Study: Global warming to heighten risk of Southwestern megadroughts

Houseboats are dwarfed by the steep banks of Shasta Lake at Bridge Bay Resort on August 31, 2014 in Redding, California. As the severe drought in California continues for a third straight year, water levels in the State’s lakes and reservoirs is reaching historic lows. Shasta Lake is currently near 30 percent of its total capacity, the lowest it has been since 1977. (Photo by Justin Sullivan/Getty Images)

Exceptional drought, the most severe category, afflicts over 58 percent of California. The current drought may be a mere sneak preview of longer lasting and more devastating drought in the coming century, according to a study published in the Journal of Climate.

The study – by Cornell University, University of Arizona and U.S. Geological Survey researchers – examines the likelihood of so-called “megadroughts” and how they may change as the climate warms. Megadroughts are distinguished by their duration, lasting a decade or more.

Megadroughts have occurred throughout history, roughly every 400 to 600 years.

The last megadrought to plague the U.S. occurred in the western U.S. in the mid-12th century when temperatures were more than one degree C. above the long-term average.

The fear is that future climate warming could make such megadroughts more common, as rising temperatures speed up evaporation, intensifying drying in areas where precipitation doesn’t fall.

“The convergence of prolonged warming and arid conditions suggests the mid-12th century may serve as a conservative analogue for severe droughts that might occur in the future,” concluded a 2009 study in the Proceedings of the National Academy of Sciences.

The new Journal of Climate study, in its forward-looking analysis, says climate models, alone, underestimate the megadrought risk – not fully capturing the range of precipitation variability likely in a warming climate. For this reason, its analysis of future megadrought risk also incorporates historical data, endeavoring a more realistic assessment.

“In the US Southwest, for instance, state-of-the-art climate model projections suggest the risk of a decade-scale megadrought in the coming century is less than 50%,” the study says. “[O]ur analysis suggests that the risk is at least 80%, and may be higher than 90% in certain areas.”

“For the southwestern U.S., I’m not optimistic about avoiding real megadroughts,” said Cornell professor Toby Ault, and lead author of the paper in a news release. “As we add greenhouse gases into the atmosphere – and we haven’t put the brakes on stopping this – we are weighting the dice for megadrought.”

While the study projects the U.S. Southwest to become more vulnerable to megadrought, its results show a decreased risk in the Northwest. Other areas around the world that may see elevated megadrought prospects in the coming decades? Southern Africa, Australia and the Amazon basin, according to the study.

“These findings are important to consider as adaptation and mitigation strategies are developed to cope with regional impacts of climate change, where population growth is high and multidecadal megadrought—worse than anything seen during the last 2000 years—would pose unprecedented challenges to water resources in the region,” the study says.

The present circumstances of California may offer a glimpse of the future. Some climate scientists believe the drought plaguing the West, including California, has already reached megadrought levels.

“The current drought could be classified as a megadrought — 13 years running,” Columbia University paleoclimatologist Edward Cook said at a lecture last year, according to Climate Central.

And Columbia University bioclimatologist Park Williams told USA Today: “When considering the West as a whole, we are currently in the midst of a historically relevant megadrought.”

For some additional valuable reading on megadrought risk, see:

In the Parching West, It’s Beginning to Feel like 1159 (DotEarth, Andrew Revkin)
California’s 100-year drought (Doyle Rice, USA Today)