Under the influence of climate change, the ocean is inexorably losing oxygen. This phenomenon, known as deoxygenation, has become a key indicator of the transformations underway in the ocean. But while scientists see ocean deoxygenation as a marker of climate upheaval, the general public’s understanding of it is often clouded by misinformation. Under the auspices of the Programme Prioritaire de Recherche (PPR) Océan & Climat, Mathieu Delteil, a doctoral student at LOCEAN IPSL (Paris, France), is exploring the impacts of ocean deoxygenation, and looking in more detail at how knowledge about this phenomenon is circulated in the public sphere.
By Carole Saout-Grit and Laurie Henry
Ocean deoxygenation, a marker of climate upheaval
Ocean deoxygenation is a major alarm signal for marine ecosystems. This phenomenon, characterized by a steady decline in oxygen levels in marine waters, is mainly driven by two interdependent processes. On the one hand, rising surface temperatures reduce the solubility of oxygen. Secondly, it intensifies ocean stratification, preventing vital exchanges between the deep layers, where oxygen is breathed in, and the oxygen-rich surface layers.
These upheavals are particularly pronounced in oxygen minimum zones (OMZs), regions where oxygen concentrations are naturally so low as to prevent the survival of breathing organisms such as fish. Between 1970 and 2010, this oxygen loss reached up to 3.3% for the first 1,000 meters of ocean depth, and the volume of these OMLs increased by up to 8% over the same period, significantly reducing the extent of habitable zones for marine species.
These transformations are indicators of ongoing climate change and raise critical issues for ocean biodiversity. Yet the general public’s understanding of the phenomenon remains hampered by misconceptions, notably the simplistic view that the oceans are the “lungs” that produce atmospheric oxygen. This image masks the true impact of ocean deoxygenation: the loss of essential marine habitats.
A scientific and societal study of the issues
This is the background to Mathieu’s thesis work at the Laboratoire d’Océanographie et du Climat : Expérimentations et Approches Numériques (LOCEAN), a laboratory of the Institut Pierre-Simon Laplace (IPSL) in Paris. Co-supervised by Marina Lévy, biogeochemical oceanographer and physicist at LOCEAN IPSL and Sébastien Dutreuil, historian and philosopher of Earth sciences at the Centre Gilles Gaston Granger (CGGG), this work is supported by the PPR Océan & Climat over the period 2023-2026.
The thesis aims to refocus the debate on the ecological and social repercussions of an increasingly uninhabitable ocean, by responding to a two-fold challenge: to deepen scientific understanding of the mechanisms and impacts of ocean deoxygenation, and to raise public and decision-maker awareness of this complex issue.
© Mathieu Delteil
To achieve these objectives, Mathieu adopts a transdisciplinary approach combining marine biogeochemistry, climate modelling and human sciences.
From a scientific point of view, the aim is to identify the temporal threshold at which the expansion of ZMOs will exceed natural variability, thus becoming a clear indicator of anthropogenic impact. Using simulations from theCoupled Model Intercomparison ProjectPhase 6 (CMIP6) and the latest version of the IPSL climate model (IPSL-CM6A-LR), Mathieu is analyzing ocean oxygen dynamics over the period 1850-2060. This model, which integrates marine biogeochemistry via PISCES*, differentiates natural variations from anthropogenic influences. The challenge is to accurately predict the future evolution of MPAs and to determine the year of emergence, i.e. the moment when the effects of climate change will overtake natural processes. This work will provide crucial information for anticipating the reduction of marine habitats and their impact on ecosystems.
At the same time, Mathieu is studying how deoxygenation has been perceived, studied and discussed in scientific and public spheres. Drawing on archival research and interviews, he will trace the history of awareness of this phenomenon, particularly through the key role played by the international Global Ocean Oxygen Network (GO2NE). This group of experts, created under the aegis of UNESCO, strives to advance research and disseminate its findings to a wide audience.
The contributions of NGOs campaigning for ocean preservation will be analyzed in detail, in particular to better understand their strategies for integrating the issue of deoxygenation into public debates on climate. The aim is to deconstruct misunderstandings, such as the erroneous idea of an ocean as the “lungs of the planet”, and to refocus the discourse on the real ecological consequences: the progressive loss of habitats for breathing marine species.
Ultimately, this ambitious research should help to formulate concrete recommendations for ocean management, while reinforcing the integration of deoxygenation into environmental policies. By combining numerical modeling, historical analysis and scientific communication, Mathieu aims not only to advance our understanding of this phenomenon, but also to raise collective awareness, which is essential if we are to protect a vital ecosystem in the face of climate upheaval.
* PISCES is a biogeochemical model integrated with the IPSL climate model, simulating the oxygen cycle in the ocean, biological and chemical interactions, and the impact of climate change on marine ecosystems.
The IPSL climate model simulates atmosphere-land-ocean-sea-ice interactions, studying the climatic impact on ecosystems.
3 Questions to Mathieu Delteil
Why did you decide to do a thesis in marine sciences?
My initial training was in teaching physics and chemistry, and doing a thesis seemed the obvious way to deepen my scientific knowledge and skills. This will enable me to pass on high-level knowledge to my students. Marine sciences, coupled with climate sciences, emerged as a field of study that matched my desire to link teaching and research with current environmental issues.
What made you want to apply for this thesis? What were your motivations?
What first attracted me to this thesis topic was its transdisciplinary nature, combining marine sciences, climate sciences and the history of science. My interest in teaching and scientific mediation also played a key role: this subject allows me to explore all the stages involved in the effective and successful transmission of scientific knowledge, from its production to its dissemination. This subject also seems important to me, as it contributes to a better understanding of the consequences of climate change on the oceans, while raising awareness of the urgent need to protect them.
How do you see your future after this thesis?
After my thesis, I’d like to devote myself fully to teaching, an activity that is particularly close to my heart. This experience will enrich my scientific knowledge and skills, and I’m keen to share them. I want to make young people aware of the changes that the ocean is undergoing, as well as the crucial issues linked to climate change. Teaching is also a way of making research more visible, better understood and more accessible to new generations.
Reference : Mathieu Delteil, « Ocean deoxygenation: New information and old myths », thesis 2023-2026
Contact : mathieu.delteil@locean.ipsl.fr