Paleoenvironmental Change

Long-term climate change causes shifts in plant, animal and human populations, but the patterns are never simple. At the end of the Pleistocene individual plant species moved independently of each other, making plant communities transitory. Human communities, when confronted with difficulties, make social and technological adjustments, giving Southwestern archaeologists reason to question whether environmental change should be given primary credit for shifts in native populations and culture. Controversy over the so-called Anasazi collapse is the primary example from this region.

What is clear is that over a long period of occupation the Anasazi did experience profound climate shifts in an already difficult environment. In the relatively benign climate of the late Holocene (700-1130 A.D.) Anasazi culture and population advanced. Water levels in streams were at a maximum, precipitation was increasing, and crop yields became more predictable. Dry farming on the mesa tops expanded dramatically, giving regional farmers roughly twice the arable land than is available today in the Four Corners area. The Anasazi became dependent on good harvests.

Historical records from 900 to 1300 A.D. in Europe indicate that this was a time of changes in atmospheric circulation known as the Medieval Warm Period. In high-latitude regions this was largely beneficial: grapes were grown in England and the Norse founded colonies first in Iceland and then in southern Greenland. But in arid regions a warmer climate, especially when accompanied by drought, can cause significant difficulties for farmers. A fifty-year drought occurred between 1130 and 1180 A.D. It was during this period that soil and water conservation features such as grid borders, terraces and check dams began to be built in the Four Corners area.

The prolonged droughts of the subsequent period must have created great stress for the Anasazi. The so-called Great Drought, a sharp decline in precipitation from 1276 to 1299 A.D., had to have been particularly devastating. The elevational zone for upland dry farming began to shrink rapidly, and may have disappeared altogether by 1300 A.D. Settlement began to aggregate in large cliff dwellings in canyons with more reliable sources of water. Food storage became paramount as did ceremonial space. But even these adaptations were short-lived; by the end of the century, Mesa Verde and other magnificent aggregations of cliff dwellings had been abandoned.

Too simple? Click here for a New York Times article on the current debate over the Anasazi "collapse."

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