Ocean Acidification

Climate change is nowhere more apparent in its disruptions than in the world’s oceans. The vast bodies of water that make our planet unique are currently undergoing fast and extensive transformations that are unlike anything scientists have seen before, according to a study published Tuesday in the journal Nature Climate Change.

Researchers from the Institute of Atmospheric Physics at the Chinese Academy of Sciences in Beijing, Mercator Ocean International in Toulouse, France, and the Laboratoire de Météorologie Dynamique at the École Normale Supérieure in Paris created a framework and tool to standardize and assess ocean variables and figure out when those variables are changed due to the warming climate.

The scientists examined how “compound changes” — the simultaneous effects of the Earth’s oceans losing oxygen, acidifying and becoming saltier or fresher — are pushing ecosystems past the point where adaptation is possible. And different layers of the pelagic world — a place where scientists have estimated over two million species reside, with approximately only 250,000 known to humanity — are affected to various degrees.

“For example, some specific species, e.g. some kind of fish, may be okay if their living environment is only saltier, but may become more vulnerable if they are exposed to the ‘compound change,’” Zhetao Tan, lead author of the study and a researcher at the Laboratoire de Météorologie Dynamique at the École Normale Supérieure, said in an email.

New research from the university of St Andrews has found that some coastal areas will become much more acidic than previously anticipated.

Because atmospheric CO₂ and ocean pH (acidity) are tightly coupled, the more CO₂that is released into the atmosphere, the more is absorbed by seawater, making the ocean progressively more acidic.   However,  in a paper published in Nature Communications,  researchers, using the California Current as an example, show that oceanic upwelling systems actually amplify ocean acidification.   

Upwelling is where nutrient- rich and already acidic  waters from deep in the oceans rise along the coast. When organic matter from the surface ocean sinks to the deep ocean, microbes gradually break it down in a chemical reaction that releases CO₂ and increases seawater acidity. When this deep water upwells, it brings the acidity to the surface, where it further reacts with the atmospheric CO₂, which makes these water masses even more acidic.  

The researchers used historic coral samples and boron isotope signatures recorded in their skeletons to reconstruct how acidity changed over the 20^th century, and then applied a regional ocean model to predict how acidity will change during the 21^st century.   The study showed that in these upwelling regions of the ocean, ocean acidification outpaces the level “expected” from rising atmospheric CO₂alone. This is because the upwelled water masses are acidic to start with and anthropogenically rising CO₂ exacerbates the acidity.   

Upwelling systems are among the most productive systems on our planet and support much of the world’s fisheries. Understanding how they respond to rising CO₂₂ is therefore not only critical for ocean science, but also carries major implications for fisheries and their potential vulnerabilities.  

Co Author Dr Hana Jurikova, Senior Research Fellow in from the School of Earth and Environmental Science, said: “Predicting how upwelling systems will respond to climate change is highly complex, as anthropogenic influences interact with natural sources of ocean acidification. Our research shows that such interactions can amplify environmental change in the California Current System, highlighting the need for similar studies in other regions to better anticipate future change.” 

The California Current can be used as an example of other upwelling systems. Other important areas of coastal upwelling around the world include the Humbold Current off the coast of Peru or the Benguela and Canary Currents off the coast of west Africa.   

Co Author Dr James Rae, Reader in the School of Earth and Environmental Science, said: “the ocean becoming more acidic poses major risks to marine ecosystems and the communities and economies they support. The solutions we now have for climate change, like heat pumps and electric vehicles, also fix ocean acidification, so it’s critical that we support them”.   

Human greenhouse gas emissions are raising temperatures and sea levels, collapsing ice sheets and acidifying oceans. Now, research maps out the range of emissions pathways that can limit these changes.

