A new CU Boulder-led study has revealed that recent changes in North Pacific Ocean temperatures—long believed to be the result of natural variability—are in fact being driven by human-generated greenhouse gas and industrial aerosol emissions. These oceanic shifts are directly linked to the prolonged megadrought gripping the American Southwest, and this research published August 13th in Nature suggests it may not ease for another 30 years.
"Our results show that the drought and ocean patterns we're seeing today are not just natural fluctuations—they're largely driven by human activity," said Jeremy Klavans, postdoctoral researcher in CU Boulder's Department of Atmospheric and Oceanic Sciences and lead author of the study.
Cracking a long-standing climate puzzle
For over a century, scientists have tracked a climate cycle in the North Pacific known as the Pacific Decadal Oscillation (PDO), which alternates every few decades between a warm "positive" phase and a cool "negative" phase. In its negative phase—where cooler waters hug the U.S. West Coast—storm tracks shift northward and rainfall in the western U.S. decreases significantly.
Until now, the PDO was assumed to be governed almost entirely by natural internal processes, such as air-sea interactions. Even the most recent Intergovernmental Panel on Climate Change (IPCC) report stated with high confidence that the PDO is not influenced by human activity.
But using a new suite of over 500 climate model simulations, Klavans and his team found that since the 1950s, over half of the variability in the PDO can be attributed to human emissions, including both greenhouse gases and aerosols. Prior to 1950, these patterns were mostly driven by natural processes.
The PDO is stuck—and that's a problem
The PDO has remained locked in a negative phase since the 1990s—an unusually long period for what is typically a fluctuating cycle. This prolonged cool phase has been a major driver of the ongoing megadrought in the western U.S., drying out the region by pushing precipitation-bearing storms farther north and reducing overall moisture in the air.
"If the PDO were purely natural, we would have expected it to shift back to positive after the strong El Niño in 2015," Klavans said. "Instead, it flipped briefly and then reverted—suggesting something deeper and undiscovered is holding it in place."
New tools, new understanding
This breakthrough was made possible by advances in climate modeling that helped scientists correct for an issue in models known as the "signal-to-noise paradox." This study finds that most climate models have historically overestimated natural variability while underestimating the effects of human-driven external forcing.
"Once we corrected for that imbalance, it became clear that human emissions are the dominant factor behind the current PDO pattern and the West's extreme dryness," Klavans said.
Pedro DiNezio, a professor at the University of Colorado and co-author of the study, added: "Our work shows that on decadal timescales, climate models have been underestimating how sensitive regional climates are to external forcing."
Worst drought in over 1,200 years—and no relief in sight
The implications of this work are sobering. Research shows the American Southwest is in the midst of its driest 20-year period in at least 1,200 years. About 93% of the western U.S. is currently in drought, with 70% experiencing severe conditions.
The study warns that if greenhouse gas emissions continue at current rates, the PDO will likely stay in its negative phase, and the region's water crisis will deepen.
"This isn't a temporary dry spell," Klavans said. "It's a climate-driven transformation of the region's water system. Planners and policymakers need to treat it as such."
Global consequences
The findings may also extend beyond the Pacific. Similar patterns exist in other ocean basins—like the North Atlantic Oscillation, which is tied to droughts in parts of Europe, including Spain.
"Our methods have the potential to drastically improve predictions of climate impacts – including precipitation trends across the globe" Amy Clement, a professor at the University of Miami and co-author of this study added. "That kind of foresight is critical for planning and adapting to a changing climate."
