Ocean acidification: has Earth crossed another safety line?

For more than a decade, scientists have been sounding the alarm: the Earth is approaching its limits. In 2009, an international team of researchers, led by Johan Rockström, defined nine so-called planetary limits – processes crucial to maintaining safe conditions for life on our planet. One of them is ocean acidification, a long-term change in the chemistry of sea water caused by its absorption of carbon dioxide (_CO2) from the atmosphere.

The increase in ocean acidification levels since the pre-industrial era was the subject of a study by a British-American team of scientists led by Helen S. Findlay, the results of which were published June 9 in the journal Global Change Biology . With their in-depth analysis, the study’s authors have brought an interesting voice to the discussion of Earth’s planetary boundaries.

A chemical reaction with global implications

When_CO2 dissolves in seawater, carbonic acid is formed, which leads to a decrease in the pH of the water, as well as a decrease in the concentration of carbonate ions (^CO32-) and the saturation state of aragonite (ΩArag). This seemingly technical phenomenon has dramatic consequences: the lower the value of ΩArag, the more difficult it is for marine organisms – such as corals, worms and clams – to build and maintain their calcareous skeletons and shells. When ΩArag drops below unity, calcium carbonate structures begin to dissolve.

How far are we from the dangerous border?

To determine whether acidification exceeds safe limits, the researchers took the global average level of ΩArag at the ocean surface as an indicator. The pre-industrial state – about 3.44 – was considered the reference value. According to the original assumptions, the safety limit was to be exceeded only when the average drop in ΩArag reached 20 percent of the reference value. This threshold was intended to protect both organisms living in polar waters and tropical coral reefs.

However, the latest data show that the global average ΩArag has already fallen to around 2.8 by 2022, a nearly 19 percent reduction from pre-industrial levels. Although formally the limit has not yet been crossed, four key regions – the Arctic, North Pacific, Southern Ocean and North Atlantic – have already reached or exceeded the 20 percent threshold. In the Arctic, for example, the reduction in ΩArag is already 26 percent.

An ocean outside your comfort zone

Analyzing data from 2020, the researchers found that more than 40 percent of the ocean surface is now outside the safe limit for acidification. In the Southern Ocean and the North Pacific, exceedances of the limits are observed in up to 85 percent of the area, which means a serious threat to the ecosystems there. And the problems are not limited to the surface of the waters – significant chemical changes have also been found in the deeper layers, which often overlap with other adverse phenomena, such as oxygen deficiency and rapid temperature changes.

How ocean acidification is hurting indicator species

The biological consequences of these changes are already apparent. Reef-forming corals, whose presence is a prerequisite for maintaining biodiversity in the tropics, require ΩArag higher than 3.0 for normal growth. At lower values, the growth of their skeletons slows, and environmental stress increases the risk of bleaching and death.

In the polar oceans, hindworms – tiny snails that play an important role in the food chain – lose their shells through dissolution as early as at ΩArag of 1.5, and at 1.2 the process becomes very intense. These types of conditions are observed in Arctic waters.

Bivalves, although it is more difficult to set a specific threshold for them, also suffer from deteriorating chemical conditions. This is important not only for ecological reasons, but also for economic reasons – their global market value is more than $20 billion a year.

New, more cautious border is bad news for Earth

Given the biological effects of ocean acidification and the uncertainties associated with measuring and modeling ΩArag, the study’s authors call for a new, more conservative limit of a 10 percent reduction from pre-industrial levels.

In purely theoretical terms, such a threshold would make ecosystems much safer: it would reduce the spread of extreme conditions in the Arctic, reduce the risk of dissolving winged shells, protect more than 70 percent of tropical reefs, and maintain favorable conditions in coastal areas where bivalves are found. Unfortunately, the 10 percent limit was already exceeded in the 1980s, and by the beginning of the 21st century the entire ocean, according to this more conservative criterion, was outside the safe zone.

It’s high time to take action

The authors of the study, which was supported by the European Space Agency and NOAA’s Global Ocean Monitoring and Observing and Ocean Acidification Programs, among others, point out that ocean acidification is not a problem of the future, but of the present. Reducing_CO2 emissions and developing technologies to sequester it are a priority today. At the same time, we need better monitoring, especially in the deeper layers of the oceans, and strategies to protect the most endangered regions and species.

Chemical changes in the oceans are permanent and reversible only on a long time scale. If we want to keep the Earth in a livable state, we must act now, before the next frontier is irreversibly crossed.

Bibliography:

Findlay, H.S., Feely, R.A., Jiang, L.-Q., Pelletier, G. and Bednaršek, N. (2025), Ocean Acidification: Another Planetary Boundary Crossed. Glob Change Biol, 31: e70238. https://doi.org/10.1111/gcb.70238