How much greenhouse gas can be emitted before the Earth changes beyond the natural world’s ability to adapt? One approach to answer this involves looking at the characteristics of the world humans evolved in and establish limits to preserving these ecosystems¹. There are many of these ‘planetary boundaries’ proposed, and previous work has mapped out how to stay within them². However, in reality, many of the targets interact in complex ways — for instance, the amount of carbon dioxide that can be released while staying below a temperature target can be increased if more cooling sulfates are emitted³, but these sulfates can increase acid rain. Now, writing in Nature Climate Change, Gasser and colleagues⁴ propose a framework to address these interconnections that assesses a range of climate targets and produces a set of compatible emissions pathways with different degrees of climate uncertainty.

The ocean’s smallest engineers, calcifying plankton, quietly regulate the Earth’s thermostat by capturing and cycling carbon. However, a new review published this week in Science by an international team led by the Institute of Environmental Science and Technology at the Universitat Autònoma de Barcelona (ICTA-UAB) (Spain) finds that these organisms, coccolithophores, foraminifers, and pteropods, are oversimplified in the climate models used to predict our planet’s future.

By omitting these plankton, current models may underestimate key processes in the global carbon cycle and the ocean’s capacity to respond to climate change. Calcifying plankton build minute shells of calcium carbonate (CaCO₃), a critical component of the ocean’s carbon cycle. These organisms influence seawater chemistry and facilitate the transfer of carbon from the atmosphere to the deep ocean. This “carbon pump” helps regulate Earth’s climate and influences everything from ocean chemistry to the fossil record. 

“Plankton shells are tiny, but together they shape the chemistry of our oceans and the climate of our planet,” said Patrizia Ziveri, ICREA research professor at ICTA-UAB and lead author of the study. “By leaving them out of climate models, we risk overlooking fundamental processes that determine how the Earth system responds to climate change.”

The Deep Acoustic Lander recorded sound at the Challenger Deep, nearly 11,000 meters below sea level, in 2021. David Barclay

The ocean is a noisy place. Ship propellers and whale songs reverberate at the lowest pitches, while at higher tones dolphins click and shrimp snap their claws. Between these frequencies are the sounds of the churning sea itself, generated as waves, wind, and rain roil its surface. Researchers have now used this ambient noise to probe the rising acidity of the ocean. The acoustic technique, published last week in the Journal of Geophysical Research: Oceans, could make it easier to measure this key parameter of ocean health across vast distances rather than relying on point measurements.

…

The carbon emissions that are warming the globe also acidify seawater. The ocean naturally absorbs about one-third of annual carbon dioxide emissions; as this gas dissolves and reacts, it creates bicarbonate and hydrogen ions. The hydrogen lowers the pH of seawater, increasing ocean acidity, which can harm sea life and slow future carbon uptake. Ship-based measurements in shallow parts of the ocean have found that, since 1985, pH has already dropped from 8.11 to 8.04.

The waters of the ocean are layered, however, and measurements at one depth may not apply to others. Adding pH sensors to the thousands of robotic Argo floats that patrol the seas, diving as deep as 2000 meters, is one way to get a broader picture of acidity. But David Barclay, an acoustical oceanographer at Dalhousie University, and his co-authors found a way to measure average pH across even greater depth ranges, by taking advantage of the intrinsic physics of sound.

In the vast and intricate ecosystems of coral reefs, a hidden danger lurks, posing threats not just to the colorful corals themselves but to entire marine environments. Recent research spearheaded by Liu, PY., Chiu, WC., Lim, S.L., and their collaborators has shed light on the mysterious and pervasive sponge known as Terpios hoshinota. This sponge, infamous for its destruction of coral reefs, exhibits a remarkable ability to thrive under extreme environmental stressors, raising crucial questions about the future of coral ecosystems worldwide.

The study culminated from a comprehensive genomic analysis that aimed to unravel the underlying mechanisms behind the resilience and adaptability of T. hoshinota. As climate change continues to push marine environments to their limits, understanding how this sponge flourishes in conditions that would otherwise be detrimental to many marine organisms is not just interesting—it’s essential.

The research focuses on the genetic underpinnings that allow T. hoshinota to prosper in the face of rising sea temperatures, ocean acidification, and various pollutants. It is now well established that climate change has dire implications for marine biodiversity. The stressors these ecosystems endure can catalyze shifts that drastically alter their composition. As corals struggle, T. hoshinota capitalizes, spreading across coral reefs and frequently leading to mass coral die-offs.

