Optimization of UV-Vis spectrophotometer (OCaPI) parameters for measuring the pH and pCO2 of the ocean carbonate system in seawater to assess ocean acidification (Mediterranean Sea)
Automating the measurement of carbonate system parameters is essential for improving our understanding of biogeochemical processes in marine regions. The portable OCaPI (Ocean Carbon Parameters Instrument) is designed to perform simultaneous and accurate measurements of hydrogen ion concentration (pH) and partial pressure of carbon dioxide (pCO2)1 in the ocean environment. Optimizing the parameters of the UV-Vis spectrophotometer (integration time, scan-to-average, boxcar) facilitates the quantification of ocean acidification, with significantly improved measurement accuracy and reliability. The results obtained are consistent with existing techniques and offer a simplified approach to data collection, even under challenging conditions. This work, based on design principles, performance, and preliminary results obtained in the Mediterranean Sea, paves the way for the integration of these optimized techniques into long-term monitoring programs. This will contribute to a better understanding of the impacts of climate change on marine ecosystems and to improved management in the face of ocean acidification.
Coastal marine ecosystems are increasingly threatened by multiple stressors such as ocean acidification and deoxygenation, but how these co-occurring stressors interact is often poorly understood. This is especially true for tropical coral reefs where deoxygenation is an emerging yet understudied threat. Using hypoxia response curves combined with rigorous pH control, we show that acidification alters hypoxia sensitivity and oxyregulation of reef-building corals in a species-specific manner: three species exhibited increased sensitivity to various degrees, while the fourth showed enhanced tolerance. Consequently, acidification pushes critical hypoxia thresholds into oxygen regimes already prevalent on reefs today, potentially driving shifts in community composition and accelerating risks to reef resilience as these stressors intensify in the future. Our findings challenge assumptions of uniform coral vulnerability under multi-faceted climate change, emphasizing the need for trait-based approaches and to account for stressor interactions in predictive models to better anticipate coral reef futures under rapid climate change.
Sampling on the acidified reef. In contrast to the structurally complex control reefs, the seafloor here is relatively flat and dominated by turf algae — habitat that supports far fewer fish and much smaller shoals. Photo by Manabu Ooue and provided by the authorship team.
Watch a reef long enough and you realise that fish are almost never alone. They move in groups, feed in groups, and react to danger as a group. For small reef fish, being part of a shoal is a survival strategy. More eyes spot predators sooner. More bodies mean any one fish is less likely to be the unlucky one. And fish in bigger groups tend to be bolder, as they forage more efficiently, stay out in the open more, and spend less time hiding.
When we started looking at how climate change affects fish behaviour on reefs experiencing ocean acidification and warming in Japan, we assumed we would find the usual story. Warmer water and rising acidity would alter fish behaviour, make them more cautious, or accelerate their activity levels. That seemed like the obvious prediction.
It turned out to be different — or at least, for schooling species.
…
Warming and acidification had little direct effect on behaviour
Across all reef types, even during the heatwave, the fish behaved in much the same way. They kept feeding. They did not suddenly become more nervous. The direct effects of warming, acidification, and heatwave stress on individual behaviour were mostly minimal.
You could read that as good news. Fish holding their own against climate change. But when we looked at what actually caused how the fish were behaving, the answer was not temperature or water chemistry at all. It was how many fish were in the shoal.
Fish in bigger shoals foraged more and hid less. Fish in smaller shoals were more cautious, regardless of the reef conditions around them. Shoal size, not climate stress, was mediating the behaviours we observed.
That sent us back to ask a different question: why were shoals so much smaller on the acidified reef?
What acidification actually does to a reef
On non-acidified reefs, the benthos is structurally complex, a mix of algae, encrusting organisms, and vertical relief that gives reef fish the three-dimensional habitat they rely on. On our acidified reef, that structure was largely gone. The seafloor was dominated by short turf algae, flat and featureless.
There were far fewer fish. Not because the fish were sick or behaving strangely, but because the habitat could not support the same densities we observed on non-acidified reefs. With fewer fish around, the shoals that did form were much smaller: up to 79% smaller than shoals on nearby control reefs. And with smaller shoals came more cautious behaviour across the board.
