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Mega-drought, the Anasazi, and Us

Updated: Jul 7, 2021

The 19-year period from 2000-2018 was the second driest such period in the history of the American southwest since the year 800. Human activity is thought to account for half of the drought's severity.

Anasazi (Ancestral Puebloan) farmers living in the region now preserved as the Bears Ears National Monument, were largely dependent on rainfall for their crops. Largely self-sufficient in food, most families relied on maize supplemented with squash and (after AD 500) beans, Hunting (deer, bighorn sheep, rabbits) provided nutrition when available, as did gathering piñon nuts and prickly pear cactus.

From Williams et al in Science The red ziz-zag line indicates fluctuations in soil moisture over time. Moisture figures below the zero line correlate with drought. For a full description of the figure, please see the original article.

Rainfall amounts and locations vary from year to year, but tree ring data suggest there were reliable summer rains in the Bears Ears region in the late 1000s and early 1100s. A severe drought occurred in the mid-1100s, but rainfall returned in the late 1100s and early 1200s. After 1270, severe drought again seized the region and coincided with the end of the Ancestral Puebloan occupation of the entire four corners region.

A recent report in Science analyzes 1200-year tree ring data to determine soil moisture content and concludes "the 2000–2018 SWNA drought was the second driest 19-year period since 800 CE, exceeded only by a late-1500s megadrought.".

Moreover, although exaggerated periods of drought in the past appear to have been triggered by natural variations (cooling eastern tropical Pacific ocean surface temperatures, for example) the contemporary severe drought has been exacerbated significantly by human activity.

Graph illustrating the severity and duration of major droughts since AD 800. The red line is the current 19-year period. The blue line shows what the current drought would have looked like without the effect of global warming from human activity. For more commentary, please see the original paper.

The Colorado is the region's only significant river. Approximately 40 million Americans depend on it for water. However, the Colorado River's flow has declined by more than 20% in this century as compared to the last according to the Washington Post. Lake Mead and Lake Powell, the giant reservoirs on the Colorado, are less than half full, and with global warming things are projected to get worse.

The ancestral Puebloan people moved away from their homes to deal with their prehistoric megadrought. In the current one, people are still moving towards the worsening drought—an unsustainable situation that will require as-yet-unknown adaptations.

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