Monsoon variability and the decline of Angkor: A modeling study of human responses to environmental change

Thursday, June 18, 2015: 11:00 AM
Jed O Kaplan, University of Lausanne, Institute of Earth Surface Dynamics, Lausanne, Switzerland, Kevin J Anchukaitis, Woods Hole Oceanographic Inst, Woods Hole, MA, United States and Benjamin Cook, NASA Goddard Institute for Space Studies, New York, NY, United States
Abstract:
Climate variability and projected future climate change has led to growing interest in understanding societal responses to environment. To investigate the vulnerability or resilience of societies to climate variability it is instructive to look to the past, where we have examples of civilizational decline that may be linked to environmental change. The Khmer Empire of the late 1st and early 2nd Millennium AD is an example of a society that may have been affected by climate: After constructing the world’s largest urban area in pre-modern time at Angkor, the site was largely abandoned after ~600 years of occupation. To test hypotheses about the importance of climate variability on societies, we developed a new process-based model of human subsistence and demography for preindustrial Southeast Asia. The model is driven by topography, soils, vegetation and climate, and accounts for the range of subsistence lifestyles that were present at the time, from semi-nomadic shifting cultivators to complex urban polities. To simulate the effects of climate variability on potential food production and carrying capacity, we created an estimate of annual rice yields in the Angkor region based on high-resolution gridded climate reconstructions from the Monsoon Asia Drought Atlas. The model outputs include estimates of human population based on environment, subsistence lifestyle, and availability of labor.

Beginning with an initial condition that the population of the Angkor region was close to carrying capacity at the end of the 13th century AD, our model results demonstrate the dramatic impact of subsequent climate variability on the potential population of the region. A series of major flood (AD 1309, 1316) and drought (AD 1362-1363, 1402-1403) years result in large reductions in rice yield. Large flood years further reduce the total area available for rice cultivation, by inundating important paddy growing areas in deep water. This sequence of extreme climate events can explain a major drop in the population of the Angkor region, from close to 4 million at the beginning of the 14th century to less than 500,000 by AD 1425. A combination of floods that damaged infrastructure and reduced cultivable land area, followed by drought that affected food production may have been at least in part responsible for the decline of the Angkor society.

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