One of the surprising findings of the research was that T. hoshinota possesses a unique set of genes that facilitate the breakdown of harmful substances in its environment. These genes effectively enable the sponge to withstand conditions that would typically weaken or kill other marine organisms. The genomic data indicates that this sponge has evolved sophisticated biochemical pathways, granting it a metabolic edge in nutrient acquisition even when resources are scarce.

Perhaps more alarming is the sponge’s ability to adapt rapidly to changing environmental conditions. The study highlights the sponge’s remarkable genomic plasticity, allowing for quick responses to stress. While many coral species take years or decades to make adaptations, T. hoshinota seems to have a genetic toolkit that allows for swift modifications. This adaptability could mean that the sponge will remain a dominant presence within marine ecosystems, further complicating conservation efforts targeting coral health.

Quantifying the threat of ocean acidification to predict its impact on shellfish larvae

1. Key Points

• As ocean acidification worsens, concerns over its effects on calcifying organisms^※1, including shellfish and coral, are increasing. To date, research has been unable to quantify the impact of ocean acidification on calcifying organisms, largely because of their incredibly small shells.
• In the present study, mollusk larvae with tiny shells measuring approximately 0.1 mm were raised in an environment designed to simulate severe ocean acidification. Subsequently, a high-resolution microfocus X-ray computed tomography (MXCT)^※2 scanner was used to obtain three-dimensional measurements of the shells. For the first time, globally, changes in shell morphology, in terms of reduced shell thickness, size, and density, were quantified precisely.
• In addition, gene expression in genomic domains involved in shell formation was reduced significantly, which marks a significant step toward comprehensive understanding of the effects of ocean acidification on organisms, with respect to biological responses and the impact on shells.
• The methodology employed in the present study could also be applied to other organisms with calcified shells or skeletons, such as bivalves and corals. Such studies could facilitate environmental impact assessment, marine conservation, and fisheries resource management in future.

Fig. 1. Predicting the future using a technique to measure the shell density of shellfish larvae.

…

2. Overview

As global warming intensifies, so does ocean acidification. Such ocean acidification poses a serious threat to marine ecosystems. It not only lowers the pH of seawater but also reduces the “aragonite saturation state (Ω_aragonaite)^{※3, ※4} ”. Aragonite is a crystalline form of calcium carbonate. When the “aragonite saturation state” is 1 or higher, it indicates a supersaturated state; conversely, values below 1 indicate an unsaturated state. The value is as an indicator of how easily organisms can form aragonite-based shells or skeletons. When the aragonite saturation state decreases, it is more difficult for organisms to form aragonite shells or skeletons. However, evaluating the effects of ocean acidification on early developmental stages of organisms such as plankton and mollusk larvae with aragonite shells, has proven challenging. Their shells are exceptionally small (approximately 0.1 mm in diameter and only a few micrometers^※5 thick), which makes precise quantitative assessment more challenging than with mature specimens.

To address the challenges above, Keisuke Shimizu (Associate Researcher) and Katsunori Kimoto (Acting Group Leader) from the Japan Agency for Marine-Earth Science and Technology Research Institute for Global Change Earth Surface System Research Center, alongside Masahide Wakita (Associate Researcher, Mutsu Research Institute), carried out joint research with Takenori Sasaki (Associate Professor, University Museum at the University of Tokyo). The team analyzed the morphology of the shells of shellfish larvae, using high-resolution MXCT and scanning electron microscopy (SEM)^※6, as well as gene expression. Globally, the research team is the first to successfully visualize and quantify the effects of decreased aragonite saturation on the growth and structure of extremely small shells (approximately 0.1 mm) composed of aragonite crystals, using shell density (which is analogous to human bone density) as a novel growth marker. In addition, the findings suggest that a decrease in aragonite saturation may both directly impact shells and influence gene expression domains involved in shell formation in shellfish larvae. The findings could facilitate prediction of the effects of environmental change (such as global warming and acidification) on calcifying organisms such as shellfish and corals.

