Drought in Western U.S. may become the worst in history

Over the last couple of decades, drought has plagued much of the western United States and northern Mexico. Scientists have predicted that climate change is pushing the region toward an unprecedented long-term drought.

The results of a new study indicate that a megadrought is already in progress, and the event could become as bad or worse than any known drought in history. 

Based on weather observations, 1,200 years of tree-ring data, and dozens of climate models, the analysis suggests that climate warming is a key contributor to the extreme drought conditions. 

The investigation is the most comprehensive of its kind, covering a region that spans from Oregon and Montana down through California and New Mexico into northern Mexico.

Study lead author Park Williams is a climatologist at Columbia University’s Earth Institute.

“Earlier studies were largely model projections of the future,” said Williams. “We’re no longer looking at projections, but at where we are now. 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.”

Modern observations only date back to around the beginning of the 20th century, but tree rings hold valuable scientific data for centuries.

The researchers used tree ring data from thousands of trees to analyze environmental factors such as soil moisture. They charted dozens of droughts across the region, beginning in 800 AD. 

The study pinpointed four events that stood out as being megadroughts during the late 800s, mid-1100s, the 1200s, and the late 1500s. 

The team compared the data from ancient megadroughts to soil moisture records from 2000 to 2018. They concluded that the drought in the Western U.S. is already exceeding the earliest three megadroughts in terms of severity.  

All of the ancient droughts lasted longer than 19 years, but they also started out with conditions similar to those that are now emerging. 

According to the researchers, the current drought is affecting wider areas more consistently than any of the early droughts, which can be attributed to global warming.

Overall, rising temperatures are responsible for about half the extreme conditions, while natural fluctuations are also playing a large role. 

Without the influence of manmade climate change, the ongoing drought would rank as the 11th worst in history, while now it is well on its way to ranking as the all-time worst.

“It doesn’t matter if this is exactly the worst drought ever,” said study co-author Benjamin Cook. “What matters is that it has been made much worse than it would have been because of climate change.” 

Since temperatures are expected to continue rising, the drought will persist for a long time, even if it briefly fades. 

“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.” 

Williams said it is conceivable the region could stay arid for centuries. “That’s not my prediction right now, but it’s possible.”

The drought has already produced devastating effects, ranging from worsening California wildlifes to shrinking water reservoirs in Colorado. Although 2019 was a relatively wet year, there are some indications that 2020 will be much less cooperative.

Angeline Pendergrass, a staff scientist at the U.S. National Center for Atmospheric Research, said that it is too early to say whether the region is on the cusp of a true megadrought, considering that natural weather fluctuations are also heavily involved. She added that “even though natural variability will always play a large role in drought, climate change makes it worse.”

The study is published in the journal Science.

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By Chrissy Sexton, Earth.com Staff Writer