Observed Large-Scale and Deep-Reaching Compound Ocean State Changes Over the Past 60 Years
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A 2025 study finds that multiple ocean stressors — warming, deoxygenation, salinity shifts, and acidification — are emerging across up to 32% of the global subsurface ocean, creating compound climate hotspots with growing risks for ecosystems and fisheries.
The ocean does not face the consequences of climate change one stressor at a time. Warming, acidification, deoxygenation, and salinity shifts are all unfolding simultaneously — and the regions where they converge are experiencing something qualitatively different from, and potentially more damaging than, any single change in isolation.
Tan et al. (2025) present the first comprehensive, observation-based global assessment of these compound climatic impact-drivers (CIDs) in their study — tracking not just individual ocean stressors, but where and when multiple stressors have simultaneously and persistently emerged from background variability across both surface and deep ocean layers over the past 60 years. The results reveal an ocean in the midst of a broad, deep, and accelerating state transformation.
Key Findings
A New Framework: Compound CIDs and Time of Emergence
The study introduces a formal framework for detecting and characterizing compound CIDs — defined as the simultaneous, persistent emergence of multiple ocean stressors (temperature, salinity, dissolved oxygen, and surface pH) beyond the noise of natural variability, sustained over periods exceeding 25 years.
Emergence is determined using a time of emergence (ToE) methodology, in which a long-term signal (extracted via a 25-year LOWESS filter) is compared to background short-term variability (noise). The ToE is defined as the first year in which the absolute signal-to-noise ratio (SNR) exceeds 1 and permanently remains above that threshold — representing a persistent, irreversible departure from the historical baseline.
The framework distinguishes three levels of concurrent change: single emergence (one CID), double emergence (two CIDs simultaneously, e.g., warming and deoxygenation), and triple emergence (all three physical/biogeochemical stressors simultaneously). Each level of emergence is further characterized by three metrics — duration, intensity, and magnitude — which together define the ocean exposure category (low, medium, or high) for any given region.
The study draws on multiple independent observational datasets for each variable, including IAP, Ito, Ishii, and EN4 products for temperature, salinity, and dissolved oxygen, as well as surface pH products, providing robust cross-validation of the results.
Individual CID Emergence: Already Widespread
Surface pH has the fastest and most geographically extensive emergence of all CIDs studied: nearly the entire global ocean surface (~100%) has already experienced significant acidification emergence since 1995, relative to a 1985–1989 baseline. This is attributed to the continuous increase in anthropogenic CO₂ emissions driving a net positive flux of CO₂ from the atmosphere into the ocean.
Ocean warming emergence in the epipelagic (0–200 m) and mesopelagic (200–1,000 m) zones has been detected since the early 1990s in roughly 20–60% of the global ocean area, driven by enhanced radiative forcing from rising greenhouse gas concentrations and the well-documented acceleration of ocean heat uptake since the 1960s.
Salinity changes — salinization in the Atlantic and Indian Oceans, freshening in much of the Pacific — have emerged across substantial ocean areas, driven by the intensification of the global hydrological cycle under the "wet-get-wetter, dry-get-drier" paradigm. Local temperature and salinity changes reflect both perturbed air–sea fluxes and redistribution through ocean circulation.
Dissolved oxygen decline has emerged across significant fractions of both the epipelagic and mesopelagic zones, attributable to a combination of warming-driven reductions in oxygen solubility and changes in ventilation and stratification that limit oxygen replenishment from the surface.
Compound CID Emergence: Growing and Deep-Reaching
In the case of double emergence (warming combined with either salinity change or deoxygenation), the percentage of global ocean area affected has grown from roughly 7% at the surface to approximately 32% at the bottom of the mesopelagic zone since the 2000s. Triple emergence (all three physical stressors simultaneously) has reached about 8–11% of ocean area from the epipelagic zone to the mesopelagic floor.
Crucially, the incidence of compound emergence increases with depth — from surface waters down to 1,000 m — reflecting the deep-reaching nature of these concurrent state changes and the importance of subsurface ocean dynamics in propagating long-term signals downward through subduction, mixing, and ventilation.
The results are robust across multiple observational datasets and are not sensitive to the choice of data product, providing strong confidence in the spatial and temporal patterns identified.
Regional Hotspots: Where Multiple Stressors Converge
Five regions stand out as especially prominent compound CID hotspots, where simultaneous emergence of multiple stressors is most advanced and most intense:
Mediterranean Sea: The highest proportion of significant double and triple emergence in the epipelagic zone of any region (~96%), driven by its semi-enclosed, evaporative character, warm and saline inflows, and oxygen dynamics linked to deep convection and the overflow from the Red Sea and Persian Gulf.
