The oceans are seeing a gradual shift towards acidity, an invisible change with far-reaching consequences. Kerry MacKay reports
ENVIRONMENT
The acid test
The oceans are seeing a gradual shift towards acidity, an invisible change with far-reaching consequences. Kerry MacKay reports
Climate change is usually discussed in terms of temperature. However, many other aspects of our planet are changing too. Ocean acidification is only occasionally discussed. It is happening, but the potential impacts are poorly understood.
Let’ s start with a quick refresher of acidity. Acidity is a measure of hydrogen ion concentration. A lot of hydrogen ions make things acidic, and fewer ions mean less acidic. The pH scale is how we express this. It runs from 0 to 14, with 7 representing neutral pH. Anything lower than 7 is acidic( for example, vinegar), and anything higher is alkaline( an example being bleach).
It has been about 200 years since the Industrial Revolution began. Since then, we have burned fossil fuels, releasing carbon dioxide into the atmosphere. Pushing the average atmospheric carbon dioxide( CO2) concentration from 313 parts per million( ppm) in 1958, when modern recording began, to 429.4 ppm in 2026.
Despite all this talk of ocean acidification, the oceans are actually alkaline. Over the last million years, ocean surface pH has been relatively stable between 8.2 and 8.3. The oceans absorb about 30 % of CO2 from our atmosphere. Water and CO2 combine to form a weak carbonic acid. Pushing the average pH down to 8.04 in 2025. Around 40 % more acidic compared to pre-industrial levels.
This change in acidity is already impacting many ocean species. Species particularly at risk are those that create calcium carbonate shells and skeletons.
For example, shellfish, sea snails, echinoderms, corals, and crustaceans.
These creatures create their shells and skeletons using calcium and carbonate ions from the seawater. Calcium is readily abundant in the oceans. Carbonate, however, is in limited supply and will preferentially bond to the hydrogen ions in a more acidic ocean. Making it much more difficult for creatures to get the carbonate they need.
Science remains uncertain as to the fate of these critters. If the water is too acidic, these creatures’ shells and skeletons can even begin dissolving. It is possible that creatures could adapt, such as the shell-inhabiting creatures found living in the highly acidic waters around hydrothermal vents. Or maybe they will become smaller, so they don’ t need as much carbonate to begin with. Or they could grow thicker shells to withstand increased acidity. Whatever happens, it will be harder for them to survive.
Ocean acidification could prove beneficial to some ocean life. Algae and seagrasses use CO2 for photosynthesis, just like plants on land. It is thought this is a factor in the record-breaking 40 million-tonne sargassum inundation being experienced in the Caribbean and West Africa.
If we do not reduce our CO2 emissions, the oceans are predicted to reach a pH of 7.8 by the year 2100. The last time our oceans were that acidic was 14-17 million years ago, during a major extinction event. What can we do about it? Dumping a bunch of alkaline material, such as limestone, into the sea is being considered, and capturing carbon from the atmosphere for long-term storage could help in the future. Better still is addressing the root of the problem, global CO2 emissions.
Anything we can do to reduce CO2 emissions will help. Switch your electricity to a 100 % renewable tariff, stop eating beef and lamb, switch to greener transport, campaign for sustainable corporations and countries … All these efforts really do matter. If you want something a bit more handson, try getting involved with BSAC’ s Great Seagrass Survey or Operation Oyster. Any kind of coastal restoration helps on numerous levels, so protect those kelp forests, salt marshes, seagrass beds, and mangroves. �
Low pH levels make shell creation harder
1994 2026
The maps show the levels of ocean acidity for 1994 compared to 2026
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