Ocean Acid Limit Breached: 60% of Seas Now in Danger By David Freeman - June 28, 2025
The oceans are no longer chemically stable. According to a new study published in Global Change Biology in June 2025, Earth’s surface and subsurface ocean systems have already breached the accepted planetary boundary for acidification. This is not a projection. It is not tied to emissions targets, future warming, or any climate model. It is a direct measurement of chemical change. The ocean’s aragonite saturation state, a vital metric for marine life, has declined by more than 20 percent compared to pre-industrial levels across large parts of the globe.
This threshold matters because it was originally set to prevent the collapse of marine ecosystems that rely on calcium carbonate, including corals, molluscs, and microscopic shell-forming plankton that support entire food chains. That boundary was never meant to be symbolic. It was a physical tipping point, beyond which marine chemistry begins to impair biological function across multiple species.
By the year 2020, over 40 percent of the ocean’s surface and 60 percent of the subsurface (down to 200 metres) had already crossed this chemical boundary. In polar regions, which naturally hold more dissolved carbon due to colder temperatures, the damage is further advanced. In the Arctic, nearly 80 percent of surface waters are now within the range of undersaturation during certain seasons. This condition dissolves shells, disrupts food chains, and alters ecosystem stability.
Researchers used the most advanced ocean biogeochemical models available, combining global satellite data, ship-based observations, and CO₂ absorption patterns, to track the saturation state of aragonite from pre-industrial times through to 2020. Their conclusion is stark. The global ocean’s mean surface aragonite saturation state has dropped from a pre-industrial baseline of 3.51 to 2.90, with a margin of error of plus or minus 0.06. That is a 17.3 percent decline. This places current conditions directly within the uncertainty band of the planetary boundary.
But this is not just a surface-level crisis. At 100 and 200 metres below the surface, the data show even greater declines, with 58 percent and 61 percent of ocean area now having crossed the boundary, respectively. These zones are home to deep corals, larval stages of many commercial fish, and key layers in the oceanic carbon cycle. What was once a stable buffer zone has become a silent casualty of accelerated CO₂ absorption.
Across multiple regions, the results are no longer ambiguous. Four of the seven major ocean basins have already passed the 20 percent acidification boundary. The Arctic, North Pacific, Southern Ocean, and North Atlantic have all crossed the threshold. Every other basin, including the Indian and central Pacific, has entered the boundary’s uncertainty band. These are not projections. They are conditions already measured.
Tropical coral reefs are no longer operating within safe chemical conditions. Their ability to build and maintain skeletons, reproduce, and form viable habitat has already been compromised. The new analysis confirms that surface aragonite saturation levels in nearly all low-latitude coral reef regions have already fallen below the threshold of 3.5, the recognised tipping point for reef stability. This figure marks the lower boundary of suitable growth conditions. Once crossed, the reefs shift into a state of marginal function and slow erosion.
By 2020, this threshold had been breached across most of the tropical Pacific, Indian Ocean, and central Atlantic. The average surface values in these regions now range between 3.36 and 3.49, below the level needed to support sustained coral calcification. In practical terms, this means many reefs are no longer growing. Some are dissolving. The biological infrastructure that supports over one billion people through food, storm protection, and local economies is being chemically dismantled in real time.
The study measured the extent of this decline by comparing current data to pre-industrial conditions. Between 1750 and 2020, the area of tropical surface ocean with aragonite saturation below 3.5 has grown by 43 percent. That zone is continuing to expand. Warm-water coral systems, which once thrived in large belts across equatorial waters, are now shrinking toward isolated refuges where chemistry has not yet tipped past critical limits.
Coral reefs support a quarter of global marine species. They stabilise coastlines. They buffer extreme wave energy. They serve as nurseries for countless fish species, including those crucial to subsistence fisheries. The decline of reef habitat is triggering broader ecological losses. The saturation level that allows coral to form calcium carbonate is no longer present across large parts of their natural range.
The 3.5 saturation threshold was selected based on decades of laboratory and fieldwork. It reflects the lower limit at which reef calcification can outpace dissolution. Below this point, reef growth halts. What follows is erosion and ecosystem collapse. This level is not theoretical. It is already being exceeded in most of the low-latitude ocean, where coral reefs are concentrated.
