know?
In response to growing scientific and political debate about the effects of human activities on Earth's climate, the United Nations Environmental Program and the World Meteorological Organization created the Intergovernmental Panel on Climate Change (IPCC) in 1988. The Panel's purpose is not to conduct research or produce new data, but to evaluate climate data available primarily in peer-reviewed, published scientific journals. The IPCC's third assessment report (TAR), issued in 2001, is a four-volume examination of the state of the world's climate and is the basis for much of the material in this month's series of articles on climate change. TAR documents recent climate changes, examines the effects of projected change worldwide on a region by region basis, and reviews options and costs for the future. The fourth report is scheduled for release in 2007.
You can find the complete IPCC third assessment report, Climate Change 2001, on the Web at
—Patricia Kellogg
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