Evidence of a Roman-era Southwest "megadrought" has University of Arizona climate researchers again warning water managers that Western rivers have not been historically reliable in their flow.
Tree-ring samples collected by Cody Routson from the oldest trees in the San Juan Mountains of southern Colorado record a drought that spanned half of the second century.
That, combined with previous studies that showed a drought of similar duration in the Middle Ages, are worrisome signs for states such as Arizona that depend on water from mountain watersheds. It raises a fear that our current, persistent drought may be far from over.
Because the evidence for the Roman-era drought comes from a single location and a small number of bristlecone pines, the "analysis should be viewed as exploratory," says a paper published in Geophysical Research Letters by Routson, Connie Woodhouse and Jonathan T. Overpeck.
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Routson, a graduate student in geosciences, created a 2,200-year record of tree-ring growth from 26 living and remnant bristlecones in the higher elevations of the San Juan Mountains, which feed both the Rio Grande and San Juan rivers.
Only six remnant specimens were old enough to cover the time period of the newly discovered drought, but the record of that early drought shows up in incomplete records across the West, stretching all the way to Idaho, Routson said.
"Our record shows very strong evidence that this drought occurred at this particular site," Routson said. The other records show that "it definitely was regional," he said.
Analysis of the severity and exact duration requires further study. The period of most severe drought appears to range from A.D. 148 to A.D. 173.
Routson's research also adds further evidence to climate studies done by UA researchers that previously uncovered a period in medieval times when the region underwent a drought that lasted for six decades. The Roman-era drought was at least that severe, if not worse, the paper says.
When uncovered by a team led by Woodhouse and UA tree-ring researcher David Meko, the medieval drought became a cautionary tale for Colorado River water managers.
The Colorado River and its 246,000-square-mile watershed is a water source for seven Western states, including Arizona. Its waters were divided by federal compact during what is now recognized as a wet period in the region's climate.
The message is that such mega-droughts can occur, even without the warming of the climate that is predicted for the globe.
Warming could make future megadroughts even worse, the paper concludes:
"Until the climate dynamics of megadrought are thoroughly understood, managers of water and natural resources in the Four Corners, Rio Grande and Colorado regions should take note that megadroughts as long, or longer, than 50 years could reoccur with the caveat that future droughts will likely be even warmer than those in the past."
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Did you know?
Dendrochronology, the science of reading patterns in tree rings, was invented by astronomer A.E. Douglass, who originally searched for correlations between tree growth and sunspot activity. Douglass founded the University of Arizona Laboratory for Tree-Ring Research in 1937.
Contact reporter Tom Beal at tbeal@azstarnet.com or 573-4158.
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