FEATURE

What Happened to Angkor?

Columbia tree-ring scientists journey to a remote forest in Cambodia to search for clues about the demise of a lost civilization.

by David J. Craig Published Spring 2011
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Brendan M. Buckley / Photo by Andrew Nelson

“These rings hold a lot of secrets,” he says. “Not just the tree’s age, but also its annual growing conditions. In a year when there’s little rain, you’ll get a skinny ring.”

Since the mid-1990s, Buckley has collected cores from thousands of trees across Cambodia, Vietnam, Thailand, and Laos, generating insights into the region’s climate history that could have been achieved no other way: Just as ice cores provide a glimpse of past atmospheric conditions and coral reefs indicate historic ocean temperatures, tree rings document annual precipitation levels.

And it doesn’t hurt the tree. After Buckley takes a core, he doesn’t even need to plug up the hole. “It’s actually better for the tree if you don’t plug it,” he says. “Trees are very good at compartmentalizing their wounds, which means they physically and chemically wall off the injured area to prevent pathogens from seeping in.”

Over the course of this three-day field expedition, Buckley will collect dozens of cores, slide each one into a clear plastic tube that resembles an oversized drinking straw, and ship them to his lab at the Lamont-Doherty Earth Observatory, in Palisades, New York. Once he’s back at the lab, he will sandpaper each core until it’s shiny smooth, which enables him to take microscopic measurements of its rings. Then, by analyzing the rings from many trees of the same species — looking for years in which all of the trees grew a skinny ring or a wide ring, for instance — he will identify common patterns in their year-to-year growth variations. With this information, he can estimate past rainfall levels.

“Weather stations started taking routine measurements of rainfall in this area in 1951,” Buckley says. “So we start by correlating the newest tree rings against these precise rainfall measurements. Then, we can extrapolate backward into the distant past, based simply on the rings.”

Photo: Patrick Brown

Buckley didn’t start this work with Angkor in mind. As a climate scientist, he has always had a broader goal: to help fellow scientists design computer models that can predict future rainfall patterns in Asia, based on past monsoon cycles. He has already made important contributions in this area, showing, for instance, that when water temperatures in the Pacific and Indian Oceans have changed over the past millennium, monsoons have typically been disrupted, triggering wild variations in the amount of rainfall they bring.

A few years ago, however, Buckley, who is widely regarded as the foremost tree-ring researcher working in the Asian tropics, started receiving phone calls from historians and archaeologists. Word had spread that he was routinely coring trees as old as 750 years, dating back to Angkor’s heyday. Soon, Buckley was collaborating with archaeologist Roland Fletcher, a professor at the University of Sydney and an expert on Angkor’s medieval civilization. Buckley began looking for old-growth forest as near as possible to Angkor and helping his colleague interpret the data.

Their big discovery came last spring, when Buckley, Fletcher, and fellow Columbia tree-ring specialists Edward Cook and Kevin Anchukaitis published a paper showing that Angkor, during the century before it is thought to have collapsed, experienced two long and severe droughts. The first lasted an astonishing 30 years, the next 20 years. Each of these dry periods was punctuated by several years of heavy monsoons that, according to Buckley, likely caused devastating floods.

“We’re talking about dry spells the likes of which we’ve never seen in modern history,” he says. “And then, the skies open up and the rain won’t stop.”

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Comments

As a Columbia University alum and Cambodian-American, it was great to see scientists exploring Cambodia and studying about what may have happened to the city of Angkor. Never knew how much climate change can impact a country, society and significant period of time. Thanks for the read.

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