Ocean Acidification: A Hidden Threat the World Can No Longer Ignore

Global Change Biology, reveal that the planetary security threshold linked to acidification was crossed in 2020, much earlier than anticipated. This discovery questions the real extent of the danger for marine ecosystems and coastal savings. It occurs while six of the nine planetary limits had already been exceeded according to the latest scientific assessments.

A planetary threshold already crossed for acidification

The breakup occurred earlier than announced. According to the researchers, the global ocean has exceeded a critical threshold in terms of acidification from 2020. Without that triggering the expected political alarm. This threshold is based on a specific indicator: saturation in Aragonite, a mineral essential for the formation of shells and skeletons of many marine organizations. When this level drops more than 20 % compared to the pre -industrial era, the balance of marine ecosystems is considered in danger.

The study reveals that this organic and chemical barrier is crossed on more than half of the waters located between the surface and 200 meters deep. An area where marine life is particularly dense. Unlike a widespread idea, most of the biodiversity is not concentrated on the surface, but in these intermediate waters. Professor Helen Findlay points out that the species that live there are just as crucial as the corals visible in shallow waters.

Overcoming this limit does not represent a simple scientific measure. This is a concrete alert. The oceans no longer effectively twist the CO₂. And water chemistry is already altered to the point of compromising vital functions such as calcification or reproduction. This implies an acceleration of biodiversity losses, a destabilization of food chains and an increasing difficulty in restoring marine habitats. This silent crossing signs a major rocking in the state of health of the oceans.

Lost habitats, endangered species

The effects of acidification are no longer theoretical. They are now observed in the most sensitive marine habitats. The study led by the Plymouth Marine Laboratory reveals that 43 % of tropical and subtropical coral reefs have already lost chemical conditions compatible with their survival. However, these reefs house more than a quarter of known marine biodiversity. Although they only cover 1 % of the surface of the oceans. Their accelerated decline directly compromises the reproduction, food and protection of thousands of species.

Further north, in the cold waters of the polar regions, it is the pterropods that are affected. These tiny planktonic molluscs, nicknamed “sea butterflies”, have seen disappear up to 61 % of their viable habitat. However, they constitute a fundamental link in trophic networks in Arctic and Antarctica. They serve as food for species such as salmon, cod, or whales.

Coastal bivalves, such as oysters and mussels, also lose ground. Globally, they have seen a 13 % reduction in coastal areas where their essential biological functions – growth, calcification, reproduction – can still be exercised correctly.

These losses are directly linked to the fall of the pH, caused by the dissolution of carbon dioxide in sea water. This CO₂, absorbed massively by the ocean (around 25 % of annual human emissions), turns into carbonic acid. The environment becomes more acidic, which reduces the availability of calcium carbonate, essential for the formation of shells and skeletons. As this availability decreases, marine organizations become more vulnerable, threatening the balance of ecosystems and coastal economies that depend on it.

The challenge of visibility and reaction

One of the great paradoxes of ocean acidification lies in its invisibility. Unlike a dark tide or a laundering of coral, this threat does not jump out. “” There is no obvious alarm signal », Notes Steve Widdicombe, scientific director at the Plymouth Marine Laboratory. On the coast, nothing suggests that the pH of the water has dropped critically. This lack of direct perception complicates public and political awareness, despite already measurable impacts.

A striking example occurred in the northwest of the United States, around 2010. The oyster industry, however well established in this region, suddenly experienced an unexpected collapse of its production. The investigations have shown that deep waters, naturally more loaded in CO₂, had rose to the surface by Upwelling. This rise in waters has been amplified by global acidification. Corrosive waters then disrupted the development of oyster larvae, unable to properly train their shell.

The professionals' response was immediate and technical. The hatching has installed sensors to measure the pH of pumped water in real time. They then added alkaline solutions to basins to neutralize acidity. If this system has saved the sector locally, it remains expensive and inaccessible to many countries.

For Jessie Turner, director of the International Alliance to Fight Ocean Acidification, this case illustrates a crucial reality: “ Adaptation is possible. But it requires means, surveillance and governance committed ».

Acidification: a silent emergency, a still possible turn

The acidification of the oceans therefore progresses without noise, but its effects are concrete, documented and irreversible in the short term. It is not a distant scenario, but a reality measured by robust data. By crossing the planetary safety limit from 2020, the oceans reached a critical point. And the consequences are already affecting food chains, coral reefs and economic sectors such as fishing or conchyliculture.

Researchers call for a clear answer. On the one hand, it becomes essential to reduce global carbon dioxide emissions, the only way to slow the phenomenon permanently. On the other hand, local adaptation measures must develop, such as chemical surveillance of water, the protection of resilient habitats or the limitation of organic pollution.

The difficulty remains to integrate this subject, too often secondary, in climatic and marine policies. As Jessie Turner reminds us, the risk is due to the inaction of governments. It leaves the field open to poorly supervised private initiatives, such as certain forms of oceanic geo-engineering still little scientifically evaluated.

The holding of the UN summit on the oceans in Nice, at the time of the publication of these key data, offers a concrete action window. By integrating acidification in international discussions, decision -makers can still change the trajectory. This meeting could mark the start of a coordinated response to a long-shaped crisis.

Source: Findlay, HS, Feely, Ra, Jiang, L.-Q., Pelletier, G. and Bednaršek, N. (2025). “Ocean Acidification: Another Planetary Boundary Crossed”. Glob Change Biol, 31: E70238.