Social context is a critical yet underexplored determinant of behavioural resilience to climate change. Group living can buffer individuals against environmental stress through enhanced vigilance, reduced predation risk and improved foraging efficiency.
However, whether these behavioural expressions persist under chronic (warming, acidification) and acute (marine heatwaves) climate stressors remains unclear. Using natural climate analogues spanning present-day, ocean warming and combined warming–acidification reefs, we quantified how shoal size influences behavioural expression in a range-extending reef fish (Pomacentrus coelestis).
Across all climate conditions, fish in larger shoals consistently exhibited higher foraging and activity levels and reduced risk-avoidance behaviours, whereas direct effects of warming, acidification and heatwaves on behaviour were negligible.
In contrast, ocean acidification most likely constrained collective behaviour indirectly by simplifying benthic habitats, where fish densities were 84% lower than at the warming reef, resulting in shoals that were up to 79% smaller than the Warming and Control reefs.
Combined, our data suggest that shoal size mediates behavioural expression between foraging and predator avoidance and that acidification-driven habitat simplification can alter behavioural expression indirectly by reducing fish densities and the formation of large shoals.
We conclude that climate change can indirectly modify behavioural expression in shoal-forming fishes through habitat-driven erosion of social structure.
Systematic review of chemistry educational strategies and curriculum integration in ocean acidification
This systematic literature review examines the trends and developments in ocean acidification education research from 2011 to 2025. Using the PRISMA methodology, 30 articles from the Scopus database were analyzed to identify key themes, research gaps, and future directions in teaching and learning about ocean acidification. The findings reveal a growing interest in integrating ocean acidification into science education curricula, with a significant emphasis on inquiry-based learning, technology-enhanced instruction, and interdisciplinary approaches. The United States leads research production (51 authors), followed by Spain, Sweden, and Greece. Key educational innovations include virtual reality applications, computational modelling, hands-on laboratory experiments, and collaborative learning strategies. With an average of 23.37 citations per document, this field has a substantial academic impact. However, challenges persist in terms of public awareness, teacher preparation, and curriculum integration. The review identifies the critical need for enhanced pedagogical resources, professional development programs, and assessment tools to effectively teach ocean acidification as a climate change issue. These findings provide valuable insights for educators, curriculum developers, and policymakers seeking to strengthen ocean and climate change education in formal and informal settings.
Over the past decade, the East Ross Sea has experienced a significant decline in sea ice, enabling direct observational studies of regional carbon dynamics. The accumulation rate of anthropogenic CO2 in the East Ross Sea is up to six times higher than the long-term Industrial Era mean due to the inflow of seawater from the Amundsen Sea by accelerated glacial melting. In contrast, the West Ross Sea exhibited comparatively smaller changes. Measurements of dissolved inorganic carbon and stable carbon isotope indicate that, over the period 2011–2020, changes in δ13C (Suess effect) and anthropogenic CO2 were 0.20 ± 0.06‰ and −5 ± 2 μmol kg−1 in the West Ross Sea, and −0.15 ± 0.01‰ and 9 ± 1 μmol kg−1 in the East Ross Sea. These findings suggest rapid acidification in the East Ross Sea, with aragonite undersaturation likely to occur by the mid-2030s, accompanied by an expected pH decrease of ∼0.2 units by the end of the century.
Climate change is a major threat to the small-scale fishing communities, particularly in South Asia, a region characterized by widespread coastlines, extreme population density, and high dependence on marine resources for food, income, and employment. This review synthesizes findings from peer-reviewed articles published between 2000 and 2025 to analyse the climate change impact on socioeconomic conditions and well-being of the small-scale fishermen. The review identifies a range of climatic stressors, such as rising sea surface temperatures, ocean acidification, sea level rise, and extreme weather events, that are severely disrupting marine ecosystems and fish availability. These ecological shifts directly affect the livelihoods, income stability, and food security of fishing communities, escalating existing vulnerabilities like poverty, indebtedness, and limited occupational mobility. The study categorizes the impacts into physical, economic, and social dimensions, highlighting issues such as declining catch volumes, increased operational costs, infrastructure destruction, and disruptions in food supply and nutrition. It also examines local and regional adaptation responses, ranging from ecosystem-based solutions, such as mangrove restoration and cage aquaculture, to institutional and behavioural shifts, including migration, livelihood diversification, and early warning systems. While some adaptations enhance resilience, others pose sustainability challenges. This review highlights the pressing need for targeted policy interventions that support sustainable adaptation, enhance institutional frameworks, and prioritize vulnerable fishing communities in climate resilience planning.
