The world’s oceans are becoming increasingly acidic—a shift that scientists warn may already be threatening marine life and global food supplies. Growing levels of carbon dioxide (CO₂) in the atmosphere, largely a result of burning fossil fuels such as coal, petrol and natural gas, are being absorbed by the sea. Once absorbed, CO₂ forms carbonic acid, gradually lowering the pH of ocean water. While this process does not make the ocean dangerous for swimming, it disrupts the delicate chemical balance that marine ecosystems depend on.
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Ocean acidification has now breached what experts call its “planetary boundary”—a scientifically defined threshold beyond which the risk of irreversible environmental damage becomes significantly higher. Alarmingly, new data suggest this limit was surpassed as early as 2015.
Speaking recently, United Nations Secretary-General António Guterres described the oceans as “the ultimate shared resource” and warned that humanity is failing to protect them. Among the most pressing concerns he highlighted: illegal fishing, plastic pollution, warming sea temperatures, and the accelerating acidification of marine environments.
“Ocean acidification is climate change’s equally evil twin,” some scientists have said, pointing to its potentially devastating impact on biodiversity and coastal economies. The Smithsonian Institution reports that seawater has already become 30% more acidic in the past two centuries.
Although some marine species—such as seaweed and algae—may thrive under more acidic conditions, others, including shellfish like mussels and oysters, are at risk. Lower pH levels make it harder for these animals to form and maintain their calcium-based shells, leaving them more vulnerable and stunting their growth.
Dr Helen Findlay, a biological oceanographer at Plymouth Marine Laboratory, published research in 2023 indicating that shell integrity is already being compromised. “Fewer survive,” she explained, “and those that do have weaker shells and grow more slowly.”
In UK waters, the North Atlantic Ocean—especially areas to the west of Britain and Ireland—has been identified as a hotspot for CO₂ absorption due to natural deep-water circulation. This region now contains more dissolved carbon dioxide than any other ocean on Earth. As a result, vulnerable cold-water coral reefs and local shellfish populations may be exposed to rapidly worsening conditions.
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“There is also evidence of ocean acidification affecting the shell integrity of plankton calcifiers,” said Dr Findlay. “This raises concern over the long-term health of bivalve and crustacean populations in the North Sea, and the future of the UK’s shellfish and aquaculture sectors.”
Dr Yuri Artioli, a marine ecosystem modeller at Plymouth Marine Laboratory, warned that by 2030, corrosive conditions in UK shelf seas may become a seasonal occurrence. “Under high-emission scenarios, around 60% of bottom waters on the North-West European Shelf could become permanently corrosive to more soluble forms of calcium carbonate such as aragonite.” Certain areas, such as the Bristol Channel, are expected to experience faster acidification than others, according to Plymouth University researchers.
The global picture is equally stark. A billion people worldwide rely on the ocean for their primary source of protein. As more waters pass the acidification threshold—defined as a 20% reduction in calcium carbonate from pre-industrial levels—the impacts could be far-reaching.
Research from 2023 indicates that by 2020, 60% of global waters at 200 metres depth had already exceeded the safe limit. Moreover, acidification is expected to continue at the current rate and may accelerate further after 2050.
A treaty to govern international waters—focused on sustainable use and marine conservation—is expected to come into force in 2026. While this marks a step forward, scientists emphasise the need for urgent, global reductions in CO₂ emissions to slow the pace of ocean acidification and safeguard marine biodiversity for future generations.
