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Ocean Acidification: What Is It?

by diana — last modified August 14, 2013 09:58 PM

In the past 200 years the oceans have absorbed about 525 billion tons of carbon dioxide (CO2), almost half of the CO2 produced by human activities like fossil fuel burning. As CO2  dissolves in seawater, it makes the water more acidic. The average pH of the global oceans has already dropped from 8.21 to 8.1 since the industrial revolution. On the pH scale, this seemingly small change of 0.1 indicates an increase of 30% in acidity.  If global CO2 emissions continue on current trends, then by the end of the century the oceans will see a pH drop of 0.4, a more acidic ocean environment than in the past 20 million years.

The Process of Ocean Acidification

Ocean acidification has been called a sister problem to climate change because it also results from human activities that release CO2. Its impacts are additional to, and may exacerbate, the effects of climate change.

rkckelp_Braxton_Dew_NOAA.jpgSeawater pH is a critical variable in marine systems. Today’s surface ocean water is slightly alkaline, with a pH ranging from 7.5 to 8.5 and it is saturated with calcium carbonate, a very important organic molecule for organisms like corals, mollusks and crustaceans that make shells. As CO2 reacts with the seawater, it lowers the pH and releases hydrogen ions. These ions bind strongly with carbonate, preventing it from forming the important calcium carbonate molecules.  If the pH of the global oceans drops 0.4 by the end of the century as predicted, the levels of calcium carbonate available for use by marine organisms will decrease by 50%.

Ocean acidification is likely to alter the biodiversity of the world’s marine ecosystems and may affect the total productivity of the oceans. Previously it was thought that these changes would take centuries, but new findings indicate that an increasingly acidic environment could cause problems in high-latitude marine ecosystems within just a few decades. 


Ocean Acidification Will Affect Polar and Sub-polar Regions First

Currently, the oceans’ surface water layers have sufficient amounts of calcium carbonate for organisms to use (known as saturated conditions). This calcium carbonate rich layer is deeper in warmer regions and closer to the surface in colder regions. Because calcium carbonate is less stable in colder waters, marine life in the polar oceans will be affected by calcium carbonate loss first.  A study published in Nature by 27 U.S. and international scientists stated, “Some polar and sub-polar waters will become under-saturated [at twice the pre-industrial level of CO2, 560 ppm], probably within the next 50 years” (Orr et al. 2005). Under-saturated refers to conditions in which the seawater has some calcium carbonate remaining, but it does not have enough available for the organisms to build strong shells.

Effects of Ocean Acidification on Marine Life

pteropod_limacina_helicina_NOAA.jpgResearch has shown that lowered ocean pH will affect the processes by which animals such as corals, mollusks and crustaceans make their support structures. Because these organisms depend on calcium carbonate, increasing acidity threatens their survival.  Pteropods are small planktonic mollusks that are at the bottom of the food chain and because of their dependence on calcium carbonate, they will be one of the first casualties of increasing acidity in Alaska's marine waters. In recent experiments exposing live pteropods to the conditions predicted by “business-as-usual” carbon emission scenarios the pteropod shells showed evidence of dissolution and damage within only 48 hours. Pteropods are a key food source for salmon and other species.

white_red_bubble_gum, Robert Stone, NOAA.jpgIn addition to pteropods, marine organisms at risk from increasing acidification include the corals and coralline algae commonly found in reef communities. Cold water coral communities along the Aleutian Islands form important fish habitat. Forminifera and coccolithophorids, planktons abundant in most surface waters, are also at risk.  Some commercial species like clams and crabs will be directly impacted by reductions in calcium carbonate. Others, like most fish populations, will be affected indirectly as acidification impacts their key prey species.

An urgent research question now is "to what extent will ocean acidification alter marine ecosystems and biodiversity?"

Predictions for the consequences of ocean acidification are based on the Intergovernmental Panel on Climate Change’s scenarios for future carbon dioxide emissions. Under the “business-as-usual” scenario where we just continue adding carbon to the atmosphere without regulation (also known as the continually increasing scenario), by 2050 organisms in the polar oceans will be facing a pH low enough to cause calcium carbonate shortages. Under the “stabilization” scenario where future carbon emissions are reduced, the polar and sub-polar oceans may continue to have calcium carbonate present, but at a much lower concentration and in a much thinner surface layer.

Because the water in the oceans cycles very slowly, the effects of increasing acidity on the oceans will last for thousands of years. According to researchers from the Royal Society (the British equivalent of the U.S. National Academy of Sciences) “Reducing CO2 emissions to the atmosphere appears to be the only practical way to minimize the risks of large-scale and long term changes to the oceans” (Raven et. al 2005).

Feedback to Climate Changes

The changes in ocean chemistry described above will reduce the ocean’s ability to absorb CO2 from the atmosphere. The ocean is an important carbon sink, but it is not an infinite one.  Without the ocean taking nearly half of the human produced CO2  out of the atmosphere, the human influence of carbon on the greenhouse effect and global warming will accelerate.



Further reading:

The March 2006 edition of Scientific American magazine has an article, “The Dangers of Ocean Acidification,” by Scott Doney, one of the authors of the study published in Nature.

Elizabeth Kolbert wrote an article for The New Yorker, November 20, 2006 about ocean acidification titled "The Darkening Sea: What carbon emissions are doing to the ocean."



1. Orr et al. Anthropogenic ocean acidification over the twenty-first century and its impact on calcifying organisms, in Nature, 437, 681-686 (29 September 2005).

2. Kleypas et al. "Impacts of Ocean Acidification on Coral Reefs and Other Marine Calcifiers: A Guide for Further Research." Report of a workshop held April 18-20, 2005.

3. Raven et al. "Ocean Acidification due to increasing carbon dioxide." The Royal Society. June 2005.

4. Freely et al. "Impact of Anthropogenic CO2 on CaCO3 System in the Oceans." Science 305: 362-366. June 16, 2004.




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