Ocean Acidification

Ocean acidification has reached critical levels, crossing a "planetary boundary" about five years ago, according to recent research. This is damaging to coral reefs and marine life, particularly at depths of 200 meters, where 60 percent of global waters have breached safe limits. We are running out of time to protect marine ecosystems.

Ocean acidification, often called the “evil twin” of the climate crisis, is caused when carbon dioxide is rapidly absorbed by the ocean, where it reacts with water molecules leading to a fall in the pH level of the seawater. It damages coral reefs and other ocean habitats and, in extreme cases, can dissolve the shells of marine creatures.

The planetary boundaries are the natural limits of key global systems – such as climate, water and biodiversity – beyond which their ability to maintain a healthy planet is in danger of failing. Six of the nine had been crossed already, scientists said last year. The seventh boundary, ocean acidification, has now been shown to have been reached about five years ago.

The study drew on new and historical physical and chemical measurements from ice cores, combined with advanced computer models and studies of marine life, which gave the scientists an overall assessment of the past 150 years. It found that by 2020 the average ocean condition worldwide was already very close to – and in some regions beyond – the planetary boundary for ocean acidification. This is defined as when the concentration of calcium carbonate in seawater is more than 20% below preindustrial levels.

The deeper in the ocean they looked, the worse the findings were. At 200 metres below the surface, home to many more different types of plants and animals, 60% of global waters had breached the “safe” limit for acidification, compared to over 40% of the global surface ocean. These changes result in significant declines in suitable habitats for important calcifying species, including 43% reduction in habitat for tropical and subtropical coral reefs, up to 61% for polar pteropods, and 13% for coastal bivalves.

Since these deeper waters are changing so much, the impacts of ocean acidification could be far worse than first thought. This has huge implications for important underwater ecosystems such as tropical and even deep-sea coral reefs that provided essential habitats and nursery grounds for the young of many species. As pH levels drop, calcifying species such as corals, oysters, mussels and tiny molluscs known as sea butterflies struggle to maintain their protective structures, leading to weaker shells, slower growth, reduced reproduction and decreased survival rates.

Decreasing CO₂ emissions is the only way to deal with acidification globally, but conservation measures could and should focus on the regions and species that are most vulnerable.

The research shows we are running out of time. This is an existential threat, and much suitable habitat for key species has already been lost. Governments can no longer afford to overlook acidification in mainstream policy agendas.

SOURCES: based on Lisa Bachelor, The Guardian, 9 June 2025, https://www.theguardian.com/environment/2025/jun/09/sea-acidity-ecosyst…

Original paper in Global Change Biology https://onlinelibrary.wiley.com/doi/10.1111/gcb.70238