According to a 2025 report from the Potsdam Institute for Climate Research (PIK), ocean acidification has now exceeded a safe boundary for ecosystems. It is the first time it is placed in the ‘danger zone’. This also means that seven out of the nine ‘planetary boundaries’ have now been crossed.
This concept of ‘planetary boundaries’ was defined in 2009 by some thirty researchers to determine thresholds for Earth-system processes that humanity should not cross to remain within a ‘safe operating space’. The article was entitled “A safe operating space for humanity”.
Stress on aquatic ecosystems
At the time, they believed that humanity had transgressed at least three planetary boundaries, but since then, the annual reports of PICR have shown a continuous deterioration. The 2025 report indicates that the threshold of ‘ocean acidification’ has now been crossed, placing us in a dangerous condition with a worsening trend.
While acidity is measured using pH, the reference for this limit is the concentration of aragonite, a mineral essential to the life of corals and shelled marine animals. The more acidic the ocean, the more aragonite disintegrates.
Since the beginning of the industrial era, ocean acidity levels have increased by 30 to 40 percent. This change threatens organisms that form calcium carbonate shells or skeletons, such as corals, molluscs, or crucial plankton species.
For instance, pteropods, tiny free-swimming marine snails, are already showing signs of shell damage in parts of the ocean. The gradual disappearance of these organisms can disrupt the food chain, scientists warn.
The authors also note that marine oxygen levels are trending downward and marine heatwaves are increasing, which compounds the stress on aquatic ecosystems. Multiple stressors affect oceans simultaneously: acidification, warming, biodiversity loss, and nutrient runoff all interact.
Six limits crossed
The main cause of ocean acidification is the absorption of carbon dioxide (CO2) emitted by the burning of fossil fuels. The degradation of the ocean’s ability to regulate carbon and climate is also a concern: weakening of the ocean as a carbon sink can have feedbacks that worsen climate change.
The other six limits that have already been exceeded concern climate change (CO2 in the atmosphere), the integrity of the biosphere (extinction of species and appropriation of resources by humanity), land use change (deforestation), the freshwater cycle (, water cycle disruptions, areas affected by drought or floods), biogeochemical cycles (addition of fertilizers and pesticides) and the introduction of new entities into the biosphere (plastics, chemical pollutants, synthetic compounds, and other industrial chemicals).
Hazardous to the stability of Earth systems
The two planetary boundaries not crossed and still within ‘safe operating space’ are the level of aerosols in the atmosphere (air pollution) and the level of ozone in the stratosphere.
The authors of the report warn that crossing these boundaries is hazardous to the stability of the Earth system, as the boundaries are interconnected; exceeding one can amplify stresses on others.
Dr Sylvia Earle, oceanographer and Planetary Guardian, said, “The Ocean is our planet’s life-support system. Without healthy seas, there is no healthy planet. For billions of years, the ocean has been Earth’s great stabiliser: generating oxygen, shaping climate, and supporting the diversity of life. Today, acidification is a flashing red warning light on the dashboard of Earth’s stability. Ignore it, and we risk collapsing the very foundation of our living world. Protect the ocean, and we protect ourselves.”
