page_title: Ocean acidification
section_title: Carbon cycle
hierarchical_section_title: Ocean acidification / Carbon cycle
caption_attribution_description: Air - sea exchange of carbon dioxide
context_page_description: Ocean acidification is the ongoing decrease in the pH of the Earth's oceans, caused by the uptake of carbon dioxide from the atmosphere. Seawater is slightly basic, and ocean acidification involves a shift towards pH-neutral conditions rather than a transition to acidic conditions. 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. 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.
context_section_description: The carbon cycle describes the fluxes of carbon dioxide (CO
  ₂) between the oceans, terrestrial biosphere, lithosphere, and the atmosphere. Human activities such as the combustion of fossil fuels and land use changes have led to a new flux of CO
  ₂ into the atmosphere. About 45% has remained in the atmosphere; most of the rest has been taken up by the oceans, with some taken up by terrestrial plants.
  The carbon cycle involves both organic compounds such as cellulose and inorganic carbon compounds such as carbon dioxide, carbonate ion, and bicarbonate ion. The inorganic compounds are particularly relevant when discussing ocean acidification for they include many forms of dissolved CO
  ₂ present in the Earth's oceans.
  When CO
  ₂ dissolves, it reacts with water to form a balance of ionic and non-ionic chemical species: dissolved free carbon dioxide (CO
  2(aq)), carbonic acid (H
  ₂CO
  ₃), bicarbonate (HCO⁻
  ₃) and carbonate (CO²⁻
  ₃). The ratio of these species depends on factors such as seawater temperature, pressure and salinity (as shown in a Bjerrum plot). These different forms of dissolved inorganic carbon are transferred from an ocean's surface to its interior by the ocean's solubility pump.
  The resistance of an area of ocean to absorbing atmospheric CO
  ₂ is known as the Revelle factor.
caption: The CO
  2 cycle between the atmosphere and the ocean
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