Acidity is pleasantly tart for people’s taste buds, but it is not usually good for sea creatures that have shells or other hard parts: They are especially sensitive to changes in acidity.

Acidic foods taste sour from vinegar, which is acetic acid; fruit acids, which contain citric and ascorbic acids; or carbonation. Acid in food rounds out the flavors, and it can make the difference between a bland dish and one with lots of taste.

On the other hand, corals and other organisms must work against a chemical potential in order to extract the calcium and carbon dioxide needed to build their structures from seawater.

Water that is neither acidic nor alkaline always contains a certain amount of hydrogen ions. We think of the water molecule as being stable, and it is over time, but on the nanoscale, water molecules are constantly breaking apart and re-forming due to electric field fluctuations caused by molecular interactions. This happens billions of times per second.

When awater molecule dissociates, it leaves a positively charged hydrogen ion and a hydroxide ion. The latter is composed of a single atom of oxygen bonded to a single atom of hydrogen with an overall negative electric charge.

Neutral water has a pH of 7. This means that the concentration of hydrogen ions is 10-7. For every 10 million water molecules, only one will be dissociated at a given instant.

The term pH stands for the negative logarithm of the concentration of hydrogen. That sounds complicated but it is not, really. It is necessary to simplify when the concentration is other than an even power of 10. For example, the current ocean pH of 8.07 is equivalent to a concentration of 8.5 x 10-9, or 85 dissociated out of every 10 billion molecules.

The pH of the ocean fell from 8.18 to 8.07 since the original Challenger expedition first measured it in 1751. That does not seem like much, but it is a 30 percent increase in acidity.

When seawater dissolves carbon dioxide from the atmosphere, the ocean absorbs it. It then reacts to release hydrogen ions. To date the ocean has absorbed about one-third of the carbon dioxide produced from human activities since 1800.

Corals exist in an environment where calcium carbonate, which forms the hard parts of their bodies, cannot naturally precipitate in seawater. Just a small addition of hydrogen ions can throw off the equilibrium and make it impossible for the corals to crystallize the necessary calcium carbonate out of the seawater.

Corals are not the only organisms affected by acidification, but they are the canaries in the cages that serve as a warning that the ocean ecosystem is shifting toward a new equilibrium.

We cannot predict what the final equilibrium will be or how it will affect the world ocean, but we know that any change to an ecosystem, especially one as complex as the world ocean, will produce unpredictable and unexpected results.

Any change in an ecosystem requires adaptation. Unlike ocean organisms, humans are good at cultural adaptation, but we do not like to adapt, and the more extreme the adaptation, the more difficult it will be for us.

Richard Brill is a professor of science at Honolulu Community College. His column runs on the first and third Fridays of the month. Email questions and comments to brill@hawaii.edu.