Ancient “Megadroughts” Struck U.S. West, Could Happen Again, Study Suggests
Richard A. Lovettfor National Geographic News
May 24, 2007

Much of the western U.S. may be headed into a prolonged dry spell-a “perfect drought,” scientists say, that could persist for generations.

The West already has been dry for six years and is looking to be dry again in 2007, said Glen Macdonald, an ecology professor at the University of California, Los Angeles (UCLA).

(Related: “U.S. Southwest Drought Could Be Start of New Dust Bowl” [April 5, 2007].)

But that’s nothing compared to what has happened in the region in the past, according to Macdonald and other scientists.

In a study published today in the journal Geophysical Research Letters, a team from Arizona and Colorado found that the Southwest suffered a six-decade megadrought from 1118 to 1179.

For 62 years mountain snows-one of the area’s main sources of water-were frequently diminished, reducing the river’s flow during the heart of the drought by an average of 15 percent.

And for an extended period there were no high flows at all, said Connie Woodhouse, a study co-author from the University of Arizona in Tucson.

This is grim news for today’s Westerners, who rely on wetter years interspersed through a drought to fill reservoirs, the scientists said.

Water Data From Dead Trees

The new findings came from a study of growth rings in trees from the upper Colorado River dating back to A.D. 762. These rings indicate year-by-year moisture conditions that can be used to estimate long-ago river flows.

Prior studies hadn’t gone this far back in the history of the Colorado River headwaters because there aren’t enough living old trees to analyze.

But Woodhouse’s team discovered that there are lots of ancient logs, stumps, and standing dead trees that can provide data.

“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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