Fig.1. A pteropod (sea butterfly) shell placed in seawater with pH and carbonate levels projected for year 2100. The shell dissolved over 45 days. (Credit: David Liittschwager/National Geographic Stock)
A routine survey off the U.S. West Coast conducted by the NOAA Ocean Acidification Program in 2011 found frightening results. More than half of their collected pteropods (sea butterflies) had severely dissolved shells. The ocean’s absorption of human-caused carbon dioxide emission created an acidic environment that has only recently caught the public eye. Now dubbed as “climate change’s evil twin,” ocean acidification represents a serious problem brought about by global warming. As more carbon dioxide dissolves into the ocean, marine organisms and the humans that depend upon them are put at risk.
The costs of acidityOcean acidification is the process by which the uptake of carbon dioxide from the atmosphere lowers the ocean’s pH. As surface ocean water mixes with carbon dioxide from the air, carbonic acid is formed. In addition to lowering the pH, an overabundance of carbonic acid causes a reduction in available carbonate ions -- an important part of calcium carbonate that is needed for growth of marine structures like shells and coral.
This trend has the potential to harm the $100 million a year shellfish industry as well as other marine life that depends upon the availability of calcium carbonate. When ocean environments become under-saturated of calcium carbonate, animals with shells and skeletons lack the necessary elements to grow these structures. Even worse, those already formed can dissolve.
(Another) historical disasterA collections record stretching back millions of years tells the same worrying story. According to soil and rock core samples from Earth’s crust, ocean acidification has happened before. Core samples are obtained from sediment or rock. Their fossil and chemical contents shed light on air composition and ecosystem makeup millions of years ago.
Researchers found that a similar sudden rise in atmospheric carbon dioxide and ocean acidity happened about 56 million years ago during the Paleocene-Eocene Thermal Maximum (PETM). This period saw a temperature increase by 9℉ (5℃) and a sudden drop in ocean pH. The fossil record shows a similar significant drop in shelled sea life. These species either went extinct or experienced the same structural damage found during the U.S. West Coast Survey in 2011 (Fig.2).
Fig.2. Calcium dependent nano-fossils under a Scanning Electron Microscope. (A) is before PETM and (B) is after PETM (Credit: http://www.realscience.us/2012/03/06/acid-oceans-spell-trouble-for-sea-life/)
Ocean acidification has increased by about 30 percent since pre-industrial times, nearly ten times the rate experienced during PETM. Recovery from that ancient decrease in pH took about 100,000 years, which indicates an even longer recovery in our future, both in terms of sea life survival and ocean acidity.
In addition to lower pH, the ocean is also experiencing the highest global mean surface temperatures ever recorded. These record-breaking temperatures and pH levels are certainly a transformation for our planet’s largest bodies of water. Scientists are only now realizing the true extent of how a different ocean chemistry affects growth, behavior, productivity, nitrogen-carbon ratios in food webs, and more. Although ocean acidification does not harm all marine organisms, it threatens ecosystem health and the 10-12 percent of the world’s population whose livelihoods depend upon fisheries.
As our shell and core sample collections indicate, the road to recovery is long. The immediate cut of carbon emissions is essential for controlling global warming and therefore, ocean acidification. The recent agreement between the U.S. and China to decrease carbon pollution is an important step in a global effort to reduce emissions. Just as we saw an increase in ocean acidification in pre-industrial records, we might see a decrease of acidification in future core and shellfish or coral collections. Hopefully it will be shorter than 100,000 years.
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