Oyster farming lines at Lambert Shellfish on the Eastern Shore in Virginia on May 28, 2026. Katherine Hafner/WHRO News
Shellfish farmers are working with the Virginia Institute of Marine Science to better understand the looming issue.
Shellfish such as clams, crabs and oysters need tiny building blocks called calcium carbonate ions to grow and thrive.
These particles are essential to build sturdy shells that protect marine creatures from predators – and make them more appealing for human diners.
But rising acidity tied to climate change is making it harder for shellfish to access those fundamental building blocks.
The issue caught officials’ attention in the late 2000s when acidic corrosion caused mass die-offs of baby oysters in the Pacific Northwest.
Researchers at William & Mary’s Batten School and Virginia Institute of Marine Science have teamed up with local shellfish farmers to learn more about how changing conditions could impact the aquaculture industry in coastal Virginia.
“We know the threat exists,” said Emily Rivest, an associate professor in ecosystem health at the Batten School and VIMS. “This gives us the opportunity to try to get ahead of it to understand how to build our resilience and prevent those kinds of dramatic negative impacts.”
The project is funded by a $1.2 million grant from the National Oceanic and Atmospheric Administration’s Ocean Acidification Program.
The ocean absorbs about a third of the climate-warming carbon dioxide humans release into the atmosphere. That causes a cascade of chemical changes underwater, including reducing pH levels.
For millions of years, the ocean’s pH remained relatively stable at 8.2 on the pH scale, which ranges to 14, the most basic. Since the start of the Industrial Revolution, it has dropped to 8.1. Because the scale is logarithmic, that represents a nearly 30% increase in acidity, according to NOAA.
In much of the Chesapeake Bay, the rate of change is happening even more quickly, particularly in the middle, VIMS previously found.
One factor is freshwater. When freshwater surges into the bay, it lowers the amount of salt in the water, which makes it more susceptible to acidity, Rivest said. That has sometimes affected restored oyster reefs on the western side of the Eastern Shore.
Pollution from nutrients washing off land into the bay compounds the issue by triggering algae that produce carbon dioxide when eaten by bacteria, Rivest said.
Luckily, natural variability in the Chesapeake Bay “has shaped the eastern oyster to be a very tough and resilient species,” Rivest said.
Lab tests indicate local oysters are “much more tolerant of acidification” than out West. The question is: What is their breaking point?
Temperature rise reduced European sea bass and Senegalese sole sperm motility.
Gilthead seabream sperm showed lower variation under acidification and warming.
Challenge tests allowed differentiation among males based on sperm performance.
Approach provides a screening framework for sperm performance.
Abstract
We aimed to develop a screening approach to differentiate among males of European sea bass (Dicentrarchus labrax), gilthead seabream (Sparus aurata), and Senegalese sole (Solea senegalensis) based on sperm performance under environmental acidification and temperature increase. Sperm samples were selected using a CASA system, and three challenge tests were applied. The first one consisted of sperm activation with artificial seawater (ASW) across a pH range (7.6–8.2). The second assessed activation at species-specific temperatures. The third test evaluated the combined effect of ASW pH (7.8 and 8.2) and different temperatures. Results from the third challenge test revealed differences in sperm performance under environmental variations, allowing differentiation among males. For this purpose, sperm motility values obtained for each sample under species-specific natural environmental conditions were used as references, and variations in motility were compared across challenge conditions. Different levels in the criteria (regarding the different percentages of motility variation) were applied to differentiate among males. The temperature increase affected the sperm kinetic parameters of European sea bass and Senegalese sole, while gilthead seabream sperm showed lower variation under seawater acidification and rising temperatures. The challenge test allowed differentiation among males based on sperm performance under environmental variations and represents a preliminary screening approach. However, these results are based on in vitro conditions and should be interpreted as a first proxy, requiring further validation to establish links with reproductive performance in vivo.
