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Climate History: Exploring Climate Events and Human Development
The Past 1000 Years: Climate & Culture in the American Southwest
Image of Spruce Tree House, Mesa Verde Around 850-900 AD in the mesas and canyons of the American Southwest, tribal peoples now known as the Ancestral Pueblos (formally the "Anasazi" which means "ancient enemies" in the Navajo language) began to build and live in above ground structures with contiguous rooms known by the Spanish word "pueblos" meaning village. Their agricultural society revolved around corn that was well-adapted to the arid climate, and some large communities, such as those that were built in Chaco Canyon, were built in open areas. But about 1200 AD some of these native peoples moved into cliff dwellings in the Mesa Verde area and in Tsegi Canyon.

Image of Anasazi potteryWas the move into the cliffs for defense from enemies? Perhaps, but there are other plausible explanations, like the fact that less labor would be needed to maintain structures that were protected from the elements, or that moving into rock shelters kept the population off the best agricultural lands. Moving into alcoves at the canyon heads also provided reliable sources of water for the community.

Imge of Cliff PalaceThe Ancestral Pueblo peoples at Chaco Canyon and Mesa Verde developed elaborate water cachment systems (Vivian, 1990 and Wilshusen, 1997), but the water supply wasn't always reliable when climate turned dry. According to the National Park Service World Heritage Program, drought was a factor in the abandonment on Chaco during the 12th Century:
"The decline of Chaco apparently coincided with a prolonged drought in the San Juan Basin between 1130 and 1180. Lack of rainfall combined with an overtaxed environment may have led to food shortages. Even the clever irrigation methods of the Chacoans could not overcome prolonged drought. Under these pressures Chaco and the outliers may have experienced a slow social disintegration. The people began to drift away."

During the 13th Century, the Ancient Pueblo peoples of Mesa Verde and nearby regions also abandonded their masonry homes. For many decades the conventional wisdom was that severe drought pushed them from the region due to crop failures. Paleo proxy data from tree rings and packrat middens have been used as evidence that a severe drought had hit the region. Analysis of bones from the inhabitants which showed malnutrition seemed to confirm the drought theory. (See Johnson, 1996.)

Image of drought reconstruction from northeast New Mexico

From Grissino-Mayer's 2129-Year Reconstruction of Precipitation for Northwestern New Mexico, USA. Units are of standard deviation. Red indicates periods of drought. Click on image for more detailed graphs of climate variability in the American southwest. Also see tutorial on drought.

The more scientists study the situation, the more complex the problem actually becomes. Yes, there was a drought (see the chart above which is based on tree ring data from Northeast New Mexico collected by Henri Grissino-Mayer, 1996), but was it really severe enough to force the Ancient Pueblos from their homes? Some researchers were skeptical. Evidence of cannibalism and human sacrifice was found in the region, adding new questions to the mix. Were the Ancestral Pueblos pushed out of the region by other tribes, such as the Apache and Navajo, or threatened by bands from the central valley of Mexico intent on human sacrifice? (White, 1992). Or did disease, perhaps something akin to the modern day hanta virus that can be triggered by sudden shifts from dry to wet climates, causing increases in disease carrying deer mice populations, run through the communities? (Martin, 1994)

Image of Mt. St. Helens from USGSOther researchers took another look at climate just to make sure they weren't missing something, and Matthew Salzer (2000) noted there was significant volcanic activity, with one particular event-- likely the largest of the Holocene that occurred in 1259 A.D. -- that may have chilled the atmosphere, thereby shortening the growing season and perhaps disrupting normal rainfall patterns. (See Volcanic Aerosols for access to data).

As with many studies of how humans relate with and are impacted by the environment, the reasons for the Ancestral Pueblo people's migration south to become the Pueblo peoples in modern day New Mexico and Arizona are complex and likely not solely related to one single cause. But because their culture was agriculturally based and the region was relatively arid, periods of drought and shorter growing seasons may well have played a significant role in their southward move. It is known from early Spanish records that later droughts in the 17th century had a devastating impact on the Pueblo villages. (Cordell, 1994).

In other parts of the world, changes in climate, such as the "Little Ice Age" have impacted human activities. See Resources 1000 years for more.

Graphs of paleoclimatic data in the Southwest offer more on the regional climate variability . For more on the Ancestral Pueblo people and how they responded to climate change, visit the Colorado Plateau- Land Use History of North America's website on Climate Change on the Colorado Plateau, the Bureau of Land Management's Anasazi Heritage Center and Mesa Verde National Park.

Other cultures where extended drought has played a role in the collapse of societies include the Mayan, Egyptian and Persian civilizations.

Image of Little Ice Age from Ruddiman
Image from Ruddiman, 2001 used by permission of W. H. Freeman & Co.

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