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New explanations for Western drought

For the past five years or so, the West has undergone what scientists think may be one of the most extreme droughts in 400 years. Since 2000, water levels in lakes Mead and Powell on the Colorado River have been below average. At the end of June, the U.S. Drought Monitor showed swaths of “exceptional” drought throughout Idaho, Montana and Wyoming, with “extreme” to “abnormally dry” conditions in parts of the rest of the West. Although wildfires this year have been below average so far, they are expected to blossom in the dry conditions across the region, which have weakened trees and left them susceptible to insect damage.

A pair of images taken from the same point in Utah shows the upper Colorado River delta emptying into Lake Powell. The lake is about half empty after several years of drought. The image on the top was taken in June 2002, and the image on the bottom, May 2003. Image courtesy John Dohrenwend.

While the current drought is severe, particularly following several years of anomalously high precipitation, this dry spell is nothing new for the region, researchers say. “You have to look at the whole climate system,” says Robert Webb of the U.S. Geological Survey (USGS) in Tucson, Ariz., who has looked at the recent drought’s effects on the Colorado River Basin. People tend to isolate drought, Webb says, without considering the larger picture of how the climate system works in North America, where “you can have severe drought in the Colorado River Basin and you can have flooding in the panhandle in Texas” at the same time. To get the full picture, some unexpected sources are necessary: trees and oceans — and most unexpectedly, the North Atlantic.

For more than a decade, people have looked to the behavior of the Pacific, particularly the El Niño Southern Oscillation, or ENSO, for harbingers of future precipitation (or lack thereof). “The first thing they saw with the Pacific,” with regard to ENSO over a decade ago, says Greg McCabe, of the USGS in Denver, were “statistical relationships.” Similar associations today are now making it clear that the North Atlantic also has an impact halfway around the planet, but the exact connections remain uncertain, McCabe says.

Using sea-surface temperature data from a record that stretches back to 1856, McCabe and others recently published results in the Proceedings of the National Academy of Sciences of models of the two dominant regimes of the North Atlantic and the Pacific, from 1900 through 1999. Both oceans regularly undergo sea-surface temperature flip-flops. For the Pacific, such oscillations are almost decadal, but the North Atlantic may take several decades to switch between a warm or cool surface.

Taken together, the researchers found, the two oscillations account for just over half of the variation in drought in the entire United States, when matched to a century’s worth of instrumental data collected across the continent. The team’s results also indicated that the North Atlantic multi-decadal oscillation is a potentially dominating factor in the current drought. If that ocean’s surface remains warm for another decade or so, then the severe drought in the West may not relent. In fact, it could possibly kick the drought into a state similar to the 1930s Dust Bowl or the 1950s droughts.

But other droughts from well before the historic record in North America were greater than the 1930s event. In an effort to figure out exactly what is considered “normal” for the American West, paleoclimatologists are looking as far back as possible using trees.

Modern climate station data, collected over the past century or so, may provide a short-term record of wet and dry periods in the West. But to determine what is normal, “you have to go back to the tree ring record,” Webb says, where dry periods show up as thin rings from trees growing under stressed conditions. “If you look at the tree ring record,” he says, “for climate in the 20th century, the most abnormal are the wet periods.”

Tree ring core data extending back about 750 years have allowed a team of researchers from USGS and the Big Sky Institute at Montana State University to reconstruct paleoclimate for several basins in the western United States, with results they have presented in Geophysical Research Letters and elsewhere. Although the current drought does not qualify as a 500-year drought, they found that it seems to be quite similar to several regional droughts, including one in particular that occurred during the mid- to late 1500s, as well as several other multi-decadal periods throughout the past five centuries.

“The paleorecord tells us that long-duration droughts … even of decadal length are a common part of the climate system across western North America as a whole,” says Stephen Gray, a paleoecologist and paleoclimatologist at the Big Sky Institute. The tree ring data also allowed the team to track the “epicenter” of droughts, noting that some areas received momentary relief, while a larger and longer drought shifted around the continent.

Gray says that the 30-year models used locally by water managers are not long enough, as they only consider the instrumental record — and miss the trees. He also says that more modeling work is necessary to link global sea-surface temperatures to changes in precipitation across North America over the longer term.

“We’re only just now getting to the point where we can get a grasp on how the whole system interacts to produce these droughts,” Gray says. “What we see here is really a lot of promise for the ability to forecast [drought] over many seasons, even years in advance.”

Naomi Lubick

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