While the term megadrought has no strict definition, it is generally considered to be a severe dry period persisting for several decades or longer. Many climate researchers and hydrologists have long thought that a Southwestern megadrought was highly likely. A 2016 study put the probability of one occurring this century at 70 percent or higher.

Dr. Williams and his colleagues reconstructed drought conditions in the Southwest for every year back to 800 A.D., using tree growth as a proxy for soil moisture content. Examining nearly 1,600 tree-ring records, they found four periods of more than two decades each during which soil moisture content was far below the baseline for the entire 1,200 years, indicating severe drought conditions of lack of precipitation and increased dryness. One of these megadroughts, in the 13th century, lasted more than 90 years.

Their analysis showed that, as measured by soil moisture content, the current drought is more severe than three of the ancient ones. Only one in the late 1500s was worse, and not by much, the researchers said.

“Ancient megadroughts have always been seen by water managers as worst-case scenarios,” Dr. Williams said, “and we just have to hope that there’s some kind of protection measure in the climate system that’s not going to allow one of those to repeat itself. And what we’re seeing is that we’re actually right on track for one.”

Water temperatures in the tropical Pacific Ocean are thought to play a large role in determining drought conditions in the Southwest, both now and in the past. Cooler sea-surface temperatures — what is commonly called La Niña — affect the jet stream, which pushes storms approaching Western North America to the north, leaving the Southwest drier.