In 2009, scientists defined nine areas that are crucial to the stability of the Earth system. Since then, they have been continuously calculating the extent to which human activities influence these areas. And to what extent these influences damage these important areas to such an degree that the risk to the entire system increases. From this, they deduce how far we can go. They define the degree of damage at which it becomes critical as planetary boundaries.
In September 2025, the Potsdam Institute for Climate Impact Research published its second “Planetary Health Check” report. (Planetary Boundaries Science 2025) Seven of the nine planetary boundaries have already been exceeded. Here, we largely adopt the wording of the “Planetary Health Check.”
"Planetary Health at a glance. Just as a blood test provides insights into a human body's health and identifies areas of concern, this Planetary Health Check evaluates the 13 measured control variables across the 9 Planetary Boundary (PB) processes to report on Earth’s stability, resilience, and life‐support functions – the overall health of our planet. The 2025 assessment shows that seven of the nine PBs have been breached: Climate Change, Change in Biosphere Integrity, Land System Change, Freshwater Change, Modification of Biogeochemical Flows, Introduction of Novel Entities, and Ocean Acidification. All of these show increasing trends, suggesting further deterioration in the near future. Two PB processes remain within the Safe Operating Space: Increase in Atmospheric Aerosol Loading (improving global trend) and Stratospheric Ozone Depletion (currently stable). The Planetary Health Check Symbol summarizes all of these findings, showing the Planet’s overall health at a glance."
The health of the ocean determines our livelihood
What is new is that ocean acidification has also exceeded the planetary boundary. The foreword of the Health Check states:
«While the focus remains on the whole Earth system, this report places special emphasis on the ocean – not only in recognition of new findings, but as a reflection of the ocean’s foundational role in planetary stability.
Most of the ocean is unexplored, and many questions remain about how the ocean shapes planetary processes – but clearly, without the ocean, the blue Earth would resemble our bleak red neighbor, Mars. After all: No water, no life. (...)
Industrial extraction of ocean wildlife, addition of toxic wastes, plastics, lost or tossed fishing gear, and unprecedented noise since the 1950s have contributed to the loss of about half of the ocean’s life, from coral reefs and forests of kelp, from charismatic megafauna to microscopic organisms. The living ocean is the planet’s largest carbon sink, climate stabilizer, and source of oxygen. We need a healthy ocean to have a healthy planet, but it cannot protect us if we do not protect it. The growing scale of impacts – rising acidity, deoxygenation, warming, and biodiversity loss – threatens to disrupt these essential functions.
This year’s report brings sobering news: For the first time, we have crossed the Planetary Boundary for Ocean Acidification. This paints a grave picture – not just for marine ecosystems, but for the entire Earth system that depends on a healthy ocean. (...)
The Planetary Health Check report is more than data. It’s a call to action. With each new insight comes greater responsibility – to protect the global commons, to invest in restoration and renewals, and to empower a new generation of planetary stewards. (...) By understanding the boundaries that keep Earth stable, we can make better choices – before tipping points become points of no return."
The ocean is sick
The Planetary Health Check summarizes the most important findings:
«The ocean absorbs a substantial proportion of the CO₂ released by human activities. This oceanic uptake slows climate change but causes the seawater to become more acidic – a process known as ocean acidification. Ocean acidification has now gone beyond what is considered safe for marine life. A key indicator of ocean acidification is the aragonite saturation state. Aragonite is a form of calcium carbonate that many marine organisms – like corals and shellfish – use to build their shells and skeletons.
As more CO₂ enters the ocean, it forms carbonic acid, which lowers the pH but also reduces the availability of carbonate. This makes it harder for these organisms to grow and survive. Marine ecosystems are already feeling the effects. Cold-water corals, tropical coral reefs, and Arctic marine life are especially at risk as acidification continues to spread and intensify.»
2025 Status
The Planetary Boundary for Ocean Acidification is now assessed as transgressed. This conclusion is based on a high-quality global observational dataset of surface ocean acidification variables, combined with a revised estimate of pre-industrial conditions. Recent model simulations (...) suggest that the pre-industrial aragonite saturation state (Ω) in the year 1750 was higher than previously thought, with a value of 3.57. (Jiang et al. 2023) This indicates that current ocean conditions have diverged more significantly from the pre-industrial state than earlier assessments suggested.
