By Matt McGrath Environment correspondent
BBC | Science and Environment
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Lake Mead saw huge drop in water levels during the recent drought. Photo: GETTY IMAGES
Researchers say the megadrought is a naturally occurring event that started in the year 2000 and is still ongoing.
Climate change, though, is having a major impact with rising temperatures making the drought more severe.
Some researchers are more cautious, saying that it is too early to say if the region really is seeing a true megadrought.
How do researchers know what drought conditions were like in the past?
The key to this new study is the use of tree ring records to reconstruct soil moisture data for the past 1200 years.
The team were also able to use supporting evidence such as medieval tree stumps growing in normally wet river beds, the abandonment of settlements by indigenous civilisations at the peak of the 13th century drought, plus evidence from lake deposits indicating wildfire activity was enhanced during these drought periods.
What did the study find?
The researchers discovered that when they compared the worst 19-year drought events in the past to soil moisture records from 2000-2018, the current period is already worse than three of the four megadroughts recorded.
The fourth one, which ran from 1575 to 1603 was likely the worst one of all, but the difference with the present event is slight.
"The first two decades of this drought look just like the first two decades of all of the mega droughts," said lead author Dr Park Williams, from Columbia University in New York.
"In fact, it is essentially tied with the worst two decades of the worst of the mega droughts."
Is the current megadrought a natural event or was it caused by climate change?
The authors say that undoubtedly the current drought situation is a natural event but is being made much worse by climate change.
The key event seems to have been the El Niño/La Niña weather phenomenon.
"We know from, from many lines of evidence that when you have La Niña type conditions in the tropical Pacific Ocean, then the southwestern US and northern Mexico get dry. And that's what we've seen over the last two decades," said Dr Williams.
But climate change has super-charged the current drought.
The authors say that in the western US, temperatures have gone up by 1.2C since 2000. Hotter air holds more moisture and that moisture is being pulled out of the ground. They believe that climate change is responsible for about half of of the pace and severity of the current event. "It doesn't matter if this is exactly the worst drought ever," said co-author Benjamin Cook, who is affiliated with Columbia University and Nasa's Goddard Institute for Space Studies. "What matters is that it has been made much worse than it would have been because of climate change."