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Angkor's Ancient Drought

For hundreds of years the Khmer empire ruled in what is now Cambodia.

But in the 13th century, the capital city, Angkor, fell into ruin. A new scientific study indicates that climate, specifically decades of drought interspersed with intense monsoons, helped bring down the Khmer capital.

Brendan Buckley, the Columbia University scientist who led the study, says that in the ancient world, Angkor was known for its sophisticated water system.

"Well, Angkor was really the dominant civilization in that part of the world without any question. It was the center of their universe. And it was called the 'hydraulic city' because it had really remarkably massive arrays of barays, which are these giant water tanks and a series of canals and interconnected waterways that was really unparalleled in the ancient world in that part of the world."

Buckley isn't an archaeologist. He studies tree rings, which record the growth history of trees that can be hundreds of years old, or even older. A new ring is added every year, and thicker rings represent a kind of savings account, when the tree collects more nutrients than it can use. Thin rings show the tree is barely getting along, like during a drought year.

Using samples from around Southeast Asia, Buckley and his colleagues saw this pattern in tree rings from recent years, when he could corroborate the rings with other historical climate information. His newest tree ring samples, from [a rare cypress, Fokienia hodginsii, in] southern Vietnam, enabled him to take the climate record back much further.

"We realized we have trees that are more than 1,000 years old. And we started seeing these big, giant periods of drought that took place around that time. And as I started to get more interested in the history of Southeast Asia I realized that that was the time of the collapse of Angkor."

The research team used what are called core samples from hundreds of trees throughout Southeast Asia. Using a hollow tube, they drill into the tree and extract a 5-mm wide cylinder that shows each ring starting with the most recent, just under the bark.

By comparing rings from different trees and with other historical data, you can often identify particular rings with the exact calendar year that they grew.

"We were able to match up the narrow and wide rings exactly so that we can assign the exact calendar dates to the exact rings of every tree. In the tropics, a lot of tree species don't even form rings that we can see. So to be able to get a tree that, first of all, has very clear rings that we're able to visually match to each other and then go through and produce these long records was remarkable."

The rings tell a story of decades of drought, which dried up Angkor's extensive water works, followed by monsoons that overwhelmed the 'hydraulic city.'

But the climate shifts weren't the only factor at work in the decline of the Khmer capital, which was a long time coming. Buckley quotes his co-author, Daniel Penny, as saying the climate was the "final nail in a coffin that took about 200 years to build," as Angkor and the Khmer empire were being buffeted by political, social, and economic stress.

"The times were changing, [shifting] toward an economic system that was taking them more to the coastline so they could trade with Chinamore readily. I believe that drought was one of the things that piled onto the pile of things that were affecting the Angkorians at that time. And it may very well have provided that final impetus to really kind of kill off this inland agricultural system."

The idea of a civilization being pushed over the edge by climate change resonates in the modern world, of course, and Brendan Buckley says his research on 13th century Southeast Asia has some lessons for us today.

"We probably have more abilities to adapt than they did at the time. But one of our biggest problems is a very large agrarian based population in places like India or Southeast Asia. And it's very hard to adapt to that giant population being in an area that is likely to be hit by these kinds of problems. The other thing is that rising sea level, which we're already seeing the evidence of in places like Ho Chi Minh City is a great example. So this is becoming an actual problem that we can see in real time on the ground. I guess that remains to be seen how we're going to cope with it."

Prof. Brendan Buckley's paper on climate's contribution to the demise of Angkor in present-day Cambodia was published online this week in the Proceedings of the National Academy of Sciences.