\u2022 How acidified is the ocean as of 2025?<\/a>
\u2022 Why does this matter?<\/a>
\u2022 What will happen to ocean animals with shells?<\/a>
\u2022 Do we need to worry about humans and other animals, too?<\/a>
\u2022 What is the ocean acidification planetary boundary?<\/a>
\u2022 Is every part of the ocean affected in the same way?<\/a>
\u2022 Can we learn from acidic oceans of the past?<\/a>
\u2022 What can be done about it?<\/a><\/p>\n\n\n\n
How does CO2 in the atmosphere make the ocean acidic?<\/h2>\n\n\n\n
Ocean acidification is part of the global carbon cycle.<\/p>\n\n\n\n
When carbon dioxide dissolves in water, it forms carbonic acid. This acid releases hydrogen ions, which lower the seawater’s pH balance.<\/p>\n\n\n\n
pH balance<\/p>
This sliding scale of 14 points indicates the acid\/alkaline balance of a solution. Position 1 indicates the highest acidity, 14 the highest alkalinity. It stands for \u201cpotential of hydrogen\u201d, because the scale is determined by the concentration of hydrogen ions.<\/p><\/div><\/div>\n\n\n\n
Carbon dioxide emitted by human activities may be largely released into the atmosphere, but it does not all stay there. Huge amounts are absorbed by the ocean. A study published in 2023 determined that the ocean absorbed 25%<\/a> of anthropogenic CO2 emitted from the early 1960s to the late 2010s. This has so far saved humanity from greater global warming.<\/p>\n\n\n\n Because of the rise in atmospheric CO2 concentrations over the past century, more CO2 has been taken up by the ocean, causing it to acidify.<\/p>\n\n\n\n Earth\u2019s ocean has become roughly 30% more acidic than it was in the pre-industrial age, according to data from the European Environment Agency published<\/a> in October. A huge part of that seawater pH decrease has come in recent decades. Just before the industrial revolution began in around 1750, Earth\u2019s mean surface seawater pH was 8.2. By 1985 it had fallen to 8.11. By 2024, it was down to 8.04. <\/p>\n\n\n\n This data indicates that pH at the surface of the ocean will decline even further by 2100, by between 0.15 and 0.5, depending on how much emissions are curtailed.<\/p>\n\n\n\n Also in October, researchers working at the Norwegian University of Science and Technology (NTNU) authored a study<\/a> of ocean acidification models to quantify the global ecological and economic consequences of future acidity rises. \u201cIf in the years to come we continue on the current emission level, our models show that for most regions the ocean is on a trajectory toward worst-case scenarios,\u201d says Sedona Anderson, lead author of the paper.<\/p>\n\n\n\n In May, a new study found<\/a> that as acidification intensifies, the ocean\u2019s capacity to absorb atmospheric CO2 weakens, reducing its ability to slow global warming. The explanation lies in chemistry: as the sea accumulates more carbon dioxide, its \u201ccarbonate buffering\u201d capacity is reduced, making CO2 more chemically challenging to absorb. <\/p>\n\n\n\n In March, researchers reported<\/a> that the speed at which people are releasing CO2 into the atmosphere is much faster than the ocean’s natural capacity to absorb it without becoming acidic. This is profoundly altering the ocean\u2019s chemistry. The consequences of the change are not yet fully clear.<\/p>\n\n\n\n A study published in July explored<\/a> the effects of ocean warming and acidification on two bryozoan species (invertebrate marine calcifiers) living in and around volcanic CO2 vents in the Tyrrhenian Sea, off Italy\u2019s western coast. It found the populations of these animals are being increasingly depleted as the ocean warms in this particularly acidic environment. Calcifiers, such as corals, molluscs and terrapods, are small creatures with skeletons formed of calcium salts. Crucially, these creatures are foundational to food chains. They constitute, for example, about half of the diets of Arctic salmon.<\/p>\n\n\n\n In a collaborative study<\/a> between Greenland and Spain published in 2016, it was concluded that calcifiers can meet the greater energy demands of processing calcium salts in more acidic environments by increasing their nutrient uptake; this has been seen on White Island in New Zealand. However, current models<\/a> demonstrate that the ocean is becoming less hospitable, making it hard to provide the nutrients organisms need to cope with an already acidic environment.