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Ocean acidification

Large bamboo coral is a "canary in the coal mine" – declining health is one of the early indications of acidification. Dickins Seamount, Pacific Ocean.

Photo credit: Image from NOAA Photo Library, BY-2.0

Lead author: Carol Turley

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.

Ocean acidification is changing the basic chemistry of the ocean at a speed not experienced by marine life for millions of years

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.

If CO2 emissions continue unabated, ocean acidity could increase by 100-150% by the end of this century

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.

Nowadays ocean acidity

The average pH of ocean surface waters has fallen by about 0.1 units, from 8.2 to 8.1, since the beginning of the industrial revolution. This corresponds to a 26% increase in acidity. Image courtesy: IGBP

Projected ocean acidity by 2100

Compared with preindustrial levels shown here, the projected increase in ocean acidity is about 170% by 2100 if high CO2 emissions continue (RCP 8.5). Image courtesy: IGBP

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.

The rate and extent of ocean acidification are potentially very serious to all marine ecosystems, especially polar, upwelling and coral reef ecosystems

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.

Risk of impacts will depend on the severity (of which strength and speed are components) of ocean acidification, and the exposure and vulnerability of organisms to ocean acidification especially those playing key roles in ecosystems. The higher the severity, exposure and vulnerability the greater the risk of impact to numbers of organisms and therefore to foodwebs and ecosystems