Ocean Acidification

Unique identifier / Notation

MH0021

Ocean acidification refers to a reduction in the pH of the ocean over an extended period, which is caused primarily by uptake of carbon dioxide from the atmosphere and can also be caused by other chemical additions or subtractions from the ocean (IPCC, 2011).

Primary reference(s)

IPCC, 2011. IPCC Workshop on Impacts of Ocean Acidification on Marine Biology and Ecosystems. Intergovernmental Panel on Climate Change (IPCC). Accessed 1 October 2020.

Additional scientific description

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 alters the carbonate chemistry of the seawater, resulting in a fall in pH and accompanying declines in dissolved carbonate ion concentration and an increase in the partial pressure of CO2 (pCO2) as well as an increase in the concentration of dissolved bicarbonate ions. The decrease in the concentration of dissolved carbonate ions lowers the saturation state of biogenic forms of calcium carbonate minerals, including calcite and aragonite (IPCC, 2019; UNESCO, no date).

Metrics and numeric limits

The seawater carbon system has four measurable variables and at least two of these are needed in order to ‘constrain’ the carbon system relative to ocean acidification: pH, pCO2, total alkalinity (TA), and total dissolved inorganic carbon (DIC). Temperature and salinity data are also required.

The units of these parameters are pH on total scale; pCO2 [μatm]; DIC [μmol/kg]; and TA [μmol/kg] (IOC, 2018; IPCC, 2019).

Key relevant UN convention / multilateral treaty

United Nations 2030 Agenda on Sustainable Development (UNGA, 2015).

Examples of drivers, outcomes and risk management

Ocean acidification occurs when the CO2 absorbed by the ocean reacts with seawater, resulting in a shift in the dissolved carbonate chemistry, including increased acidity levels in the marine environment (i.e., decreased seawater pH). The observed changes have been shown to cause a range of responses at the organism level that can affect biodiversity and ecosystem structure. Direct consequences for marine life can propagate through the food web and affect ocean-related services and uses, including food security in terms of fisheries and aquaculture, livelihoods, coastal protection, tourism, and cultural heritage.

For example, decreases in dissolved carbonate ion concentrations have been shown to impact the growth and larval survival of key marine calcifying organisms, including oysters, mussels and corals. The impact on corals is likely to result in weaker reefs making them prone to storm damage and altering the structural complexity that supports biologically diverse reef ecosystems. Effects on growth, reproduction, and predatory avoidance have also been observed for many other marine organisms from changes in pH and/or pCO2. Food production from marine calcifiers in both wild and aquaculture fisheries is expected to be adversely impacted by ocean acidification, with some aquaculture facilities already implementing monitoring and adaptation strategies.

It may be possible to lessen the impacts of ocean acidification on ocean services through mitigation and adaptation strategies informed by appropriate monitoring and improved understanding of natural and human-induced variability, as well as the rate of change, combined with studies of biological impacts. Further investigation is also required as ocean acidification may act as a stress multiplier when combined with other changes in the marine environment, including changes in temperature and oxygen concentrations.