The ocean acts as a massive sink for atmospheric CO₂, absorbing roughly a quarter of human-generated emissions annually. While surface waters have long been studied to understand the impacts of acidification—characterized primarily by decreasing pH levels and carbonate ion concentrations—the subsurface layers have remained relatively enigmatic. Leveraging a rare and invaluable dataset collected over 35 years by the Hawai‘i Ocean Time-series (HOT) program, the research team embarked on a detailed analysis of carbon chemistry throughout the entire water column, extending nearly three miles deep at Station ALOHA, a remote site 60 miles north of O‘ahu.

Remarkably, their findings reveal that the intensification of ocean acidification is not confined to surface waters alone. Instead, acidification indicators exhibit even more rapid changes in subsurface layers. This vertical intensification was consistently observed across all measured parameters—pH, partial pressure of CO₂ (pCO₂), and carbonate alkalinity—marking a novel and consequential discovery. The data indicate that these deeper waters, naturally more acidic due to respiration processes and long water mass residence times, are becoming disproportionately altered by increasing anthropogenic carbon inputs and shifts in water mass properties.

Underlying this subsurface acceleration of acidification is a complex interplay of biogeochemical and physical processes. Organic matter derived from plankton and other surface organisms sinks and decomposes throughout the water column, releasing CO₂ which contributes additional acidification beyond what atmospheric equilibrium alone would predict. Furthermore, changes in temperature and salinity in these layers suggest the influence of advected waters sourced from higher latitudes in the North Pacific, where environmental and climatic changes have remodeled water mass characteristics before these currents deliver them to Hawai‘i’s open ocean.

The implications of these findings are profound for marine ecosystems. Many planktonic species and benthic organisms that inhabit subsurface environments rely on stable carbonate chemistry to maintain their physiological processes, including shell and skeleton formation. Enhanced acidification threatens to destabilize these processes, potentially leading to declines in population and shifts in community structure that cascade through the food web. These chemical changes also bear consequences for nutrient cycling and biological productivity, integral components of ocean health and global carbon flux.

Lucie Knor, the lead author and postdoctoral researcher at SOEST, emphasized the unexpected magnitude of this discovery, noting that previous global-scale studies had hinted at subsurface acidification but none had documented such uniform and rapid change across all indicators. The meticulous examination of nearly four decades of data from Station ALOHA not only affirms the continuation of this trend but also highlights regional sources and oceanographic processes that exacerbate the acidification cycle.

Compounding these challenges, the recent era has witnessed an unprecedented frequency of marine heatwaves and severe El Niño events. These anomalies disrupt ocean temperature profiles and circulation patterns, potentially interacting synergistically with acidification processes. The overlap of thermal stress and chemical stressors on marine organisms raises new concerns about ecosystem resilience and adaptability, underscoring the urgency for comprehensive monitoring and mitigation efforts.

A critical aspect of the study is the identification of the role regional-scale circulation and water chemistry transformations play in shaping the subsurface acidification trends observed at Station ALOHA. Christopher Sabine, a co-author and SOEST professor, reveals that the evolving environmental conditions in distant North Pacific source waters propagate downwards through ocean currents, highlighting a connectivity between remote oceanographic events and the local ocean environment surrounding Hawai‘i. This insight reshapes the framework through which ocean acidification is both studied and managed.

Looking ahead, the research team is advancing investigations to isolate and quantify the anthropogenic carbon component within the layered water column, discerning human-driven influences amid complex natural variability. This endeavor is crucial for refining predictive models and informing global climate mitigation strategies. Understanding the vertical distribution and temporal shifts in anthropogenic carbon uptake will enable more accurate assessments of the ocean’s capacity to serve as a carbon sink in a rapidly changing world.

This study’s robust observational approach, grounded in one of the longest continuous oceanographic datasets in existence, exemplifies the critical value of sustained monitoring efforts. Without persistent measurements extending beyond the surface, such comprehensive recognition of subsurface acidification trends would remain elusive. The enduring commitment of programs like HOT lays the foundation for informed ocean stewardship, ensuring that emerging threats to ocean health are detected and addressed in a timely manner.