Scientists have shown conclusively for the first time that tiny marine organisms in polar oceans survived the mass extinction event that wiped out prehistoric dinosaurs because they needed less energy and were more tolerant to darkness.
The study, led by the University of Bristol and published in the journal Nature, sought to solve a longstanding evolutionary enigma: what factors determined whether marine species would survive a mass extinction event known as the Cretaceous-Paleogene (K-Pg) boundary some 66 million years ago? Findings revealed that being small and accustomed to darkness proved to be the vital attributes.
Image shows microscopic marine plankton, which are larger in size and went extinct. Credit: Brian Huber Smithsonian
Study lead author Dr Rui Ying said: “It’s an exciting breakthrough. For so many years scientists have been unable to test what actually decided whether a species prevailed or perished because the extinction event involves multiple environmental changes like ocean acidification and darkness.”
“It is difficult to understand the causality because of the lack of fossil data and environmental proxy data, especially at century timescale. Using a numerical model, I looked at the base of the food chain – plankton – which helped us to identify the most likely cause and the best survival strategies for plankton.”
The Cretaceous–Paleogene (K–Pg) boundary is an ancient and much-studied geological signature marking the mass extinction that wiped out non-avian dinosaurs, separating the Mesozoic Era (the age of reptiles) from the Cenozoic Era (the age of mammals). It is thought that the impact of an asteroid, called Chicxulub, caused the extinction of around 75% of species in the fossil record by triggering catastrophic environmental changes.
Despite decades of research, the mechanisms linking the environmental changes to the selective extinction patterns observed in the fossil record have until now been unresolved. But by creating and deploying a unique model which maps ecosystem traits globally, the scientists have been able to establish what attributes resulted in the marine plankton community’s survival.
Dr Ying, who is now a Senior Research Associate at the University of East Anglia, said: “The model is based on trades and the trade-off of how often they are eaten by predators and what they can eat against specific attributes, such as temperature, light level and body size.”
Study co-author Dr Fanny Monteiro, Associate Professor in Ocean Sciences at the University of Bristol, explained: “The body size and abundance of small plankton mean the organisms rely on less energy, increasing their likelihood of survival. An ability to deal with lower light and darkness and turbulent waters in higher latitudes also makes them more adaptable to polar regions. In contrast, species adapted to higher light and warmer waters were more vulnerable to this type of mass extinction.”
The model allowed the traits of millions of organisms to be analysed and quantified with unprecedented accuracy, providing important insights into the physical and chemical changes linked to diversity. Besides shining a light on the distant past of marine life, the research can also help inform forecasts of how ecosystems might respond in future.
Study co-author Professor Daniela Schmidt, Professor of Earth Sciences at the University of Bristol, said: “This study not only demonstrates how trait-based models can help us better understand biodiversity crises in ancient history, but it also has potential to indicate how less light and hotter environments, as a result of global warming, might impact current and future ecosystems.”
Building upon previous research, this study examines potential relationships between septal thickness in Devonian ammonoids from the Anti-Atlas of Morocco and isotopic proxy data from the literature for atmospheric CO2, sea surface temperature, oceanic pH, and weathering (δ18O, δ13C, δ11B, 87Sr/86Sr). Recent studies have demonstrated that various mollusc groups show some growth sensitive to environmental factors. Our results indicate no significant correlation between septal thickness and the examined proxies, except for significantly thinner septa in the genus Phoenixites following the environmental perturbations during the Kellwasser Event, which included anoxic conditions and possibly ocean acidification. This supports the hypothesis that a positive selection for reduced shell material occurred in response to changing seawater chemistry. Additionally, our results align with published data and may support a correlation between septal thickness and palaeolatitude. This study contributes to our understanding of the evolutionary impacts of environmental stressors such as ocean acidification on ammonoids and their adaptive strategies to changing environmental conditions.
