PopSci's Fourth Annual Brilliant 10

Hope Jahren

Johns Hopkins University

She extracts secrets from ancient trees that shed light on global warming

Hope Jahren slips a crumbly hunk of tobacco-colored wood from a Ziploc bag. "Looks like driftwood from the beach," she says. But her delicate grip says otherwise-it's actually a precious 45-million-year-old fossil of the redwood-like Metasequoia tree. This specimen and others chilling in Jahren's lab are clues to a major puzzle: How did a lush forest once flourish a snowball's toss from the North Pole?

At 36, Jahren is already considered a master at prying loose secrets about the Earth's climatic history by scrutinizing the carbon, oxygen and hydrogen inside plants. Her skill at this technique-known as stable isotope analysis-is yielding insights into the Eocene epoch, a potentially instructive period from 57 million to 36 million years ago when a global heat wave melted most of the planet's ice cover. It's also earning her kudos. In December, Jahren will pocket the American Geophysical Union's prestigious Macelwane medal, making her one of only four researchers-and the only woman-to snag both this and the other coveted prize for young earth scientists, the Donath award.

Jahren's work has taken her from the rain forests of Puerto Rico to Axel Heiberg Island, 700 miles from the North Pole. Today Axel is a glacier-glazed landscape. During the Eocene, however, the island was a veritable Eden, crawling with crocodile-like beasts and forested with Metasequoia and deciduous conifers. Incredibly, mummified branches, cones and pollen remain. In 2003 Jahren's analysis of these fossils revealed that the conditions on Axel during the Eocene were a result of more moisture in the air.

That finding is especially relevant today, because the role of water vapor in global warming is fiercely debated. "It's the wild card in the whole greenhouse story," notes University of Michigan geochemist Philip Meyers. As the global thermostat rises, it causes more water to evaporate. Some models predict that added humidity will counteract the heating effects of greenhouse gases; others predict that it could amplify the gases' warming effect.

Jahren prefers to leave the global-warming debate to others. She's too caught up with yet another Axel enigma: How did such a forest survive without sunlight for three months a year? "It's equivalent to discovering ancient humans who could live underwater," she says.

-Michael Stroh