- Scientists have known for decades that soaring atmospheric carbon dioxide emissions are causing changes in ocean chemistry, threatening marine life and ecosystems.
- In June 2025, a study found that ocean acidification has passed a safe threshold across large swathes of the world’s marine environment, not only near the sea surface, but also up to 200 meters (656 feet) deep. The effect is especially severe in polar regions.
- Ocean acidification is an added stressor to marine life already facing pressure from multiple threats connected to climate change (including marine heatwaves and reduced oxygen levels in seawater), along with other direct human impacts including pollution, overfishing and deep-sea mining.
- Carbon emissions need to be deeply slashed and ocean protections greatly enhanced to allow ecosystems time to adapt and one day recover, say experts.
Ocean health is moving into a danger zone, with rampant human-caused carbon dioxide emissions having already pushed ocean acidification levels beyond safe limits in large swaths of the marine environment, according to a recent study. The new findings underline the urgent need to ramp up protection of the world’s oceans, while simultaneously slashing CO2 emissions, say experts.
But from a scientific perspective, worsening ocean acidification is not an overly surprising finding, considering that carbon dioxide emissions remain high, says lead author Helen Findlay, a biological oceanographer at the Plymouth Marine Laboratory in the U.K.
Researchers have known for decades that humanity’s CO2 emissions are being absorbed by seawater, triggering chemical reactions that release hydrogen ions, in turn reducing the abundance of carbonate ions. This ocean acidification process — which has escalated in tandem with atmospheric emissions — has implications for a large number of ocean-dwelling calcifying species that rely on calcium carbonate for their shells, with harm to those species potentially reverberating throughout marine ecosystems.
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Researchers found that 60% of the world’s oceans have passed safe limits for ocean acidification to 200 meters (656 feet) depth, compared to 40% of them at the surface. These findings are concerning because most marine life is found within this zone, and because the impacts of acidification on biodiversity at deeper depths are not fully known. Image by Katerina Katopis / Ocean Image Bank.
Rockström’s international team — known for their groundbreaking planetary boundary research — is working on an updated Planetary Health Check, due out in September. Last year’s inaugural report found ocean acidification on the cusp of transgression. But evidence from multiple sources is now pointing to this boundary being crossed, he says. “It’s too early to say conclusively, but I think this [new] paper is important in that context.”
Mounting evidence of worsening ocean acidification should trigger a “much more ambitious level of ocean protection,” along with rapid climate action, Rockström says. “When you add one additional stressor, like ocean acidification, you have an even stronger argument to protect [marine systems] because they’re getting weaker under the pressure of multiple stressors.”
Acidification widespread, and runs deep
Findlay’s team found that four of seven ocean basins have crossed the planetary boundary for ocean acidification — with polar waters and ocean upwelling areas particularly affected.
They also found that this acidification picture becomes worse when one looks deep below the ocean’s surface. The authors found that 60% of the world’s ocean has crossed the safe limit down to 200 meters (656 feet) depth, compared to 40% of surface waters.
That’s concerning for marine life, Findlay says, as this part of the water column is where much of Earth’s marine biodiversity thrives.
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The researchers found that rising ocean acidification levels have already caused “significant declines” in habitat for some calcifying creatures. According to their study, tropical and subtropical coral reefs have lost 43% of their suitable habitat. Polar pteropods, a free-swimming form of planktonic marine snail, and a key part of the food chain, have lost 61% of their suitable habitat, while coastal bivalves have lost 13%.
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Urgency for ocean protection
Globally, marine protection lags far behind land efforts, say conservationists. But there is building momentum on the back of the just-concluded 2025 UN Ocean Conference. In recent weeks, a raft of new Marine Protected Areas (MPAs) and ocean protection commitments were announced, as nations moved toward ratifying the Agreement on Marine Biological Diversity of Areas beyond National Jurisdiction, also known as the High Seas Treaty.
This international agreement aims to push forward protection of 30% of the world’s oceans by 2030 while establishing legal mechanisms for protection of high seas areas. Fifty countries have ratified the treaty so far, but 60 are needed for it to come into force.
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Addressing the root cause: Fossil fuels
All experts Mongabay interviewed for this story agree: The number one solution to address ocean acidification is to aggressively tackle its root cause — the continuing carbon emissions driving the uptake of CO2 in the oceans.
Researchers recently warned that the window to keep warming below the 1.5°C (2.7°F) Paris Agreement target is rapidly closing, with only three years left. Several analyses have concluded that, barring a drastic course correction, the world is rushing toward a catastrophic 2-3°C (3.6-5.4°F) rise in temperature over preindustrial levels by 2100.
But even if atmospheric CO2 levels were reduced today, the consequences of ocean acidification will remain with us for centuries as the oceans continue to soak up the CO2 altering the sea chemistry and pH.
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Mongabay, 26 June 2025. Full article.
