On the first day of the United Nations Ocean Conference in Nice, France, the findings of a new study have revealed that ocean acidification (OA) — which damages ecosystems like coral reefs — is not only getting worse, but crossed its “planetary boundary” roughly five years ago.

The findings came as a surprise, as scientists in a report published last year said OA was “approaching a critical threshold,” but had not yet crossed the boundary.

The nine defined planetary boundaries in which Earth can operate safely include climate change, freshwater use and OA.

“OA is the term given to the long-term shift of marine carbonate chemistry resulting primarily from the uptake of carbon dioxide (CO2) by the oceans, leading to an increase in ocean acidity and a decrease in carbonate ion (CO32−) concentration,” the authors of the study wrote. “OA can severely affect marine organisms through its direct impact on physiology, growth, survival and reproduction.”

The researchers pointed out that ocean conditions vary widely across the world, with OA levels in tropical regions over two times as high as in polar regions.

“Looking across different areas of the world, the polar regions show the biggest changes in ocean acidification at the surface. Meanwhile, in deeper waters, the largest changes are happening in areas just outside the poles and in the upwelling regions along the west coast of North America and near the equator,” said lead author of the report Helen Findlay, a professor at the United Kingdom’s Plymouth Marine Laboratory (PML) and chair of the North-East Atlantic Ocean Acidification Hub, in a press release from PML.

The study, led by PML, the National Oceanic and Atmospheric Administration in the United States and Oregon State University’s Cooperative Institute for Marine Resources Studies (CIMERS), said OA’s threat to marine ecosystems globally is much more widespread than previously believed.

The research team used the most recent chemical and physical measurements of the upper ocean, along with studies of marine life and advanced computer models to conclude that by 2020, average ocean conditions globally were already very near — and in some regions had already surpassed — the OA “danger zone.”

“Most ocean life doesn’t just live at the surface – the waters below are home to many more different types of plants and animals. Since these deeper waters are changing so much, the impacts of ocean acidification could be far worse than we thought. This has huge implications for important underwater ecosystems like tropical and even deep-sea coral reefs that provide essential habitats and nursing refuge for many species, in addition to the impacts being felt on bottom-dwelling creatures like crabs, sea stars, and other shellfish such as mussels and oysters,” Findlay explained.

Professor Helen Findlay working in the laboratory to investigate the impacts of climate change and ocean acidification on marine organisms and ecosystem functioning. Plymouth Marine Laboratory

The team discovered that roughly 60 percent of deeper ocean waters — down to approximately 656 feet — had crossed the planetary boundary for OA — compared with 40 percent at the surface. The increase in acidification has enormous implications for the survival rates of many sea creatures, particularly those who build their skeletons or shells from calcium carbonate.

Some subtropical and tropical coral reefs have already lost 43 percent of suitable habitats, while pteropods, a main food web species in the polar regions — also known as “sea butterflies” — have lost as much as 61 percent of their habitat. Additionally, 13 percent of the habitats of coastal shellfish species have been lost worldwide.

Based on the findings, the scientists recommended that a change be made to the previous safety limit of a 10 percent deviation from pre-industrial levels being harmful to ocean ecosystems, as the entire surface of the ocean had already exceeded the stricter limit by about the year 2000.

“Ocean acidification isn’t just an environmental crisis – it’s a ticking time bomb for marine ecosystems and coastal economies. As our seas increase in acidity, we’re witnessing the loss of critical habitats that countless marine species depend on and this, in turn, has major societal and economic implications.” said Steve Widdicombe, a professor at PML who is co-chair of the Global Ocean Acidification Observing Network, one of the focuses of the UN’s sustainable development goal of addressing and minimizing the effects of OA.

“From the coral reefs that support tourism to the shellfish industries that sustain coastal communities, we’re gambling with both biodiversity and billions in economic value every day that action is delayed,” Widdicombe added.

The authors of the study suggested conservation measures should be directed toward to species and regions that are most vulnerable to acidification. They also emphasized the importance of suitable management measures or protection of areas that are the least compromised by acidification to ensure their longevity.

The newly identified subsurface water impacts highlight a pressing need to safeguard mid-water habitats and their marine life. The researchers stressed the importance of improved approaches to addressing OA, as well as other ocean pressures, to better support stronger ecosystem resilience.

“This report makes it clear: we are running out of time and what we do – or fail to do – now is already determining our future,” said Jessie Turner, director of the International Alliance to Combat Ocean Acidification, who was not part of the study, as The Guardian reported. “We are coming to terms with an existential threat while grappling with the difficult reality that much suitable habitat for key species has already been lost. It’s clear that governments can no longer afford to overlook acidification in mainstream policy agendas.”

The study, “Ocean Acidification: Another Planetary Boundary Crossed,” was published in the journal Global Change Biology.

TOMORROW Join the launch of the #COP30 Virtual Ocean Pavilion during #UNOC3!
10 June | 10–11 AM CEST | Zoom
Join via Zoom here: us02web.zoom.us/j/8955059069…

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— Plymouth Marine Laboratory (@plymouthmarine.bsky.social) June 9, 2025 at 2:11 PM

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