U.S. megadrought worst in at least 1,200 years, researchers say

The megadrought that has gripped the southwestern United States for the past 22 years is the worst since at least 800 A.D., according to a new study that examined shifts in water availability and soil moisture over the past 12 centuries.

The research, which suggests that the past two decades in the American Southwest have been the driest period in 1,200 years, pointed to human-caused climate change as a major reason for the current drought's severity. The findings were published Monday in the journal Nature Climate Change.

Jason Smerdon, one of the study's authors and a climate scientist at Columbia University's Lamont-Doherty Earth Observatory, said global warming has made the megadrought more extreme because it creates a "thirstier" atmosphere that is better able to pull moisture out of forests, vegetation and soil.

"It's a slow-motion train wreck," he said. "What we showed in the paper is that increasing temperatures in the Southwest contributed about 42 percent to the severity of this drought."

Over the past two decades, temperatures in the Southwest were around 1.64 degrees Fahrenheit higher than the average from 1950 to 1999, according to the researchers. Globally, the world has warmed by about 2 degrees Fahrenheit since the late 1800s.

In the study, Smerdon and his colleagues analyzed tree rings to piece together water availability throughout history, using data that stretches back to the year 800 A.D. The scientists focused on a swath of North America from southern Montana to northern Mexico, and from the Pacific Ocean east to the Rocky Mountains.

Tree rings are one way that scientists can estimate soil moisture conditions in past climates. Researchers can build detailed chronologies by looking at the thickness of tree rings, with thinner rings indicating drier years with less moisture, and wider rings signifying wetter years.

The results can then be compared to other paleoclimate signatures, including from sediment samples and archaeological records, as well as observational data throughout history, to forensically fill out a timeline of Earth's climate.

The researchers found that several significant megadroughts have occurred in the region over the past 12 centuries, some even lasting up to 30 years. Before the current megadrought, the region had not experienced such dry conditions since medieval times, in the late 1500s.

But while droughts occur naturally throughout history, climate change is making them both more frequent and more intense, the scientists said. And compared to other megadroughts in the historical record, what's surprising is that this current drought shows no signs of letting up, said A. Park Williams, a climate scientist at the University of California, Los Angeles, and the study's lead author.

"Twenty-two years in, some of these big megadroughts in the past were starting to peter out," he said. "This drought is not petering out. Instead, it's in full swing and is as strong now as it ever was before."

Based on current climate trends, the study found that there's a 75 percent chance that the current drought will hit the 30-year mark.

During that time, precipitation levels may fluctuate, with some years being wetter than others, Williams said. With such extreme dryness in the region, however, it will likely take multiple consecutive wet years to end the drought.

"When you have worse and worse droughts that are closer and closer together, there's just no time to recover," Williams said.

Isabel Monta?ez, a geologist and geochemist at the University of California, Davis, who was not involved with the research, said the study confirms the grim outlook that models have been projecting but adds a new component of isolating the impact of human-caused climate change.

"Being able to conclude that around 40 percent of this is probably driven by climate change is not surprising to anyone who does this work, but it's now been tested," she said. "And it’s quantitative.”

Monta?ez's own research focuses on studying deposits in caves, such as stalagmites, to understand the incidence and intensity of droughts and wildfires throughout California's history. She said that scientists have long known about the impacts of global warming on droughts, but that communities are only now grappling with the consequences.

The megadrought has already depleted water supplies in two of the largest reservoirs in North America, forcing officials across states to rethink water management strategies for the years ahead. Dry conditions have also increased the risk of wildfires year-round and threatened the livelihoods of farmers and ranchers in the region.

"It's going to transform how we operate," Monta?ez said. "Big changes are coming."

While the study's findings are alarming, Smerdon said it's still not too late to avert the worst impacts of climate change. He said he hopes the research compels communities and governments to act.

"If you're on a boat getting tossed around by waves, you don't go up to the captain and ask, 'Are we screwed?' You find a bucket and you start bailing water," he said. "That's the attitude we all have to have about this. We need to all go pick up a bucket and get to work."