In a groundbreaking study led by oceanographers at the University of Hawai'i at Mānoa, new insights have emerged revealing an alarming acceleration of ocean acidification beneath the surface of the North Pacific Ocean near Hawai'i. While scientists have long understood that atmospheric carbon dioxide (CO₂) dissolving into ocean surface waters increases acidity -- a process that has steadily intensified since the dawn of the industrial revolution roughly two centuries ago -- this new research unveils that subsurface waters are acidifying at an even more rapid pace. The findings, recently published in the Journal of Geophysical Research: Oceans, challenge previous assumptions and provide critical data that could fundamentally alter current models of ocean chemistry and climate interactions.

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

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