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.

“We have really good data sets, and the data sets and this paper really just emphasize that we’re just watching the system crash.… [W]e need to be making real change now so that we don’t make things worse,” Findlay says.

“In my assessment, [this new paper] confirms what we’ve been expecting,” agrees Johan Rockström, director of the Potsdam Institute for Climate Impact Research in Germany, who was not involved in the current study. “[W]e are unfortunately moving beyond the safe boundary on ocean acidification.”

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.

Rockström and an international team of planetary boundary scientists working out of Sweden’s Stockholm Resilience Centre previously set the safe limit for ocean acidification at 20% aragonite saturation (aragonite being a form of calcium carbonate). But Findlay and her team, after assessing the acidification tolerances of a wide range of species and inspecting regional data, propose a 10% aragonite saturation safe limit to ensure functional integrity of ocean ecosystems.

Concerningly, that would push the crossing of the safe space back to 2000.

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%.

“It’s quite frustrating to now be at a point where we’re saying, well actually, if we wanted a really good system — to be healthy and safe [for] all these ecosystems — we need to have gone back and kept [ocean acidification] at year 2000 levels,” Findlay says.

However, experts emphasize that current levels of acidification are not an immediate death knell for ocean life. But they do note that these rising levels are greatly concerning when factoring in the bombardment of the world’s oceans by other stressors including climate change-induced marine heatwaves, declining ocean oxygen levels, eutrophication, and more direct anthropogenic impacts such as overfishing and pollution from far-ranging sources including microplastics and raw sewage.

We need to see ocean acidification “as a component of other challenges that can make things worse,” says Hans-Otto Pörtner, a marine biologist at Germany’s Alfred Wegener Institute, former co-chair of the Intergovernmental Panel on Climate Change (IPCC) Working Group II who wasn’t involved in the recent study. He emphasizes that the risks posed by interactions between acidification, heating and oxygen loss are all driven by human carbon emissions. “My more holistic view would be that the interaction of these three [factors], in terms of Earth history … have been a driver for evolutionary mass extinctions.”

“The findings are certainly concerning for coral reefs and bivalves and many other organisms that rely on calcium for their shells and their skeletons,” adds Helen Fox, a senior scientist at the Coral Reef Alliance, California, who wasn’t involved in the recent research. “Corals are already suffering from an onslaught of impacts. We are still in the midst of the fourth global bleaching event. So there has already been a lot of death and habitat loss from bleaching.”

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.

Findlay says her team’s acidification findings should add impetus for nations who have yet to ratify the accord. “Any additional protection that’s placed on the ocean, provides an opportunity for ecosystem resilience against harder-to-solve issues, such as [ocean acidification],” she says. “However, [our findings] should also be a motivation to cut emissions, given that [acidification] and climate change are a [dual] threat to marine biodiversity, which this treaty aims to protect.”

In a June 2025 Nature commentary, Rockström and other scientists, including eminent marine biologist Sylvia Earle, warned that the high seas treaty, though important, will likely take years to put into action. They argue that urgent steps are needed immediately to protect the world’s oceans from all forms of exploitation.

“Given the urgency of addressing the climate and biodiversity crises, the world can’t wait another decade to fix the problems humans have created,” they write. “Ocean life is too precious and important to lose, and shifts in the chemical and physical environments of the sea, once made, will be irreversible on timescales of centuries to millennia.”

“For the high seas, 30 by 30 is not enough,” Rockström tells Mongabay. “We should halt 100% of the high seas overexploitation with industrial fishing [and] industrial trawling and forbid all forms of deep-sea mining.”

Others say ocean protection must become far more adaptive in the face of rapidly changing ocean chemistry. That includes identifying and targeting conservation action in marine areas that could act as climate refugia for particularly vulnerable ecosystems.

“We talk about refugia in terms of heat. We do not talk about them in terms of other aspects, such as ocean chemistry, and we need to,” says Daniela Shmidt, a professor at the School of Earth Science at the University of Bristol, England, who wasn’t involved in the recent study. In depth metrics on ocean acidification, warming and other marine changes could help identify key areas to protect, she adds. “We can’t protect all the world. So we need to know where our efforts are best placed.”