These results have been published in the Journal of Molluscan Studies on September 18 (Japan time). The research was conducted with the support of a Grant-in-Aid for Scientific Research from the Japan Society for the Promotion of Science (KAKENHI; JP23H02299).

…

Une nouvelle limite planétaire a été franchie pour la première fois en 2025: l’acidification des océans. Ce processus, directement lié à nos émissions de CO2, est délétère pour les écosystèmes marins.

L’acidité des océans perturbe la reproduction, la croissance et les fonctions métaboliques de nombreuses espèces. WIKIMEDIA COMMONS

Après celle sur le cycle de l’eau en 2023, une nouvelle limite planétaire a été franchie pour la première fois en 2025: l’acidification des océans. Provoquée par nos émissions de CO2, elle vient de dépasser un seuil alarmant. C’est la conclusion centrale du rapport sur les limites planétaires publié le 24 septembre par le Planetary Boundaries Science Lab, un laboratoire allemand dépendant de l’Institut de recherche de Potsdam sur les effets du changement climatique.

La notion de limites planétaires est développée depuis 2009 par plusieurs scientifiques à la pointe des sciences du «système Terre», autour notamment du chercheur suédois Johan Rockström. Ils définissent ces limites comme autant de seuils dans des processus planétaires à ne pas franchir, au risque de déstabiliser l’ensemble du système de manière irréversible, avec des effets majeurs pour le vivant. L’humanité, entre autres, dépend depuis 12 000 ans de cette stabilité pour «vivre, grandir et prospérer en toute sécurité», répète avec insistance le rapport.

7 limites dépassées sur 9

Les signaux rouges clignotent de toutes parts. Sur neuf limites planétaires identifiées par les chercheurs, l’acidification des océans est la septième à être franchie. Les six premières (changement climatique, cycle de l’eau, biodiversité, perturbations du cycle de l’azote et du phosphore, déforestation et changement d’utilisation des sols, pollution terrestre par des milliers de substances synthétiques) sont non seulement déjà dépassées, mais leur situation continue de s’aggraver.

Seules deux limites sont respectées et ne se détériorent pas: la pollution aux aérosols atmosphériques et le maintien de la couche d’ozone.

L’acidité de l’eau à la surface de l’océan a augmenté de 30 à 40% depuis l’ère préindustrielle, alertent les auteurs du rapport. Un processus directement lié à nos émissions de gaz à effet de serre puisque l’océan a la capacité de dissoudre une partie du CO2 atmosphérique. Il est même un puits de carbone essentiel, qui absorbe environ le quart de l’ensemble des émissions anthropiques.

Revers de la médaille: ce CO2 dissout dans l’eau conduit, par une suite de réactions chimiques, à augmenter l’acidité de l’océan. Un phénomène extrêmement délétère pour les organismes marins. Beaucoup d’espèces – coraux, mollusques et certains crustacés notamment – ont de plus en plus de difficulté à fabriquer leur coquille et leur squelette lorsque l’acidité augmente.

Working from a dock on St. Helena Island, S.C., on a sweltering day this summer, Ed Atkins pulled in a five-foot cast net from the water and dumped out a few glossy white shrimp from the salt marsh.

Mr. Atkins, a Gullah Geechee fisherman, sells live bait to anglers in a shop his parents opened in 1957. “When they passed, they made sure I tapped into it and keep it going,” he said. “I’ve been doing it myself now for 40 years.”

These marshes, which underpin Mr. Atkins’s way of life, are where the line between land and sea blurs. They provide a crucial nursery habitat for many marine species, including commercial and recreational fisheries.

But these vast, seemingly timeless seascapes have become some of the world’s most vulnerable marine habitats, according to a new study published on Thursday in the journal Science that adds up and maps the ways human activity is profoundly reshaping oceans and coastlines around the world.

Soon, many of Earth’s marine ecosystems could be fundamentally and forever altered if pressures like climate change, overfishing, ocean acidification and coastal development continue unabated, according to the authors.