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France and the nine planetary boundaries
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Ocean acidification

A PLANETARY BOUNDARY THAT HAS NOT BEEN TRANSGRESSED, BUT FOR HOW LONG?

The ocean is at the heart of climate and biodiversity issues. Like forests, it is considered to be the "lungs" of the Earth. It produces over 50% of the oxygen we breathe, and absorbs around 25% of the carbon dioxide (CO2) in the atmosphere. It is therefore a major carbon sink and plays a major role in climate regulation. However, since the industrial revolution, the increase in the level of CO2 emitted into the atmosphere by human activities (around 40%) has disrupted the biogeochemical balance of the oceans, with serious consequences for marine ecosystems and biodiversity.

As atmospheric CO2 dissolves on contact with the ocean, a series of chemical reactions take place. In particular, carbonic acid is formed and hydrogen ions (H+) are released, reducing the hydrogen potential (pH) - (see glossary) of seawater. This is known as "ocean acidification". The released hydrogen ions then combine with carbonate ions (see glossary) to form bicarbonate, which reduces the quantity of carbonate ions available to many marine organisms (shellfish, corals, molluscs, plankton) to build their shells or calcareous skeletons, in aragonite or calcite.

Since the start of the industrial revolution, the average pH of ocean surface waters has fallen from 8.2 to 8.1229. Since the pH scale is logarithmic, this apparently moderate decrease (-0. 1) has in fact translated into a 30% rise in ocean acidity since the pre-industrial era.

As part of the work on planetary boundaries, scientists adopted as a control variable "thesaturation state of global surface ocean in aragonite" (Ω), one of the forms of calcium carbonate (CaCO3) produced by marine organisms. The planetary boundary corresponds to an average of 80% of the aragonite saturation state of global surface ocean in the pre-industrial era (Ω= 3.44 in 1850). When the saturation state is less than 1, this means that the seawater is undersaturated and can become corrosive to calcareous shells and most coral systems (Table 8).

29 As the ocean's pH is above 7, the ocean is basic. However, a decrease in pH indicates that the ocean is less basic, and this is referred to as acidification.

Table 8: Control variable and planetary boundary for ocean acidification

Control variable

Threshold and zone of uncertainty

Global value

Aragonite saturation state of global surface ocean (% of pre-industrial value) - (Ω)

80% average aragonite saturation of global surface ocean in the pre-industrial era (Ω= 3.44 in 1850) - (≥80 - ≥ 70%)

84% of pre-industrial value

(Ω= 2,9)

Source: based on Steffen et al., 2015

In 2015, the aragonite saturation state of global surface ocean was estimated at 84% (Ω= 2.9) of the pre-industrial level (Ω= 3.44), i.e. below the planetary boundary. Continuing at the same rate until 2050, it could reach 2.80, or around 80% of the pre-industrial level. This saturation rate evolves with CO2 emissions, but also with temperature and glacier melt.

Consequences and challenges

Ocean acidification is detrimental for several reasons. It disrupts the development of some marine plankton, such as coccolithophores, whose calcareous skeleton is sensitive to the acidity of the environment. For the same reasons, it also affects the development and survival of corals. Marine plankton and corals are the basis of many marine ecosystems on which human populations are highly dependent. The disappearance of coral reefs could jeopardize the protection of coastlines against storms, the development of aquatic fauna30 and, as a result, the food supply of a significant proportion of the planet's population, as well as tourism and underwater leisure activities.

Worldwide, more than a billion people derive direct benefits from coral reefs. The French Coral Reef Initiative (Ifrecor) estimates the annual value of ecosystem services associated with coral reefs, associated ecosystems, mangroves and seagrass beds in the French overseas territories at €1.3 billion.

30 Coral reefs cover less than 1% of the ocean's surface, but are home to a third of all known marine species.

The IPCC has drawn up projections grouped into four possible trajectories, known as "RCP scenarios" (see glossary), for the 21st century and beyond, depending on the emissions profile. According to the high CO2 emissions scenario (RCP 8.5), if CO2 emissions continue to rise at the same rate, the surface pH of the ocean could fall by 0.3 pH units before the end of the century, leading to a pH close to 7.7 by 2100. Furthermore, aragonite saturation levels could fall below 3 in surface waters around tropical reefs by 2100. More specifically, in Antarctica, 60% of surface waters could become corrosive to organisms whose shells are made from aragonite, such as pteropods. Some regions of the Arctic are already corrosive for certain marine species, and most will be within a few decades.

