As the ocean takes up carbon dioxide from burning fossil fuels, its surface has become about 18% more acidic since 1985, and in 2025 ocean acidification was assessed as the seventh planetary boundary to be crossed, putting corals, shellfish and other shell-building life at risk.
Written from the sources listed below, not independently peer-reviewed.
Global average surface ocean pH fell from 8.11 in 1985 to 8.04 in 2024. Roughly how much more acidic did the surface ocean become over that period?
8.04
Global average surface ocean pH
Annual global mean surface seawater pH in 2024, down from 8.11 in 1985, an increase in acidity of about 18%. The value is a reconstruction from observations by the Copernicus Marine Service, reported by the European Environment Agency.
Data year 2024. Source: European Environment Agency, 2026
Over 40%
Surface ocean past the acidification boundary
Share of the global surface ocean where aragonite saturation had fallen by more than 20% from pre-industrial levels by 2020 (± 9.7 percentage points), according to a 2025 study. Up to 60% of the ocean between the surface and 200 m depth had crossed the same line. This is a modelled estimate based on observation-based data products.
Data year 2020. Source: Findlay H.S., Feely R.A., Jiang L.-Q., Pelletier G., Bednaršek N., Global Change Biology 31(6), 2025, 2025
45
Countries reporting acidification data to the UN
Number of countries reporting ocean acidity data towards SDG indicator 14.3.1 in 2026, from 906 stations. Only 13 of them have submitted records covering more than a decade.
Data year 2026. Source: United Nations Statistics Division, 2026
Unit: stations
Number of measurement stations whose data countries submitted for SDG indicator 14.3.1, as reported by the UN Statistics Division. This tracks monitoring effort, not acidification itself, and growth partly reflects more countries reporting existing data. The years in between are not plotted. Global surface pH declined throughout this period.
SDG target 14.3 asks countries to minimize and address the impacts of ocean acidification, including through stronger scientific cooperation. It sets no numerical goal or deadline. The UN reports that observations now document acidification in all ocean basins and seas, with global average pH still falling, and the IPCC expects acidification to keep increasing under globally agreed climate scenarios. Reporting has grown, but in 2026 only 45 countries submitted data and most of them had records of just one to five years.
Surface pH has fallen fastest in the Indian Ocean, the Southern Ocean, the eastern equatorial and northern tropical Pacific and parts of the Atlantic, areas that together make up 47% of the sampled global ocean.
Source: World Meteorological Organization, 2026Acidification has gone further below the surface than at it: up to 60% of the ocean down to 200 m had crossed the boundary by 2020, compared with over 40% of the surface.
Source: Findlay H.S., Feely R.A., Jiang L.-Q., Pelletier G., Bednaršek N., Global Change Biology 31(6), 2025, 2025Polar pteropods have lost up to 61% of their suitable habitat, the largest loss among the groups studied, and uncertainty about pre-industrial conditions is largest in the Arctic, where data are sparse.
Source: Findlay H.S., Feely R.A., Jiang L.-Q., Pelletier G., Bednaršek N., Global Change Biology 31(6), 2025, 2025Measurements are thinnest in coastal Asia and Africa and in most of the open ocean outside the North Atlantic, so many of the coastal communities that depend on fisheries have little local data on how their waters are changing.
Source: United Nations Statistics Division, 2026Each country is shaded by its latest available value, so you can see where the gap is widest. Hover or tap a country, search for one, or open the full table.
Loading country values…
Values in t CO₂ per person.
Acidification happens in the shared ocean and cannot be mapped by country, so this map shows a driver: each country's annual CO₂ emissions per person in 2024, excluding land use and forestry. It does not show where acidification is worst. Values are modelled inventory estimates; several small island territories are reported as zero, and a few very small states show extreme values.
Source: EDGAR (European Commission Joint Research Centre), via World Bank Open Data. Latest year available per country; years differ.
Values load from World Bank Open Data. Simplified Natural Earth borders; they imply no position on disputed territories.
Instead of one “percent solved” score, DO GOOD rates three separate dimensions. A problem can have mature solutions that still reach too few people. These ratings are an editorial reading of the sources on this page, not a measured index.