The paper stresses that even if the global average acidification has not yet exceeded the 20 percent reduction boundary, regional hotspots have. The tropical reef belt is one of them. The global average hides local severity. What the data show is a 15 percent drop in saturation levels across reef zones compared to pre-industrial conditions. That alone is enough to reduce suitable coral habitat significantly. It is not a future threat. It is present-day loss.
The polar oceans are entering a phase of rapid chemical instability. Conditions in the Arctic and Southern Ocean are shifting beyond the tolerance limits of key species. These waters have always been cold and carbon-rich, but the rate of change has accelerated. The data show that vast areas of the polar seas are now dipping below safe saturation levels during certain seasons, with permanent undersaturation zones emerging in some regions of the Arctic.
Between 1750 and 2020, the surface waters of the Arctic saw a fourfold increase in the area experiencing aragonite undersaturation. This metric reflects a tipping point where seawater can no longer maintain calcium carbonate structures. Once crossed, shell formation becomes difficult, and existing shells begin to dissolve. In 2020, roughly 21 percent of Arctic surface waters had already fallen below this threshold during parts of the year. That figure was just 5 percent in the pre-industrial period.
Pteropods, tiny floating snails that form the base of many polar food chains, are now showing signs of severe stress. Their shells are visibly dissolving. The study uses aragonite saturation levels of 1.5 and 1.2 as thresholds for mild and severe shell degradation. Across the polar regions, the proportion of ocean water falling below these levels has surged. In total, 80 percent of the polar ocean’s upper 200 metres is now classified as marginal for pteropod survival. That includes regions critical for krill, seabirds, and commercial fish species that depend on these planktonic organisms.
Even without complete undersaturation, marine life in these regions is being forced into conditions it has never experienced before. The chemical changes affect reproduction, feeding, and energy balance. At the surface, the Arctic and Southern Ocean remain just above the 1.0 saturation threshold on annual averages, but this masks the increasing frequency and duration of exposure to unsafe conditions during specific months. These seasonal pulses of corrosive water are now widespread, and their biological effects are compounding.
The original acidification boundary was meant to prevent this. It was set to stop polar waters from dropping below aragonite saturation levels where dissolution begins. But the average decline across the Arctic and Southern Ocean has already reached 26 percent and 22 percent respectively. These numbers exceed the global boundary by a wide margin. The damage is no longer theoretical or distant. It is happening at depth, across seasons, and across species.
The deeper layers of the ocean, often overlooked in surface-based monitoring, are showing signs of advanced chemical degradation. The new study highlights that acidification is not confined to the surface. Between 50 and 200 metres below sea level, the decline in aragonite saturation is even more severe, affecting zones that support larval fish, deep corals, and long-range carbon cycling.
By 2020, over 58 percent of ocean water at 100 metres depth and 61 percent at 200 metres had already crossed the planetary acidification boundary. These layers, once chemically stable, are now sliding into corrosive conditions that threaten organisms adapted to low light and cold temperatures. Unlike surface regions, which experience seasonal variability, the deepening acidification in these strata is constant. It is chemically embedded and slow to reverse.
The analysis draws on new model–data fusion products that map the carbonate chemistry of the ocean column in unprecedented detail. The results are clear. Acidification increases with depth in most regions. Carbon dioxide absorbed at the surface is transported downward through mixing, respiration, and biological cycling. As it accumulates in deeper water, it drives down pH and aragonite saturation. This shift is particularly severe in the North Pacific and the Southern Ocean, where organic matter decomposition adds an additional acidifying layer.
The implications for deep-sea ecosystems are profound. Many species that live or reproduce in these layers rely on stable carbonate conditions to form shells and internal structures. Deep-water corals, which grow slowly and live for centuries, are among the most vulnerable. They are already experiencing exposure to undersaturated waters in some regions, including the North Atlantic and Southern Ocean. These corals are key habitat builders for deep-sea biodiversity and long-lived fish species. Once damaged, recovery is measured in decades, not years.
