On different sides of the planet, two civilizations shared many similarities, including a vulnerability to regional climate changes.
The classical period Mayans in Central America and the Khmer in Southeast Asia both hacked a space for their people out of tropical forests and constructed impressive stone cities with sophisticated water storage systems. But their Achilles heel was a dependence on seasonal rains for their crops and drinking water. When regional climate changes caused erratic rainfall, their cities and fields may have dried out and left them vulnerable to collapse.
Between the 9th and 15th centuries, the Khmer Empire dominated much of what is now Laos, Cambodia, Thailand, Vietnam and Burma (Myanmar). The empire's famous capital, Angkor, may have been one of the largest pre-industrial city complexes in the world. The empire was supported by an extensive water management system, including lake-sized reservoirs called barays.
But reservoirs only work if there is rainwater to fill them. Research recently published in the Proceedings of the National Academy of Sciences suggests that a series of erratic monsoons may have destabilized the once-mighty Khmer.
Researchers led by Mary Beth Day, an earth scientist with the University of Cambridge, found evidence of a series of failed monsoons in 14th and 15th century that coincided with the Khmer's collapse. Sediments in the largest Khmer resevoir, the West Baray, showed that extremely heavy downpours were followed by drought. The heavy rains may have washed away crops, and those that survived shriveled in the drought. During the dry spells, drinking water may have run low as well.
Although many other factors came into play, such as the threat of Mongol invasion and social upheaval brought on by the spread of Theravada Buddhism, the researchers note that an inability to feed their people could have weakened the Khmer to the point of collapse.
The Mayans of the classical period (c. 250 – 900 AD) could have warned the Khmer. While the Khmer were rising, the classical Mayans were falling, possible because the rains weren't.
The Mayans too seem to have been dependent on seasonal rains. They often built their cities near natural reservoirs called cenotes or near rivers, then augmented nature with their own water management systems, such as the dam at Kinal and reservoirs at Uxul. Mayans even had pressurized water.
But when the rains failed, so did some of the Mayan city states, suggest some researchers including Jared Diamond in the book Collapse.
Hacking a home out of the forest may have had an effect on rainfall, as well. Forests help to create rainfall when they respire moisture and help keep the ground moist. When large amounts of forest were cleared for agriculture, it may have reduced rainfall. When the rains do come, they wash away the soil which is no longer protected by the trees.
The causes of a civilizations' failure are complex, and the verdict is still out on what ultimately toppled the Mayans and Khmer. But evidence suggests decades long changes in rainfall patterns may have had a serious effect.
Wall, Tim. 2012. "Khmer, Like Mayans, Fell Under the Weather". . Posted: January 5, 2012. Available online: http://news.discovery.com/earth/khmer-collapsed-under-climate-pressure-120105.html