BURSTGreenhouse 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,” lead author Jeremy Klavans, a postdoctoral researcher at the University of Colorado Boulder, said 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.
“The PDO has been locked in a consistent downward trend for more than three decades, remanding nearby regions to a steady set of climate impacts,” the authors stated.
To explore why the negative PDO has persisted, Klavans and his colleagues turned to more than 570 climate simulations that reflected past conditions and help predict potential future circumstances.
They found that between 1870 and 1950, changes in the PDO were predominantly fueled by internal forces, but that since the mid-20th century, greenhouse gas and aerosol emissions have been responsible for more than half the variations.
And when they added in a new tool that corrected for previous climate model errors — which had left some shifts undetected — the authors learned that human emissions drove nearly all the forced changes to the PDO.
While this study focused on megadrought in the U.S. West, Klavans said that their methods could be applied to climate pattern fluctuations in other parts of the world. For example, he referred to the North Atlantic Oscillation, which is driving drought in Spain.
As for the U.S. West, Klavans warned that if greenhouse gas emissions continue to escalate, the PDO could remain in its negative phase and drought could persist for at least three decades.
Bearing that in mind, the authors expressed hope that water managers could use their results to help develop better water management programs.
“Water planners could set new expectations and make proper investments in water infrastructure now, knowing this drought is here to stay,” Klavans added.