The world’s oceans have failed a key planetary health check for the first time, primarily due to the burning of fossil fuels, a report has shown.
In its latest annual assessment, the Potsdam Institute for Climate Impact Research said ocean acidity had crossed a critical threshold for marine life.
This makes it the seventh of nine planetary boundaries to be transgressed, prompting scientists to call for a renewed global effort to curb fossil fuels, deforestation and other human-driven pressures that are tilting the Earth out of a habitable equilibrium.
The report, which follows earlier warnings about ocean acidity, comes at a time of record‑breaking ocean heat and mass coral bleaching.
Oceans cover 71% of the Earth’s surface and play an essential role as a climate stabiliser. The new report calls them an “unsung guardian of planetary health”, but says their vital functions are threatened.
The 2025 Planetary Health Check noted that since the start of the industrial era, oceans’ surface pH has fallen by about 0.1 units, a 30-40% increase in acidity, pushing marine ecosystems beyond safe limits.
Cold-water corals, tropical coral reefs and Arctic marine life are especially at risk. This is primarily due to the human-caused climate crisis. When carbon dioxide from oil, coal and gas burning enters the sea, it forms carbonic acid. This reduces the availability of calcium carbonate, which many marine organisms depend upon to grow coral, shells or skeletons.
Near the bottom of the food chain, this directly affects species like oysters, molluscs and clams. Indirectly, it harms salmon, whales and other sea life that eat smaller organisms. Ultimately, this is a risk for human food security and coastal economies.
Scientists are concerned that it could also weaken the ocean’s role as the planet’s most important heat absorber and its capacity to draw down 25-30% of the carbon dioxide in the atmosphere. Marine life plays an important role in this process, acting as a “biotic bump” to sequester carbon in the depths.
In the report, all of the other six breached boundaries – climate change, biosphere integrity, land system change, freshwater use, biogeochemical flows, and novel entities – showed a worsening trend. But the authors said the addition of the only solely ocean-centred category was a alarming development because of its scale and importance.
Levke Caesar, co-lead of Planetary Boundaries Science Lab, noted that it took a year for newly emitted gases to mix into the atmosphere, but a thousand years to settle into the ocean. “To be a good scientist, I have to take emotions out of work. Still, I would say looking at this data, when I allow myself to connect to it emotionally, then I am afraid. This really scares me,” she said.
She stressed that there was still much that could be done to tackle this problem – particularly in reducing fossil fuels, but also cutting pollution and managing fisheries more assiduously: “If were a planetary doctor, I would sit down with my patient and say: ‘Last year, I suggested you change something but you didn’t. You haven’t changed to healthier habits or moved to a better diet. So your health is declining further. Now it is really time to consider something.’”
The report notes that good policies and international cooperation have made a difference in ensuring that two planetary boundaries – ozone layer and aerosol emissions – have not been breached: “Decades of international action, like the Montreal protocol and shipping regulation, show that policy can turn the tide.”
The Potsdam Institute director, Johan Rockström, said: “We are witnessing widespread decline in the health of our planet. But this is not an inevitable outcome. The drop in aerosol pollution and healing of the ozone layer, shows that it is possible to turn the direction of global development,. Even if the diagnosis is dire, the window of cure is still open. Failure is not inevitable; failure is a choice. A choice that must and can be avoided.”
