"It's so arid that wood can remain on the landscape for hundreds of years," Woodhouse said in a statement. "The outside [layers] of some of our remnants date to 1200, meaning the tree died 800 years ago."
(See a related photo of a Colorado reservoir dried up by drought.)
The tree-ring data indicate that the West has experienced droughts that lasted ten times longer than anything the modern U.S. has ever seen.
Today millions of people living in Southern California are dependent on water from the Colorado as well as from local rainfall and snowmelt in the Sierra Nevada.
The latest study focused only on water levels in the Colorado River.
But previous work has suggested that all three sources were significantly reduced twice in the past thousand years: once from 1012 to 1075 and again from 1130 to 1192.
Bye-Bye El Niño?
The most likely causes of the megadroughts, Macdonald said, are changes in the temperature of the eastern Pacific Ocean that, in essence, "inoculate" it against wet El Niño years and lock in dry La Niña years.
El Niño and La Niña are the extreme ends of a regular ocean temperature shift called the Southern Oscillation. The events affect global weather in almost opposite ways, with La Niña creating warmer, drier conditions in the U.S. West.
Overall warmer waters "doesn't mean you won't get [El Niño] from time to time," Macdonald said, "but it will make it harder to achieve."
A thousand years ago such a change was likely caused by natural alterations in volcanism and solar radiation.
Today global warming may be producing similar results, Macdonald said this week during a meeting of the American Geophysical Union in Acapulco, Mexico.
Is the current drought the start of another Big One? Nobody knows, he said.
"What we can say is that we are putting the pieces in place to develop such a long drought."
And if this is the first stage of a superdrought, it isn't likely to be limited to California and the Southwest.
The tree ring data suggest that the ancient droughts extended all the way from Canada's Yukon Territory to southern Mexico, said Edward Cook of Lamont-Doherty Earth Observatory in Palisades, New York.
In addition, studies of fossil diatoms, a common type of algae, at Moon Lake, North Dakota, have revealed traces of long droughts in the Great Plains about a thousand years ago.
"The northern Great Plains is not immune to these multidecadal changes in moisture," Cook said. "That dry period shows up all the way into Alberta [Canada]."
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