The American Southwest is prone to drought, and the summer of 2011 proved no exception, when a severe drought extended from Arizona to Florida. But a sediment core from New Mexico suggests that today’s droughts—even the 1930s Dust Bowl—are fleeting events compared to conditions of the ancient past. Hundreds of thousands of years ago, some droughts could persist for centuries. Researchers find one ancient period of warm, dry conditions especially intriguing because it was, in many ways, similar to conditions on Earth during the last 10,000 years.
Clues about this ancient period are preserved in a dry lakebed in New Mexico named Valle Grande. On May 25, 2011, the Advanced Land Imager (ALI) on NASA’s Earth Observing-1 (EO-1) satellite captured this natural-color image of the lakebed. It is an unevenly shaped expanse of beige grassland situated inside the larger Valles Caldera.
Researchers extracted a 260-foot (80-meter) sediment core from this lakebed in 2004, and published their analysis in 2011. Ancient lake muds in the core document the region’s climate between 360,000 and 550,000 years ago. During that time, glaciers advanced over North America in recurring ice ages, and conditions warmed in interglacial periods. The core includes sediments from two warm interglacials.
One interglacial covered in the sediment core that particularly interested the research team is known as Marine Isotope Stage 11 (MIS 11), which occurred around 400,000 years ago. Our planet’s orbit around the Sun has varied over geologic time, but the 50,000-year period comprising MIS 11 experienced an orbital configuration similar to that of the last 10,000 years and, consequently, the amount of solar radiation reaching the Earth was similar.
During MIS 11, the researchers found, the climate of the American Southwest underwent a series of changes. As ice-age conditions gave way to warming, plant life thrived in a seasonally wet climate. But the warming continued, withering grasses and shrubs, and drying out lakes. Mud cracks in the sediment core illustrate the aridity. The drought documented by the sediment core lasted thousands of years—a megadrought.
The sediment core suggests that the megadrought occurring in MIS 11 not only began abruptly, but also ended abruptly, replaced by cooler, wetter conditions. As geologists explain, we live in an interglacial today; Earth’s most recent ice age ended only about 10,000 years ago. The similarity of the Earth’s orbital configuration between MIS 11 and now suggests that, as happened hundreds of thousands of years ago, the Southwest might eventually experience cool, wet conditions again. Such a transition could be derailed, however, by warming caused by increased concentrations of greenhouse gases.
- Fawcett, P.J., Werne, J.P., Anderson, R.S., Heikoop, J.M., Brown, E.T., Berke, M.A., Smith, S.J., Goff, F., Donohoo-Hurley, L., Cisneros-Dozal, L.M., Schouten, S., Sinninghe Damste, J.S., Huang, Y., Toney, J., Fessenden, J., WoldeGabriel, G., Atudorei, V., Geissman, J.W., Allen, C.D. (2011). Extended megadroughts in the southwestern United States during Pleistocene interglacials. Nature, 470, 518–521.
- Rickman, J.E. (2011, February 28). Dry lake reveals evidence of southwestern “megadroughts.” Los Alamos National Laboratory. Accessed July 18, 2011.
- U.S. Drought Monitor. (2011, July 14). Conditions for July 12, 2011. (PDF file) University of Nebraska, Lincoln. Accessed July 16, 2011.
- University of California Museum of Paleontology. The Pleistocene. Accessed July 18, 2011.
- Williams, J. (2011). Climate change: Old droughts in New Mexico. Nature, 470, 473–474.