Reef-building corals form the calcium-carbonate frameworks that underpin tropical coral reefs, yet global coral cover has declined by ~50% in recent decades, due to marine heatwaves and other stressors. Identifying refugia environments, such as upwelling systems, that buffer stress, promote recovery, and enhance resilience by promoting physiological plasticity that supports thermotolerance is therefore critical. Here, we compared benthic community composition, coral percent cover, and photo-physiology between an upwelling location in the Gulf of Papagayo and a non-upwelling location in Sámara on the Pacific coast of Costa Rica. Waters in Papagayo were cooler, more acidic, and richer in chlorophyll a. Reefs at this location exhibited higher crustose coralline algae, higher sea urchin cover, and lower macroalgae cover, compared to Sámara. Papagayo also showed higher stony coral cover, driven by Pocillopora spp., while Sámara was dominated by massive, heat-tolerant Porites spp.. When significant, photophysiological measurements showed 9.7 – 44.5% higher photosynthetic efficiency (Fv’/Fm’) in Papagayo corals and 19.94 – 42.75 % higher maximum photosynthetic rates (Pmax) in Sámara corals. These results highlight how contrasting environmental regimes within a relatively small geographic area can shape distinct coral community compositions and photophysiological strategies, with implications for identifying areas of reef persistence or refugia.
An updated version of the OA-ICC bibliographic database is available online.
The database currently contains 9,859 references and includes citations, abstracts and assigned keywords. Updates are made every month.
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In the Northeast Pacific, the marine carbonate system’s variability across timescales is not well constrained. Here, we quantify observed seasonal and non-seasonal variability in Dissolved Inorganic Carbon (DIC), partial pressure of carbon dioxide () and aragonite saturation state and discuss potential drivers. We used three decades of observations from four Line P time series stations, the longest marine carbonate system time series in the Northeast Pacific (1990–2019). To gauge the spatial extent of the variability patterns, we used output from a global ocean model representing the observed period. In the Northeast Pacific, seasonal and non-seasonal variability at 10 m was minimal, mostly damped by the opposing influence of DIC and temperature changes at both seasonal and interannual timescales. For DIC and , the seasonal cycle dominated total variability in the top 60–70 m, with mean-transect 10 m seasonal amplitudes of 35 3 μmol and 0.31 0.04, respectively. In the upper 60–70 m, the magnitude of non-seasonal variability was at least half that of the seasonal variability for most variables. From five climate indices examined, we focused on the basin-scale Pacific Decadal Oscillation index (PDO) to investigate potential drivers of non-seasonal variability, with 20%–40% of the non-seasonal variability in DIC and associated to this index. In the Northeast Pacific, positive PDO periods were linked to a mean reduction in 10 m DIC of 5 μmol and an increase in 10 m of 0.04 for each PDO unit increase, which could potentially reduce the occurrence and severity of ocean acidification events. The opposite could be expected during negative PDO periods.
Plain Language Summary
Using 30 years of observations from the Northeast Pacific, we characterized sources of variability for three marine carbonate system variables: , dissolved inorganic carbon (DIC) and the saturation state of aragonite (an common indicator of ocean acidification). The seasonal and non-seasonal variability was minimal in the top 10 m. The seasonal cycle of DIC and aragonite saturation state was the major contributor to total variability in the top 60–70 m, and not detectable below. Also, in the top 70 m of the water column, up to 20%–40% of the DIC and aragonite saturation state non-seasonal variability was associated to the Pacific Decadal Oscillation index (PDO). The PDO is a statistics-derived index that captures variability patterns influencing the whole Pacific basin and has a positive and negative phase. We found that a warmer than usual upper water column in the Northeast Pacific during a positive PDO phase, potentially driven by reduced mixing, was linked to a lower DIC and higher values of aragonite saturation state. The opposite could be expected during negative PDO periods. Knowing the magnitude of natural variability in the marine carbonate system is important to identify the emergence of ocean acidification and other human-driven changes in the ocean.
