A groundbreaking study led by researchers at the University of Colorado Boulder has fundamentally reshaped our understanding of climate variability in the North Pacific Ocean, linking recent temperature changes to anthropogenic greenhouse gas and aerosol emissions rather than natural causes alone. Published on August 13th in the prestigious journal Nature, this research elucidates the mechanisms underpinning the Pacific Decadal Oscillation (PDO) and its unprecedented persistence in a negative phase, a phenomenon now tied closely to the historic megadrought affecting the American Southwest. The implications of this work extend beyond academic curiosity, signaling a dire water crisis for the region that may extend for decades to come.
For over a century, climatologists have tracked the cyclical patterns of the PDO, an ocean-atmosphere phenomenon characterized by shifts in North Pacific sea surface temperatures roughly every 20 to 30 years. These oscillations vacillate between a warm, positive phase and a cool, negative phase, the latter marked by cooler waters along the U.S. West Coast. During these negative periods, storm tracks move poleward and inland rainfall declines significantly, often heralding drought conditions across the western United States. Until recently, the scientific consensus held that these phases were predominantly natural oscillations driven by internal variability in the ocean-atmosphere system.
The newest findings challenge this long-held assumption by presenting robust evidence that human activity, specifically emissions of greenhouse gases and industrial aerosols, has gained substantial influence over the PDO since the mid-twentieth century. Leveraging an innovative array of more than 500 climate model simulations, the research team demonstrated that anthropogenic factors account for over 50% of PDO variability from the 1950s onward, a dramatic shift from the primarily natural drivers observed prior to that period. This revelation forces a reevaluation of the PDO’s role in modern climate dynamics and water resource forecasting in drought-prone regions.
What sets this study apart is the application of enhanced modeling techniques that address the notorious “signal-to-noise paradox” in climate science—a challenge wherein models tend to overestimate natural variability while underplaying the impact of human-induced external forcings. By implementing corrections for this bias, researchers achieved greater fidelity in simulating past climate patterns and unraveling the dominant forces shaping the PDO. According to Jeremy Klavans, lead author and postdoctoral researcher in CU Boulder’s Department of Atmospheric and Oceanic Sciences, this methodological breakthrough allowed them to confidently attribute the prolonged negative phase of the PDO—and consequently the enduring megadrought—to anthropogenic emissions rather than natural cycles.
Since the 1990s, the PDO has been anomalously stuck in its negative phase, a duration that far exceeds its usual oscillation period. This stuck phase has amplified the already severe hydrological deficits across the American Southwest by rerouting precipitation-laden storm systems northward and suppressing moisture availability. Such prolonged dry conditions have culminated in the region’s current status as one of the driest in over 1,200 years, with approximately 93% of the western United States experiencing drought and 70% confronted by severe levels. The persistence of this dry trend poses significant challenges for water management, agriculture, ecosystems, and urban planning in an area already experiencing heightened climate vulnerability.
Intriguingly, the team found that natural climatic resets, such as the strong El Niño event in 2015—which typically would shift the PDO back toward a positive phase—failed to produce lasting change. Instead, the PDO briefly flipped positive only to revert swiftly to its negative state, signaling an underlying yet poorly understood anthropogenic forcing mechanism effectively anchoring the oscillation. This insight suggests that the PDO’s behavior in the coming decades cannot be assumed to follow historical natural rhythms, posing a formidable obstacle to conventional climate forecasting and resource management strategies.
By uncovering the extent of human influence on the PDO, the study invites a broader reconsideration of decadal climate variability as not merely internal fluctuations but as signals increasingly shaped by external forcings. This reframing holds profound implications for climate science, underscoring the importance of refining global climate models to better capture the sensitivities of regional climates to anthropogenic drivers. Co-author Pedro DiNezio emphasizes that existing models historically underestimated the sensitivity of regional systems like the PDO to such forcing, thus highlighting the necessity for improved simulation techniques in predicting climate extremities.
Beyond the American Southwest, the findings carry potential global significance. The research posits that similar oceanic modes, such as the North Atlantic Oscillation—which influences drought patterns in Europe including Spain—may likewise be modulated by human activity to a greater extent than previously recognized. If so, this might herald a new paradigm in understanding the anthropogenic imprint on ocean-atmosphere variability across multiple basins, with far-reaching consequences for regional climate risk assessments and adaptation planning worldwide.
The American Southwest’s water crisis today must therefore be viewed not as a transient meteorological anomaly but as a manifestation of a fundamental climate-driven reconfiguration of the hydrological cycle. As Klavans cautions, the ongoing drought is effectively a lasting transformation of the region’s water system, necessitating urgent, science-informed policy responses. Planners and decision-makers need to incorporate this expanded understanding into water resource management, infrastructure development, and conservation initiatives to mitigate the escalating risks posed by this anthropogenically locked PDO phase.
Looking forward, the study’s methodological innovations also provide a powerful toolset for improving long-term predictions of regional climate impacts, including precipitation patterns crucial for agriculture, ecosystems, and urban water supply. Amy Clement, co-author and professor at the University of Miami, highlights that enhanced model accuracy in capturing human-driven external forcings will enable more reliable climate projections, a critical capability for societies worldwide facing the multifaceted challenges of climate change. The integration of these new insights into climate modeling thus promises to reshape how scientists, policymakers, and communities prepare for and respond to the intensifying extremes of a warming planet.
In summary, the CU Boulder-led research marks a seminal advance in climate science, upending previously entrenched beliefs about the origins and persistence of the Pacific Decadal Oscillation. It definitively connects human emissions to the prolonged negative phase of the PDO, a key driver of the longest and most severe megadrought in over a millennium in the American Southwest. Far from temporary natural variability, the region’s drought is deeply rooted in human-induced changes to ocean-atmosphere dynamics, casting a long shadow over water security in the coming decades. This realization demands that climate mitigation and adaptation efforts account for these altered baseline conditions as our planet’s climate system increasingly responds to anthropogenic influence.
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News Publication Date: August 13, 2025
Web References: http://dx.doi.org/10.1038/s41586-025-09368-2
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Keywords: Pacific Decadal Oscillation, megadrought, anthropogenic forcing, greenhouse gas emissions, aerosols, North Pacific Ocean, climate modeling, signal-to-noise paradox, climate variability, water crisis, American Southwest, El Niño, climate adaptation