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.

Experts stress that net zero and net negative approaches will be needed to protect oceans, including some geoengineering solutions that involve CO₂ removal (CDR) on land and sea — even though these technologies remain in their infancy, requiring much testing and scaling up.

Reducing ocean acidification will likely require novel methods, such as ocean alkalinity enhancement or electrochemical approaches. But these techniques come with still poorly understood consequences for marine life and ecosystems.

“We all know that reduction in CO₂ [emissions] is … the most important step. But maybe for some of the … really critical [marine] ecosystems, we might need additional adaptation or mitigation approaches,” says Nina Bednarsek, assistant professor of senior research at Oregon State University and a co-author on the recent paper.

CDR technologies will be necessary to a degree, agrees Rockström. “There’s not one climate scenario that you see that can hold 1.5° Celsius [2.7° Fahrenheit] and still giving us a little sliver of a remaining carbon budget for an orderly phase out of coal, oil and gas, without assuming very optimistic scaling of CDR.”

But he draws the line at more aggressive geoengineering approaches. “These are technologies that involve such a large-scale manipulation of the Earth system, with not well understood and potentially catastrophic side effects, that they should, under all circumstances, be avoided,” he says. “Many of them will not even solve the problem, because [while] they could temporarily reduce the temperature, they wouldn’t get rid of the stress of acidification in the ocean.”

Pörtner is skeptical of geoengineering methods that add material to the ocean at a global scale, and instead emphasizes the need to slash emissions and raise ocean protections. “For all efforts to protect the ocean, it would be beneficial to stop emissions,” he says. “The way forward is … to stop CO₂ emissions, and then the natural systems over time, over a long time, will help us bring things back into balance.”

“This doesn’t have to be all doom and gloom in terms of all ocean life … dying,” says Bednarsek. Instead, the passing of the safe threshold for ocean acidification should be viewed as an “early warning” spurring us to act. “This sort of knowledge is absolutely critical. It’s not just … to be alarmist. It’s … so we can do something about this.”

Banner image: A ray swims over seagrass in Australia. Experts say that protecting marine ecosystems, such as seagrass, could help mitigate ocean acidification in some areas. Image by Jordan Robins / Ocean Image Bank.

Electrochemical removal of ocean CO2 offers potential — and concerns

Citations:

Findlay, H. S., Feely, R. A., Jiang, L., Pelletier, G., & Bednaršek, N. (2025). Ocean acidification: Another planetary boundary crossed. Global Change Biology, 31(6). doi:10.1111/gcb.70238

Caesar, L., Sakschewski, B., Andersen, L S., Beringer, T., Braun, J., Dennis, D., Gerten, D., Heilemann,. A., Kaiser, J., Kitzmann, N.H., Loriani, S., Lucht., W Ludescher, J., Martin, M., Mathesius, S., Paolucci, A., te Wierik, S., & Rockström, J. (2024). Planetary Health Check Report 2024. Potsdam Institute for Climate Impact Research, Potsdam, Germany.

Roberts, C. M., Dyer, E., Earle, S. A., Forrest, A., Hawkins, J. P., Hoegh-Guldberg, O., … Lynas, M. (2025). Why we should protect the high seas from all extraction, forever. Nature, 642(8066), 34-37. doi:10.1038/d41586-025-01665-0

FEEDBACK: Use this form to send a message to the author of this post. If you want to post a public comment, you can do that at the bottom of the page.

Credits

Glenn Scherer Editor

Topics

Carbon ConservationCarbon DioxideCarbon EmissionsChemicalsClimate ChangeConservationConservation SolutionsCoral ReefsEnvironmentGeoengineeringGlobal Environmental CrisisImpact Of Climate ChangeMarineMarine ConservationMarine CrisisMarine EcosystemsOcean AcidificationOcean CrisisOcean WarmingOceansOceans And Climate ChangePlanetary BoundariesResearchGlobal

See Topics