Emissions are driving the West’s worst drought in 1,200 years

New evidence shows that the North Pacific is no longer drifting on its own. Human-driven greenhouse gases and industrial aerosols are steering ocean temperatures in ways that dry out the American West. The same forces may keep the region parched for decades.

Researchers at the University of Colorado Boulder and collaborators used hundreds of climate simulations to revisit a famous ocean pattern and link its recent behavior to the West’s record-breaking drought.

Human hand in Pacific cycle

For more than a century, scientists have watched the Pacific Decadal Oscillation (PDO), which seesaws over decades between a warm, “positive” phase and a cool, “negative” phase.

In the negative phase, cooler waters line the U.S. West Coast, storm tracks shift north, and rain and snow across the Southwest decline.

The standard view held that this cycle was almost entirely internal to the climate system. The latest IPCC report even judged, with high confidence, that human influence was negligible. However, the new study overturns that view.

Using over 500 model runs, the team found that since the 1950s, human emissions explain more than half of PDO variability, while before 1950, natural swings dominated.

“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 lead author Jeremy Klavans, a postdoctoral researcher in CU Boulder’s Department of Atmospheric and Oceanic Sciences.

Pacific stuck in dry mode

The PDO has lingered in its negative state since the 1990s. That persistence is unusual. It matters because it pushes moisture north and leaves the Southwest with less precipitation and drier 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.”

That “something deeper” is the external shove from greenhouse gases and industrial aerosols, the authors conclude. The ocean pattern is still a cycle, but it is being nudged into one gear.

Human forcing clear in models

The team’s rethink relied on advances in climate modeling. For years, many models suffered from a “signal-to-noise paradox.”

They inflated the role of random swings while muting the effect of external forcing from human activity. The researchers corrected that imbalance across a large ensemble.

“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.

“Our work shows that, on decadal timescales, climate models have been underestimating how sensitive regional climates are to external forcing,” said co-author Pedro DiNezio, a professor at the University of Colorado.

Worst drought on record

Tree ring records show the Southwest’s past two decades are the driest since at least 800 CE. Across the West, most areas are in drought, and a large share is in severe drought. Reservoirs, soils, forests, and rivers reflect the strain.

The study warns that if emissions continue on their current path, the PDO is likely to stay negative for many years, which would lock in the megadrought. This would constrain water supplies, heighten wildfire risk, and pressure farms and cities.

“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.”

Adapting to a changing climate

The methods in this paper may sharpen forecasts far from the Pacific. Other ocean patterns, such as the North Atlantic Oscillation, shape rainfall in Europe and beyond.

Better separating human forcing from internal swings could improve projections of wet and dry trends on the time horizons that matter for planning.

“Our methods have the potential to drastically improve predictions of climate impacts – including precipitation trends across the globe,” said co-author Amy Clement, a professor at the University of Miami. “That kind of foresight is critical for planning and adapting to a changing climate.”

For the American West, the message is blunt. Managing a water system built for a wetter past will require new rules, new investments, and, above all, time. The PDO may eventually flip. But the study argues that external forcing has tilted the odds against a quick reversal.

Cut emissions, calm the Pacific

The science also underscores the leverage of emission cuts. Less forcing means less push on the ocean cycle and better odds that the Pacific can slip out of its drought-favoring mode sooner.

This is not a story of the ocean alone. It is the story of how greenhouse gases and aerosols have rewired a familiar Pacific rhythm, with cascading effects on storms, snowpack, and rivers. The pattern is still oscillating – it is just doing so on a drought-shifted baseline that is reshaping the West.

The study is published in the journal Nature.

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