Ocean Acidification

The latest review of planetary boundaries in 2023 showed that six of the nine boundaries had been crossed. Now a seventh planetary boundary has been breached: ocean acidification. The health of the oceans is worse than previously thought, and we are running out of time to protect marine ecosystems.

Carbon dioxide is not only a greenhouse gas warming the atmosphere. It is rapidly absorbed by the ocean, where it reacts with water molecules leading to a fall in the pH level of the seawater, making it more acidic. This makes it harder to form calcium carbonate that all shells and coral skeletons are made of. It damages coral reefs and other ocean habitats and, in extreme cases, can dissolve the shells of marine creatures.

Until now, the science suggested that the planetary boundary for ocean acidification, the natural limit beyond which the ocean's role in maintaining a healthy planet might be in danger, had not been crossed. However, a new study as found that ocean acidification’s “boundary” was reached about five years ago. 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.This is not just an environmental crisis, it is a fundamental threat to marine ecosystems and coastal economies. We are running out of time to protect marine ecosystems.

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, 60% of global waters had breached the “safe” limit for acidification. These deeper waters are home to many more different types of plants and animals, which could make the impacts of ocean acidification far worse. This has huge implications for important underwater ecosystems such as tropical and even deep-sea coral reefs that provide 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 report makes it clear that we are running out of time and what we do – or fail to do – now is already determining our future. This is an existential threat since the reality is that 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…

See also Helen S. Findlay, Richard A. Feely, Li-Qing Jiang, Greg Pelletier, and Nina Bednaršek. “Ocean Acidification: Another Planetary Boundary Crossed”. Global Change Biology, 9 June 2025. https://doi.org/10.1111/gcb.70238 https://onlinelibrary.wiley.com/doi/10.1111/gcb.70238