What does our oceans become when they swallow too much CO2?

The ocean absorbs approximately 30 % of carbon dioxide (CO2) that we reject in the atmosphere. This role of “shield” against warming has long been hailed. However, By storing too much CO2, seawater becomes more and more acidic. This acidification of the oceans is a discreet but deep phenomenon which considerably modifies marine chemistry.

When CO2 dissolves in water, it forms carbonic acid which lowers the ocean pH and reduces the quantity of carbonatean element Essential to the formation of limestone shells and skeletons. For organizations such as corals, mussels, oysters or certain plankts, This decrease amounts to lacking cement to build their houses. And the more acidic the ocean becomes, the easier these structures dissolve.

Another outdated limit!

Scientists from around the world use the concept of planetary limits to assess If our planet remains in conditions favorable to life. Among these nine borders, that of ocean acidification has been Officially crossed in 2020according to a study published in 2025 in the journal Global Change Biology.

Many believe that the situation is not so serious, explains Nina Bednaršek, co-author of the study and principal researcher at the State University of Oregon.

This observation is based on a chemical indicator called saturation en aragonitewhich measures the capacity of water to support calcification. A reduction in 20 % of this level compared to the pre -industrial era (before 1850) Mark a Danger zone. Or, Over 40 % The global ocean surface has already crossed this threshold. And that’s not all: 60 % of water layers between 50 and 200 meters deep are also concerned.

Researchers have shown that This rocking started in the 2000smuch earlier than we thought. It means that We already live in a modified oceanwhose chemistry no longer resembles that that marine organizations have known for millennia.

How did scientists come to this conclusion?

To measure this crossing, researchers have Compared the current conditions of the oceans with those before the industrial era. They used data from Ice carrots to reconstruct the atmospheric CO2 levels of 1750 to 1850which allowed them to estimate a reference level for saturation in Aragonite, located between 3,44 et 3,57. They then calculated the variations using International climate models (CMIP6), crossed with observation data collected at sea.

They also took into account the Uncertainties related to measuresby integrating an error margin of ± 0,18 in their calculations. This rigorous method made it possible not only to identify the overcoming of the overall threshold, but also to measure the effects region by region and at different depths.

Finally, researchers have crossed these chemical data with the real biological needs of species. For this, they used established thresholds by scientific experiences: for example, we know that tropical corals need a saturation level greater than 3.5 To grow normally.

However, this level is already exceeded in the three major regions where the reefs are the most abundant: the Pacific, the Atlantic and the Indian Ocean.

The impacts are already visible!

The consequences of this acidification are far from theoretical. According to the study, 43 % of habitats adapted to tropical corals have disappeared. For them polar pterropodssmall molluscs that play a crucial role in the food chain, the loss reached 61 %. And Coastal bivalveslike oysters and mussels, have seen 13 % of their favorable areas deteriorate.

These figures are alarming because they affect both biodiversity, Food resources et The means of subsistence of millions of people, especially in coastal and island regions.

A new security threshold is essential

The authors of the study believe that the Current limit of 20 % reduction is no longer sufficient. They offer a more prudent threshold, to 10 % reduction in Aragonite compared to the pre -industrial era. But even this new, more protective limit has already been exceeded in the year 2000 For all the upper layers of the ocean, generally up to approximately 200 meters deep.

This revision is essential if we want to take into account the biological effects already observed. Because beyond the global averages, these are Local ecosystemsTHE sensitive species and services rendered by the ocean to humanity who are at stake.

What can we do?

The good news is that everything is not frozen. Unlike certain irreversible changes, ocean acidification can still be slowed down, even stabilizedprovided quickly and strongly reduce our CO2 emissions. Scientists show that only an ambitious transition scenario, called SSP1-2.6would allow Limit damage.

On the other hand, if we remain on our current trajectories (such as scenarios SSP3-7.0 or SSP5-8.5), 100 % of the ocean surface will cross the critical threshold from here 2100and near 95 % will have a drop in more than 40 % of saturation in Aragonite.

Reference of the article

Findlay, H. S., Feely, R. A., Jiang, L.-Q., Pelletier, G., & Bednaršek, N. (2025). Ocean acidification : Another planetary boundary crossed. Global Change Biology. https://doi.org/10.1111/gcb.70238