Impacts: ocean acidification

Ocean acidification

(Image: David Littschwager/NOAA)

Ocean Acidification


“Ocean acidification has happened before. Carbon dioxide from Siberian volcanoes caused the world’s oceans to acidify 252 million years ago, generating the greatest ever extinction of life on this planet.NIWA

  • Ocean acidification is happening today faster than at any time in the last 300 million years, making it unlikely that marine organisms will be able to adapt before becoming extinct.
  • The ocean has absorbed around 50% of the carbon dioxide (CO2) emitted from burning fossil fuels, manufacturing, agriculture, and other human-generated sources.
  • Absorbing so much CO2 has already changed the chemistry of the ocean, making it 30% more acidic than pre-Industrial levels.
  • The IPCC estimate acidity will be 100-150% higher by the end of the century as CO2 emissions (Fig. 4) continue to climb.
  • Colder waters hold more CO2 than warm waters. Colder sub-polar and polar regions are key nurseries for creatures at the bottom of the oceanic food web, so the entire oceanic ecosystem is threatened.
  • Around New Zealand, run-off from the land has excessive nutrients (nitrogen) from farming. This is exacerbating ocean acidification.
  • Ocean acidification will impact people who depend on seafood for kai, our commercial fishing industry including aquaculture, and seabirds and mammals that depend on these ecosystems for food (Video 1).
  • Plankton absorb 50% of CO2 as part of the carbon cycle. But because plankton are under threat from ocean acidification, their ability to absorb carbon is also under threat.
Video 1: How ocean acidifcation affects all marine life and the availability of kai.
Fig. 1: The chemistry of acidification – how increasing CO2 leads to ocean acidification. (Image: NIWA/Nicky Barton/ Arie Ketel)

Changing the chemistry of the ocean

“Plankton are responsible for at least half of the oceans’ CO2 uptake, so they help to regulate climate.”  NIWA

Video 2: How sea butterflies, the base of the ocean food chain, are being affected by ocean acidification. This was filmed in 2015. Since then, more than 50% of the Great Barrier Reef has died due to warming and acidification.

All animals including humans need biogenic calcium carbonate (CACO3). We use it to grow bones and teeth, fingernails, muscles (including our heart) and our nervous system. On the land, we get CACO3 from calcium-rich foods.

In the ocean everything from fish, crustaceans, kina, shellfish like mussels, oysters and paua, sea butterflies to the tiny coccolithophores that form the base of the food chain need CACO3 to built shells and exoskeletons, muscles and nervous systems (Video 2). The oceans have absorbed around 50% of the excess CO2 we have emitted into the atmosphere. While some of that CO2 stays as dissolved gas, most combines with water (H2O) to produce H2CO3, or carbonic acid, causing the oceans to become more acidic and reducing the availability of marine creatures to absorb CACO3 (Figs. 2 & 3) This has several impacts:

  • Zooplankton can’t develop properly during their embryonic and planktonic phases
  • Acidic water dissolves shells and corals and can make adults prone to disease
  • Fish are becoming less fearful of predators and can’t navigate as well (possible issues with nervous systems), reducing the changes of juvenile fish reaching adulthood.
  • The muscles of mussels and other shellfish such as oysters and paua are not as strong. This makes it harder for them to hold tight against waves and rough seas (storms)
  • It also makes free-swimming oceanic creatures from algae to fish, harder to swim.
  • With less carbon able to be taken up by marine organisms, less carbon can be stored at the bottom of the ocean when they die, ie, this nature carbon sink may be reduced (see the carbon cycle).

“Ocean acidification is changing the productivity and composition of phytoplankton communities at the base of the aquatic food web. Now a study shows that acidification impairs the swimming ability of flagellated microalgae, suggesting that their capacity to survive is threatened in a high CO2 world.”Jolanda Verspagen, Institute for Biodiversity and Ecosystem Dynamics, University of Amsterdam

Fig. 2 Sea butterflies collected in Antarctic waters 2011 showing the impact of acidification (right). Sea butterflies form the basis of the oceanic food web. (Image: from Bednarsek et al)

“We found that living in an acidic environment makes small reef fish become attracted to the smell of their potential predators. Their sense of smell was acutely affected in CO2 rich waters in ways that gravely threaten their survival.” Alistair Cheal, Australian Institute of Marine Science

Fig 3: In a lab experiment, a sea butterfly shell placed in seawater with increased acidity slowly dissolves over 45 days. (Image: David Littschwager/NOAA)

Fig. 3: Instructions for this interactive graph (Credit: The Institute.)

  • Mouse over anywhere on the graph to see the changes in global atmospheric carbon dioxide over the last thousand years.
  • To see details for time periods of your choice, hold your mouse button down on one section then drag the mouse across a few years, and release it.
  • To see how this compares to the past 771,000 years, click on the ‘time’ icon on the top left.
  • Compare this to rising global temperatures by clicking the planet/thermometer icon at the top left corner.
  • To return the graph to its original position, double-click the time icon to the left of the thermometer/planet icon

The annual ups and downs in the graph are because plants accumulate carbon in the spring and summer and release some back to the air in autumn and winter. As the northern hemisphere has more land and more plants, carbon dioxide levels go up in winter. Annual measurements of carbon dioxide are an average of these ups and downs.

References and further reading