Ocean Acidification
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Current State
Impacts
Ocean acidification affects both marine ecosystems and the planet’s ability to regulate climate. As the ocean absorbs more carbon dioxide from the atmosphere, its chemistry changes, reducing the availability of carbonate ions needed by shell-forming organisms. This disrupts marine food webs, weakens coral reefs, and even limits the ocean’s capacity to absorb CO₂, creating feedbacks that intensify climate change.
Marine life under stress
Shell-forming organisms such as corals, mollusks, and tiny sea snails (pteropods) struggle to grow and survive as seawater becomes more acidic. This disrupts entire food webs, as many fish and larger species depend on these organisms for food or habitat. Coral reefs — already stressed by marine heatwaves — face additional challenges, as acidification slows recovery after bleaching and reduces reef resilience.
Deep-sea and regional impacts
In deeper waters, acidification is progressing faster because of lower buffering capacity. The aragonite saturation horizon — the depth where calcium carbonate begins to dissolve — is rising closer to the surface, threatening deep-water corals and other cold-water ecosystems. Regionally, the Arctic is acidifying fastest due to its cold temperatures and freshwater inputs, while tropical and coastal waters face added pressures from nutrient runoff, upwelling, and pollution.
Weakened ocean buffering and CO₂ uptake
Acidification reduces the ocean’s natural capacity to absorb carbon dioxide, slightly slowing one of Earth’s key climate regulation mechanisms. As carbonate ions decline, the ocean’s buffering system weakens, allowing more CO₂ to remain in the atmosphere and reinforcing global warming.
Key Drivers
Rising atmospheric CO₂ and climate change
Human-caused carbon dioxide emissions are the dominant driver of ocean acidification. As more CO₂ dissolves in seawater, it forms carbonic acid, which lowers pH and reduces the availability of carbonate ions essential for marine organisms.
Warming oceans and changes in circulation patterns influence how CO₂ is absorbed and mixed through the water column. These processes can accelerate acidification in certain regions, especially in colder or poorly mixed waters.
Local and regional pollution
In coastal areas, nutrient runoff from agriculture and wastewater, along with upwelling and high biological activity, can locally intensify acidification. These pressures add to global CO₂-driven trends, creating strong regional and seasonal variability.
Control Variables
The Planetary Boundary for Ocean Acidification is defined by the aragonite saturation state of surface seawater — a measure of how easily marine organisms can build shells and skeletons from calcium carbonate. The Ocean Acidification boundary was been formally assessed as transgressed in 2025, making it the seventh Planetary Boundary to be outside of its Safe Operating Space and confirming that human CO₂ emissions have pushed ocean chemistry beyond safe levels.
Aragonite Saturation State (Ω)
Source: Data from Lan et al., 2024 and Lan & Keeling, 2024. • This figure shows the annual global mean of atmospheric CO2 concentration from 1979-2023, and the marginally different dataset of CO2 concentration as measured at Mauna Loa in Hawaii, which integrates data over a longer timespan (from 1959-2023). The red line shows the PB of 350 ppm, while the green line represents the pre-industrial baseline of 280 ppm.
Definition
Aragonite is a form of calcium carbonate that many marine organisms – such as corals and shellfish – use to build their shells and skeletons. The aragonite saturation state (Ω) reflects the availability of carbonate in seawater relative to the amount needed for stable aragonite formation, with values below 1 indicating corrosive conditions. Ω is closely tied to CO₂ levels in the atmosphere, making it a useful indicator of the impact of rising CO₂ on both ocean chemistry and marine organisms.
Unit
Dimensionless.
Historical Range
Ω varies by region, predominantly ranging from 3.3 to 4.0 in tropical regions to 1 to 2 in polar regions.
Planetary Boundary (PB)
The PB for global mean surface Ω is set at 2.86, which is 80% of the pre-industrial value of 3.57. The 80% threshold was selected with the aim of preventing large-scale aragonite undersaturation in high-latitude waters, while maintaining well-oversaturated conditions in low-latitude regions, thereby limiting harmful impacts on marine calcifiers.
