Ocean acidification

Ocean acidification is the ongoing decrease in the pH of the Earth's ocean. Between 1950 and 2020, the average pH of the ocean surface fell from approximately 8.15 to 8.05.^[2] Carbon dioxide emissions from human activities are the primary cause of ocean acidification, with atmospheric carbon dioxide (CO₂) levels exceeding 410 ppm (in 2020). CO₂ from the atmosphere is absorbed by the oceans. This chemical reaction produces carbonic acid (H₂CO₃) which dissociates into a bicarbonate ion (HCO−3) and a hydrogen ion (H⁺). The presence of free hydrogen ions (H⁺) lowers the pH of the ocean, increasing acidity (this does not mean that seawater is acidic yet; it is still alkaline, with a pH higher than 8). Marine calcifying organisms, such as mollusks and corals, are especially vulnerable because they rely on calcium carbonate to build shells and skeletons.^[3]

A change in pH by 0.1 represents a 26% increase in hydrogen ion concentration in the world's oceans (the pH scale is logarithmic, so a change of one in pH units is equivalent to a tenfold change in hydrogen ion concentration). Sea-surface pH and carbonate saturation states vary depending on ocean depth and location. Colder and higher latitude waters are capable of absorbing more CO₂. This can cause acidity to rise, lowering the pH and carbonate saturation levels in these areas. Other factors that influence the atmosphere-ocean CO₂ exchange, and thus local ocean acidification, include: ocean currents and upwelling zones, proximity to large continental rivers, sea ice coverage, and atmospheric exchange with nitrogen and sulfur from fossil fuel burning and agriculture.^[4][5][6]

Decreased ocean pH has a range of potentially harmful effects for marine organisms. These include reduced calcification, depressed metabolic rates, lowered immune responses, and reduced energy for basic functions such as reproduction.^[7] The effects of ocean acidification are therefore impacting marine ecosystems that provide food, livelihoods, and other ecosystem services for a large portion of humanity. Some 1 billion people are wholly or partially dependent on the fishing, tourism, and coastal management services provided by coral reefs. Ongoing acidification of the oceans may therefore threaten food chains linked with the oceans.^[8][9]

The United Nations Sustainable Development Goal 14 ("Life below Water") has a target to "minimize and address the impacts of ocean acidification".^[10] Reducing carbon dioxide emissions (i.e., climate change mitigation measures) is the only solution that addresses the root cause of ocean acidification. Mitigation measures which achieve carbon dioxide removal from the atmosphere would help to reverse ocean acidification. The more specific ocean-based mitigation methods (e.g. ocean alkalinity enhancement, enhanced weathering) could also reduce ocean acidification. These strategies are being researched, but generally have a low technology readiness level and many risks.^[11][12][13]

Ocean acidification has occurred previously in Earth's history.^[14] The resulting ecological collapse in the oceans had long-lasting effects on the global carbon cycle and climate.