The Planetary Boundary for Ocean Acidification defines a "safe operating space" as 80% of the preindustrial Ω value – a 20% decline. Applying this threshold to the revised pre-industrial estimate of 3.57 yields a boundary value of 2.86. The observed global surface aragonite saturation state in 2024 was 2.84 which is below the revised Planetary Boundary threshold. This is supported by an independent study published earlier this year, which also found that the Planetary Boundary for ocean acidification has been transgressed. (Findlay et al., 2025)"
"Ocean Acidification has transgressed its Planetary Boundary. This figure shows how the global surface aragonite saturation state has changed over time (...). (Ford et al. 2025) It has declined significantly in recent decades and has now breached the Planetary Boundary. The baseline (green line) is a revised estimate of the pre-industrial aragonite saturation state around the year 1750, based on Jiang et al. (2023). Because it is higher than previous estimates, the revised Planetary Boundary (set at 80% of the pre‐industrial state; red line) is also higher – meaning today’s ocean is even further from its pre-industrial state than previously thought.
Key takeaway: Ocean acidification has gone beyond safe limits, increasingly endangering marine ecosystems."
Key DriversThe control variable for the Ocean Acidification Planetary Boundary is the global mean aragonite saturation state of the surface ocean. While the term “acidification” refers specifically to a decrease in pH (i.e., an increase in hydrogen ion concentration and acidity), ocean acidification involves a broader set of chemical changes in seawater driven by CO₂ uptake. (Ma et al., 2023) As the ocean absorbs more CO₂, it forms carbonic acid, which breaks apart and releases hydrogen ions. These hydrogen ions bind with carbonate, making less of it available. This lowers the aragonite saturation state, making it harder for shell-building organisms to maintain their shells and skeletons. Therefore, the aragonite saturation state indicates changes in ocean chemistry as well as the increasing pressure on marine life. Humancaused CO₂ emissions are the main driver of ocean acidification, causing a long-term decline in aragonite saturation state.
Biological Impacts of Ocean Acidification
Ocean acidification affects marine life in multiple ways. Calcifying organisms – such as corals, shelled mollusks, and some crustaceans – require more energy to build and maintain their calcium carbonate structures, a requirement that can hinder their growth and survival. High-latitude pteropods, tiny drifting snails also known as sea butterflies, are already showing signs of shell damage. (Bednaršek et al. 2021And 2012) Since these snails play an important role in the marine food web, their decline could cause ripple effects that impact other species, including those without shells, and potentially harm the wider marine ecosystem. The degradation of tropical coral reefs, biodiversity hotspots that also serve as critical habitat for early life stages, leads to the loss of essential shelter and resources for many marine species. These reefs are facing increased loss of suitable environmental conditions, including favorable pH levels. (Richardson et al. 2023) While increasingly frequent marine heatwaves are the main cause of shallow-water coral reef degradation in tropical regions, ocean acidification adds further pressure by impairing the recovery of bleached reefs.
Deep-water corals are particularly vulnerable to ocean acidification because they live near the aragonite saturation horizon – the depth where aragonite, the mineral that forms their skeletons, begins to dissolve. Acidification progresses faster in deep waters due to lower buffering capacity, causing this aragonite saturation horizon to rise closer to the surface. (Müller & Gruber 2024) As a result, deep-water corals face increasing difficulty building and maintaining their skeletons, making acidification one of their greatest threats.
