Indicador assessment: ocean acidification

Fig. 1: Decline in ocean pH measured at the Aloha station and yearly mean surface seawater pH reported on a global scale (Copernicus Marine)

Note: A decline in pH corresponds to an increase in the acidity of ocean water. – Green line: data based on in situ measurements at Aloha station. – Blue line: pH calculated at Aloha station from dissolved inorganic carbon concentrations and total alkalinity, at in situ temperature. – Red line: global yearly mean surface seawater pH, calculated by Copernicus Marine Environment Monitoring Service. Data source:

Over the last million years, average 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 emitted CO2 is partially absorbed by the ocean, causing a decline in pH and ocean acidification. A goal of the 2030 Agenda for Sustainable Development is to address the impacts of this (UN, 2015).

The global annual mean atmospheric CO2 concentration exceeded 400 ppm in 2016 (Kahn, 2016), which is more than 40% above the pre-industrial level (280 ppm); half of that increase has occurred since the 1980s. Over the same period, ocean pH decreased from 8.11 to below 8.06, corresponding to an approximately 30% increase in acidity. This decrease in pH occurred at a rate about 100 times faster than any change in acidity experienced during the past 55 million years (Rhein et al., 2013).

This indicator looks at the longest time series of measured pH values available, from the Aloha station, offshore of Hawaii, and also calculated data on global average surface ocean pH from the Copernicus Marine Environment Monitoring Service (CMEMS). The reduction in pH measured in surface mixed-layer depths (up to 100 m) is consistent with that calculated from atmospheric CO2 concentrations, assuming thermodynamic equilibrium between the ocean surface and atmosphere. The northernmost seas, i.e. the Norwegian and Greenland Seas, have seen significantly larger decreases in pH than the global average.

Average surface open ocean pH is projected to decline further, with the largest projected decline representing more than a doubling in acidity (Joos et al., 2011; Helcom, 2013; Bindoff et al., 2019). This will affect many marine organisms and could alter marine ecosystems and fisheries. Such rapid chemical changes are an added pressure on marine calcifiers and Europe’s sea ecosystems.

Without substantial reductions in CO2 emissions, it will take thousands of years for the Earth system to re-establish balanced ocean chemical conditions and recover from human-induced acidification, and millions of years for coral reefs to return, based on records of natural coral reef extinction events (Orr et al., 2005; Archer and Brovkin, 2008).

European Environment Agency, 30 August 2021. Full article.

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