The massive dust storm that engulfed Phoenix last week was unusual for the 20th century, but could become more common in the 21st.

The massive dust storm that engulfed Phoenix last week was unusual for the 20th century, but could become more common in the 21st.

The storm resulted from thunderstorm-cooled air plummeting into the ground like mist pouring from an open freezer, only exponentially more powerful. Combine those winds with extremely dry conditions, and the result was a wall of dust 100 miles wide and 5,000 feet high.

Dust storms are common in the U.S. southwest, but not storms this big. No formal records are kept, but meteorologists said it was the largest such storm in at least 30 years. It was on par with storms seen in China’s Gobi desert and Australia. Some commentators invoked the apocalyptic storms of the 1930’s Dust Bowl.

As dry as it’s been in the southwest this year, with precipitation 50 percent below mid-20th century levels, there’s reason to think that extra-dry conditions will continue to prevail. Extreme, century-long droughts are not uncommon in the southwest, and the drought that started in the late 1990s appears to fit that pattern. Whether it’s a consequence of climate change or would have happened regardless is debated, but the bottom line is a dry one.

A dust cloud approaches a small town in Oklahoma. Kansas State University

Moreover, temperatures in the southwest, which have already risen by 1.5 degrees Fahrenheit since the mid-20th century, are expected to rise by another 4 to 10 degrees by this century’s end.

Researchers have studied what this will mean for dust. In January, scientists from the United States Geological Survey released the results of a 20 year-long survey (.pdf) of relationships between temperature, vegetation and soil loss on the Colorado Plateau, a region spanning Utah, New Mexico, Colorado and Arizona. They found that higher temperatures reduce plant density, which in turn removes roots that hold soil together.

“Our results suggest that increased temperatures associated with climate change will indirectly lead to increased wind erosion and dust emission on the Colorado Plateau,” they concluded.

The researchers also found that so-called biological soil crusts, a little-studied layer of lichen and bacteria found in arid western regions, play a surprisingly large role in preventing erosion. Just a few millimeters thick, these crusts are extraordinarily sensitive, and can take decades to recover from a mere footprint. They’re also widely compromised by livestock, off-road vehicles and mining.

But in one important way, biological soil crusts are pwerfully resilient. The researchers found them to be mostly unaffected by changes in temperature and rainfall. If the crusts can be protected, a 21st century Dust Bowl might be averted.www.wired.com