New Zealand's environmental reporting series: environmental indicators - ocean acidification

Our oceans have become more acidic by absorbing and storing the high levels of atmospheric carbon dioxide (CO₂) emitted mainly from human activities. Ocean acidification is projected to continue for generations if substantial emissions of CO₂ from human activities continue.

Ocean acidification may cause widespread harm to our marine ecosystems. More acidic waters make shell-building harder for species with carbonate shells, affecting their survival, growth, and reproduction. These organisms include plankton, which form the base of the marine food chain, and other species harvested for customary, commercial, or recreational purposes. Ocean acidity also affects the behaviour and physiology of some fish and invertebrates.

We classified Ocean acidification as a national indicator.

Key findings

Increasing trend (declining state)

Globally, the oceans are becoming more acidic.

The trend of the pH of New Zealand subantarctic waters decreased 0.0015 units a year between 1998 and 2016. This trend was assessed using linear regression. The upper and lower 95 percent confidence intervals are -0.0020 and -0.0011 pH units per year, respectively.

Definition and methodology

The oceans are a large carbon sink, with almost all marine habitats capturing and storing some carbon. The oceans have absorbed about 30 percent of the CO₂ emitted by human activities since pre-industrial times, significantly reducing atmospheric greenhouse gas concentrations relative to what otherwise might be expected and minimising some of the impacts of global warming (Intergovernmental Panel on Climate Change (IPCC), 2013). However, this has led to oceans becoming more acidic.

The pH of New Zealand subantarctic waters is calculated from pCO₂ (dissolved carbon dioxide) and alkalinity measurements using refitted Mehrbach constants (see Mehrbach et al, 1973; Dickson & Millero, 1987), and in-situ temperature taken from the Munida time-series transect off the Otago coast. Measurements of pCO₂ are taken approximately every two months.

The Munida transect, in the subantarctic waters off Otago, is the Southern Hemisphere’s longest-running record of pH measurements (NIWA, 2015).

Increased acidity observed in subantarctic waters is consistent with changes measured elsewhere in the world (Bates et al, 2014). The average global ocean surface water pH has fallen by about 0.1 units since the start of the industrial era (IPCC, 2013). While this may seem low, the pH scale is logarithmic – a 0.1 pH unit decrease is equivalent to a 26 percent acidity increase (IPCC, 2013).

Ocean acidification is ranked as the most serious human-based threat to New Zealand’s marine habitats (Macdiarmid et al, 2012). Organisms with calcium carbonate shells, such as plankton, corals, crustaceans, and molluscs are particularly at risk. Many overseas studies show that acidification can reduce growth and survival rates in molluscs (eg shellfish) and echinoderms (eg kina). Any impact on shell-forming plankton, a direct or indirect food source for almost all marine animals, could have widespread effects on marine ecosystems (Fabry & Seibel, 2008). Acidification also affects the sensory systems and behaviour of some fish and invertebrates (Secretary of the Convention on Biological Diversity, 2014).

Data quality