Ocean acidification, a key indicator of planetary health, has exceeded safety thresholds on a global scale, according to a major new study published in Global Change Biology.

The research, led by an international team from the Plymouth Marine Laboratory (PML) in the UK, the US National Oceanic and Atmospheric Administration (NOAA), Oregon State University and the University of Maryland, presents compelling evidence that the world’s oceans passed a critical “planetary boundary” for acidification around five years ago.

This threshold is defined as a 20% drop in aragonite saturation compared to pre-industrial levels – a measure crucial for shell-building marine organisms. The study finds that by 2020, this limit had already been crossed in multiple regions of the ocean, with some areas now showing aragonite saturation levels closer to 17% of historical baselines.

“Ocean acidification isn’t just an environmental crisis, it’s a ticking time bomb for marine ecosystems and coastal economies,” said Steve Widdicombe, director of science at PML and co-chair of the Global Ocean Acidification Observing Network, in a press release. He underlined the growing threat to habitats that marine species depend on, including coral reefs and coastal ecosystems.

The oceans are absorbing more CO₂ because of emissions and the climate crisis, and the resulting acidification reduces the availability of carbonate ions, a key building block for marine organisms such as coral, oysters and crabs for forming their shells and skeletons.

The paper’s release coincides with the opening of the United Nations Ocean Conference in Nice, France, from June 9-13, amplifying concerns among scientists and policymakers.

The findings are based on a combination of recent ocean chemistry data, physical measurements, biological surveys and advanced computer models, allowing researchers to assess conditions both at the surface and in deeper waters up to 200 metres below the surface.

In these deeper layers, conditions are worsening even more rapidly, the scientists said. About 60% of subsurface waters have now crossed the acidification boundary, compared with 40% at the surface.

These changes have profound implications for marine biodiversity, particularly for organisms like corals, oysters, mussels and pteropods that depend on calcium carbonate to build shells and skeletons. As acidity rises, these species experience weakened structures, reduced reproduction and declining survival rates.

“The waters below the surface are home to a much greater diversity of life than many realise,” said lead author Helen Findlay of PML and chair of the North-East Atlantic Ocean Acidification Hub. “Since these deeper waters are shifting so significantly, the repercussions could be even greater than we anticipated.”

The impacts are already visible. Tropical and subtropical coral reefs have lost an estimated 43% of their suitable habitats. In polar regions, sea butterflies – a vital food web species – have lost as much as 61% of their habitat. Coastal shellfish have seen a 13% decline in viable habitats for sustaining basic biological functions.

The research team argues that the existing boundary of a 20% decline is no longer an appropriate safety marker. They propose that even a 10% reduction from pre-industrial aragonite levels could be harmful – a threshold that was surpassed globally around the year 2000.

“This isn’t limited to one bay or coast,” Nina Bednaršek, study co-author and Oregon State University researcher, told Inside Climate News. “We’ve altered the chemistry of the entire ocean, from untouched regions to the places closest to home.”

Regional disparities are particularly striking. The most severe changes at the ocean’s surface are occurring in polar regions, while deep-water impacts are concentrated in upwelling zones along the west coasts of the Americas and around the equator – areas typically rich in marine productivity.

Richard Feely, NOAA oceanographer and co-author, pointed out that these zones are among the most biologically active and sensitive to acidification, making the findings especially concerning for global food webs and fisheries.

The study adds to the growing alarm over planetary boundaries. These are nine or ten critical environmental thresholds beyond which Earth’s systems may become unstable. By 2023, several had already been breached. With this new evidence, ocean acidification is another boundary that has been crossed, said the scientists.

The authors urge immediate conservation and management interventions, prioritizing both the most affected regions and those that remain relatively intact. Protection of mid-depth habitats, often overlooked in conservation planning, is now seen as essential.

“We are gambling with both biodiversity and billions in economic value every day that action is delayed,” Widdicombe warned.