Has the situation improved in the real world?
LowSurface pH has fallen without interruption since records began in 1985, at about 0.017 units per decade, and in 2025 the acidification boundary was assessed as crossed for the first time.
How mature and validated are the available solutions?
ModerateThe cause is well understood and the only lasting remedy, cutting CO₂ emissions, is known. Monitoring methods and a common UN reporting methodology are established, but there is no proven way to reverse acidification at sea, and methods to counter it directly are still experimental.
How widely are effective solutions actually used?
LowAtmospheric CO₂ reached 422.1 ppm in 2024 and is still rising, so the root cause is not yet being addressed at the scale needed. Monitoring has expanded but covers only 45 reporting countries, with large gaps in coastal Asia and Africa.
The global decline in open-ocean pH is well established from decades of observations. Estimates for coasts, the Arctic and the biological thresholds of species are much less certain, and the authors of the 2025 boundary study say coastal uncertainties are likely underestimated.
Surface pH kept falling through 2025, the boundary was crossed in 2025, and the UN expects acidification to continue increasing under agreed climate scenarios because the ocean keeps absorbing CO₂.
The ocean takes up around a quarter of the carbon dioxide (CO₂) that human activity releases into the air each year. In seawater, this CO₂ changes the water's carbonate chemistry and lowers its pH, a process called ocean acidification. The surface ocean has not become acidic in absolute terms, but it is moving in that direction: global average surface pH fell from 8.11 in 1985 to 8.04 in 2024, an increase in acidity of about 18%, and the World Meteorological Organization (WMO) reports a steady decline of about 0.017 pH units per decade from 1985 to 2025. Since the start of the industrial era, surface pH has fallen by around 0.1 units, or 30–40% more acidity, and the Intergovernmental Panel on Climate Change (IPCC) has very high confidence that today's values are unprecedented for at least 26,000 years. The main cause is CO₂ from fossil fuels; atmospheric CO₂ reached 422.1 parts per million in 2024, more than 50% above the pre-industrial level.
Acidification makes it harder for organisms that build shells and skeletons from calcium carbonate, such as corals, oysters, mussels and tiny swimming sea snails called pteropods, to form and keep them. Pteropods, an important food source in marine food webs, already show signs of shell damage. A 2025 study estimates that between pre-industrial times and 2020 the change in ocean chemistry cut suitable habitat by 43% for tropical and subtropical coral reefs, by up to 61% for polar pteropods and by 13% for coastal bivalves such as oysters and clams. The only documented population die-offs so far are of Pacific oyster larvae at hatcheries on the west coast of North America, where acidification added to naturally corrosive upwelled water. Cold-water corals, tropical reefs and Arctic marine life are considered especially at risk, with knock-on effects for coastal fisheries, food security and the communities that depend on them. Acidification acts together with ocean warming and loss of oxygen, and the effects of the three combined are still poorly understood.
Acidification is tracked through changes in seawater chemistry, measured at fixed stations, from ships and floats, and combined into global reconstructions. The main measures are:
Surface ocean pH
pH measures how acidic or alkaline water is. The scale is logarithmic, so a fall of 0.1 units means a 30–40% increase in acidity. SDG indicator 14.3.1 asks countries to report average marine acidity (pH) at an agreed set of representative sampling stations; WMO and the Copernicus Marine Service publish a global average reconstructed from observations since 1985.
Aragonite saturation state
Aragonite is the form of calcium carbonate that corals and many shellfish use to build their skeletons and shells. Its saturation state shows how easily they can do so; as CO₂ rises, it falls. A 2025 study estimates the global surface average at about 2.90, down from a pre-industrial value of about 3.51, a decline of about 17% by 2020.
The planetary boundary
The planetary boundaries framework sets the safe limit for ocean acidification at a 20% fall in surface aragonite saturation from pre-industrial levels. The 2025 Planetary Health Check assessed this boundary as crossed for the first time. Some researchers argue for a stricter limit of 10%, which the whole surface ocean had already passed by 2000.
