Ocean acidification

Ocean acidification is the reduction in the pH value of the Earth’s ocean. Between 1751 and 2021, the pH value of the ocean surface is estimated to have decreased from approximately 8.25 to 8.14.[3] The root cause of ocean acidification are the human-caused carbon dioxide emissions which have led to atmospheric carbon dioxide (CO2) levels of more than 410 ppm (in 2020). The oceans absorb CO2 from the atmosphere. This leads to the formation of carbonic acid (H2CO3) which dissociates into a bicarbonate ion (HCO3) and a hydrogen ion (H+). The free hydrogen ions (H+) decrease the ocean pH of the ocean, causing "acidification" (this does not mean that seawater is acidic yet; it is still alkaline, with a pH higher than 8). The lowered pH causes a decrease in the concentration of carbonate ions, which are the main building block for calcium carbonate (CaCO3) shells and skeletons. It also lowers carbonate mineral saturation states. Marine calcifying organisms, like mollusks, oysters and corals, are particularly affected by this as they rely on calcium carbonate to build shells and skeletons.[4]

Ocean acidification means that the average ocean pH value is dropping over time.[1]
Spatial distribution of global surface ocean pH (Panel a: the annually-averaged surface ocean pH to be approximate for the year 1770; Panel b: the difference between pH in 2000 and 1770 in the global surface ocean).[2]

The change in pH value from 8.25 to 8.14 represents an increase of almost 30% in hydrogen ion concentration in the world's oceans (the pH scale is logarithmic, so a change of one in pH unit is equivalent to a tenfold change in hydrogen ion concentration).[5] Sea-surface pH and carbonate saturation states can vary depending on ocean depth and location. Colder and higher latitude waters have the capacity to absorb more CO2. This can increase acidification, lowering the pH and carbonate saturation states in these regions. Other factors that affect the atmosphere-ocean CO2 exchange, and therefore impact 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.[2][6][7]

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.[8] 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 future food chains linked with the oceans.[9][10] Ocean alkalinity is not changed by ocean acidification, but over long time periods alkalinity may increase due to carbonate dissolution and reduced formation of calcium carbonate shells.[9][11]

The United Nations Sustainable Development Goal 14 ("Life below Water") has a target to "minimize and address the impacts of ocean acidification".[12] 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 actively being researched at present but generally have a low technology readiness level and many risks.[13]:12–36

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.

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