Animation created by Sarah Cooley, Woods Hole Oceanographic Institution, showing a computer recreation of surface ocean pH from 1895 to the present and a forecast of ocean pH between now and 2100 under current emission rates. The pH scale is shown on the right.
Carbon dioxide is naturally present in tiny amounts in the atmosphere; however, since humans started burning fossil fuels at the beginning of the industrial age, carbon dioxide levels have increased dramatically. Carbon dioxide is a gas that plants combine with energy from the sun to create food. It is also a greenhouse gas: it traps the heat of the sun and keeps it from reflecting back into space. Today the oceans absorb about 25 percent of the CO2 released every year. Initially, scientists focused on the benefits of removing the greenhouse gas from the atmosphere, but in the last decade research has shown that the excess CO2 absorbed by the oceans is changing the chemistry -- the pH -- of the seawater in a process called ocean acidification.
Scientists use the pH scale to measure the concentration of hydrogen (H) ions in a solution on a scale of 0 to 14. A pH of 7 is considered neutral, numbers smaller than 7 indicate increased acidity, and numbers greater than 7 indicate increased alkalinity. The pH scale is logarithmic, so one pH unit corresponds to a tenfold increase in the concentration of H ions.
Ocean acidification takes place when excess CO2 from the atmosphere dissolves in the ocean and combines with seawater to form carbonic acid. Carbonic acid breaks up into hydrogen ions and bicarbonate ions -- the increase in hydrogen ions reduces the pH of the seawater, causing it to become more acidic, and interferes with the uptake of calcium carbonate by shelled ocean organisms. Many shelled organisms are affected by a decrease in pH, including oysters, scallops, abalone, and even the smallest plankton that makes up an important part of the ocean food web. When there is less carbonate available, shells become thinner and growth slows down. Organisms have to spend more energy building their shells, which means less energy for growth. Larval stages are especially at risk because they need to grow shells before they start to eat. Shelled organisms provide food for a multitude of other creatures in the ocean, and so their decline has the potential to affect the entire ocean ecosystem.
One of the major concerns of ocean acidification is the drastic rate at which it is happening. Scientists have determined the most rapid CO2 increase in Earth’s history occurred 55 million years ago, and that change was likely not as rapid as the change we see today. It is uncertain whether marine organisms will be able to adapt to the rapidly changing chemistry in such a short time. As organisms are compromised, the biodiversity of the ocean may decrease, as only certain species will be able to adapt to the changing chemistry.