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Ocean acidification

The ocean absorbs around 30% of carbon dioxide (CO2) released to the atmosphere as a result of human activities. As CO2 dissolves in seawater, it forms carbonic acid, decreasing the ocean’s pH. This is called ocean acidification. The acidity of the ocean has increased by 26% since the beginning of the industrial era.

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Pre-industrial pH levels and predicted pH levels for 2100.
GBP, IOC, SCOR (2013). Ocean Acidification Summary for Policymakers – Third Symposium on the Ocean in a High-CO2 World.
International Geosphere-Biosphere Programme, Stockholm, Sweden

Ocean acidification is also changing other aspects of seawater carbonate chemistry. The saturation of calcium carbonate minerals, such as calcite and aragonite, is lowered, reducing the availability of these minerals. Organisms using calcium carbonates as the main building blocks of their shells and skeletal structures, such as mussels, crustaceans and corals, are struggling or unable to form and maintain the shells and carapaces they need.

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Atmospheric CO2 concentrations and ocean pH values. Atmospheric CO2, shown in red, is measured at Mauna Loa, Hawaii. Seawater pCO2 (green) and pH values (blue) are from the ocean to the north of Hawaii (Station Aloha). As CO2 accumulates in the ocean, the water becomes more acidic (the pH declines).
Adapted from: Dore et al. 2009. PNAS 106:12235-12240.

Ocean acidification has been shown to affect organisms and ecosystems, impacting ecosystem services such as food security, by endangering fisheries and aquaculture. It also impacts coastal protection (for example by weakening coral reefs shielding the coastline), transportation and tourism. The ocean’s capacity to store carbon dioxide and help regulate the climate will be affected, as the capacity of the ocean to absorb CO2 decreases as ocean acidification increases. Regular observations and measurements of ocean acidification in open oceans and coastal areas are necessary to improve our understanding of the effects, enable modelling and predictions and help inform mitigation and adaptation strategies.

Under the United Nations’ 2030 Agenda, the IOC-UNESCO is the custodian agency for the Sustainable Development Goal (SDG) Target Indicator 14.3.1, calling for the “average marine acidity (pH) measured at agreed suite of representative sampling stations”. The methodology provides detailed guidance to scientists and countries about how to carry out measurements following the best practices established by experts in the ocean acidification community and explains how to report the collected information.


UNESCO’s Intergovernmental Oceanographic Commission (IOC) and its Ocean Science Section are involved in the coordination of several programmes to monitor and study ocean acidification and its effects.

The IOC is part of the Global Ocean Acidification Observing Network (GOA-ON). The network aims to improve understanding and measurements of global ocean acidification and its effects by coordinating regional and national research efforts, improving data quality and comparability by organising targeted workshops and providing data visualisation tools.
The IOC and the Scientific Committee on Oceanic Research (SCOR) sponsor the International Ocean Carbon Coordination Project (IOCCP), an ocean carbon monitoring and research programme. The IOCCP promotes the development of a global network of ocean carbon and biogeochemistry observations, coordinates the development of globally acceptable strategies and provides technical coordination by developing operating methodologies, practices and standards for the carbon research community.
The IOC Sub-Commission for the Western Pacific, IOC WESTPAC, is located in one of the most productive oceanic regions, home to more than 600 coral species and around 53% of the world’s coral reefs. As these organisms are particularly vulnerable to ocean acidification, IOC WESTPAC aims to establish regional research and monitoring networks on ocean acidification in the Western Pacific and its adjacent regions and to develop a regional program to monitor the impacts of ocean acidification on coral reef ecosystems. IOC WESTPAC has hosted and organised several workshops on ocean acidification research and monitoring, strengthening awareness and scientific capacities in the region.


The IOC participates in and supports the Ocean Acidification international Reference User Group (OA-iRUG)

The IOC is part of the Advisory Board of the Ocean Acidification International Coordination Centre (OA-ICC) of the International Atomic Energy Agency

The IOC takes part in and contributes to the ‘Bridging the Gap Between Ocean Acidification Impacts and Economic Valuation’ workshops


  • As the acidity of the ocean increases, its capacity to absorb CO2 from the atmosphere decreases. This decreases the ocean’s role in moderating climate change.
  • The biological impacts of ocean acidification will vary, because different groups of marine organisms have a wide range of sensitivities to changing seawater chemistry.
  • Since the industrial revolution, the surface ocean’s pH has dropped by 0.1 pH units and is projected to drop 0.3-0.4 units (equivalent to a 100-150% increase in acidity) by the end of the 21st century. Note that pH is measured on a logarithmic scale, meaning that a change of one unit represents a tenfold change in acidity.
  • 50% of marine life is expected to be affected by ocean acidification, with the consequences propagating through the food web, impacting most aspects of ocean related services.


Kirsten Isensee (
Programme Specialist – Ocean Carbon Sources and Sinks