Ocean acidification

World map showing the varying change to pH across different parts of different oceans


Ocean acidification is the ongoing decrease in the pH of the Earth's oceans, caused by the uptake of carbon dioxide (CO
2) from the atmosphere. The main cause of ocean acidification is the burning of fossil fuels. Seawater is slightly basic (meaning pH > 7), and ocean acidification involves a shift towards pH-neutral conditions rather than a transition to acidic conditions (pH < 7). The issue of ocean acidification is the decreased production of the shells of shellfish and other aquatic life with calcium carbonate shells. The calcium carbonate shells can not reproduce under high saturated acidotic waters. An estimated 30–40% of the carbon dioxide from human activity released into the atmosphere dissolves into oceans, rivers and lakes. Some of it reacts with the water to form carbonic acid. Some of the resulting carbonic acid molecules dissociate into a bicarbonate ion and a hydrogen ion, thus increasing ocean acidity (H+ ion concentration). Between 1751 and 1996, surface ocean pH is estimated to have decreased from approximately 8.25 to 8.14, representing an increase of almost 30% in H+ ion concentration in the world's oceans. Earth System Models project that, by around 2008, ocean acidity exceeded historical analogues and, in combination with other ocean biogeochemical changes, could undermine the functioning of marine ecosystems and disrupt the provision of many goods and services associated with the ocean beginning as early as 2100.

Increasing acidity is thought to have a range of potentially harmful consequences for marine organisms such as depressing metabolic rates and immune responses in some organisms and causing coral bleaching. By increasing the presence of free hydrogen ions, the additional carbonic acid that forms in the oceans ultimately results in the conversion of carbonate ions into bicarbonate ions. Ocean alkalinity (roughly equal to HCO3− + 2CO32−) is not changed by the process, or may increase over long time periods due to carbonate dissolution. This net decrease in the amount of carbonate ions available may make it more difficult for marine calcifying organisms, such as coral and some plankton, to form biogenic calcium carbonate, and such structures become vulnerable to dissolution. Ongoing acidification of the oceans may threaten future food chains linked with the oceans. As members of the InterAcademy Panel, 105 science academies have issued a statement on ocean acidification recommending that by 2050, global CO
2 emissions be reduced by at least 50% compared to the 1990 level. To ensure that ocean acidification is minimized, the United Nation's Sustainable Development Goal 14 ("Life below Water") aims to ensure that oceans are conserved and sustainably used.

Latest research challenges the potential negative impact of end-of-century ocean acidification level on the coral fish behavior and suggests that the effect could be negligible. Controversially, laboratory experiments in the controlled environment showed CO
2 induced growth of the phytoplankton species. Field study of the coral reef in Queensland and Western Australia from 2007 to 2012 argues that corals are more resistant to the environmental pH changes than previously thought, due to internal homeostasis regulation; this makes thermal change, rather than acidification, the main factor for coral reef vulnerability due to global warming.

While ongoing ocean acidification is at least partially anthropogenic in origin, it has occurred previously in Earth's history, and the resulting ecological collapse in the oceans had long-lasting effects for global carbon cycling and climate. The most notable example is the Paleocene-Eocene Thermal Maximum (PETM), which occurred approximately 56 million years ago when massive amounts of carbon entered the ocean and atmosphere, and led to the dissolution of carbonate sediments in all ocean basins.

Ocean acidification has been compared to anthropogenic climate change and called the "evil twin of global warming" and "the other CO
2 problem". Freshwater bodies also appear to be acidifying, although this is a more complex and less obvious phenomenon.

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References

Clarke, L.; Edmonds, J.; Jacoby, H.; Pitcher, H.; Reilly, J.; Richels, R. (July 2007). "Scenarios of Greenhouse Gas Emissions and Atmospheric Concentrations. Sub-report 2.1A" (PDF) . In U.S. Climate Change Science Program and the Subcommittee on Global Change Research (ed.). Synthesis and Assessment Product 2.1 . Washington, DC., USA: Department of Energy, Office of Biological & Environmental Research. Archived from the original (PDF) on 16 June 2013. Good, P.; Gosling, S. N.; Bernie, D.; Caesar1, J.; Warren, R.; Arnell, N. W.; Lowe, J. A. (2010). An updated review of developments in climate science research since IPCC Fourth Assessment Report (PDF) (Report). London, UK: AVOID Consortium. Report website. UK Royal Society (September 2009). Geoengineering the climate: science, governance and uncertainty (PDF) . London: UK Royal Society. ISBN  978-0-85403-773-5 , RS Policy document 10/09. Report website. UNEP (November 2010). The Emissions Gap Report: Are the Copenh...
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