Biological impacts
Effects on living things are estimated from laboratory experiments, field observations and the chemical conditions at which species such as corals, pteropods and bivalves start to suffer. These thresholds are less certain than the chemical trend itself, because few studies test how species responded under pre-industrial conditions.
What works
Acidification is driven mainly by CO₂ from fossil fuels, so reducing emissions is the only way to slow it across the whole ocean. Every tonne not emitted means less CO₂ for the ocean to absorb.
Limits
Ocean chemistry responds slowly, and the CO₂ already absorbed will keep affecting it for a long time. Even under a stricter 10% limit, the entire surface ocean had already passed the line by 2000, so some damage is locked in.
What works
Coordinated measurements show where and how fast waters are changing. The Global Ocean Acidification Observing Network (GOA-ON) has grown from 150 members in 35 countries in 2013 to over 1,300 members in more than 100 countries in 2026, with 11 regional hubs.
Limits
Most national records are short, and only 13 of the 45 countries reporting to the UN in 2026 had more than a decade of data. Coastal Asia, Africa and much of the open ocean remain poorly covered.
What works
A common methodology lets countries submit comparable data on ocean acidity to the UN, which builds a shared global picture. The number of stations reported rose from 178 in 2021 to 906 in 2026.
Limits
Reporting is voluntary and depends on national capacity, so the countries most exposed to the impacts are often those with the least data.
What works
Corals, shellfish and plankton face warming and oxygen loss as well as acidification. The authors of the 2025 boundary study argue that protecting them also means making them more resilient to these other stressors, for example through measures that limit additional harm to the habitats where they live.
Limits
Such measures do not change ocean chemistry, and how acidification interacts with warming and oxygen loss is still poorly understood, so their benefit is hard to predict.
What works
Training, mentoring between experienced and new researchers and starter measurement kits, such as GOA-ON's Pier2Peer mentoring and 'GOA-ON in a Box', help scientists in countries without established programmes begin their own measurements.
Limits
Short training and starter kits do not by themselves secure the long-term funding, staff and quality control needed for decade-long records.
Promising, but not proven at scale. The stage shows how far each approach has come; it is not a recommendation.
Adding finely ground alkaline minerals or similar substances to seawater could raise pH locally and help the ocean store more CO₂. Effects on marine life, how well the extra carbon storage can be verified and how it could be governed are still open questions; it is being tested in laboratories and small field trials.
Seagrass meadows and seaweed take up CO₂ as they grow, and researchers are studying whether they can create small zones of less acidic water around shellfish farms or reefs. The effect varies with time of day, season and water flow, and is local at best.
Some aquaculture programmes select oysters and other shellfish that grow better in more acidic water. This could help farmers, but tolerance may come with trade-offs in growth or disease resistance and does not help wild species.
Cheaper sensors on buoys, floats and gliders could fill gaps in coastal and remote waters where ship-based sampling is rare. Keeping their measurements accurate enough to detect small pH changes over many years remains a challenge.
Coordination gap
Atmospheric CO₂ reached 422.1 ppm in 2024, more than 50% above the pre-industrial level of 280 ppm. As long as it keeps rising, the ocean will keep absorbing more and acidifying further, and no regional action can offset that.
Implementation gap
Observations are particularly scarce in coastal Asia and Africa and in most open ocean areas outside the North Atlantic, which limits global models and assessments of who is most vulnerable.
Knowledge gap
Most countries reporting to the UN have between one and five years of data, and only 13 of 45 have more than a decade. Detecting a decline of less than 0.02 pH units per decade requires long, carefully calibrated time series.
Knowledge gap
Few biological studies show how species responded under pre-industrial conditions, which makes it hard to set safe limits. Researchers call for better coordination between chemists and biologists.
Technology gap
Global models poorly represent river inputs and sea ice, and uncertainty about pre-industrial conditions is largest in the Arctic (±13%). Coastal estimates are likely to be more uncertain than current figures suggest.
Coordination gap
SDG target 14.3 asks countries to minimize and address acidification but sets no numerical goal or deadline, so there is no agreed yardstick for judging national progress beyond reporting data.
Unresolved questions, not vetted breakthroughs. Answering any of them would change what is possible.
