Facts of the Matter
Modern era sees acidity of sea rise
The solid Earth is surrounded by a fluid envelope of atmosphere and ocean, a coupled system of direct interaction.
Unlike our Venusian and Martian neighbors, which have very high concentrations of carbon dioxide in their atmosphere, most of Earth's carbon dioxide is stored in the ocean as carbonate rocks, such as coral and shells of microscopic plankton.
The carbonate cycle is a complex dynamic equilibrium between carbon dioxide in the atmosphere and in sea water that controls the pH (acidity) of the oceans and the amount of carbonate deposits.
Carbon dioxide reacts with water to form a molecule of carbonic acid, which is a weak acid. About three out of every hundred carbonic acid molecules break apart to release a bicarbonate ion and a hydrogen ion. The presence of those excess hydrogen ions increases the acidity of the water.
In sea water this initiates a series of chemical reactions that ends with formation of calcium carbonate, removing hydrogen and calcium ions and acting as a buffer to keep the acidity of the sea water at nearly constant levels over time.
Microscopic plankton use bicarbonate ions and calcium ions from sea salt to build shells, and coral use it to build reefs from calcium carbonate.
Increases in acidity inhibit formation of calcium carbonate. Extreme acidity can even cause existing calcium carbonate to dissolve, pushing the reaction backward and releasing carbon dioxide into the atmosphere.
Because the surface water is well mixed and then mixes with deep water, the effect is slow, but absorption of carbon dioxide will eventually make the water more acidic, with a lower pH.
Approximately 45 percent of the carbon dioxide that has been emitted into the atmosphere since the start of the industrial era has been absorbed by the ocean, as its overall pH has dropped from 8.18 to 8.10.
This does not sound like much, but pH is a logarithmic scale so that a change of this size represents a 16 percent increase in hydrogen ions. Although calculations are not precise because of the number of variables involved, the best figures predict a change of 40 percent by the middle of the century, dropping pH to 7.95.
Absorbing carbon dioxide from the atmosphere will cause the pH of ocean water to become more acidic, which in turn will cause coral reefs to shrink, and cause a decrease in carbonate-based plankton production.
This represents a potential environmental crisis as great or greater than global warming.
Coral reefs are one of the most biologically productive ecosystems on the planet, producing food for commercial fisheries as well as for blue-water fish such as swordfish and tuna.
Plankton produce about one-half of all atmospheric oxygen by photosynthesis. The other half comes from land-based photosynthesis, most of which is from tropical rain forests, which are being systematically deforested.
There is no way to know the exact effects of increased carbonic acid on plankton and coral.
One thing is certain: Increased human output of carbon dioxide affects the entire fluid envelope, and it puts more at stake than just the greenhouse effect.
, professor of science at Honolulu Community College, teaches earth and physical science and investigates life and the universe. E-mail questions and comments to firstname.lastname@example.org