Although this boundary has not yet been crossed, the planetary limit of ocean acidification poses an increasing threat : its level has already risen by 30% over the past two centuries, profoundly affecting marine ecosystems.
In this article, Laurent Bopp, Deputy Director of the IPSL-Institut Pierre-Simon Laplace and member of the French Academy of Sciences, shares his expertise. Specialist in climate and ocean science, he focuses in particular on ocean acidification and the impacts of climate change on marine ecosystems. He also contributed to the writing of the IPCC‘s, published in 2022. Structured around five key questions, this article sheds light on the ocean’s central role in climate regulation, the ecological and economic challenges linked to the pressures it faces, and concrete avenues for better protecting it.
1. What role does the ocean play in regulating the global climate?
Although people often speak of the oceans in the plural, Laurent Bopp prefers to use the singular, because “the different ocean basins, Atlantic, Pacific, Indian, and the Southern Ocean, are all interconnected”. It is therefore the ocean, in a global sense, that regulates the climate.
This vast reservoir stores an enormous amount of energy and carbon, which it exchanges with the atmosphere. The ocean absorbs a very large share of the energy generated by the enhanced greenhouse effect (i.e., the accumulation of heat) in the Earth’s climate system, more than 90%, and thus acts as a buffer against global warming. The ocean also absorbs nearly 25% of our carbon dioxide emissions. In this way, it slows the acceleration of climate change.
2. What are the effects of ocean acidification, and what causes it?
Ocean acidification is a phenomenon directly linked to the increase in atmospheric CO₂. CO₂ enters the ocean at the air-sea interface because it is rising faster in the atmosphere than at the ocean surface. This creates a chemical imbalance that nature attempts to correct by drawing more CO₂ into the water. Once dissolved, the CO₂ reacts with water to form carbonic acid, a weak acid, which dissociates and releases H⁺ ions, thereby lowering the ocean’s pH.
“An ocean that absorbs CO₂ is an ocean that acidifies,” summarises Laurent Bopp. The more atmospheric CO₂ increases, the more acidification progresses. This process is already well underway: “The concentration of H⁺ ions (protons) has already increased by 30%, and an even greater rise is expected in the coming decades.”
3. What are the ecological and economic consequences of ocean acidification?
This acidification profoundly alters the chemistry of seawater, with major effects on ecosystems. The most affected are calcifying organisms, those that build shells or skeletons from calcium carbonate, such as mussels, oysters, corals, and certain plankton species. “In a more acidic ocean, it becomes much more difficult to calcify,” explains the researcher.
Table-type Acropora coral, bleached. Coral bleaching and mortality caused by excessive warming of seawater, a direct consequence of climate change.
Corals observed in the Red Sea, near Safaga, Egypt. Photo: Andriy Nekrasov (2023)
This imbalance spreads throughout the food chain. By affecting phytoplankton, the first link in the ocean’s trophic network (food chains), the entire foundation of marine resources can be compromised. The fish we consume depend directly on it.
Moreover, coral reefs, which are also highly sensitive to acidification, provide numerous ecosystem services. They are vital for tourism but also for coastal protection: they shield our shores from coastal flooding caused by major tropical cyclones. The degradation of reefs could therefore weaken coastal communities and the blue economy
Finally, this phenomenon is interconnected with other planetary boundaries, such as those related to the nitrogen and phosphorus cycles. These nutrients, mainly from fertilisers, can lead to algal blooms and dead zones by depleting coastal waters of oxygen.
4. What restoration and adaptation strategies are being considered to protect marine ecosystems?
Scientists are exploring several avenues to try to regenerate marine ecosystems, despite the complexity of how they function. Heat-resistant coral species have been identified, particularly in the Red Sea. This has led to the idea of transplanting them to more vulnerable regions in order to help strengthen weakened reefs.
Underwater coral restoration structure, where healthy, colorful corals are attached to submerged metal frames. Photo: Mbarmawi (2025)
Another approach involves reducing biological pressures—for example, by removing predators such as starfish that threaten weakened corals. From a chemical standpoint, experiments have been conducted on small reefs using the addition of alkaline material (bases) to neutralise acidification and promote calcification. Early results show improved coral calcification under these conditions.
Finally, nature-based solutions are also being considered, such as the restoration of mangroves, which enhance coastal resilience by protecting human populations from extreme events.
For more information on climate change adaptation, check out our article (in french) : AXA Climate and the Calanques National Park Join Forces to Co-Develop a Climate Change Adaptation Plan
5. How can individuals, businesses, and scientific research help protect the ocean?
The priority is clear: reducing CO₂ emissions, which are the root cause of both climate change and ocean acidification. “We are all responsible, though to varying degrees, of course,” emphasises Laurent Bopp. He stresses the need for action at all levels: individual, collective, regional, and national.
This involves concrete actions related to transportation, diet, heating, and housing. To identify where efforts are needed, both individuals and organisations should carry out a carbon footprint assessment.
Other threats, such as plastic and chemical pollution, must also be addressed at the source. “A significant portion of the plastic we use today ends up in the ocean,” he points out, which calls for a reassessment of our consumption habits and packaging choices.
Businesses also have a responsibility: they can assess their own emissions, reduce the use of plastics and chemicals, and rethink their operations. Some are already taking steps, but they also need to work more closely and effectively with scientists and adapt their strategies accordingly.
Finally, science has a fundamental role to play in assessing the proposed solutions. Researchers are currently studying the potential effects of interventions such as ocean alkalinisation or fertilisation while remaining cautious about the ecological consequences, which are still poorly understood. As Laurent Bopp warns, “In trying to fix the major problem of climate change, we must be careful not to disrupt ecosystems in ways we do not yet fully understand.”
Listen to the podcast and check out the full interview in its entirety (in french) :
Weekly Dose of Science – Planet boundaries : ocean acidification
A climate scientist and oceanographer, Laurent Bopp is Deputy Director of the IPSL–Pierre-Simon Laplace Institute and a member of the French Academy of Sciences. His research focuses on the ocean carbon cycle, with particular attention to the evolution of the oceanic carbon sink. He also studies ocean acidification and the impacts of climate change on marine ecosystems. Bopp is the author or co-author of over 200 scientific publications in international journals and served as a lead author for both the 5th and 6th Assessment Reports of the Intergovernmental Panel on Climate Change (IPCC). He has received the Marine Sciences Prize from the French Academy of Sciences and is a member of the Academia Europaea.
