Ocean acidification is an ongoing decrease in the pH of global seawaters. Scientists use the pH scale to describe the strength of acids and bases. The scale goes from 0 to 14, with lower numbers indicating increasing acidity. The ongoing decrease in seawater pH results from human activity. Since modern industry began to develop in the late 1700’s, average pH in the surface oceans has declined from about 8.21 to about 8.10 in 2012. This change in pH corresponds to a 28.8 percent increase in acidity. The pH of the surface ocean is believed to be declining faster than at any other time in Earth’s past. Scientists are concerned about this trend because lowering pH can disrupt important chemical processes that maintain the overall health of the oceans.
Acidification occurs when ocean waters absorb increasing carbon dioxide (CO2) in the atmosphere. In the ocean, CO2 reacts with seawater to form carbonic acid (H2CO3). Since the mid-1800’s, CO2 levels in the atmosphere have risen about 50 percent. The burning of fossil fuels by human beings is a major contributor to the rise in CO2. The surface ocean has absorbed roughly one third of the CO2 emitted by burning fossil fuels.
Scientists expect ocean acidification to have a profound impact on living things. Acidification of seawaters can disrupt such biochemical processes as calcification, photosynthesis, reproduction, and respiration. Calcification is the formation of calcium carbonate shells and skeletons. Photosynthesis is the manufacture of food by plants using the energy in light. Acidification can alter these processes by changing important chemical balances in the ocean.
One key effect of ocean acidification results from an associated decrease in carbonate ions (CO3 2-) and increase in bicarbonate ions (HCO3-) in seawaters. The carbonate ion is an important building block in the formation of calcium carbonate (CaCO3), a key component of skeletons and shells. The decrease in the availability of calcium carbonate ions thus makes it more difficult for marine organisms to calcify shells or grow skeletons. Scientists have observed unusually thin, small, or deformed shells on such marine animals as clams, mussels, oysters, and coral in several places around the world.
Some species may be able to adapt to ocean acidification. But certain species may also become extinct. The decline of certain species could disrupt entire ecosystems within the oceans. Such a disruption would not only harm ocean wildlife. It would also result in a reduction of food from fishing, income from beach and marine tourism, coastal protection by coral reefs, and cycling of oxygen and other important nutrients.