In an era marked by accelerating environmental change, the revelation that subsurface waters are acidifying more rapidly than surface layers adds a new and alarming dimension to our understanding of the oceanic carbon cycle. It invites a reassessment of marine ecosystem vulnerability and reinforces the necessity for integrated scientific inquiry spanning chemistry, biology, and physical oceanography. These insights bear weighty ramifications for climate models, fisheries management, and conservation efforts not only in Hawai‘i but across global oceanic systems.

Ultimately, this research calls for intensified scientific focus on the vertical stratification of ocean acidification—a complex and dynamic facet of anthropogenic climate influence. As the ocean continues to absorb vast quantities of CO₂, tracking the intricate evolution of its chemistry through the depths will be essential to safeguard marine biodiversity and the broader planetary health that depends on it. The sustained acidification of subsurface waters stands as a clarion warning of the pervasive and multifaceted reach of humanity’s footprint beneath the waves.

Subject of Research: Not applicable

Article Title: Drivers and Variability of Intensified Subsurface Ocean Acidification Trends at Station ALOHA

News Publication Date: 27-Jun-2025

Web References:

• Journal of Geophysical Research: Oceans
• University of Hawai‘i School of Ocean and Earth Science and Technology (SOEST)

References:
Knor, L., Sabine, C., et al. (2025). Drivers and Variability of Intensified Subsurface Ocean Acidification Trends at Station ALOHA. Journal of Geophysical Research: Oceans. DOI: 10.1029/2024JC022251

Image Credits: Carolina Funkey

Keywords: Ocean acidification, subsurface acidification, Pacific Ocean, Station ALOHA, carbon cycle, biogeochemistry, marine heatwaves, El Niño, ocean circulation, Hawai‘i Ocean Time-series, anthropogenic carbon, marine ecosystems

Ocean acidification arises when CO₂ from the atmosphere reacts with seawater, forming carbonic acid and thereby lowering pH levels. This phenomenon poses existential risks to marine ecosystems, particularly organisms dependent on calcium carbonate for their shells and skeletons, including corals and various plankton species. The research team, spearheaded by postdoctoral researcher Dr. Lucie Knor, meticulously analyzed a comprehensive dataset spanning 35 years, collected by the Hawai‘i Ocean Time-series program at Station ALOHA—an open ocean site located approximately 60 miles north of O‘ahu, Hawai‘i. Unlike most previous studies focused primarily on surface waters, this investigation spans the entire water column, extending to nearly three miles deep, offering an unprecedented vertical profile of changing ocean chemistry.

Dr. Knor expressed profound surprise at the uniformity of the acidification intensification across multiple parameters throughout the entire water column. “We anticipated that some indications of acidification would accelerate more quickly below the surface, as global models have suggested localized intensifications. However, seeing every single ocean acidification indicator change at a faster rate below the surface was an unexpected and concerning revelation,” she detailed. These indicators include measures such as pH, carbonate ion concentration, and total dissolved inorganic carbon, each demonstrating escalating shifts that highlight the multi-dimensional nature of ocean acidification.

Underlying this rapid intensification is a complex interplay of biogeochemical processes. The research highlights that an increase in carbon content throughout the water column corresponds to the natural decomposition of sinking organic matter, a phenomenon that releases CO₂ as microbes break down plankton and other organisms that perish and descend from the sunlit surface. This decomposition not only contributes to the carbon pool but also exacerbates acidification processes by increasing local acidity in subsurface layers. Furthermore, the study identifies associations between accelerated acidification and changes in water temperature and salinity, with fresher and colder waters in some layers intensifying the chemical shifts.

The consequences of these transformations run deep in both literal and ecological senses. Subsurface waters of the North Pacific are naturally more acidic compared to surface waters, and this baseline acidity is worsening at an accelerating rate. Scientists warn that such conditions could seriously disrupt the foundational planktonic species that underpin marine food webs, potentially triggering cascading effects across broader oceanic ecosystems. As Dr. Knor emphasizes, “The rapidly increasing acidity in these deeper waters might imperil species that have adapted to relatively stable chemical environments, potentially leading to profound shifts in biodiversity and ecosystem function.”

