A new assessment finds that the world’s oceans crossed the safe threshold for acidification in 2020, breaching a key planetary boundary and posing serious threats to marine life.
Ocean acidification is caused when excess atmospheric carbon dioxide, resulting from human activities like burning fossil fuels, dissolves in seawater, forming carbonic acid that increases the water’s acidity. The reduced availability of carbonate ions can affect the survival of marine species that build calcium carbonate shells and skeletons, including coral, shellfish and crustaceans.
For this study, researchers looked at a key indicator of ocean acidification called aragonite saturation state, a measure of how well seawater supports the formation of aragonite, a form of calcium carbonate. They estimated aragonite saturation state over time at different depths in the ocean.
They also compared that information with biological tolerance thresholds for species like coral and sea snails, or the levels of aragonite saturation below which the marine animals experience stress.
Previously, scientists established that a 20 per cent drop in aragonite saturation, compared with preindustrial levels, was the threshold for breaching the ocean acidification planetary boundary.
The last global assessment in 2023, led by Katherine Richardson with the Globe Institute at the University of Copenhagen, Denmark, found a 19 per cent decrease. The new study’s findings confirm that this boundary has now been crossed.
Richardson, who wasn’t involved with the latest research, told Mongabay by email she was “not at all surprised” by the new finding. “We said it was on the edge in our last assessment and, as atmospheric CO2 concentrations have risen since then, it is hardly surprising that it should be transgressed now.”
The new study found four of the world’s seven ocean basins have crossed the planetary boundary for ocean acidification. Polar regions have been most widely affected: Nearly 87 per cent of the Southern Ocean, 84 per cent of the North Pacific and 78 per cent of the Arctic Ocean surface waters have passed the safe threshold.
Overall, 40 per cent of ocean surface waters and 60 per cent of subsurface water up to 200 meters (660 feet) deep passed the planetary boundary threshold by 2020.
Subsurface acidification “is of similar seriousness to surface acidification, given that many species live in these zones,” Helen Findlay, the study’s lead author and biological oceanographer with the Plymouth Marine Laboratory, UK, told Mongabay by email.
The researchers also found that suitable habitats for key marine species — where there is still adequate aragonite saturation — have declined significantly. Habitat for coral, which form crucial marine ecosystems, has shrunk by approximately 43 per cent.
Habitat for polar sea snails, an essential food source for whales, fish and seabirds, has decreased by 61 per cent. Coastal bivalves, food and natural storm protection for coastal communities, have lost about 13 per cent of their viable habitat.
In light of this study, Findlay said policy makers need to take “real action to reduce emissions” and incorporate “ocean acidification monitoring, mitigation and adaptation in national policies.”
This story was published with permission from Mongabay.com.