Ancient "Megadroughts" Struck
National Geographic News, May 24, 2007
Much of the western
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
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
For 62 years mountain snowsone of the area's main sources of waterwere 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
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
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."
The tree-ring data indicate that the West has experienced droughts that lasted ten times longer than anything the modern
Today millions of people living in Southern California are dependent on water from the
The latest study focused only on water levels in the
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
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
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
The tree ring data suggest that the ancient droughts extended all the way from
In addition, studies of fossil diatoms, a common type of algae, at