Sea butterlfies impacted by acidification David Littschwager/NOAA
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Changing the chemistry for the ocean
The ocean has absorbed ~24% of the excess carbon dioxide (CO2) emitted into the atmosphere. While some CO2 stays in the ocean as dissolved gas, most combines with water (H2O) to produce carbonic acid (H2CO3), causing the oceans to become more acidic. The ocean is now ~30% more acidic than pre-Industrial levels Fig. 1).
Around New Zealand, run-off from the land has excessive nutrients (nitrogen) from farming, exacerbating acidification. This will impact those who depend on seafood for kai, commercial fishing industry including ocean-based aquaculture, and seabirds and mammals that depend on these ecosystems for food (Video 1).
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 fish, crustaceans, kina, shellfish like mussels, oysters and paua, and sea butterflies—the tiny coccolithophores that form the base of the food chain—all need CACO3 to built shells and exoskeletons, muscles and nervous systems.
Increasing acidification reduces the amount of available CACO3 in the ocean, which 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
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. – Cheal, Australian Institute of Marine Science 2014
- 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, so this nature carbon sink is being reduced.
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. – Verspagen 2020
Ocean acidification is happening 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.
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
Marine life also absorbs CO2 as part of the carbon cycle. Plankton, for example, absorb 50% of the CO2 in the ocean. But:
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. – 2023 State of the Cryosphere
A combination of marine heatwaves and extreme acidification had already caused population crashes by 2023. By 2025 a tipping point had been breached:
We have breached 7 out of 9 Planetary Boundaries. In this 2025 edition, we assess for the first time that Ocean Acidification is the seventh transgressed Planetary Boundary – Planetary Health Check 2025
Ticking timebomb’: sea acidity has reached critical levels, threatening entire ecosystems – The Guardian 2025 (see the research paper here)
Because a tipping point has been crossed, there is no practical way to reverse acidification until atmospheric CO2 is reduced to ~120 ppm. Current concentration of atmospheric CO2:
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