Drop a dirty penny into a glass of Coke. If you examine the penny after a week, Abraham Lincoln’s head will be gleaming. The carbonic acid in the cola has dissolved the organic grime on the copper-plated coin. It’s a science-class experiment many of us will remember from childhood. Now consider that the ocean is becoming corrosive, like Coke.

The phenomenon is called ocean acidification, and, like climate change, it is a result of increasing carbon dioxide (CO₂) emissions. The oceans have absorbed about one third of the CO₂ released into the atmosphere by humans over the past 200 years, and that is changing the waters’ chemistry. The oceans are not fizzing like that glass of Coke--the chemical change is not that extreme--but they are becoming more acidic, with ominous consequences. A shift in the pH balance of seawater is under way, and it threatens shell-building creatures, corals, fisheries such as salmon, oysters, mussels, and sea urchins, and entire marine ecosystems.

The chemical reactions involved in acidification are well understood. There is also no controversy over the fact that acidification is happening on a global scale. And what can be done to slow it down is simple from a scientific view: eliminate all sources of human CO₂ emissions, immediately. Even if it were possible for this to occur, however, the harm will likely continue. Enough CO₂ may have already entered the ocean to cause hundreds of years of damage to millions of years’ worth of evolutionary progression.

Acidity is measured on a pH scale, which ranges from 0 (strong acid) to 14 (strong base), with pure water a neutral 7. Like the seismic scale for earthquakes, the pH scale is logarithmic, so each additional 0.1 is in fact an increase of 30 percent. Depending on the marine habitat and depth, seawater can range in pH from 7.5 to 8.4, so it’s slightly basic. When CO₂ dissolves in seawater, it forms carbonic acid. A few chemical reactions later, extra hydrogen ions are released; it is those ions that lower the pH level, making the water more acidic.

Ken Caldeira, an ocean acidification researcher at the Carnegie Institution for Science, recently described a demonstration experiment simple enough for a middle-school science fair. First, place a beaker of water inside an airtight bell jar and start pumping CO₂ into the jar. The water begins to absorb the gas as the bell jar fills. A few dips of litmus paper into the water over a short amount of time will give pH readings. "You can test the pH of the water and watch it shift," Caldeira said.

Unlike pure water, seawater contains many dissolved ions that help maintain a stable pH, including carbonate ions. The more CO₂ is forced into the water, the more carbonate ions are needed to keep the system balanced. If seawater were used in Caldeira’s experiment, it would take longer to see the pH change because of the carbonate ions--at first. Once those carbonate ions run out, however, the pH would plummet and the water would acidify. In the oceans, whatever carbonate ions are used to equilibrate the seawater chemistry are no longer available for corals and other animals to build their protective shells.