The study also emphasises that this subsurface degradation is not uniform. Tropical waters are affected less at depth, but polar and subpolar zones have already tipped past the 20 percent threshold. These regions are also home to cold-water fisheries and carbon drawdown zones that regulate global climate. As these deeper layers grow more acidic, the ability of the ocean to store carbon and sustain biodiversity may be compromised.
Subsurface change is harder to see, but not less important. The upper 200 metres of the ocean contain a complex web of interactions between physical chemistry, planktonic life, and long-term carbon sequestration. As the saturation boundary continues to move downward, these systems are losing resilience. No visible warning is likely to emerge before the biological effects become locked in. By then, the chemical signals will already be decades old.
Coastal regions are now experiencing acidification levels that directly threaten global aquaculture, shellfisheries, and nearshore ecosystems. The chemical changes observed in the upper 25 metres of coastal waters have pushed large stretches of habitat into conditions classified as marginal or unsuitable for the development of key bivalve species, including oysters and mussels.
The aragonite saturation threshold for bivalves has been set at 1.8. Below this point, larval development falters, shell formation is stunted, and survival rates drop sharply. The study shows that, by 2020, over 12 percent of global coastal waters had already crossed into this danger zone. That figure represents a 13 percent increase from pre-industrial conditions. For species with early life stages sensitive to calcium carbonate availability, these changes strike at the most vulnerable point in their life cycle.
The findings draw particular attention to the Pacific oyster, Magallana gigas, a cornerstone species for aquaculture. Its larvae have been shown to experience total production failure when saturation states fall near 1.75. In parts of the North Pacific and along the west coast of North America, saturation levels now frequently approach or fall below that threshold during key spawning periods. Hatchery failures in these regions have already been linked to sudden shifts in carbonate chemistry.
The risks are not confined to one species. The study highlights similar thresholds for the Olympia oyster, the Eastern oyster, and the blue mussel. Each shows a sharp decline in growth and calcification when saturation drops between 1.4 and 1.9. These are not speculative values. They are based on repeated field observations and laboratory testing under conditions now being matched in open water.
This chemical shift also threatens the ecological role that bivalves play. Beyond food production, they filter water, stabilise shorelines, and build reef-like structures that shelter other marine life. Once saturation drops below functional levels, the loss of bivalves is not just economic. It reduces water quality, breaks down habitats, and accelerates erosion in already vulnerable coastal zones.
The geographic scale of this transition is expanding. Coastal systems within 300 kilometres of the shore now show a clear trend toward increased acidity, particularly in regions with high nutrient runoff, stratification, and organic decay. These factors combine with atmospheric CO₂ to drive saturation downward. In many regions, including parts of the Gulf of Mexico, the North Sea, and Southeast Asia, the early stages of aquaculture disruption are already in motion.
According to the analysis, if coastal saturation declines another 10 percent, nearly one fifth of the world’s shellfish-producing waters will become unsuitable for reliable harvests. The margin for error is narrowing. These are not slow shifts measured over centuries. The last twenty years have seen saturation fall at a rate that outpaces adaptation, both for marine species and for the industries that depend on them.
Acidification is not unfolding uniformly across the world’s oceans. Some regions are already well past the established chemical boundary. Others are rapidly approaching it. The study separates the world’s seas into seven major basins and tracks their individual progression toward the critical 20 percent decline in aragonite saturation from pre-industrial conditions. The results show a pattern of uneven but accelerating chemical collapse.
The Arctic Ocean is the most affected. By 2020, it had already experienced a 26 percent drop in aragonite saturation on average. That figure includes wide areas that now fall below the threshold for safe shell formation. Seasonal undersaturation events are spreading in both frequency and geographic range. Nearly 80 percent of Arctic surface waters have now entered the zone of marginal conditions during parts of the year.
The North Pacific follows closely behind, with a 22 percent decline. This region has deep water upwelling systems that transport low-saturation water from the depths to the surface. That process now pulls up water that is already below the boundary. It mixes with surface layers and pushes saturation lower than surface conditions alone would suggest. Commercial fisheries in the region are already experiencing disruptions tied to these shifts.
