What is ocean acidification?

Anthropogenic CO2 emissions arising from fossil-fuel combustion, land-use practices, and concrete production during and since the industrial revolution first enter the atmosphere, but a large proportion of are absorbed into the ocean by physical and biological processes that are normal parts of the natural carbon cycle. These emissions have resulted in a ~40% increase in atmospheric CO2 concentrations over pre-industrial levels. Over the same period, the ocean has absorbed approximately 30%-50% of these emissions. Although the absorption of anthropogenic CO2 by the ocean has provided a degree of buffering against global warming (which results largely from increased CO2 in the atmosphere), the increase in dissolved CO2 is accompanied by chemical reactions that increase oceanic hydrogen ion concentrations (thus reducing seawater pH) and bicarbonate ion concentrations while reducing carbonate ion concentrations.

The result is more CO2 dissolved in the world’s oceans. Seawater is a weakly-alkaline solution (with a pH of ~ 8.1), but this extra CO2 changes the carbonate chemistry of the surface ocean, driving ocean pH and carbonate ion concentrations lower. While the relative acidity or alkalinity of seawater (typically measured as pH) shows significant spatial and temporal variation throughout the world’s oceans, seawater is on average a weakly-alkaline solution (with a mean pH of ~ 8.1). Ocean acidification is estimated to have lowered the mean pH of the ocean from its pre-industrial state by about 0.1 pH units.

The process of ocean acidification is already underway and discernible in the ocean. By the end of this century pH levels are likely to drop 0.2 – 0.3 units below pre-industrial pH. The level of atmospheric CO2 is now higher than at any time in at least the past 650,000 years, and probably has not been as high as present levels for approximately 4-5 million years . The current rate of increase of CO2 in the atmosphere is one hundred times greater than the most rapid increases during major climate changes over the last 650,000 years, and the concomitant rate of carbonate chemistry change in the ocean is similarly rapid . Nearly half the fossil-fuel CO2 emitted to date has now dissolved into the ocean .

CO2- driven acidification, in addition to lowering seawater pH, shifts the proportion of dissolved carbon dioxide away from carbonate ion and to bicarbonate ion, and thus towards lower saturation states for carbonate mineral. Calcium carbonate precipitation at a decreased saturation state requires higher energetic demands from shell-making organisms.

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