AURA

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Scott A. Elias, The Ice-Age History of Southwestern National Parks (Washington: Smithsonian Institution Press, 1997), pp. 153, 160, 163:

"Tree ring data from the Colorado Plateau region, analyzed by Jeffrey Dean from the Laboratory of Tree-Ring Research in Tucson, indicate that droughts occurred throughout this region at A.D. 1150, and again between A.D 1250 and 1450.… (Historical records dating from A.D. 900 to 1300 in Europe indicate that this was a time of longer growing seasons (more frost-free days), milder winters, and warmer summers.…

"Atmospheric circulation that ushered in the Medieval Warm Period in Europe may have brought drought to the American Southwest.

"Prior to the droughts (of A.D. 900-1150), advancements in Anasazi culture coincided with the most benign climate of the late Holocene giving regional farmers roughly twice the arable land that is available today in the Four Corners region. Unfortunately, this advantageous climate was setting up the Anasazi for the devastating droughts that were to come, because as the population grew so did the people’s dependence on good harvests.…

"Tree ring data developed by Jeffrey Dean and others indicate that drought conditions forced the evacuation of Chaco Canyon. Starting in A.D. 1150, a series of droughts ruined crop after crop. These conditions lasted for 50 years. The people of Chaco fought back by building elaborate water-control systems consisting of dams, canals, ditches, reservoirs, terraces, and grid borders around farming plots. The object of these systems was to catch … the runoff from the mesa tops.… By A.D. 1150, the people of Chaco Canyon were out of firewood and very low on water. Their crops were failing. Perhaps the need to import more and more supplies to keep Chaco pueblos going created additional stresses on the people. It was time to move on.

"In the end, the Chaco community fled, in search of arable land with more reliable moisture. Some members migrated north to the San Juan River Valley. Others moved southeast, to the Rio Grande Valley. These were the ancestors of the modern Pueblo clans of New Mexico, who were well established in villages along the Rio Grande from Taos to south of Albuquerque by the time the Spanish arrived four centuries later.…

[The lesson?] When the number of people on the land approaches the ecological limits of sustainability, even a slight change in the physical environment can bring catastrophe.

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