Emerging research shows that greenhouse gas emissions may have forced the Pacific Decadal Oscillation into a decades-long dry phase—turning natural cycles into human-imposed drought.

Greenhouse gas emissions generated by humans may have “locked in” a climate pattern responsible for long-term megadrought across the U.S. West, a new study has found.

Human-induced shifts to the pattern, called the Pacific decadal oscillation (PDO), are directly linked to the persistent aridity plaguing California and the Colorado River basin states, according to the study, published in Nature.

“The drought and ocean patterns we’re seeing today are not just natural fluctuations — they’re largely driven by human activity,” said lead author Jeremy Klavans, a postdoctoral researcher at the University of Colorado Boulder, in a statement.

The PDO, a fluctuation of the North Pacific Ocean, usually waxes and wanes every couple decades, the authors explained. Its positive phase usually involves warmer waters in the eastern Pacific and colder waters near Japan. In the negative phase, the reverse happens, with colder conditions along the U.S. West coast and warmer ones in Japan.

Since the 1990s, however, the PDO has been stuck in an uncharacteristically long negative phase, according to Klavans. That prolonged status quo has generated colder air, which holds less moisture than warm air and caused precipitation declines across the U.S. West, the authors explained.

As a result, about 93 percent of the U.S. West has been enduring drought, with 70 percent experiencing severe dryness, the researchers noted. Previous studies, they added, have indicated that the Southwest is its driest in at least 1,200 years.

While scientists have long viewed the PDO as driven by natural fluctuations, the authors explained that if that theory was accurate, the pattern should have flipped to positive in 2015 after a strong El Niño warming event.

Instead, they observed, the PDO shifted only briefly to positive before quickly reverting to the negative phase.