A drought, equal to the worst to have hit the western US in recorded history, is already under way, say scientists.
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.
According to the authors of this new paper, a megadrought in North America refers to a multi-decade event, that contains periods of very high severity that last longer than anything observed during the 19th or 20th centuries.
The authors say there have been around 40 drought events over the period from 800-2018 in the western US.
Of these, only four meet the criteria for a megadrought.
These were in the late 800s, the mid-1100s, the 1200s and the late 1500s.
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.
Read more at BBC