The California Current, a crucial marine ecosystem along the western coast of North America, has long been recognized for its dynamic interplay of oceanographic processes, including upwelling—an ocean phenomenon where nutrient-rich deep waters rise to the surface, fueling productivity. However, recent research published in Nature Communications by Stoll, Deutsch, Jurikova, and colleagues unveils a sobering transformation over the past century: the upwelling system in this region not only sustains biodiversity but now dramatically amplifies ocean acidification, a process with far-reaching implications for marine life and coastal economies.
Ocean acidification is a by-product of increased atmospheric carbon dioxide (CO2) concentrations, with oceans absorbing roughly a quarter of anthropogenic CO2 emissions. This absorption alters seawater chemistry, lowering pH and reducing carbonate ion availability, vital for calcifying organisms such as shellfish and corals. In the California Current, acidification is being exacerbated by the upwelling of deeper waters naturally richer in carbon dioxide, creating hotspots of intensified chemical stress beyond baseline global ocean trends. The research team deployed an integrative approach, combining long-term observational data with advanced biogeochemical modeling, to reconstruct changes in the carbonate system and unravel the mechanisms driving these shifts over the last century.
At the core of their findings lies the discovery that the California Current’s upwelling system heightens acidification beyond what would be expected solely from atmospheric CO2 increases. The process of upwelling, typically bringing cool, nutrient-dense water from depths, also transports waters with elevated CO2 concentrations and lower pH. Over time, intensified climatic and oceanographic changes have modified the timing, intensity, and biogeochemical signatures of these upwelled waters, creating an amplified acidification scenario which has escalated since the early 20th century. This subtle but insidious process threatens the foundational species of this rich coastal ecosystem.
The study meticulously analyzes historical data spanning multiple decades, including surface pH observations, total alkalinity, dissolved inorganic carbon, and other carbonate parameters measured at various locations along the California coast. Such long-term datasets are rare but critical, enabling a temporal context to shifting ocean chemistry patterns. Through this analytical lens, the authors discern a noteworthy trend: the magnitude of acidification episodes caused by upwelling events is increasing. Moreover, these acidification spikes tend to coincide with seasonal upwelling periods, suggesting that organisms reliant on these environments face not just a gradual decline in pH but acute, cyclical acid stress.
One of the profound implications concerns marine calcifiers, which depend on carbonate ions to build their shells and skeletons. The California Current harbors many economically and ecologically significant species, including oysters, mussels, and pteropods, that form the base of marine food webs. Amplified acidification disrupts their ability to mineralize calcium carbonate efficiently, rendering them more vulnerable to predation, disease, and reproductive failure. This cascade threatens the fisheries and communities that rely on these resources, signaling an urgent need for mitigation and adaptation strategies based on robust scientific understanding.
Interestingly, the research also highlights that upwelling-driven acidification is not uniform but exhibits spatial heterogeneity influenced by local physical and biological factors. Coastal geomorphology, wind patterns, biological uptake and release of CO2 from respiration and photosynthesis all modulate seawater chemistry at scales ranging from kilometers to tens of kilometers. This complexity underscores the challenge in predicting localized acidification impacts and designing marine protected areas or conservation frameworks to shield vulnerable ecosystems effectively.
Methodologically, the authors leveraged coupled physical-biogeochemical models calibrated with historical observations. These models simulate seasonal and interannual variability in upwelling strength and associated carbonate chemistry, enabling exploration of future scenarios under continued anthropogenic CO2 emissions. Simulations reveal that without significant mitigation efforts, the amplifying effect of upwelling on acidification could intensify further by the mid-21st century, placing additional stress on marine organisms during critical life stages, such as larval development and settlement.
Further complicating the picture is the interaction of acidification with other concurrent stressors such as warming, hypoxia (oxygen depletion), and nutrient loading from terrestrial sources. These combined stressors may act synergistically, exacerbating physiological challenges for marine species. The California Current is thus emerging as a microcosm exemplifying how climate change can drive multiple overlapping impacts on ocean ecosystems through interconnected physical and chemical pathways.
The authors emphasize the importance of continuous monitoring and improved mechanistic understanding of biogeochemical cycles in upwelling systems. Enhanced observational networks encompassing autonomous sensors, ship-based surveys, and remote sensing technologies are critical for resolving fine-scale heterogeneity and temporal dynamics in ocean chemistry. Such data integrated with high-resolution models offer the best prospects for forecasting ecosystem responses, informing fisheries management, and devising adaptive strategies that sustain ecosystem services in the face of climate change.
This century-scale analysis of the California Current serves as a clarion call about the complex and often underappreciated feedbacks between physical oceanographic processes and biogeochemical changes. The amplification of acidification by upwelling processes highlights the need to consider local and regional ocean dynamics when assessing global ocean health. It also showcases the value of leveraging historical data archives combined with cutting-edge computational tools to reveal long-term trends that may otherwise remain obscured.
In conclusion, this pioneering research provides comprehensive evidence that upwelling systems, traditionally viewed as natural drivers of ocean productivity, are paradoxically accelerating the deleterious impacts of ocean acidification by transporting CO2-rich waters to the surface. The findings underscore an urgent imperative for the scientific community, policymakers, and resource managers to collaborate in monitoring, modeling, and mitigating acidification impacts—protecting both marine biodiversity and human livelihoods dependent on these dynamic coastal ecosystems. As climate change intensifies, understanding such critical ocean processes and their consequences is paramount to safeguarding the future of our oceans.
Subject of Research: Changes in ocean acidification and biogeochemistry in the California Current upwelling system over the past century
Article Title: A century of change in the California Current: upwelling system amplifies acidification
Article References:
Stoll, M.M.V., Deutsch, C.A., Jurikova, H. et al. A century of change in the California Current: upwelling system amplifies acidification. Nat Commun 16, 9661 (2025). https://doi.org/10.1038/s41467-025-63207-6
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