Navigating the Souring Seas explores how ocean acidification (OA)-a significant yet under-governed environmental threat-is being addressed on the global stage. Bridging science, law, and international policy, this interdisciplinary book introduces global experimentalist governance as an innovative and adaptable framework for tackling complex and uncertain issues like OA. It provides a clear overview of the scientific background of OA and maps the international governance landscape, identifying it as a regime complex. Through detailed interview-based case studies of the Ocean Acidification Alliance and the International Maritime Organization, the book evaluates real-world efforts to govern OA and highlights how experimentalist features, such as flexibility, learning, and multilevel collaboration, can enhance their effectiveness. Accessible and timely, this book is essential reading for scholars, students, policymakers, and environmental practitioners seeking practical, forward-looking governance strategies for ocean and climate challenges. It offers both theoretical insight and concrete recommendations for improving global environmental governance.
Tropical coral reefs exhibit high variability in coral metabolism, driven by complex interactions among physical, chemical, and biological processes. Understanding the spatiotemporal patterns of coral metabolism and their drivers is critical, as such variability may underpin corals’ adaptive capacity to withstand a warming and acidifying ocean. Here, we use a coupled hydrodynamic–biogeochemical–physiological model to investigate spatial and diel variations in coral metabolic processes (photosynthesis, respiration, and calcification) across Moorea’s north shore reef system under three prevailing wave regimes. We find that photosynthesis varies little across the reef, whereas respiration and calcification show pronounced spatial heterogeneity. These spatial patterns closely mirror the ones in seawater carbonate chemistry and depend strongly on wave-driven flow. Hydrodynamics regulate diffusive exchanges between coral tissues and surrounding seawater, and eventually generate distinct internal chemical environments (in the coelenteron and calcifying fluid) across the reef. Landward reef regions exhibit the greatest spatial and diel variability in coral metabolism. Low-wave, slow-flow conditions amplify metabolic fluctuations throughout the reef, but more strongly in the landward regions. Overall, our results highlight how interactions among transport processes, carbonate chemistry, and coral physiology produce strong day-night fluctuations and spatially heterogeneous but structured metabolic patterns across the reef, which vary systematically with wave conditions.
Ocean acidification (OA) is reshaping marine biogeochemistry and threatens microbial communities that regulate carbon and nutrient cycling. Although existing bibliometric reviews have examined OA in relation to coral reefs, calcifying organisms, and broader marine ecosystems, no study has systematically mapped the specific sub-domain of OA impacts on microbial ecology, a gap that hinders identification of methodological blind spots, collaboration imbalances, and under-explored research frontiers unique to microbial systems. Meanwhile, research progress on OA-driven microbial change remains fragmented and lacks a systematic analysis of the field’s evolutionary trajectory and emerging frontiers. This study presents a comprehensive bibliometric analysis of 495 publications retrieved from the Web of Science Core Collection (2005–2025), utilizing CiteSpace to map the knowledge domain of OA impacts on microbial ecology. Temporal analysis reveals three distinct developmental phases: emergence (2005–2010), exponential growth (2011–2021), and recent stabilization (2022–2025). The global collaboration network spans 53 countries, characterized by a triadic leadership structure involving China, the United States, and Germany, with the GEOMAR Helmholtz Centre serving as a central institutional hub. Keyword co-occurrence and burst detection analyses uncover a significant paradigm shift: the research focus has transitioned from foundational carbonate chemistry parameters to complex ecosystem-relevant microbial processes, including community structure, functional genes, and biogeochemical cycling. Notably, “responses” emerges as the most active contemporary research frontier with the strongest recent citation burst, reflecting a consolidated focus on how microbial communities adapt to acidification stress at physiological, community-structural, and functional levels. However, network analysis also reveals structural blind spots: archaea and viral ecology remain conspicuously absent from high-frequency keyword clusters despite their recognized ecological importance, and research contributions from Africa, Southeast Asia, and Small Island Developing States are markedly limited. Based on these findings, we propose four evidence-linked strategic directions centered on multi-omics integration, spatiotemporal expansion through global observatory networks, factorial multi-stressor experimental designs, and bridging molecular processes to ecosystem-scale biogeochemical cycles. This study provides a data-driven roadmap for next-generation research on OA-microbe interactions, essential for predicting ecosystem resilience in a changing ocean.
