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History’s Wake-up Call for the Greenhouse Century

In early December, as I motored away from Sky Harbor International Airport in Phoenix, Arizona, I glimpsed a sign for the Hohokam Expressway. The road was named after the Hohokam culture that had thrived in south-central Arizona for more than 1,000 years–and then abruptly disappeared around 1450 A.D. As I proceeded to drive past countless...

In early December, as I motored away from Sky Harbor International Airport in Phoenix, Arizona, I glimpsed a sign for the Hohokam Expressway. The road was named after the Hohokam culture that had thrived in south-central Arizona for more than 1,000 years–and then abruptly disappeared around 1450 A.D. As I proceeded to drive past countless palm trees, swimming pools, green lawns, and suburban housing tracts, I couldn’t help but wonder whether the fate of modern Phoenicians would ultimately mirror that of the enterprising Hohokam.


Illustration of Hohokam irrigation by Peter V. Bianchi.



Less well known than their Anasazi neighbors at Chaco Canyon and Mesa Verde, the Hohokam farmed successfully in the region for many centuries by tapping the Salt and Gila River systems to supply vast irrigation networks anchored by more than 300 miles (480 kilometers) of main canals. They grew surpluses of corn, beans, and squash, which freed them up to pursue arts, crafts, and handiworks. From a Hohokam settlement called Snaketown, southeast of Phoenix, archeologists have unearthed beautiful beads, bracelets, pottery, and painted ceramics. At its zenith, Hohokam trade reached north to Chaco Canyon, in northwest New Mexico, and south into Mesoamerica. The Hohokam population peaked somewhere between 50,000 and 200,000, and their territory spanned an area the size of Guatemala.

What made this thriving, entrepreneurial culture suddenly disappear? No one knows for sure. But a leading contender, as I recount in my book Pillar of Sand, is devastating drought followed by disruptive floods–climatic shifts for which the Hohokam were unprepared and to which they could not adapt.

(Read more about climate change’s relationship to droughts and floods.)

It’s tempting to dismiss this tale from an earlier age as having little relevance for modern-day Phoenix and its sister cities in the U.S. Southwest. But a new study suggests that this would be a big mistake: the destabilizing consequences of prolonged drought and water shortages could well hit the Southwest hard again. In fact, they may already be unfolding.

The Southwest is in the midst of a decade-long dry period. Lower-than-normal rainfall combined with higher-than-normal temperatures (likely due to greenhouse warming) has substantially altered the region’s hydrologic conditions. From 2000-2009, the Colorado River exhibited the lowest 10-year-running-average flow of any 10-year period in the last century.

(Read more about Colorado River drought in National Geographic magazine’s “Drying of the West.”)

A team of researchers led by Connie A. Woodhouse at the University of Arizona in Tucson examined paleo-climatic records to place this recent decade-long drought in a longer-term (1,200-year) context. Their findings, published in the December 13 online edition of the Proceedings of the National Academy of Sciences, are more than sobering: they are a call to assess risks and prepare for the worst.

Woodhouse and her colleagues found that as bad as the current drought is shaping up to be, it pales (so far) in comparison to one that lasted two decades during the middle of the 12th-century–and these dry conditions, they warn, could happen again. This medieval drought was more severe, widespread, and longer lasting than any other in the Southwest over the past 12 centuries. Reconstructed Colorado River flows for the period 1146-1155 averaged just 11.5 million acre-feet (14.2 billion cubic meters) per year, 22 percent lower than the river’s average annual flow during the 20th century.

Combined with high demand for water supplies, the last decade of drought has drained more than half the capacity of Lake Mead–the vast reservoir on the Colorado River that supplies 29 million people in Phoenix, Tucson, Las Vegas, Los Angeles, and other Southwestern cities, as well as 1 million acres of irrigated land. Its companion reservoir upstream, Lake Powell, has been drawn down as well.

(Read more about 2010 extreme water events in National Geographic News: “PHOTOS: 2010 a Watershed Year for Floods, Droughts?”)

The hope, of course, is that “normal” rains and snows will return and the reservoirs will re-fill. But there is no guarantee.

In fact, there is one more feature of the epic medieval drought that ups the chances of an equally debilitating dry period occurring again in the not-too-distant future: it was a warm drought. Woodhouse and her team note that temperatures in western North America during this time were about 34 degrees Fahrenheit (1 degree Celsuis) higher than the long-term average. Still, the driest decade of the medieval drought was not as warm as 2000-2009. And temperatures continue to rise from the accumulation of carbon dioxide and other greenhouse gases in the atmosphere. The 12-century “warm drought,” the researchers write, “may serve as a conservative analogue for severe droughts that might occur in the future.”

The Hohokam were farming and thriving in Arizona when the medieval drought took hold. Many changes in their culture and settlement patterns date to this time. Snaketown and a number of other population centers were abandoned. Arts and crafts enterprises–and thus trade–diminished. Somehow the inventive Hohokam muddled through, only to be hit by another drought late in the 13th century and a period of flooding early in the 14th century. By the middle of the 15th century, they were gone.

A complete rendering of the Hohokam story will never be possible, because much evidence of Hohokam life now lies buried beneath golf courses, strip malls, and, ironically, the Central Arizona Project–the vast canal system that diverts Colorado River water hundreds of miles to Phoenix and Tucson. Today, signs of the Hohokam live on in south-central Arizona mostly in name only–in the Hohokam Expressway, Hohokam Elementary School, and Hohokam Stadium, the spring-training park for baseball’s Chicago Cubs.

Whether droughts and floods caused the Hohokam’s demise may never be proved. But these hydrologic events almost certainly played an important role. And for most of the Hohokam’s 1,000 years of prosperity, they probably had no idea just how vulnerable their society was.

Today, as we enter the second decade of the greenhouse century, the Hohokam’s tale should serve as a wake-up call. It’s time to begin planning and preparing for a worst-case scenario of drought and water shortage that seems increasingly likely to become reality in the U.S. Southwest. That scenario may already be unfolding before our eyes.


Sandra Postel is director of the Global Water Policy Project and lead water expert for National Geographic’s Freshwater Initiative.  She is the author of several acclaimed books, including the award-winning Last Oasis, a Pew Scholar in Conservation and the Environment, and one of the “Scientific American 50.”

[This post has been reformatted for Water Currents.]

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Meet the Author

Sandra Postel
Sandra Postel is director of the Global Water Policy Project and author of Replenish: The Virtuous Cycle of Water and Prosperity. From 2009-2015, she served as Freshwater Fellow of the National Geographic Society. Sandra is also co-creator of Change the Course, the national water stewardship initiative awarded the 2017 US Water Prize for restoring billions of gallons of water to depleted rivers and wetlands. The recipient of several honorary degrees, she works to bridge science, policy, and practice to promote innovative ways of securing water to meet both human and ecosystem needs.