The Southern Ocean is registering a 21.8 percent drop. This basin plays a central role in global carbon regulation. Its cold waters absorb large amounts of CO₂, making it especially vulnerable to acidification. The growing undersaturation of these waters is not confined to the surface. At depths of 100 to 200 metres, the Southern Ocean shows some of the largest declines globally, with saturation levels falling well into the range where shell dissolution and biological stress occur.
The North Atlantic has reached a 20.1 percent decline. It is now classified as having crossed the acidification boundary. This basin supports major fisheries, cold-water corals, and migratory species that rely on carbonate-rich waters during different life stages. Saturation levels have already dropped enough to challenge the growth of calcifiers in the subpolar zones, with ripple effects extending into ecosystems from Greenland to the British Isles.
The central Pacific and Indian Ocean basins are both showing clear declines, although they have not yet crossed the 20 percent mark. The central Pacific shows a 17 percent drop, while the Indian Ocean is at 17.3 percent. However, both are approaching the limit and have large areas with surface values below coral viability thresholds. This is especially critical for regions such as the Maldives, Indonesia, and parts of the South Pacific where coral reefs are a primary ecological foundation.
The central Atlantic is currently the least affected, with a 16 percent drop. But even here, signs of stress are emerging. Coral reef systems in the Caribbean and along the West African coast are already showing changes in calcification rates and structure formation. The delay in boundary crossing is not protection. It is a brief reprieve.
Every basin assessed in the study has already entered the uncertainty zone around the 20 percent boundary. That means the difference between a stable system and a destabilised one is now within the margin of error. The transition has begun. In some regions, it is already complete.
While most ocean monitoring focuses on surface conditions, the deeper layers are showing the strongest signs of structural decline. Beneath the first 50 metres, acidification is accelerating, not slowing. The new model data reveal that by the year 2020, the 100 to 200 metre zone had already experienced aragonite saturation drops approaching or exceeding 20 percent in nearly all major basins. This layer is where deep-sea ecosystems operate, and where key processes in the ocean’s carbon cycle take place.
Unlike the surface, these layers are not buffered by seasonal variation. The saturation state at 200 metres now reflects the full accumulation of decades of carbon dioxide uptake and biological decay. The deep ocean is absorbing the by-products of respiration, dead plankton, and sinking organic matter. These inputs are shifting the chemistry far beyond pre-industrial conditions, and they are doing so with no clear reversal path.
The effects are not hypothetical. Deep-water corals, which live between 100 and 1000 metres, rely on stable carbonate conditions to form their skeletons. These corals do not reproduce quickly. Some of them grow less than a centimetre per year. When exposed to undersaturated water, their structures begin to dissolve. That loss undermines the three-dimensional habitat used by deep-sea fish, crustaceans, and other cold-water organisms.
The study confirms that at 100 metres, more than half of the global ocean has now crossed the 20 percent saturation drop boundary. At 200 metres, it is over 60 percent. This expansion into deeper zones is silent but irreversible in human timescales. The ocean is not just acidifying at the top. It is becoming chemically unstable throughout its upper structure.
Subsurface acidification also interferes with the biological carbon pump. This process moves carbon from the atmosphere to the deep ocean through the sinking of organic material. If acidification impairs the organisms that form the base of this process, including plankton with calcium carbonate shells, the entire system slows down. That means more carbon remains at the surface, altering heat retention and air–sea gas exchange.
In polar waters, the chemical decline at depth is even more extreme. Cold water holds more carbon dioxide, and the combination of surface uptake and subsurface respiration pushes saturation toward critical lows. Even without reaching full undersaturation, the drop in saturation is enough to alter food availability, metabolism, and ecosystem structure.
This vertical expansion of acidification creates a layered threat. Marine species are not only being pushed laterally across the map but also vertically through the water column in search of safer zones. In many cases, there is no refuge left. The range of habitable depth is shrinking. As surface and subsurface zones converge on chemically hostile conditions, the complexity and stability of marine life continues to erode.
The processes driving ocean acidification are not slow, abstract, or reversible by natural feedback alone. The shift in ocean chemistry is happening on a timeline that leaves little room for recovery. Once aragonite saturation falls below key thresholds, the chemical environment begins to impose hard limits on biological survival. Reversing that change is not as simple as reducing emissions or waiting for natural buffering to catch up.