Marine life is facing acidification, warming and falling oxygen at the same time. Do these stresses add up, amplify each other, or partly offset each other for corals, shellfish and plankton?
Pre-industrial ocean chemistry is uncertain, and researchers disagree whether the boundary should be a 20% or a 10% fall in aragonite saturation. Which level actually protects the most sensitive species and ecosystems?
Rivers, upwelling, nutrient pollution and sea ice make coastal chemistry far more variable than the open ocean. How fast are the coastal waters that fisheries and aquaculture depend on acidifying, especially in poorly monitored regions?
Could ocean alkalinity enhancement counter acidification at a meaningful scale without harming marine life, and could its benefits be measured reliably?
CO₂ from fossil fuels is the main cause. Cutting your own emissions where you can and asking representatives about national climate plans addresses the root cause, although the effect of any one person's action is small.
Donating to or volunteering with established marine research and monitoring programmes, particularly those that build capacity in under-observed regions, helps fill data gaps. Check that a programme follows recognised data standards.
Asking suppliers where shellfish are farmed and whether producers monitor water conditions supports transparency in a sector exposed to acidification. It will not change ocean chemistry.
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World Meteorological Organization (WMO), WMO-No. 1391, March 2026, published 2026
WMO's annual climate report, with an ocean acidification section based on Copernicus Marine Service data. Source of the surface pH decline of –0.017 ± 0.001 units per decade over 1985–2025, the regions with the fastest decline (47% of the sampled ocean), and the IPCC finding that present-day surface pH is unprecedented for at least 26,000 years.
European Environment Agency (EEA), using Copernicus Marine Service data, 25 August 2026, published 2026
EEA indicator assessment of global and European ocean acidification. Source of global mean surface pH (8.11 in 1985, 8.04 in 2024), the roughly 18% increase in acidity since 1985, and atmospheric CO₂ of 422.1 ppm in 2024 compared with 280 ppm before industrialisation.
Stockholm Resilience Centre, news release on the Planetary Health Check 2025 (Potsdam Institute for Climate Impact Research), 24 September 2025, published 2025
Release on the 2025 Planetary Health Check, which assessed the ocean acidification boundary as breached for the first time. Source of the fall in surface pH of about 0.1 units since the industrial era (30–40% more acidity), the groups most at risk (cold-water corals, tropical reefs, Arctic marine life), shell damage in pteropods, and effects on coastal fisheries and food security.
Findlay H.S., Feely R.A., Jiang L.-Q., Pelletier G., Bednaršek N., Global Change Biology 31(6), 2025 (open access via PubMed Central), published 2025
Peer-reviewed reassessment of the ocean acidification boundary. Source of over 40% (± 9.7%) of the surface ocean and up to 60% of the ocean to 200 m past the 20% boundary by 2020, the 17.3% decline in global surface aragonite saturation (pre-industrial about 3.51, present about 2.90), habitat losses (43% coral reefs, up to 61% polar pteropods, 13% coastal bivalves), the proposed 10% boundary passed by 2000, the Pacific oyster hatchery die-offs, and the data and model gaps for coasts and the Arctic (±13%).
United Nations Statistics Division (UN DESA), 2026, published 2026
UN progress report on SDG 14. Source of the wording of target 14.3 and indicator 14.3.1, the ocean's uptake of around one quarter of annual human CO₂ emissions, the 45 countries and 906 stations reporting in 2026 (178 stations in 2021), the 13 countries with more than a decade of data, and the monitoring gaps in coastal Asia, Africa and the open ocean outside the North Atlantic.
Global Ocean Acidification Observing Network (GOA-ON), 2026, published 2026
Overview of the global observing network. Source of its growth from 150 members in 35 countries in 2013 to over 1,300 members from over 100 countries in 2026, its 11 regional hubs, and its capacity-sharing programmes (Pier2Peer mentoring, GOA-ON in a Box).
DO GOOD International (2026). Slowing Ocean Acidification. Global Challenges. Updated October 15, 2026. https://www.dogood.world/challenges/ocean-acidification
For figures, please cite the original sources listed above.