Lord of the Tree Rings

Forget reading tea leaves. To forecast the future, NASA climate modeler Benjamin Cook looks to tree rings. Some woody species live for centuries, even thousands of years, growing when times are lush and halting during dry spells. Their annual rings tell their story—the thicker the circle, the better (and wetter) the year.

“I’ve always been fascinated by the information we can get from tree rings to inform us about climate change,” Cook says.

When he says “always,” he’s only exaggerating slightly. Cook started tagging along on tree-coring trips in middle school. The outings were a family affair: His father, Edward Cook, cofounded the Tree Ring Lab at Columbia University in 1975. Summer vacations consisted of weeks-long road trips from their home in Long Island, New York, to the West, with frequent stops to take cores. Their souvenirs added to the lab’s thousands of samples from across the globe. “Those trips gave me a fascination with and love of the West,” he says. “In grad school, since I was training as a climatologist, I figured the most sensible way to incorporate the region into my work was to study drought.”

In his climate projections for the coming decades, Cook includes tree-ring data, but he doesn’t take the modeler’s usual approach of grabbing the information from a database. Sure, he spends plenty of time in the office—“an academic mess, with papers piled everywhere, stuffed bookshelves, and a whiteboard full of scribbles and ideas.” But Cook also has a passion for getting out into the field, joining expeditions to gather new cores.

“It’s an adventure,” he says. “It’s great to go hiking through these really old forests.” Well, mostly. “Sometimes it might rain for five days straight, and then it’s miserable slogging through the mud.”

It’s all worth it for Cook, though. Like the time he made an Indiana Jones–esque discovery of what turned out to be some of the oldest trees in Southeast Asia. He and a colleague were working on a project studying drought variability and were out in search of a certain tree species when they stumbled upon a stand of Fokienia hodginsii, a type of evergreen. Naturally, they started coring.

“The record turned out to be one of the longest ever. It went back to almost 1300 AD, which for the tropics is really, really good,” he says. “It was a happy accident.” And one that became the basis for a centuries-long reconstruction of drought in the region.

Most of Cook’s work, however, has been closer to home. He’s shown that the Dust Bowl was not a natural disaster, as widely believed; rather, the drought’s severity resulted from farmers’ poor planting practices. He’s also examined the effects of climate change on flora and fauna in New York. And, of course, he’s looked at drought in his beloved West.

His latest research, published last week in Science Advances, reveals that when it comes to drought, the West ain’t seen nothing yet. Sometime between 2050 and 2100, extended drought conditions in the American Southwest and central Great Plains will become more extreme than the megadroughts of the 12th and 13th centuries. Those dry spells lasted for multiple decades, possibly destroying early southwestern civilizations. Tree rings and other evidence indicate that those medieval dry periods exceeded anything seen since in the region, even the current three-year drought gripping the West.

“It’s scary,” says Cook. “Future droughts are very likely to be worse than even the driest centuries of the last 1,000 years. It’s unprecedented within the last millennium.”

Tree rings were critical to the study. Reliable weather records only go back about 150 years, but the tree-ring data in the North American Drought Atlas (based on work by Cook's dad) extend to 1,500 years ago. Cook and his colleagues compared 17 computer projections of 21st-century climate with tree records of the past millennium. The models, explains Cook, consistently predicted drought more severe than any data—temperature or tree ring—has so far measured. And if we stick to the carbon-spewing path the world is currently on, the researchers found a greater than 80 percent risk of megadrought after 2050. “This will far eclipse anything anyone in the West has experienced,” he says.

Cook hopes the research will spur policymakers to adapt or extend current drought strategies to deal with the looming threat. And his group is already working to figure out when the deadline for those policies will be. “What kind of time horizon are we looking at—20, 30, 50 years?” he says. “That’s still an open question.”

Answering it will mean spending a lot of time in his messy office. Cook says his field days are largely behind him, but there are still places he’d love to go. Top of the list? Australia—it is among the hottest, driest places on the planet, and it’s got insanely old trees. The perfect place to pick up some more souvenirs.