The pH scale is a measure of how acidic or basic a solution is. It is a measurement that compares the proportion of hydrogen ions (H+) or hydroxide ions (OH-) in a solution. Ions are charged atoms or molecules.
Water (H2O) can dissociate into hydroxide ions and hydrogen ions. In pure water, there is always one H+ for every OH-. These solutions are neutral, with a pH of 7.0.
Acids have a pH lower than 7.0 and have extra H+ ions in the solution. Bases (alkaline solutions) have a pH higher than 7.0 and have extra OH- ions in the solution.
At one end of the scale, hydrochloric acid is extremely acidic at a pH of 1, and lye (oven cleaner) is extremely basic at a pH of 13.
The pH scale is logarithmic, so even small differences in pH represent large differences in H+ ions. Every pH point is a 10-fold change in the concentration of H+ ions, with pH 6 being 10 times more concentrated than pH 7.
Normal seawater ranges from pH 8.1 to pH 8.3. Ocean acidification occurs as the water absorbs carbon dioxide (CO2) from the atmosphere. Since the Industrial Revolution, seawater has dropped from an average of pH 8.2 to pH 8.1. Considering the logarithmic pH scale, this is a 30% increase in ocean water acidity.
Even a decline in a few tenths of a pH unit can alter the ability of marine organisms to produce shells. At a pH of 7.6, most shells would quickly dissolve.
Since the Industrial Revolution, the atmosphere has been increasing in CO2 concentration. The atmosphere and oceans are continually exchanging gases, so as the atmosphere gains CO2, so do the oceans.
Between the early 1990s and mid-2000s, scientists around the world collected and analyzed nearly 80,000 water samples from a range of ocean environments. By their estimates, some 30% of all the CO2 released by humans in the last two centuries has been absorbed by the world's oceans.
The ocean pH readings have become more acidic as ocean and atmospheric CO2 concentrations have increased.
This map represents estimates of ocean pH levels as they would be before the 1800s, with an average pH of 8.2.
Ocean pH will continue to drop significantly if we do not curtail fossil fuel use and our release of extra CO2. The projected ocean pH in 2100, accounting for current CO2 emissions levels, is a pH of 7.7.