The Western US Is Facing The Worst Megadrought In Over 1,200 Years

Los Angeles, California - August 31 2009: Station Fire was the largest wildfire of the 2009 California wildfire season. ARM Photo Video/Shutterstock

A historic megadrought is parching the American West. One of the leading culprits is climate change, and with global temperatures only set to rise further, the emerging crisis is now thought to be on the “same trajectory as the worst prehistoric droughts".

Reported in the journal Science, a new study has used 1,200 years of tree-ring data, soil moisture records, and dozens of climate models to describe the ever-lengthening string of dry spells that have hit the western United States and northern Mexico over recent decades. 

They found that a chronic drought has been lurking around the southwestern US and northern Mexico since at least the year 2000. The spell of aridity has already become so severe it’s already surpassed three of the four notoriously arid megadroughts of Medieval times. The researchers were uncertain about the extent of the fourth Medieval megadrought, which spanned from 1575 CE to 1603 CE, although they note the current drought is affecting wider areas more consistently than any megadrought since 800 CE.

"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," lead author Park Williams, a bioclimatologist at Columbia University's Lamont-Doherty Earth Observatory, said in a statement.

Areas of southwestern North America affected by drought in the early 2000s; darker colors are more intense. Yellow box shows the study area. Williams et al., Science, 2020

It’s clear the drought is driven by a range of “natural” phenomena, such as fluxes of the El Niño/La Niña weather phenomenon in the Pacific Ocean, although climate change is pushing this trend into much more dangerous territory. The researchers estimate that if climate change was removed from the equation, then the current drought would only rank as the 11th worst detected, not one of the worst ever.

"It doesn't matter if this is exactly the worst drought ever. What matters is that it has been made much worse than it would have been because of climate change," said study co-author Benjamin Cook, who is affiliated with Lamont and the Goddard Institute for Space Studies.

Some researchers have pointed out the study’s definition of a superdrought is not strictly accurate, since it’s only lasted for 19 years, not the standard 20 years usually needed to meet that definition. As such, it could be too early to say whether the region is at the start of a true megadrought.

Nevertheless, regardless of the definitions, few would dispute there’s a worrying situation on the horizon.

"Whether or not the western US has crossed a threshold into an event that goes by any specific label, what's been clear this century is that water is an essential resource in the western US, and it's a precarious one, because the region can have long spells with little precipitation," Dr Angeline Pendergrass, a scientist with the US National Center for Atmospheric Research, who was not directly involved with the study, commented to BBC News.

"And it looks like climate change won't make it better – indeed, it will likely make it worse."

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