Changes in Ocean Chemistry and CO₂ UptakeIn addition to harming marine life, ocean acidification also reduces the ocean’s ability to absorb CO₂ from the atmosphere. This effect is linked to acidification’s reduction of carbonate ion availability in seawater. Since these ions are crucial to the ocean’s buffering capacity – the system that helps neutralize acidity – their decline weakens this natural buffer. Recent observations show that the ocean’s ability to absorb CO₂ has slightly declined, likely due to a combination of reduced buffer capacity and changes in ocean circulation linked to climate change. (Müller & Gruber 2024) Despite this observed decline, the ocean still absorbs about 25% of human-caused CO₂ emissions (Friedlingstein et. al. 2025) and is expected to continue playing a major role in removing humancaused CO₂ from the atmosphere and moderating climate change. (Canadell et al. 2021)
Regional Differences in Ocean AcidificationThe Arctic Ocean has experienced the most severe acidification to date, driven by its naturally low buffer capacity, cold temperatures (which increase CO₂ solubility), and rising freshwater input. Some Arctic surface waters are already undersaturated with respect to aragonite, posing serious risks to calcifying organisms. At lower latitudes, tropical and subtropical waters currently maintain relatively high aragonite saturation states. However, the absolute rate of decline in aragonite saturation state is highest in these low latitude regions. (Findlay et al. 2025; Virkki et al. 2025) Coastal areas around the world often face additional acidification pressures from local factors such as upwelling, nutrient runoff from agriculture, high biological productivity, and freshwater input. These processes contribute to strong regional and seasonal variability in acidification levels."
"Global map of Ocean Acidification, as indicated by aragonite saturation state. This map shows anomalies in surface aragonite saturation state for the decadal mean of 2015-2024 with respect to year 1750 (Jiang et al. 2023).
Key takeaway: The Ocean has significantly acidified globally, particularly in the Arctic and Southern Ocean regions."
Figure: PBScience, (2025)
References
Bednaršek, N. et al. (2012): Extensive dissolution of live pteropods in the Southern Ocean. Nat. Geosci. 5, 881–885.
Bednaršek, N. et al. (2021): Integrated Assessment of Ocean Acidification Risks to Pteropods in the Northern High Latitudes: Regional Comparison of Exposure, Sensitivity and Adaptive Capacity. Front. Mar. Sci. 8.
Canadell, J. G. et al. (2021): Global Carbon and other Biogeochemical Cycles and Feedbacks. in IPCC AR6 WGI, Final Government Distribution chapter 5.
Findlay, H. S. et al. (2025): Ocean Acidification: Another Planetary Boundary Crossed. Glob. Change Biol. 31.
Ford, D. J. et al. (2025): UExP-FNN-U full surface ocean carbonate system. Zenodo . https://doi.org/10.5281/zenodo.15801067
Friedlingstein, P. et al. (2025): Global Carbon Budget 2024. Earth Syst. Sci. Data 17, 965–1039.
Jiang, L. et al. (2023): Global Surface Ocean Acidification Indicators From 1750 to 2100. J. Adv. Model. Earth Syst. 15.
Ma, D. et al. (2023): Four Decades of Trends and Drivers of Global Surface Ocean Acidification. Glob. Biogeochem. Cycles 37, e2023GB007765.
Müller, J. D. & Gruber, N. (2024): Progression of ocean interior acidification over the industrial era. Sci. Adv. 10, eado3103.
Planetary Boundaries Science (PBScience) (2025): Planetary Health Check 2025, edited by Kitzmann, N.H., Caesar, L., Sakschewski, B. and Rockström, J. with contributions from Sakschewski, B.*, Caesar, L.*, Andersen, L. S., Bechthold, M., Bergfeld, Beusen, A., L., Billing, M., Bodirsky, B. L., Botsyun, S., Dennis, D. P., Donges, J. F., Dou, X., Eriksson, A., Fetzer, I., Gerten, D., Häyhä, T., Hebden, S., Heckmann, T., Heilemann, A., Huiskamp, W., Jahnke, A., Kaiser, Kitzmann, N.H., J., Krönke, J., Kühnel, D., Laureanti, N. C., Li, C., Liu, Z., Loriani, S., Ludescher, J., Mathesius, S., Norström, A., Otto, F., Paolucci, A., Pokhotelov, D., Rafiezadeh Shahi, K., Raju, E., Rostami, M., Schaphoff, S., Schmidt, C., Steinert, N. J., Stenzel, F., Virkki, V., WendtPotthoff, K., Wunderling, N., Rockström, J. Potsdam Institute for Climate Impact Research (PIK), Potsdam, Germany. https://planetaryhealthcheck.org
Richardson, K. et al. (2023): Earth beyond six of nine planetary boundaries. Sci. Adv. 9, eadh2458.
Virkki, V. et al. (2025): Regionally divergent drivers behind transgressions of the freshwater change planetary boundary. Preprint at https://doi.org/10.31223/X54X7M.