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Almost one quarter of human-caused carbon dioxide (CO 2 ) emissions are absorbed by oceans, resulting in ocean acidification, i.e. a decrease in the ocean water pH. Ocean acidification has increased rapidly causing pH to drop by approximately 30% since the pre-industrial era. Further decreases in pH are projected in the future. Seawater pH has decreased from 8.11 in 1985 to 8.05 in 2021. Ocean acidification has impacts on marine organisms with its effects cascading throughout the food web, modifying ecosystem services like fisheries.

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

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Over the last million years, mean surface seawater pH has been relatively stable. Oscillating between 8.3, during cold periods (e.g. during 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 emissions from human activities are now threatening this stability, as a large fraction of emitted CO2 is absorbed by the ocean, causing a decline in pH and subsequent ocean acidification.

The global annual mean atmospheric CO2 concentration exceeded 417ppm in 2022, which is more than 40% above the pre-industrial level (280ppm). Half of that increase has occurred since the 1980s. Over the same period, annual global mean surface sea water pH decreased from 8.11 to below 8.05 in 2021 corresponding to a 15% increase in acidity since 1985 and a 40% increase since pre-industrial levels.

This indicator looks at the longest time series of measured pH values available from the Aloha station, offshore Hawaii, and visualises calculated data on global mean surface ocean pH from the Copernicus Marine Environment Monitoring Service (CMEMS). The northernmost seas, i.e. the Norwegian and Greenland Seas, have seen significantly larger decreases in pH than the global mean.

Mean surface ocean pH is projected to decline further, between about 0.15 and 0.5pH units by 2100 depending on the emission scenario. This will affect many marine organisms and could alter marine ecosystems. In the North Atlantic Ocean, cold water corals are expected to face severe impacts due to acidification and losses of carbonate skeleton.

Ocean acidification reduces calcium carbonate availability, making it more difficult for organisms such as corals, molluscs and some plankton to build and maintain their structural integrity. Such rapid chemical changes are an added pressure on marine calcifiers and Europe’s marine ecosystems. 

Substantial reductions in CO2 emissions would allow the Earth system to re-establish balanced ocean chemical conditions and recover from human-induced acidification, over a very long time period, based on records of natural coral reef extinction events.

EU policies such as the European Green Deal, the Marine Strategy Framework Directive (MSFD), the EU Biodiversity Strategy for 2030 and EU climate change mitigation and adaptation policies (European Climate Law/Fit-for-55 package) aim to address ocean acidification and restore marine ecosystems. The revised MSFD focuses on strengthening implementation, improving regional cooperation, and enhancing policy coherence, including with climate policy, to achieve environmental targets and good environmental status (GES).