Southwestern megadrought worst in 1,200 years: study

The extreme weather conditions that have desiccated North America’s Southwest over the past 22 years have now become the region’s driest “megadrought” since the year 800, a new study has found.

The ongoing megadrought has supplanted the previous record-holder: a late-16th century dry spell previously considered the worst such drought in the past 1,200 years, according to the study, published on Monday in Nature Climate Change. And the study’s authors in large part attributed the severity of the current megadrought — defined as a drought lasting two decades or more — to human-induced climate change.

“Without climate change, the past 22 years would have probably still been the driest period in 300 years,” Park Williams, lead author and a geographer at UCLA, said in a statement.

“But it wouldn’t be holding a candle to the megadroughts of the 1500s, 1200s or 1100s,” he added.

The scientists focused on the area from southern Montana to northern Mexico, from the Pacific Ocean to the Rocky Mountains — finding repeat megadrought periods from 800 through 1600 that exceeded any subsequent event in severity through the 1900s.

To pinpoint particularly severe stretches of drought, the authors analyzed tree ring patterns — which reveal critical information about annual soil moisture — and cross-checked their results with historical climate data. Ultimately, they found that such dry conditions coincided with high degrees of “soil moisture deficit,” a metric that compares soil moisture with normal saturation levels.

The scientists found that since the year 2000, the average soil moisture deficit was twice as severe as the deficit in any 20th century drought — surpassing the driest periods of all the most severe megadroughts in the past 1,200 years.

Soil moisture plays a critical role in drought — impacting runoff levels, streamflow, agricultural productivity, ecosystem health and wildlife activity, according to the study.

Although the scientists agreed that climate change has become a significant driver of megadroughts, they also acknowledged that some dramatic shifts in dryness and water availability occurred in the Southwest before human-induced climate change became a factor in the 20th century.

Well before humans began inflicting their carbon footprint upon the Southwest, the region was experiencing “infamous megadroughts that occurred repeatedly from 800-1600,” the study said.

Yet while today’s conditions would likely have been dry even without climate change, it would have been much less so, the scientists contended. Human-caused climate change, they said, has been responsible for about 42 percent of the soil moisture deficit in the region since 2000, according to their study.

Climate change is making droughts more severe as warming temperatures cause increased evaporation — drying out soil and vegetation, the authors said. From 2000 to 2021, for example, temperatures in the region of study were 0.91 degrees Celsius (1.64 degrees Fahrenheit) higher than average temperatures from 1950 to 1999, the study found.

As of just last week, the U.S. Drought Monitor indicated that 95 percent of the Western U.S. was experiencing drought conditions, while in the summer of 2021, the Colorado River’s two main reservoirs — Lake Mead and Lake Powell — had plummeted to their lowest levels since record-tracking began in 1906, the authors noted.

With dry conditions likely to persist in the region, the scientists estimated that it would take multiple wet years to remediate the damage.

The authors credited some recent efforts — such as federal cuts this past summer to Colorado River water allocations, as well as California Gov. Gavin Newsom’s (D) drought emergency declaration — as important steps toward ensuring short-term recovery.

But Williams, the study’s lead author, stressed that long-term water conservation efforts must expand beyond periods of acute drought.

“It’s extremely unlikely that this drought can be ended in one wet year,” he said.