Megadrought Emerging in Western US is One of the Most Severe Since 800 A.D.

The western U.S. is in the midst of a "megadrought" comparable to the worst droughts since the year 800 A.D., and human-driven climate change is largely to blame, scientists have said.

Writing in the journal Science, researchers used centuries' worth of data collected from tree rings in the southwestern states, from Oregon to New Mexico. Using tree rings, the scientists were able to deduce the moisture content in the soil and from that, what the climate was like in years where there are no reliable weather observations.

Previous studies have involved similar techniques to link periods of extreme weather to social upheaval and unrest as far back as the pre-Hispanic Aztec era. The "famine of One Rabbit" in 1454 in Mexico, for example, was preceded by an extended drought. In Europe, scientists have linked a colder and more changeable climate to the fall of the Roman Empire.

Megadroughts are severe droughts that can last for decades. In the latest study, researchers were able to identify four periods of extreme drought since 800 A.D.—the late 800s, the mid-1100s, the 1200s and the late 1500s.

The team compared their findings to precipitation, temperature and humidity data for the period between 2000 and 2018.

Findings showed that the last 19 years are not only comparable to previous droughts, but have been drier than any 19-year period during the megadroughts of the 800s, 1100s and 1200s. It is notable, they say, that previous megadroughts lasted longer than the current drought has so far, meaning they have a higher chance of registering a severe 19-year period.

The results suggest we have not yet surpassed the severe droughts of the sixteenth century—which are thought to have contributed to the death of half of Mexico's native population post Spanish conquest by exacerbating disease—but it comes close.

"Earlier studies were largely model projections of the future," Park Williams, lead author and a bioclimatologist at Columbia University's Lamont-Doherty Earth Observatory, said in a statement. "We now have enough observations of current drought and tree-ring records of past drought to say that we're on the same trajectory as the worst prehistoric droughts."

Ryan Towell, Director of Science and Solutions at The Climate Reality Project, who was not involved in the study, told Newsweek the results show changes in the climate are having profound impacts on lives and livelihoods right now.

"Folks living in the western U.S. have experienced firsthand the impacts that the extreme drought has had since it began in 2000," he said. "This study only confirms what they've witnessed. The researchers in this case show that the climate crisis isn't some distant problem, but rather something we are already contending with."

According to the study's authors, it is likely that the atmospheric and ocean anomalies responsible for previous megadroughts were more severe than those involved in the 2000-2018 drought. In theory, they should have produced more extreme droughts. However, the impact of climate change has exacerbated natural events in the 21st century, meaning the the severity of the current drought is comparable—and, in most cases, even worse—than these megadroughts of the past.

The researchers also point out that the current drought is more consistently affecting a larger proportion of the southwest U.S., which they say is consistent with anthropogenic climate change.

The increase in global temperatures as a result of climate change increases the evaporation of water on the land and in lakes, rivers and other sources, causing more moisture to be held in the air and not the ground. Places like the southwest U.S. are left with drier soil and are more vulnerable to drought.

While the early 21st century may have been a period of drought in normal circumstances, the researchers say it is roughly twice as severe due to human activity.

"In the absence of anthropogenic climate trends, 2000–2018 would still rank among the 11 most severe prolonged droughts in the reconstruction, but anthropogenic warming was critical for placing 2000–2018 on a trajectory consistent with the most severe past megadroughts," the study's authors write.

Remains after the Blue Cut Fire
The U.S. is in the midst of a “megadrought” comparable to the worst megadroughts since 800 CE, say researchers. From insect outbreaks to crop failure to wildfire, droughts can have a devestating effect on people living in the area. Image shows the remains of a destroyed home after the Blue Cut Fire in California, August 17, 2016. ROBYN BECK/AFP/Getty

David W. Stahle, Distinguished Professor of Geography at the University of Arkansas, who was not involved in the study, described the results as "sobering." Without human-driven climate change, the period between 2000 and 2018 would have been "just another episode of reduced precipitation, low soil moisture, and poor tree growth in the U.S. Southwest during the past millennium," Stahle said in an accompanying editorial.

"Instead, the artificially increased temperature, lower relative humidity, and rising vapor-pressure deficits have killed millions of western trees and helped to make the early 21st century the second most severe and sustained period of megadrought in 1,200 years."

The researchers expect the impact of higher temperatures and climate change on weather events like drought to increase—"Because the background is getting warmer, the dice are increasingly loaded toward longer and more severe droughts," said Williams.

"We may get lucky, and natural variability will bring more precipitation for a while. But going forward, we'll need more and more good luck to break out of drought, and less and less bad luck to go back into drought."

Towell told Newsweek there is still time to avoid the worst impacts but there is no time to waste. "We have every expectation that as our planet continues to warm, the impacts, like long-term, extreme drought, will become more severe," he said. "We can and must transition to a clean energy economy and we must do it now."