Ocean acidification

The ocean absorbs around one quarter of anthropogenic CO₂ emissions, altering seawater chemistry and causing ocean acidification, i.e. a decrease in ocean pH. Global mean surface seawater pH fell from 8.11 in 1985 to 8.04 in 2024 and is projected to decline further. Ocean acidification affects marine organisms and ecosystems, with effects cascading throughout the food web and with implications for ecosystem services such as fisheries. Reducing CO₂ emissions is essential to limit further acidification, while reducing local pressures can strengthen ecosystem resilience.

Figure 1. Decline in ocean pH measured at the Aloha station and yearly mean surface seawater pH reported on a global scale

Over the last million years, mean surface ocean pH remained relatively stable, oscillating between 8.3 during cold periods (e.g. throughout the last glacial maximum 20,000 years ago) and 8.2 during warm periods (e.g. just prior to the industrial revolution). Rapid increases in atmospheric CO₂ levels due to human activities are now threatening this long-term stability. As a large fraction of emitted CO₂ is absorbed by the ocean, seawater chemistry changes, causing pH to decline and subsequent ocean acidification.

Global annual mean atmospheric CO₂ concentration reached 422.1 ppm in 2024, more than 50% above the pre-industrial level (280ppm), with half of this increase occurring since the 1980s. Global mean surface seawater pH declined from 8.11 in 1985 to 8.04 in 2024, corresponding to approximately 18% increase in acidity. Although this change may appear small, the pH scale is logarithmic, meaning that even modest declines represent substantial increases in acidity.

This indicator combines observations from the ALOHA station off Hawaii, one of the world's longest continuous ocean acidification monitoring records, with global surface ocean pH estimates from the Copernicus Marine Environment Monitoring Service (CMEMS).

Projected declines in ocean pH, estimated at between 0.15 and 0.5 pH units by 2100 depending on the emissions scenario, are expected to increasingly affect marine organisms and could alter marine ecosystems. Ocean acidification reduces the availability of carbonate ions required by calcifying organisms such as corals, shellfish and some plankton species to build and maintain shells and skeletons. These rapid chemical changes add pressure on marine calcifiers and Europe’s marine ecosystems, potentially altering species distributions, food-web dynamics and ecosystem functioning, with implications for biodiversity, fisheries and aquaculture.

Regional impacts vary considerably. According to OSPAR (2023), Arctic and Nordic waters are experiencing particularly rapid acidification, while vulnerable habitats and species across European seas, including North Atlantic cold-water coral reefs, may face increasing risks as ocean acidification interacts with other environmental pressures.

In 2025, ocean acidification was assessed as the seventh of nine planetary boundaries to have been crossed, indicating that global ocean conditions have moved beyond the proposed safe operating space. EU policies such as the European Green Deal, Marine Strategy Framework Directive (MSFD), Biodiversity Strategy for 2030 and climate policies, including the European Climate Law and Fit-for-55, support action to address ocean acidification and restore marine ecosystems.

The European Climate Law now includes a binding target to reduce EU net greenhouse gas emissions by 90% by 2040 compared with 1990, alongside the binding objective of climate neutrality by 2050. The MSFD focuses on strengthening implementation, regional cooperation and policy coherence to achieve environmental targets and Good Environmental Status (GES) for Europe's seas.

Substantial reductions in CO₂ emissions would support the gradual recovery of ocean chemistry from human-induced acidification over very long timescales, based on evidence from . Continued monitoring, harmonised data and research are needed to improve understanding of trends, regional variability and ecological impacts of ocean acidification, and to inform effective management and adaptation.

Supporting information

Definition

This indicator illustrates the global mean average rate of ocean acidification, quantified by decreases in pH, which is a measure of acidity, defined as the hydrogen ion concentration. A decrease in pH value corresponds to an increase in acidity.

The observed decrease in ocean pH resulting from increasing concentrations of CO₂ is an important indicator of change in the global ocean and the impacts of climate change.

This indicator provides information on:

trends in ocean acidity measured at the Aloha station;
yearly mean surface seawater pH levels reported on a global scale is computed from monthly pH values by CMEMS.

Methodology

The time series are based both on direct pH measurement data from the Hawaii Ocean Time-series, obtained from the Aloha station, and gap-filling calculations using data , and on a reconstruction of global yearly mean surface pH values from CMEMS.

