Ocean Acidification

Fundamental changes in seawater chemistry are occurring throughout Earth’s oceans. Since the beginning of the industrial revolution, the release of carbon dioxide (CO2) from industrial and agricultural activities has increased the amount of CO2 in the atmosphere. Ocean acidification is the term given to the chemical changes in the ocean as a result of carbon dioxide emissions.

Oceanographic measurements worldwide indicate that the pH of seawater is decreasing—that is, the ocean is becoming more acidic. This is due to the fact that seawater absorbs CO2 from the atmosphere, and the concentration of CO2 in the atmosphere has been steadily rising owing primarily to the burning of coal, oil and gas for transportation, heating, electricity generation, and other industrial activities. The amount of CO2 produced by human activities is small compared to that released naturally through biological and geological processess, but it is large enough that forests, grasslands, and aquatic plant communities can’t absorb it all, so every year the amount in the atmosphere rises. The rate of atmospheric CO2 accumulation has steadily increased in recent decades, reaching a record high of 31.6 gt in 2011 (IEA 2012).

In addition to the CO2 from natural sources that it absorbs, the ocean absorbs approximately 30% of the CO2 emitted into the atmosphere by humans (Sabine et al. 2004). Consequently, seawater worldwide is becoming more acidic.

The average pH of ocean surface water has decreased from a calculated value of 8.2 in 1750 to  a measured value of approximately 8.1 today.  Although it seems small, since the pH scale is logarithmic, this decline actually represents 30% greater acidity overall. It is important to note that the ocean is not ‘acid’, its pH is greater than 7 and will in all likelihood remain so. However, it is becoming more acidic, and this acidification will have profound biological effects.

Atmospheric Carbon Dioxide (CO2) Accumulation
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Which Goals Does This Affect?

How Was It Measured?

The Ocean Acidification layer models the difference in global distribution changes in the aragonite saturation state (Ωarag) between pre-industrial (~1870) and modern times (2000-2009) as a proxy for ocean acidification due to human influences.  Aragonite is the most common form of calcium carbonate (CaCO3), used by corals, mollusks and other sea life to form shells. In seawater, calcium carbonate must remain in equilibrium with the concentration of calcium ions and carbonate ions as can be seen in the following equation:

Ca2+ + CO32- ↔ CaCO3

Too many calcium ions or carbonate ions force the equation to the right, making calcium carbonate. Too few calcium ions or carbonate ions force it to the left, dissolving calcium carbonate to restore the chemical equilibrium.

The saturation state, symbolized as Ω, is a function of the concentrations of those three components in seawater, defined as:


When Ω = 1, CaCO3 remains stable: a calcium carbonate structure such as a shell or reef skeleton doesn’t dissolve or grow. When Ω < 1, calcium carbonate structures dissolve. When Ω > 1, calcium carbonate precipitates out of solution or adds to existing calcium carbonate structures. Note that calcifier species make shells easily when Ω ≥ 1. They can also make shells when Ω < 1, but the lower the saturation state, the more energy they must expend until a point is reached where they do not have enough energy to replace shell structure that is dissolving and they probably cannot survive.

What Are the Impacts?

Ocean Acidification Process
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Increased levels of CO2 in the ocean causes acidification, slows or prevents the formation of shells, coral reefs and other structures composed of calcium carbonate (CaCO3), and weakens or dissolves previously made structures.   

Increasing ocean acidification compromises the growth and structural integrity of coral reefs and their ability to provide shoreline protection, food, and other health related services.

500 million people worldwide depend on reefs for shoreline protection, food and income (Wilkinson 2008).
Acidification affects the larval development of certain commercial fish and shellfish. Fisheries may experience declines due to low survival rates and the inhibited development of marine calcifier species.

A 10-25% decrease in US mollusk harvest from the 2007 rate would result in a loss of US $75-187 million per year and a net loss of US $1.7-10 billion through the mid-century (Cooley and Doney 2009a). 

Get More Information

The Ocean Acidification Network    
This international symposium gathers scientists from around the world to address the impacts of ocean acidification and to discuss policy and management options.

United Nations Division for Sustainable Development
Ocean Acidification: A Hidden Risk for Sustainable Development 

Ocean Acidification - The Untold Story: a report on the impacts of ocean acidification upon marine life.

The European Project on Ocean Acidification (EPOCA)   
A helpful guidebook produced with the support of the U.S. Ocean Carbon and Biochemistry Program and the U.K. Ocean Acidification Research Programme.    

Around the Americas    
Coral Reefs and Ocean Acidification: an examination of coral reef habitat loss due to ocean acidification.    

A Sea Change
An award-winning documentary about the effects of the changing chemistry of the oceans.    

Acid Test: The Global Challenge of Ocean Acidification    
Created to raise awareness about the problems associated with ocean acidification. 

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PHOTO(S): © Keith A. Ellenbogen
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