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Impacts & Effects: Ocean acidification

Sea butterlfies dissolving image: David Littschwager/NOAA

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Ocean acidification

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Summary

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

The global average ocean over the past 250 years indicates a 26% increase in acidity. – NOAA Ocean Acidification Program, 08 January, 2025

  • 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 25% 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 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.
  • There is currently no practical way for humans to reverse ocean acidification. The only way to slow and eventually halt acidification is through rapid CO2 emissions reductions and future carbon dioxide removal. If emissions continue to rise, these more acidic conditions will persist for tens of thousands of years.
  • Experiments to introduce lime to reduce acidification in confined waterways (estuaries) are currently underway. There is a very large carbon cost to produce this lime, however it may provide some benefit for local seafood harvesting, particularly shellfish and aquaculture.
  • See NIWA’s Ocean Acidification Observing Network for the latest updates on acidification.

  Impacts

Other sections

Home > Climate wiki > Impacts > Ocean acidification

Summary

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

The global average ocean over the past 250 years indicates a 26% increase in acidity. – NOAA Ocean Acidification Program, 08 January, 2025

  • 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 25% 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 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.
  • There is currently no practical way for humans to reverse ocean acidification. The only way to slow and eventually halt acidification is through rapid CO2 emissions reductions and future carbon dioxide removal. If emissions continue to rise, these more acidic conditions will persist for tens of thousands of years.
  • Experiments to introduce lime to reduce acidification in confined waterways (estuaries) are currently underway. There is a very large carbon cost to produce this lime, however it may provide some benefit for local seafood harvesting, particularly shellfish and aquaculture.
  • See NIWA’s Ocean Acidification Observing Network for the latest updates on acidification.
  • State of the Cryosphere 2023: polar acidification
    • The Arctic and Southern Oceans are bearing the brunt of acidification impacts because they absorb CO2 faster.
    • Compound events combining marine heatwaves and extreme acidification have already caused population crashes even at today’s 1.2°C.
    • CO2 levels topped 424 ppm several times in 2023, with 424pp, the monthly average at Mauna Loa Observatory. With that 424 measurement, levels of CO2 in the atmosphere have now officially increased 50% over pre-industrial.
    • Marine heatwaves occurred worldwide in 2023. In polar and near-polar regions, perhaps the greatest high temperature anomaly took place in the north Atlantic and north Sea in May-July, when water temperatures reached record high levels  West of Ireland, temperatures peaked at 4–5°C above average, a “Category 5” occurrence classified “Beyond Extreme” with temperatures as high for nearly as high about average in several areas of the Arctic, including near Svalbard in July, and the Barents Sea in early August. unusually warm winter water of 2°C above normal also was measured in July near ice-free conditions off Antarctica.
    • The net oceanic uptake rate of CO2 will likely decrease in the future owing to several converging trends: reduced storage capacity due to acidification to date; increased outgassing of natural CO2 due to ocean warming; and changes in ocean circulation and biology. While this may slow acidification, it means that CO2 levels in the atmosphere will rise more quickly unless emissions are reduced .
    • Parts of the Arctic Ocean seafloor will be exposed to corrosive waters by mid to late century, with 27% of the Atlantic Arctic exposed even with low emissions. Under high emissions, 45% of this seafloor will face such exposure. Nearly 1.3 million km2 of the Arctic continental shelf and slope is predicted to become submerged within a mid-depth, acidified water mass that will be corrosive by the end of the 21st century.

    Reference: 2023 State of the Cryosphere

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)
Fig. 1: The chemistry of acidification – how increasing CO2 leads to ocean acidification. (Image: NIWA/Nicky Barton/ Arie Ketel)
Fig. 2: Increasing acidification means the pH of the ocean is declining. Image: Nicolas Gruber & Luke Gregor Institute for Environmental Analytic. Note: by 2025, the global average rate had increased to 26% according to NOAA. The variability around this comes from the extent to which all ocean waters are assesses, but the trend is clear.
Fig. 2: Increasing acidification means the pH of the ocean is declining. Image: Nicolas Gruber & Luke Gregor Institute for Environmental Analytic. Note: by 2025, the global average rate had increased to 26% according to NOAA. The variability around this comes from the extent to which all ocean waters are assesses, but the trend is clear.

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

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. 3 & 4) 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).
Fig. 3 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)
Fig. 3 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)
Fig. 4 and image at top of the page: In a lab experiment, a sea butterfly shell placed in seawater with increased acidity slowly dissolves over 45 days. (Image: David Littschwager/NOAA)
Fig. 4 and image at top of the page: In a lab experiment, a sea butterfly shell placed in seawater with increased acidity slowly dissolves over 45 days. (Image: David Littschwager/NOAA)

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

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

Instructions for interactive graphs (Credit: The 2°Institute.)

  • Mouse over anywhere on the graphs to see the changes over the last thousand years.
  • To see time periods of your choice, hold your mouse button down on one section then drag the mouse across a few years, then release it.
  • To see how this compares to the past 800,000 years, click on the ‘time’ icon on the top left.
  • To return the graphs to their original position, double-click the time 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 because plants become less productive. Annual measurements of carbon dioxide are an average of these ups and downs.
  • Instructions for interactive graphs (Credit: The 2°Institute.)
  • Mouse over anywhere on the graphs to see the changes over the last thousand years.
  • To see time periods of your choice, hold your mouse button down on one section then drag the mouse across a few years, then release it.
  • To see how this compares to the past 800,000 years, click on the ‘time’ icon on the top left.
  • To return the graphs to their original position, double-click the time 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 because plants become less productive. Annual measurements of carbon dioxide are an average of these ups and downs.

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