The Aloha time series data are based on in situ measurements and calculation of pH values based on dissolved inorganic carbon concentrations and .
Global mean surface ocean pH values derived from the Copernicus Marine Service are based on a reconstruction method using in situ data and remote sensing data, as well as empirical relationships. Indicator is available at annual resolution, and from the year 1985 onwards, up to 2024. Error on each yearly value varies and is added in the data file sheet. Trend and uncertainty are defined as the slope and its residual standard deviation estimated with a linear least-squares regression.

The estimated global mean surface seawater pH is based on alkalinity values (obtained using the locally interpolated alkalinity regression (LIAR) method after ), surface ocean partial pressure of CO₂ (pCO₂) (CMEMS product) and an evaluation of a gridded field of ocean surface pH values based on (see ).

Policy/environmental relevance

Ocean acidification is addressed under the United Nations 2030 Agenda for Sustainable Development. Sustainable Development Goal (SDG) 14.3 aims to minimise and address the impacts of ocean acidification, including through enhanced scientific cooperation and monitoring.

At EU level, the European Green Deal, European Climate Law and the EU climate policy framework, including the Fit for 55 package, support reductions in greenhouse gas emissions, the primary driver of ocean acidification. Together with the Marine Strategy Framework Directive (MSFD) and the EU Biodiversity Strategy for 2030, these policies contribute to protecting and restoring marine ecosystems and strengthening their resilience to climate change and ocean acidification.

Accuracy and uncertaintiesData sources and providers

Metadata

References and footnotes

Nicole S. Lovenduski, Olivier Torres, Tilo Ziehn, Hongmei Li, Jasmin G. John, Jörg Schwinger, John P. Dunne, Marion Gehlen, Alessandro Tagliabue, James C. Orr, James R. Christian, Katsuya Toyama, Akitomo Yamamoto, Olivier Aumont, Matthew A. Chamberlain, Hiroyuki Tsujino, Yeray Santana-Falcón, Julien Palmieri, Roland Séférian et al., 2020, 'Twenty-first century ocean warming, acidification, deoxygenation, and upper-ocean nutrient and primary production decline from CMIP6 model projections', Copernicus GmbH.

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IPCC, 2021, Annex VII: Glossary [Matthews, J.B.R., V. Möller, R. van Diemen, J.S. Fuglestvedt, V. Masson-Delmotte, C. Méndez, S. Semenov, A. Reisinger (eds.)]. In Climate Change 2021: The Physical Science Basis. Contribution of Working Group I to the Sixth Assessment Report of the Intergovernmental Panel on Climate Change. Cambridge University Press, Cambridge, United Kingdom and New York, NY, USA, pp. 2215–2256, doi:10.1017/9781009157896.022,

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Victor Brovkin and David Archer, 2008, 'The millennial atmospheric lifetime of anthropogenic CO2', Springer Science and Business Media LLC.

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Orr, J. C., Fabry, V. J., Aumont, O., Bopp, L., Doney, S. C., Feely, R. A., Gnanadesikan, A., Gruber, N., Ishida, A., Joos, F., Key, R. M., Lindsay, K., Maier-Reimer, E., Matear, R. J., Monfray, P., Mouchet, A., Najjar, R. G., Plattner, G.-K., Rodgers, K. B. et al., 2005, 'Anthropogenic ocean acidification over the twenty-first century and its impact on calcifying organisms', Springer Science and Business Media LLC.

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Carter, B. R., Williams, N. L. and Gray, A. R., 2016, 'Locally interpolated alkalinity regression for global alkalinity estimation', Limnology and Oceanography: Methods 14(4), pp. 268–277.

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Carter, B. R., Feely, R. A. and Williams, N. L., 2018, 'Updated methods for global locally interpolated estimation of alkalinity, pH, and nitrate', Limnology and Oceanography: Methods 16(2), pp. 119–131.

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Dore, J. E., Lukas, R., Sadler, D. W., Church, M. J. and Karl, D. M., 2009, 'Physical and biogeochemical modulation of ocean acidification in the central North Pacific', Proceedings of the National Academy of Sciences of the United States of America 106, pp. 12235–12240.

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van Heuven, S., Pierrot, D. and Wallace, D., 2011, CO2SYS v 1.1: MATLAB program developed for CO2 system calculations. ORNL/CDIAC-105b, Carbon Dioxide Information Analysis Center, Oak Ridge National Laboratory, US Department of Energy, Oak Ridge, TN.

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Copernicus Marine Service, 2021, 'Global mean sea water pH', Copernicus Marine Service ( https://marine.copernicus.eu/access-data/ocean-monitoring-indicators/global-mean-sea-water-ph ) accessed April 1, 2021.

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