A new analysis reveals that, by 2020, ocean acidification levels had pushed beyond safe planetary boundaries, with the most severe changes occurring in the upper 650 feet of the water column.
This shift marks a significant breach in the planetary boundary framework introduced in 2009, which identifies key Earth systems that must remain stable to ensure a “safe operating space” for humanity. The chemistry of the ocean—one of nine planetary boundaries—has now moved into an “uncertainty range,” raising the risk of irreversible damage to marine ecosystems and the people who depend on them.
The study was led by Professor Helen S. Findlay, a biological oceanographer at Plymouth Marine Laboratory in the United Kingdom. Focusing on acidification’s impact on the rapidly warming Arctic and other marine environments, Findlay and her team estimate that 40% of the global ocean surface and 60% of water down to 650 feet now exist outside the previously defined chemical safety zone.
A Deeper Look At A Changing Ocean
Earlier models of ocean acidification treated the ocean as a single, uniform surface layer and were based on a single global threshold, without accounting for variations by depth or region. The new analysis takes a more refined approach by including error margins, breaking down changes by region, and crucially, extending the study into subsurface layers—where the majority of marine life resides and feeds.
Ocean acidification is driven by the long-term decline in seawater pH, largely due to the ocean absorbing massive amounts of human-produced carbon dioxide. As CO₂ dissolves into the ocean, it triggers chemical changes that lower the pH, making the water more acidic.
A critical metric in this context is the aragonite saturation state, which determines how easily marine organisms can form and maintain calcium carbonate structures. When this state falls, corals, plankton, and shellfish struggle to survive. The original acidification boundary was defined by a 20% global decrease in aragonite saturation from pre-industrial levels—a level intended to protect polar surface waters from becoming corrosive and to maintain conditions supportive of tropical coral reefs.
Collapsing Habitats Under Chemical Stress
The degradation of chemical conditions in the ocean is already affecting key species. According to the study, coral reefs in tropical and subtropical regions have lost about 43% of their suitable chemical habitat compared to pre-industrial conditions. This poses a direct threat to millions of species that rely on reefs for food, shelter, and breeding.
In polar waters, small, fragile marine snails called pteropods—which have aragonite shells—are experiencing a steep decline in livable space. Their suitable habitat has shrunk by up to 61%, raising concerns for the broader polar food web, where they are a critical food source.
In coastal zones, shellfish such as oysters and mussels are also feeling the impact, with around a 13% reduction in suitable chemical habitat. These changes pose a threat not only to biodiversity, but to coastal economies, aquaculture, and food security, especially in communities dependent on shellfish industries. The compounding effects of acidification are detailed in a broader review of its impacts, which highlights the vulnerability of shellfish fisheries and the livelihoods they support.
The Need For Tighter Limits
Professor Findlay’s team argues that the original 20% boundary may be too lenient to adequately protect marine ecosystems. Their findings support a stricter threshold—a 10% decline in average surface aragonite saturation from pre-industrial levels.
Using this revised benchmark, the surface ocean would have exited the safe zone as early as the 1980s, and by around 2000, nearly the entire surface layer had breached this tighter limit. Today, more than half of the upper 650 feet of ocean lies in conditions that are marginal or worse for many calcifying organisms.
Supporting data shows that the depth at which water becomes corrosive to aragonite shells has risen more than 650 feet in some areas since 1800. This vertical shift in corrosive conditions increases exposure for marine life throughout the water column.
Maps included in the study visualize aragonite saturation levels across different time periods—pre-industrial, 2020, and under both the 10% and 20% decline thresholds—clearly showing the shrinking zones of safe habitat for coral systems. These maps include overlaid coral reef distributions to emphasize the spatial impact.
Multiple Stressors And The Road Ahead
Ocean acidification does not act alone. According to Earth.com, this chemical shift combines with rising ocean temperatures and declining oxygen levels, creating compounding stresses that intensify pressure on marine ecosystems. In many regions, organisms now face all three stressors simultaneously, which narrows survival options and increases the likelihood of ecosystem collapse.
The future of ocean chemistry largely hinges on how quickly carbon dioxide emissions are reduced. A recent IPCC assessment cited in the report confirms that continued high emissions will worsen acidification, while strong, rapid emission cuts could stabilize ocean conditions over time.
Despite the surface of the ocean appearing calm and unchanging, the study warns that its chemistry is shifting in profound ways. Ensuring that marine ecosystems remain functional—and that the climate and food services they provide are maintained—will require treating this “chemical line in the water” with the same level of urgency as atmospheric temperature targets.