If you are one of the tens of millions of people who live in the southwestern United States, get ready for drier weather. That's the message from Richard Seager, a climate scientist at Columbia University's Lamont-Doherty Earth Observatory. According to Seager, the American Southwest is soon likely to experience a condition of "permanent drought" on par with the Dust Bowl of the 1930s. It is a frightening prediction and, according to Seager's research, the most likely scenario for the region, given the global warming now underway.
Unfortunately, he says, "it is a matter of simple thermodynamics that the region will face a considerable increase in aridity over the coming decade." As Seager explains, the Southwest is dry because, like other parts of the so-called subtropics to the north and south of the equatorial tropics, the atmospheric flow already tends to move far more moisture out of the region than the amounts that storms bring into it. Now, with increasing concentrations of heat-trapping greenhouse gases, the planet's atmosphere will retain even more moisture as it warms. Evaporation from lakes and rivers is likely to increase, soil is expected to become more arid, and plants will probably shed more moisture directly to the atmosphere.
On a planetary scale, Seager and other scientists have tracked the expansion of the "Hadley cell" toward the poles. This vast atmospheric circulation system links rising air near the Equator with descending air in the subtropics, giving rise to the subtropical jet streams. In the northern hemisphere the jet stream flows west to east across North America. The rising moist air condenses and forms thunderstorms in the tropics, but most of the moisture is lost by the time the air descends at subtropical latitudes. Scientists have long noted that most of the world's deserts are situated in the subtropics, so the expansion of the Hadley cell above a larger swath of the American Southwest, along with the shifting of the jet stream and many storms northward, is a worrisome trend for those, like Seager, who are on the lookout for changes in precipitation caused by climate change.
With these trends firmly in place, Seager says, there is little chance that the Southwest can avoid becoming drier in the coming decades. In fact, when Seager's team analyzed some 49 projections of the region's likely future climate, using 19 major computer climate models, all but three scenarios concurred that drought conditions lie ahead.
Seager has been tracking changes in precipitation for nearly a quarter century. His graduate thesis at Columbia University used computer models to try to understand the role of sea-surface temperatures in driving precipitation patterns and other forms of weather variability in the tropics and around the world. But his interest in environmental science began much earlier than that. Growing up in a working-class family in Norwich, England, Seager spent a lot of time outdoors with his family, cycling and hiking the hills of England and Scotland, which helped spark his interest in the physical world around him. Plus, he says, it didn't hurt his current professional interests that he grew up with "more weather than you would wish on anyone."
While he was an undergraduate, Seager recalls, one of his tutors was the climate scientist Ann Henderson-Sellers, who had recently returned to England after working in James Hansen's renowned climate science research center in New York. "She got me interested in the field," Seager says, "and she's the one who suggested that I apply to graduate school in climate science in the U.S." That fateful suggestion came at a key time in his life. He notes that he and his brother were the first in their family to have the chance to earn college degrees, so moving to New York for a graduate degree was a big step and, as it turned out, a lasting one. Except for a stint as a postdoc at the University of Washington in Seattle, Seager has remained at Columbia to conduct climate research ever since.
Seager recognizes that the stakes of his drought research are high. "The prospect of a drought on par with the 1930s is a matter of serious concern," he says. "With some two million people displaced, the Dust Bowl was probably the worst environmental disaster in the nation's history—even counting the current oil spill in the Gulf."
Seager is quick to add, however, that many features of the Dust Bowl are unlikely to be repeated. For one thing, he says, "we have learned an awful lot about soil conservation since the 1930s." Changes in land use, from tilling procedures to advanced crop and livestock management programs that minimize soil erosion, should help prevent the kinds of dust storms that plagued the U.S. decades ago, he says.
As severe as the impending drought conditions now appear, Seager also emphasizes the vital role played by the Colorado River, which carries an enormous volume of water to the southwestern United States. The region, he notes, now diverts between 80 and 90 percent of that water to agricultural uses. At least in the near and medium term, he says, the issue is primarily one of resource allocation rather than outright water scarcity. Other regions, however, are not as lucky. Seager warns that the Colorado River is barely a trickle of water by the time it reaches the border with northern Mexico, where the potential for water shortages is more immediate and more likely to lead to the displacement of people in coming years.
Seager sees it as part of his job to inform people about the climatic changes that the Southwest faces. He is often invited to brief water managers throughout the region, including the California Department of Water Resources and the U.S. Bureau of Reclamation, which manages the Colorado River. "It's a comparatively small part of my job, because mostly I'm focused on doing the science," Seager says, "but it is an important part. Much of our funding comes from the National Oceanic and Atmospheric Administration (NOAA), and a reasonable condition of the grants is that we do something to make sure the information gets out."
Still, Seager says, he regularly hears from those in charge of water policy issues that his work is not at the level of precision and spatial scale that would be most useful for them. As he explains, "There is still so much natural variability in the weather that I cannot say with certainty what will happen in their particular neck of the woods and exactly when." He shares the water managers' desire for more specificity. The goal, he says, spurs him to improve his modeling of the region's climate. For instance, over the past several years, Seager has tried to tease apart the causes of naturally occurring droughts in the American Southwest all the way back to the Middle Ages. The large forces driving today's climate change are clear, he says, but the extent to which naturally occurring cycles might mitigate the impending drought or exacerbate it further remains uncertain.
When I first went into this field, it had little perceived practical relevance," Seager muses. "The field has developed rapidly to a point where it can offer practical predictions, and the goal now is to make these as precise as we possibly can." He is glad to be able to give people at least some of the information they need to plan ahead. As he puts it, "It feels good to be able to do research that can be useful to people." For the most part, Seager says, he is gratified that water managers in the region seem to be paying attention to his research and preparing in an appropriate manner. "They understand that there is not a lot of question about it: it's going to get drier. So it is probably not a good idea for them to sit around and wait until our models get better."