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A new analysis using a standard drought index augurs that by the end of the century devastating drought conditions will take hold over much of the populated areas of the world

Lake Mead, the massive reservoir created in the late 1930s by Hoover Dam on the Arizona–Nevada border, has dropped to its lowest level ever, it was reported earlier this month. The lake has been steadily growing shallower since drought began reducing the flow of its source, the Colorado River, starting in 2000 due to below-average snowfall in the Rockies.

It is still too early to know whether the situation at Lake Mead and recent droughts throughout the U.S. Southwest are due to anthropogenic global warming, says Aiguo Dai, an atmospheric scientist in the Climate and Global Dynamics Division of the National Center for Atmospheric Research in Boulder, Colo. There is not enough data to rule out natural variability as the fundamental cause. But the decadelong dry spell is consistent with the predictions of models used in the Intergovernmental Panel on Climate Change (IPCC) Fourth Assessment Report (AR4) in 2007, which projected that warming of the planet would lead to long-term drying over the subtropics—the climatic regions adjacent to the tropics, ranging between about 20 and 40 degrees north and south latitude, which includes the Southwest.

It’s also consistent with a new analysis, authored by Dai, which forecasts that increasing dryness over the next several decades will eventually become devastatingly severe, with long-lasting drought predicted for most of Africa, southern Europe, the Middle East, most of the Americas, Australia, and Southeast Asia. Dai computed global values through the end of the century for a commonly used index of drought severity, using data from 22 models used by IPCC-AR4, under a middle-of the-road emissions scenario that assumes human-generated greenhouse gas emissions start reducing about 2050, and that atmospheric CO2 increases to 720 parts per million (we’re currently at around 380) by the end of the century.

Dai’s projections are helpful because they begin to bring into focus some of the water-related global warming consequences that may be upon us relatively soon, says Richard Seager, a senior research scientist at Columbia University’s Lamont–Doherty Earth Observatory. Seager, who was not part of the study, was a co-author of a 2007 study inScience that analyzed the findings of the IPCC-AR4 models. “When the IPCC report came out in 2007, there was relatively little that looked at how these climate changes developed within the coming decades,” he says. But in Dai’s new figures, “you can see that even in the coming decades or so we’re already getting into some trouble in this regard.”

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