An international team of researchers, including University of New Mexico's Peter Fawcett, has drawn a strong relationship between warmer temperatures and megadroughts during the mid-Pleistocene interglacials and the possible cause.

Fawcett, an associate professor in the Department of Earth and Planetary Sciences and the lead author of a recent paper published in Nature titled, "Extended megadroughts in the southwestern United States during Pleistocene interglacials," indicates that these ancient megadroughts may have been caused by the poleward expansion of the subtropical dry zone in response to natural warming. These megadroughts had significant effects on water availability as well as ecosystem composition.

These Pleistocene arid periods may also be suggestive of future climate change in the region. This climate mechanism is similar to that predicted for the southwestern U.S. as a result of anthropogenic warming or that caused by humans. The paper suggests that in the absence of human-caused climate change in the southwestern U.S., the region might now be entering a cooler, wetter period of time.

Using a high-resolution climate record from an 82-meter sediment core that spanned two mid-Pleistocene glacial cycles, the researchers, which included several graduate students and professors from UNM in addition to Fawcett, were able to show a correlation between warmer temperatures and megadroughts, those lasting much longer than the type seen today.

Interglacials are geological intervals of warmer global mean annual temperatures or MATs that last thousands of years. The researchers reconstructed past climates by drilling lake sediment deposited in the Valles Caldera watershed from 550,000 to 368,000 ago, a time period which contained two consecutive glacial periods.

"We used a new technique involving the organic geochemistry of soil bacteria to help reconstruct the temperature during these periods," said Fawcett. "Organic compounds in the soil bacteria change in response to temperature and soil pH and give us an idea of what was happening during the 200,000 years represented by the sediment core. The results show that there's a lot of natural variablility during interglacials."

An interesting timeframe to study, coupled with summer rains that were almost non-existent, the researchers found that the MATs and mud cracks discovered in the core corresponded to the driest and warmest periods of interglacials. Although the causes of global warming today are far different, the research suggests the southwest could be headed toward similar dust-bowl like conditions lasting decades to a century that are predicted as a consequence of human-caused global warming.

"In the core, we found two intervals that coincide with the two warmest time periods in the watershed," said Fawcett. "The reconstructed temperatures and mud cracks discovered in the core show that warmest periods of interglacials were also the driest. These droughts lasted hundreds to thousands of years unlike recent droughts. In the future, the southwest will gradually become dryer."

Other minerals in the core, such as calcite, also provide a continuous indicator of closed-basin or open basins conditions or wetter versus dryer.

"Low levels of calcite were found during wet intervals," said Fawcett. "No calcite precipitated during freshwater open-basin conditions, whereas during drier or closed-basin conditions, evaporative concentration led to calcite precipitation and preservation."

The results of the research indicate that interglacial climates in the southwestern U.S. can experience prolonged droughts with profound effects on water availability and ecosystem composition, and likely to be heightened by anthropogenic causes

Other key collaborators in the research include Jeffrey Heikoop from Los Alamos National Laboratory, Josef Werne and Erik Brown from the University of Minnesota-Duluth and Scott Anderson from Northern Arizona University.

To view the paper visit: Extended megadroughts in the southwestern United States during Pleistocene interglacials.

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