Decades Without Rain: America Set for 'Megadroughts' Not Seen Since Medieval Times

America's Southwest may soon be hit "megadroughts" that last for decades—the likes of which have not been seen since the Medieval period, scientists have said. By looking at what caused the 14 megadroughts the region has experienced between 800-1600 A.D., researchers discovered the climatic conditions present then are fast approaching now, indicating megadroughts will happen again.

"Megadroughts are the most severe kinds of droughts, and they can last multiple decades. But in North America, they haven't occurred since the 1500s," Nathan Steiger, from the Lamont-Doherty Earth Observatory at Columbia University, told Newsweek.

"Several civilizations are thought to have been dramatically affected by these past droughts including the Maya and the Anasazi; there is some evidence that the collapse of their civilizations could have been partially driven by megadroughts."

Steiger is lead author on a study published in Science Advances that examined the spate of megadroughts in the U.S. Southwest during the Medieval period. Previously, megadroughts have been linked with La Nina activity—where water temperatures in the eastern Pacific are cooler than normal. La Nina and its counterpart El Nino, where ocean temperatures are warmer, occur on irregular intervals and normally last up to a year—although they can persist for many years.

In the study, Steiger and colleagues looked at how La Nina and radiative forcing, where Earth absorbs more heat from the Sun than it expels, interact to produce megadroughts. They reconstructed sea surface temperatures over the last 2,000 years and found there were three climatic elements that, when combined, would lead to a megadrought. This included high radiative forcing, a decrease in volcanic activity (which can help cool the planet) and very strong and frequent El Nina.

Megadroughts occurred when severe and frequent La Ninas forced storms northwards, drying out the Southwest. Combined with global warming, this resulted in prolonged periods of dryness. It is estimated that one megadrought in Sierra Nevada lasted for 200 years. To put that in perspective, the recent drought in California ended after seven years.

Researchers say that while the Medieval radiative forcing was the result of natural climate variability, today's global warming is not. Anthropogenic climate change, they say, may well contribute to megadroughts in the Southwest in the future.

"Based on modeling studies of future climate, we would expect to see increased drying in the American West as a result of increased CO2, which will make megadroughts more likely," Steiger said. "But the trigger for these megadroughts will most likely be severe and/or frequent La Ninas which are not very predictable at all. So we won't know what decades in the future will experience a megadrought in the American West, but we're unfortunately confident that they'll happen again."

california drought
A car in the dried out Almaden Reservoir on January 28, 2014 in San Jose, California. The drought in California lasted for seven years. Justin Sullivan/Getty Images

Steiger said preventing global warming could reduce the likelihood of megadroughts happening in the future, but that this is unlikely: "I don't think we'll be able to prevent them from happening. We need to assume that they'll be coming and adapt water policies, our water use, and water infrastructure accordingly."

As to what will happen during a megadrought, he said societies will be highly disrupted as water becomes more scarce: "Communities and farms will likely be stretched to their limits and may even collapse. This has certainly been the case for droughts in California in recent years."

The risk of megadroughts in the Southwest has been highlighted before. Toby Ault, from Cornell University, calculates the risk of megadroughts. In 2015, his research suggested the Southwest will experience a megadrought before the end of the 21st century.

Ault, who was not involved in the research, said the latest findings provide "some of the strongest and most scientifically robust insight into the causes of megadroughts in the past."

He told Newsweek the research is significant because it provides an insight into what could happen in the future—based on what happened in the past, potentially providing a better idea of when a megadrought might unfold.

"Finally, but most importantly, the study really drives home how very small changes in the balance of the climate system can have big impacts on megadrought risk," Ault said. "The changes that occurred in the past and led to megadrought conditions are actually pretty tiny compared to the magnitude of possible climate change impacts if we continue on our current path of greenhouse gas emissions this century."