What Causes Drought?

Drought is a natural part of cyclical weather patterns in North America, notes Oglesby. Physical clues about ancient weather, such as tree rings and lake sediment cores, show that dry spells such as the Dust Bowl and a similar drought in the 1950s typically occur a few times per century. The historical record also reveals a "mega-drought," longer and more severe than any recent episodes, 500 or so years ago.

More Sea surface temperature patterns corresponding to El Niño and La Niña. Red/white denotes warmer water; purple denotes cooler. The heat content of the water was calculated from TOPEX/Poseidon satellite measurements of the water's height.

In modern times "there's more to drought than simple lack of precipitation," adds Roger Pielke Sr., a state climatologist for Colorado and a professor of atmospheric sciences at Colorado State University. "You have to consider human factors like the amount of water being drained from rivers for crop irrigation and drinking water. In absolute terms, the ongoing dry spell is not yet as severe as the Dust Bowl of the 1930s, but the impacts have been relatively severe because the demands that people place on the water supply are so much greater now than they were back then."

This makes a complicated situation even more complicated. Land-use and water-use by humans; large-scale atmospheric circulation changes caused by ocean temperatures; feedbacks between the land and atmosphere: they all play a role. Climatologists can't yet put these factors together to predict what will happen many years in advance. Next winter is mystery enough. Will it bring much snow ... and relief? No one knows.

One thing seems sure, though: With levels of moisture in the soil and snow on the mountains both below average, people in the western U.S. are facing at least one more long, dry summer.

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