<\/p>\n\n\n\n While calcifiers are the first to suffer the consequences<\/a> of a more acidic ocean, many other species, including humans, are set to pay the price if we continue on this trajectory. Upcoming research areas are focusing on the impact of changes in ocean chemistry on non-calcifying organisms.<\/p>\n\n\n\n In a summary about the \u201csilent crisis\u201d of ocean acidification published<\/a> in April, experts from Spain\u2019s Institute of Marine Sciences (ICM) highlight how data collected over recent decades is demonstrating that non-calcifying species, such as fish and squid, are suffering from impaired respiration, altered behaviour and reduced reproductive success in a more acidic environment.<\/p>\n\n\n\n The behavioural impacts are thought to be due to calciferous structures within fish ears that, like corals, cannot form<\/a> properly in environments that are too acidic. This correlates with diminishing behavioural responses to danger. However, these are preliminary studies that focus on one generation of marine creatures only. More research in the field is needed.<\/p>\n\n\n\n Two other studies published this year, one observed in the Pacific<\/a> and the other in the Mediterranean<\/a>, found that the development and survival of mollusc larvae are negatively impacted by lower pH levels. A reduction in the number and characteristic variations of adult species was detected.<\/p>\n\n\n\n Though a direct correlation between increasingly acidic oceans and human health has not yet been explored, more than a billion people worldwide depend on healthy coral reefs for food and coastal protection. If these reefs become more fragile because they cannot properly build calcium shells, this will generate cascading problems. An acidic ocean will mean millions of dollars in lost fisheries and tourism revenue, because frailer coastal reefs will not provide the same level of coastal protection from storms.<\/p>\n\n\n\n Perhaps the biggest news in ocean acidification this year is the warning that a new \u201cplanetary boundary\u201d has been crossed.<\/p>\n\n\n\n In 2009, a group of scientists led by Potsdam University in Germany set out what they named planetary boundaries<\/a> \u2013 nine limits to systems that have kept Earth\u2019s environment stable during human existence. This project has become the annual Planetary Health Check report. One of the nine boundaries it investigates is ocean acidification<\/a>. This is measured via the levels of aragonite in the ocean. Aragonite is a form of calcium carbonate used by huge numbers of sea creatures to build their shells and skeletons. When the ocean acidifies, carbonate ions become less abundant, making shell formation more difficult.<\/p>\n\n\n\n When the nine planetary boundaries<\/a> were first established, the \u201csafe limit\u201d for acidification was set at a 20% reduction in the ocean\u2019s aragonite levels, known as \u201caragonite saturation state\u201d, compared to pre-industrial conditions. This figure was met with scepticism by some researchers in the field, who felt it was too high. \u201cWhen the scientific community saw the data in the first planetary boundaries report, it quickly became apparent that that was just a best guesstimate,\u201d says Helen Findlay, a professor at Plymouth Marine Laboratory in the United Kingdom.<\/p>\n\n\n\n In the years that followed, significant effort and resources were invested in analysing acidification in both pre- and post- industrial times, as well as in trying to better determine the acidification threshold after which catastrophic consequences take place.<\/p>\n\n\n\n In 2024, Findlay\u2019s team reassessed those original thresholds. In a paper published in June 2025, the team concluded<\/a> that the reduction in the ocean\u2019s aragonite saturation state needed to be much lower than previously thought to keep the ocean healthy: around 10%, rather than 20%. The team estimated that by 2020, the global ocean was, on average, within the \u201cuncertainty range of the ocean acidification boundary\u201d. In other words, the boundary may have been crossed.<\/p>\n\n\n\n The deeper into the ocean the researchers looked, the worse the findings were. At 200 metres below the surface, 60% of global waters already indicated acidification levels beyond the newly established safe limit.