More generally, at a time when the ocean is subject to multiple pressures (warming, acidification, deoxygenation, sea-level rise, overfishing, pollution, eutrophication), a major challenge is to understand the capacity of marine organisms to adapt to these unfavorable conditions, particularly over long periods. Although the state of knowledge is improving, data is still lacking in this area, especially as the scale of the changes in the ocean is recent. In addition to the knock-on effects on the living world, ocean acidification is set to drastically reduce its carbon sequestration capacity, with the risk of worsening climate change in the decades to come.

Policies and actions to preserve the oceans

As the primary cause of ocean acidification is the increase in CO2 emissions and ocean warming, any actions that help limit our greenhouse gas emissions can only be beneficial. These are described in the "climate change" section.

Beyond these actions, it is also possible to mitigate the consequences of acidification on marine ecosystems, by reducing other pressures affecting coral development and survival, such as pollution, physical disturbance and overexploitation.

At global level, the Convention on Biological Diversity(CBD), adopted at the end of the United Nations Conference on Environment and Development (the Rio "Earth Summit") in 1992, emphasized the vital role of marine ecosystems for the planet. At the 15th Meeting of the Parties to the CBD in December 2022, the International Coral Reef Initiative - a global partnership with 93 members, including governments, civil society groups and private sector companies - put forward a recommendation to recognize coral reefs as critically endangered ecosystems and give them priority in the new global biodiversity framework. This recommendation resulted in the inclusion of coral reef monitoring indicators in the global framework's reporting system.

In 2020, the United Nations launched the UN Decade of Ocean Sciences for Sustainable Development (2021-2030) and the UN Decade of Ecosystem Restoration. The associated communication and funding campaigns aim to help protect the oceans through scientific progress and restore the planet's degraded ecosystems.

In France, efforts to protect coral reefs are coordinated by the Ministries for the Environment and Overseas Territories, which rely on the work of Ifrecor. In particular, Ifrecor helps to coordinate the actions of all French territories, including the overseas territories with their own environmental protection jurisdiction (French Polynesia, New Caledonia, Wallis and Futuna, and Saint-Barthélémy).

France is the world's fourth-largest coral reef country, and as such bears a major responsibility for the preservation of these ecosystems. As a result, France's ambitions in terms of coral reef protection have been gradually strengthened:

  • In 2016, the law for the reconquest of biodiversity set a target of protecting 75% of overseas coral reefs by 2021.
  • In 2018, the biodiversity plan proposes to protect 100% of French coral ecosystems by 2025, with an intermediate target of 75% protection by 2021.
  • In 2019, the Comité interministériel de la mer adopts the national action plan for the protection of overseas coral reefs.
  • In 2021, the national protected areas strategy proposes to include 100% of coral reefs in a protected area by 2025.
  • In 2022, the third national biodiversity strategy proposes to strengthen the national action plan for the protection of overseas coral reefs.

By 2022, 67% of coral reefs were effectively protected. The Coral Sea Nature Park, created in 2014 by New Caledonia, alone covers an area of 1.3 million km2 , making it one of the largest marine protected areas in the world. By protecting 41% of New Caledonia's coral reefs, it makes a significant contribution to the goal of protecting 100% of coral reefs by 2025. In particular, in 2018 the park created vast areas of strong protection covering more than 28,000 km2 , which mainly cover reef zones harboring remarkable marine biodiversity.

In mainland France, this ambition is also reflected in a series of actions set out in the façade strategic documents, to be adopted in 2022. For example, the Mediterranean coastline plans to "enhance knowledge of the ecological status of red coral in the Mediterranean and ensure, where necessary, its preservation", or to "take greater account of the sensitivity of deep-sea habitats in the Mediterranean", notably by regulating fishing practices at reef level.

Citizens' initiatives are also helping to protect coral ecosystems. A case in point is the "SOS Corail" crowdfunding platform, supported by Ifrecor and the French Ministry of Ecological Transition and Territorial Cohesion, which collects and distributes donations from individuals for coral reef protection projects.

Further information