Unlike surface temperature, which can stabilise or even decline with rapid intervention, saturation state depends on the physical removal of excess carbon from the water. The oceans absorb carbon dioxide from the atmosphere, but they do not release it easily. Once it dissolves into seawater, it reacts to form carbonic acid, lowering pH and reducing carbonate ion availability. That reaction cannot be undone without removing the CO₂ from the ocean itself.
This makes aragonite saturation a one-way signal. Once the threshold is crossed, damage accumulates even if atmospheric CO₂ stabilises. The slow overturning of ocean layers means that the carbon-rich water now found at 100 or 200 metres depth will remain there for decades, if not centuries. The vertical mixing that might bring relief is also the mechanism that spreads acidified water into new zones.
The study confirms that even under the most conservative emissions scenario, only limited portions of the global ocean can be held within the 20 percent boundary by the end of the century. Every other scenario leads to near-complete breach. Under mid-range emissions, one quarter of the ocean will have saturation levels more than 40 percent below pre-industrial conditions. Under high emissions, that figure rises to 95 percent.
There is no evidence that ecosystems can adapt to these rates of change. Laboratory studies show that marine calcifiers begin to suffer physiological stress well before full undersaturation. Reduced feeding, slower growth, impaired reproduction, and altered metabolism all occur in conditions that now dominate wide regions of the ocean. Even small declines in saturation create cumulative pressure across generations.
The idea of a gradual transition has now been overtaken by the data. Ocean chemistry is shifting faster than many ecological models predicted. The lag between CO₂ release and carbonate collapse has narrowed. In some regions, the tipping point was crossed two decades ago. What is being measured today is not the start of the change. It is its acceleration.
This chemical reality presents hard boundaries. No organism, economy, or fishery can override the fundamental physics of carbonate equilibrium. Once saturation drops past a species’ survival limit, it does not adapt. It fails. And the structures built on that species — reefs, food webs, fisheries, aquaculture — begin to fail with it.
There is no explosion. No single moment where the ocean changed. But the measurements show that its chemical foundation is shifting. The data now confirms that the conditions needed for stable marine life have already eroded across much of the world’s oceans.
By the year 2000, surface saturation had already dropped beyond the safer 10 percent reduction level. By 2020, more than 40 percent of the surface ocean and 60 percent of the upper 200 metres had passed the 20 percent boundary. These are the zones where key species live, reproduce, and maintain population stability. Those zones are now chemically compromised.
Calcifying species are being affected first. Coral reefs are shrinking, shellfish larvae are failing to develop, and planktonic organisms in polar waters are dissolving faster than they can rebuild. These organisms are not isolated. They form the base of larger food webs that support fish, mammals, and the fisheries people depend on.
The damage spreads through networks. Coral loss changes fish distributions. Dissolving shells reduce food sources for larger species. Hatchery failures impact aquaculture harvests. Shoreline ecosystems lose their structure. These effects don’t arrive all at once. They appear gradually, across regions, and they compound over time.
The regions with the largest drops in saturation are also those responsible for some of the planet’s most essential ocean services. The Arctic regulates deep ocean carbon storage. Coral belts along the equator provide food and coastal protection for millions. These areas are seeing saturation values fall below safe biological thresholds. Some are not expected to recover within human lifespans.
The authors of the study offer a clearer definition of what it means for ocean chemistry to breach its safe boundary. Instead of relying only on global averages, they use regional analysis and biological indicators. That change in approach shows that the transition is already well underway. What was once a theoretical line is now a measured condition.
This shift in ocean chemistry is not being reversed by natural processes fast enough to restore previous stability. The acidified layers continue to expand deeper and wider, affecting both surface systems and the life below them. The longer it continues, the more species and ecosystems fall into conditions they cannot function in.
There is no reset switch. The carbonate balance of the ocean has already changed. And that shift is now being reflected in the decline of the species and systems that depend on it.
Source:
Findlay, H. S., Lenton, T. M., Barry, J., et al. (2025). Ocean Acidification: Another Planetary Boundary Crossed. Global Change Biology. https://doi.org/10.1111/gcb.17149
on June 28, 2025, 6:41 pm