Moreover, alterations in sub-surface ocean chemistry have strategic implications for the ocean’s capacity to serve as a carbon sink. Oceans currently absorb approximately 25-30% of anthropogenic CO₂ emissions, mitigating atmospheric concentrations and buffering global temperature rise. However, as acidification alters carbonate chemistry, it may reduce the ocean’s efficiency in sequestering CO₂, potentially accelerating climate change feedback loops. This dynamic underscores the far-reaching interconnectedness of subsurface ocean conditions to global climate regulation.

Environmental changes affecting subsurface ocean chemistry near Hawai‘i are not isolated phenomena; they are driven by larger-scale shifts in Pacific Ocean circulation and source water properties. Subsurface waters arriving at Station ALOHA originate farther north in the Pacific and are transported southward via complex current systems. As such, regional environmental transformations—including variations in temperature, salinity, and carbon content at source points—are propagated into Hawai‘i’s subsurface ocean environment. Co-author Christopher Sabine, a SOEST Oceanography professor, elaborates, “Our research evidences that regional shifts in source water chemistry and ocean circulation are central to the intensified acidification trends observed at depth.”

Another emerging layer of complexity stems from the interaction between acidification and marine heatwaves, which have surged in frequency and intensity over recent decades. Prolonged warming events linked to multi-year El Niño episodes exacerbate stress on marine organisms, often overlapping with periods of heightened acidity. This combination could amplify negative biological outcomes, including coral bleaching, reduced calcification rates, and disruptions to fishery resources. The convergence of these stressors necessitates integrated monitoring and management strategies tailored to a dynamically evolving oceanic environment.

The Hawai‘i Ocean Time-series program’s decades-spanning dataset—with its detailed, continuous measurements—provides an invaluable foundation for understanding these intricate processes. Station ALOHA serves as a sentinel site, offering critical long-term observational clarity that can feed into global and regional climate models, improve projections, and inform mitigation policies. This dataset empowers researchers to disentangle natural variability from anthropogenic impacts, a vital step for robust environmental assessments.

Currently, the research team is advancing their focus towards isolating the anthropogenic carbon component within the total dissolved inorganic carbon pool at various depths. This avenue aims to clarify the proportional contributions of human-made CO₂ relative to natural sources and cycles, enabling enhanced understanding of human fingerprints in ocean chemistry. Such insights could refine predictions about future acidification trajectories and their ecological implications.

Given the foundational ecological ramifications and the intersection with global climate dynamics, this study’s revelations underscore an urgent need for enhanced ocean monitoring, targeted ecological impact research, and holistic climate action. Protecting subsurface marine habitats and maintaining the ocean’s vital role in climate regulation demands coordinated international efforts informed by cutting-edge science. As ocean acidification trends grow ever more complex and rapid, the window for meaningful intervention narrows, underscoring the vital importance of this and similar research initiatives.

In sum, the discovery of rapidly intensifying subsurface ocean acidification near Hawai‘i challenges existing paradigms and calls for urgent scientific and policy attention. By expanding the scope of acidification research beyond the surface, the University of Hawai‘i team has illuminated a hidden crisis unfolding beneath the waves—a crisis that could profoundly impact marine biodiversity, fisheries, and climate regulation alike. This study provides a clarion call to the global scientific and environmental communities to deepen investigations and accelerate conservation and mitigation measures.

Subject of Research:
Not applicable

Article Title:
Drivers and Variability of Intensified Subsurface Ocean Acidification Trends at Station ALOHA

News Publication Date:
27-Jun-2025

Web References:
https://agupubs.onlinelibrary.wiley.com/doi/10.1029/2024JC022251

References:
Knor, L., Sabine, C., et al. (2025). Drivers and Variability of Intensified Subsurface Ocean Acidification Trends at Station ALOHA. Journal of Geophysical Research: Oceans. DOI: 10.1029/2024JC022251

Image Credits:
Carolina Funkey

Keywords:
Ocean Acidification, Subsurface Ocean Chemistry, Pacific Ocean, Hawai‘i Ocean Time-series, Climate Change, Carbon Dioxide, Marine Ecosystems, Ocean Circulation, Anthropogenic Carbon, Marine Heatwaves