Carbon Dioxide and the Desert Midwest

At first glance, Nebraska’s rolling Sandhills region seems unremarkable. But take a second look–two national forests, a national grassland, and several of the most revered canoe trails in the Midwest are tucked into this surprisingly rugged patchwork of prairies, ravines, and pocket wetlands.

Even more surprising is what lurks in wait just below this idyllic surface. Sand–22,000 square miles of it, the largest swath of barren desert in North America–is the region’s ecological time bomb. It’s currently covered by a thin carpet of grass, but the area has been shifting between rich prairie/marsh and windswept, uninhabitable mini-Sahara for centuries. The problem now: Scientists are linking increased carbon dioxide levels with widespread midcontinental drought, and that could tip the Sandhills back to massive desertification.

“If current emission levels continue,” says Gerald Meehl, a climatologist with the National Center for Atmospheric Research in Boulder, CO, “the Sandhills will be 10 to 20 percent drier in summer, and see a two- to three-degree Celsius [3.6-5.4°F] rise in average temperature” over the next 30 to 50 years. This, in turn, will dry the soil and lead to a dieback in grass cover–an ominous prognosis for a region already in its ninth year of drought.

Nebraska’s famous canoe trails, like the Loup, Niobrara, and Dismal Rivers, all of which have current or proposed National Wild and Scenic stretches, will be in serious jeopardy. The swift, clear 80-mile Dismal is spring-fed, recharged by rain from spring and summer thunderstorms that percolate through the sand. “The sands absorb rainfall quickly, protecting it from evaporation in this windy region, and the underground aquifers well up in low-lying areas, creating marshes that are critical for plants, streams, and migrating waterfowl,” explains Paul Johnsgard, Professor Emeritus of Biological Sciences at the University of Nebraska and author of This Fragile Land: A Natural History of the Nebraska Sandhills.

The implications of desertification are far more serious than the loss of stellar canoe trails. The Sandhills region recharges the Ogallala Aquifer, the largest natural underground reservoir in the United States. In fact, the thickest area of the aquifer underlies the Sandhills because of that recharge effect. At stake is the water supply for hundreds of cities and thousands of farms in the nation’s most productive land for livestock, corn, wheat, and soybeans.

Until this year, paleoclimatologists believed that the last dune-making drought here occurred 100,000 years ago. Now, researchers analyzing sand strata have discovered that catastrophic dry periods happened as recently as 8,000, 3,000, and 900 years ago. The first two episodes created a mini-Sahara with 400-foot-tall dunes covering an area 94 times larger than Colorado’s Great Sand Dunes National Park and Preserve.

Twice in the last century, the Sandhills nearly returned to flowing dunes–once during the 1930s Dust Bowl period, and again in the 1950s. An extensive 2004 study concluded that the only reason the dunes didn’t surface in the ’30s was that the drought didn’t last quite long enough for the grasses’ underground root systems to disappear and relinquish the sand to the wind.

Now, the new research has scientists worried that in the next century, perhaps within a decade or two, the Sandhills could return to Saharan conditions. A mere 15 percent reduction in vegetative cover will allow the dunes to move again. “We’re already seeing a 10 to 15 percent decrease in soil moisture, and it’ll be greater than that in future summers–probably 20 percent by 2100,” says Mark Svoboda, staff climatologist at the University of Nebraska’s National Drought Mitigation Center. For canoeists, birders, and waterfowl hunters, not to mention the nation’s agricultural base, a Sandhills Sahara would be devastating.