The world’s oceans have failed a major planetary health check for the first time, largely due to the burning of fossil fuels, according to a new report.

In its latest annual assessment, the Potsdam Institute for Climate Impact Research (PIK) confirmed that ocean acidity has passed a critical threshold for marine life.

This marks the seventh of nine planetary boundaries to be breached, prompting scientists to call for renewed global efforts to curb fossil fuel use, deforestation and other human-driven pressures that are pushing the Earth beyond a safe and stable state.

The report, which reinforces earlier warnings about ocean acidification, comes amid record ocean temperatures and widespread coral bleaching.

Covering 71% of the planet’s surface, the oceans play a vital role in regulating the global climate. The report describes them as an “unsung guardian of planetary health”, warning that their essential stabilising functions are now under threat.

According to the 2025 Planetary Health Check, the average surface pH of the oceans has fallen by about 0.1 units since the start of the industrial era — representing a 30–40% increase in acidity that has pushed many marine ecosystems beyond safe limits.

Cold-water corals, tropical reefs and Arctic marine life are among the most vulnerable. The main driver is the human-induced climate crisis. When carbon dioxide from burning coal, oil and gas dissolves in seawater, it forms carbonic acid, reducing the availability of calcium carbonate — a substance vital for the growth of coral, shells and skeletons.

This chemical shift affects organisms at the base of the marine food web, such as oysters, molluscs and clams, and in turn impacts larger species including salmon, whales and other predators. Ultimately, these changes threaten human food security and the livelihoods of coastal communities.

Scientists warn that increasing ocean acidity could also undermine the seas’ role as the planet’s largest heat sink and as a carbon store, currently absorbing between 25% and 30% of global carbon dioxide emissions. Marine life helps facilitate this process through a so-called “biotic pump” that locks carbon away in the deep ocean.

All six previously breached planetary boundaries — climate change, biosphere integrity, land-system change, freshwater use, biogeochemical flows, and novel entities — have continued to deteriorate, the report notes. However, the inclusion of a specifically ocean-centred category was described as an alarming milestone because of the ocean’s central role in sustaining life on Earth.

Dr Levke Caesar, co-lead of the Planetary Boundaries Science Lab, said that while greenhouse gases mix into the atmosphere within about a year, it can take up to a thousand years for them to equilibrate with 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 emphasised that there was still time to act: “Reducing fossil fuel use, cutting pollution and managing fisheries more carefully can all make a real difference.”

The report also highlights positive examples of global cooperation, noting that effective international policies have so far prevented two planetary boundaries — ozone depletion and aerosol emissions — from being breached.

“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.’”

“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.”

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At Natural World Fund, we are passionate about restoring habitats in the UK to halt the decline in our wildlife.