Subtropical North Atlantic: ~93% double/triple emergence in the epipelagic zone, driven by enhanced air–sea heat and freshwater exchange, Atlantic "salinity pile-up," deoxygenation in eastern boundary upwelling zones and tropical OMZs, and large-scale circulation variability including the AMOC and Atlantic Multidecadal Oscillation.
Tropical Atlantic: ~71% compound emergence in the epipelagic zone, with OMZ expansion and warming-driven deoxygenation playing central roles.
Arabian Sea and Northern Indian Ocean: Triple emergence is widespread in the mesopelagic zone (~58% of 0–30°N), driven by intensifying OMZ expansion, monsoon-induced circulation changes, Red Sea and Persian Gulf overflows, and strong air–sea interaction.
Subtropical Pacific: ~42% of the mesopelagic zone within 23–40°N shows significant compound emergence, tied to deep-reaching (~800 m) dynamical patterns in the subtropical gyre.
These regional patterns reflect distinct physical mechanisms operating through three types of multivariate interactions: joint relationships (e.g., concurrent warming and salinity change jointly altering stratification and density), causal relationships (e.g., warming directly driving deoxygenation via reduced oxygen solubility), and composite relationships (e.g., combined warming, salinification, and deoxygenation acting through multiple simultaneous pathways on the biogeochemical cycle).
Exposure and Ecological Implications
Approximately 25% of the global subsurface ocean is already significantly exposed (medium to high) to the emergence of more than two CIDs simultaneously, and this fraction is expected to continue growing.
The biological consequences of compound exposure are complex and non-linear. Multiple simultaneously emerging stressors can interact synergistically (combined effect greater than the sum of individual effects), additively, or antagonistically (combined effect less than the sum) across different organisms and ecosystems. Species known to be affected include corals, phytoplankton, zooplankton, invertebrates, aquatic woody plants, and marine mammals.
The biological carbon pump — the ocean's critical mechanism for transferring atmospheric CO₂ into deep waters — is already operating under compound stress. Approximately 48.28%, 13.17%, and 2.83% of current global organic carbon export at 100 m depth originates from regions experiencing medium-to-high exposure to single, double, and triple CID emergences respectively, raising concerns about the long-term integrity of the ocean's role as a carbon sink.
Global fisheries are similarly exposed: roughly 51.47%, 14.33%, and 3.01% of high-intensity fishing regions are now subject to significant single, double, and triple CID emergences, with notable affected areas including the eastern North Atlantic, Gulf Stream, Mediterranean Sea, Tropical Atlantic, Kuroshio, and the Atlantic Subtropical Gyre.
A substantial and growing fraction of Biodiversity Beyond National Jurisdiction (BBNJ) high seas areas is experiencing compound emergence — rising from ~6% at the surface to ~38% in the mesopelagic zone. This directly informs the implementation of large-scale marine protected areas under the recently adopted BBNJ Treaty.
A New Lens on Ocean Vulnerability
This study's significance lies not just in what it finds, but in how it reframes our understanding of ocean change. Individual stressor assessments can underestimate cumulative risk because they do not capture the interactions and feedbacks that occur when multiple pressures coincide. By showing that compound CID emergence has already become pervasive — from the surface to 1,000 m depth, across all major ocean basins, and in regions critical for carbon cycling and global fisheries — the study establishes that the ocean is undergoing a systemic state transition, not a collection of isolated changes.
For policy and management, the findings carry concrete implications: compound CID hotspots should be priority areas for marine protection, climate adaptation planning, and long-term ocean monitoring. Climate models should be evaluated not only for their skill in projecting individual variables but for their ability to reproduce compound emergence patterns — a challenge given the large spread among current CMIP6 simulations in subsurface salinity and dissolved oxygen. And ocean risk frameworks — from fisheries management to BBNJ governance — must move beyond single-variable assessments toward multiparameter, compound-impact approaches that more accurately reflect the ocean conditions that ecosystems and economies will actually face.
The Ocean is Changing All at Once
The ocean's response to climate change is not a sequence of individual problems arriving one by one. It is simultaneous, interconnected, and already well underway at depth. The emergence of compound CIDs across the subtropical and tropical Atlantic, the Arabian Sea, the Mediterranean, and the subtropical Pacific represents not just a scientific finding but an early warning — one whose implications for carbon cycling, marine biodiversity, and food security are only beginning to be understood.
Tan et al.'s study makes clear that the question now is not whether the ocean is changing all at once, but whether our monitoring systems, models, and governance frameworks can keep pace with the scale and speed of that transformation.
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