<\/p>\n\n\n\n A few months later, the 2025 edition<\/a> of the Planetary Health Check report confirmed this, reinforcing the idea that Earth\u2019s oceans are now operating in a risk zone. Furthermore, following data revisions and thanks to advancements in data modelling since 2009, the 2025 report says the ocean\u2019s pre-industrial aragonite saturation state was higher than originally thought. This means the drop in levels between the pre-industrial era and today has been more significant, blowing past even the team\u2019s original ocean acidification \u201csafe limit\u201d threshold of a 20% drop in aragonite.<\/p>\n\n\n\n \u201cWhen we published our first Planetary Health Check, several ocean experts told us that they could already see impacts on marine environments that didn\u2019t align with an ocean operating in a safe zone,\u201d says Levke Caesar, a member of the team that produces the report. \u201cSo, we thought that we should have reconsidered our study using more advanced data.\u201d<\/p>\n\n\n\n Ocean acidification is a widespread global problem but increasing amounts of research suggest some regions are more affected, especially the Arctic. These northern waters are very cold, and cold water dissolves CO2 more easily than warmer water.<\/p>\n\n\n\n In addition, some parts of the Arctic have major inputs of freshwater<\/a> from rivers and melting ice. This reduces the local waters\u2019 ability to buffer changes in pH, as an analysis of historical data published<\/a> in April confirms.<\/p>\n\n\n\n Studies show the Arctic Ocean naturally experiences regional and seasonal variations<\/a> in aragonite due to changes in ice and rivers. For example, before humanity started pumping out huge amounts of CO2, around 5% of polar waters became seasonally undersaturated with aragonite because of these natural seasonal shifts, according to Plymouth Marine Laboratory\u2019s boundary reassessment<\/a>. In the 2020s, however, that figure has risen to 21%. Some polar organisms are starting to exhibit the impacts of this change, Helen Findlay says.<\/p>\n\n\n\n Other studies show that coastal regions are more susceptible to acidification than offshore waters. In coastal areas, agricultural fertiliser runoff can increase the amount of nitrogen that ends up in the ocean. This can react with oxygen, causing local acidification and amplifying emissions-related acidification.<\/p>\n\n\n\n Work published<\/a> in November shows that oceanic upwelling systems, such as the California Current, can also exacerbate acidification.<\/p>\n\n\n\n More than 300 million years ago, volcanism massively raised CO2 levels in the atmosphere. This led to the end of what is known as the Late Palaeozoic Ice Age. As is happening today, the ocean sucked in much of the plentiful CO2 being held in the atmosphere, becoming more acidic in the process, as well as warmer and lower in oxygen. This led to the near-disappearance of calcifying organisms, such as coral reefs, and a mass extinction event sometimes called the Great Dying.<\/p>\n\n\n\n Scientists still debate the exact cause of that extinction event. \u201cWe know that the planet will find a way to survive. The question is if humanity will cope with the transition,\u201d ponders Hana Jurikova, a researcher at the University of St Andrews\u2019 School of Earth and Environmental Sciences, in Scotland. She was a lead author of an article on the end of the Late Palaeozoic Ice Age, published<\/a> in early 2025.<\/p>\n\n\n\n Over the course of millions of years following the Great Dying, changes in oceanic pH were brought about by rock weathering and the degradation of mineral shells. The waters became less acidic, and ocean life rebounded until the dawn of the industrial age.<\/p>\n\n\n\n \u201cWorking in this field, I’m often depressed,\u201d says Caesar. \u201cGlobal regulations are needed, but there are also many solutions that [can] benefit most of the [planetary] boundaries.\u201d<\/p>\n\n\n\nHow acidified is the ocean as of 2025?<\/h2>\n\n\n\n
Why does this matter?<\/h2>\n\n\n\n
What will happen to ocean animals with shells?<\/h2>\n\n\n\n
Do we need to worry about humans and other animals, too?<\/h2>\n\n\n\n
What is the ocean acidification planetary boundary?<\/h2>\n\n\n\n
Is every part of the ocean affected in the same way?<\/h2>\n\n\n\n
![\"queen](\"https:\/\/dialogue.earth\/content\/uploads\/2025\/12\/queen-scallop-in-Arctic-waters_Alamy_F0E93T.jpg\")
<\/div>We know that the planet will find a way to survive. The question is if humanity will cope with the transition<\/blockquote>Hana Jurikova, University of St Andrews<\/cite><\/div><\/div>\n\n\n\n
Can we learn from acidic oceans of the past?<\/h2>\n\n\n\n
What can be done about it?<\/h2>\n\n\n\n