Ocean acidification is a process that refers to major changes in the ocean chemistry, mainly caused by ocean uptake of atmospheric carbon dioxide (CO₂). These changes involve a decrease in pH (which is important for regulation of the internal acid balance and physiological health of many organisms) and a decrease in carbonate ions (important for shell and skeleton builders). Ocean acidification can also cause an increase in bicarbonate ions which are important for algal photosynthesis. It will continue as CO₂ emissions increase and it is likely to change marine ecosystems and the benefits they supply society.

Modelled global sea-surface pH from 1870 to 2100

The blue line reflects estimated pH change resulting from very low CO₂ emissions to the atmosphere (IPCC Representative Concentration Pathway, RCP 2.6). The red line reflects pH from high CO₂ emissions (the current emissions trajectory, RCP 8.5). Credit: Adapted from Bopp et al. 2013.

If CO₂ emissions continue unabated, ocean acidity could increase by 100-150% by the end of this century. The speed of these changes has not been experienced by marine organisms for at least 60 (possibly 300) million years. It is the rate as well as the extent of change that poses substantial risks to all marine ecosystems (especially polar, upwelling and coral reef ecosystems), through the impacts on the physiology, behaviour, and population dynamics of individual species.

Pteropods and tropical coral reefs were selected as potential indicators of ocean acidification impacts in Arctic and tropical waters under the different CO₂ emission scenarios for the Open Ocean TWAP study. This is due to their importance to food webs and ecosystems. They also have high sensitivity as both use aragonite – a form of calcium carbonate sensitive to ocean acidification.

Laboratory and field experiments, and field observations have shown a wide range of sensitivities and responses amongst organisms. With the exception of those that calcify, most plants and microalgae respond positively to elevated bicarbonate ions by increasing photosynthesis and growth.

However, most shell-forming molluscs, echinoderms, and reef-building corals and calcifying algae respond negatively to elevated bicarbonate ions and are more sensitive than crustaceans and fish.

Key links in food webs such as sea butterflies (Pteropoda) are already showing shell thinning in the Southern Ocean and the Californian Current System while the calcification rate of tropic coral reefs is already declining. Some species may do well in a more acidic ocean at the expense of others - observations of natural CO₂ vents in coral reef systems off Papua New Guinea show increased algal growth but declining coral reef biodiversity and loss of reef structure that provides homes to many reef dependent species as pH decreases close to the vents.

Other vents in the Mediterranean Sea also show declining biodiversity and loss of shelled organisms. Shell fisheries have already been impacted off the Pacific coast of North America due to the influx of upwelling water, already rich in CO₂ but now with extra CO₂ from the atmosphere, into its productive coastal waters.

Other stressors acting at the same time as ocean acidification, such as ocean warming and deoxygenation, and non-climate related regional or local stressors such as pollution, nutrient runoff from land, and over-exploitation of marine resources, can lead to amplified impacts for species and ecosystems. These represent risks to the productivity of fisheries and aquaculture, and therefore to regional food security and livelihoods.