For nearly a century, scientists have tried to resolve the sensory physiology of chemical communication caused by predation stress. Only recently have we evidenced that abiotic stressors from a changing world, such as heat and ocean acidification, also trigger chemical communication between aquatic organisms – which we dubbed abiotic stress communication. Generally, the behavioural and physiological response to stress-induced cues are well understood, whereas the molecular mechanisms – cue identities, pathways of release, and perception – of this stress communication remain unresolved. Here, we propose a framework to organize the existing evidence for candidate mechanisms involved in abiotic stress-induced chemical communication, focusing on heat and acidification as two major abiotic stressors with environmental relevance. Drawing on transcriptomic, metabolomic and behavioural evidence, we propose that stressor-specific communication likely involves multiple cues and parallel routes rather than a single mechanism, such as membrane-related processes. We call for integrative work that links -omics with chemical profiling and ecological function assays to uncover the mechanisms of abiotic stress communication.
For years, scientists have known that the ocean does a huge amount of the planet’s climate work for us.
The ocean absorbs a large share of the carbon dioxide, taking in roughly 20 to 30 percent of human-caused CO2 emissions since the industrial era. At the same time, tropical cyclones are among the most violent things that happen on Earth’s surface. They churn the upper ocean, stir up deep water, cool the sea surface, and leave behind lingering physical changes that can last for weeks. What has been much less clear is how those storms affect the ocean’s role in the carbon cycle. Do tropical cyclones help the ocean absorb carbon, or do they cause it to release more back into the atmosphere?
A new study suggests the answer is not simple, and it may also be changing.
A role reversal for tropical cyclones
The study was led by experts at the National University of Defense Technology (NUDT), Chinese Academy of Sciences, the NSF National Center for Atmospheric Research, and the GEOMAR Helmholtz Centre for Ocean Research Kiel.
The team pulled together a large set of observations to build a globally available daily dataset of air-sea CO2 flux. Using that, they were able to track how tropical cyclones have influenced the exchange of carbon between the ocean and the atmosphere over time. Their conclusion is that tropical cyclones have tended to push carbon out of the ocean and into the air. But that effect has been weakening in recent decades, and if warming continues under high emissions, the role of these storms may eventually flip.
A messy carbon signal
At first, the result sounds a little counterintuitive. The researchers found that tropical cyclones generally cause net ocean carbon outgassing. The main reason is that the intense winds of a cyclone greatly strengthen the transfer of CO2 from sea to air. But there is another process happening at the same time. After a tropical cyclone passes, it often leaves behind a cold wake, a patch of sea surface that has cooled because the storm mixed the upper ocean so strongly. That cooling can increase the ocean’s ability to take up carbon dioxide from the atmosphere, partly offsetting the carbon being released. Tropical cyclones are doing two things at once: they are helping CO2 escape because of their winds, while also setting up conditions that can later encourage carbon uptake.
The new study suggests that, historically, the first effect has usually won out. Even so, that balance has not stayed fixed.
The ocean is a major sink of anthropogenic carbon, absorbing 20~30% of anthropogenic CO2 emissions across the air–sea interface. Intense weather systems, such as tropical cyclones, can strongly perturb the upper ocean and thus critically influence this carbon transfer. Here we develop an approach of synthesizing various observations to quantify the role of tropical cyclones in the global carbon cycle. Two primary, but competing effects are: (1) CO2 efflux (from the ocean to atmosphere) during tropical cyclone passage and (2) CO2 influx after tropical cyclones, associated with disturbed carbon disequilibrium by mixing upwards of colder water (cold wakes). The CO2 efflux more than offsets the influx, resulting in net ocean carbon outgassing to the atmosphere. Annual tropical cyclone-induced carbon outgassing has decreased over the past three decades, from 0.09 ± 0.02 PgC in the 1990s to 0.05 ± 0.01 PgC in the 2010s. A strengthening of the vertical temperature gradient in the upper ocean due to anthropogenic climate warming leads to cold wakes with more intense surface cooling and increased carbon uptake after tropical cyclones. This has implications as the vertical temperature gradient continues to grow under high-emissions scenarios, tropical cyclones will cause net increasing ocean carbon uptake and more severe ocean acidification.
