Invisible threat to marine life
Increased carbon dioxide in the atmosphere also ends up in the ocean.
A glass of water sitting on a table will absorb carbon dioxide (CO鈧�) from the air. The CO鈧� dissolves, creating a weak solution of carbonic acid鈥攖his makes the water more acidic. So, does this happen with the ocean? Does it absorb CO鈧� from the atmosphere? It sure does.
The ocean is naturally alkaline, usually with a pH of around 8.2 (a neutral substance has a pH of 7). As the ocean absorbs CO鈧�, carbonic acid (H鈧侰O鈧�) is formed. This releases hydrogen ions (H鈦�) which makes the ocean become more acidic. Scientists estimate that the absorption of some of the additional CO鈧� in the atmosphere has already lowered oceanic pH by around 0.1 since 1750, bringing it to 8.1.
The ocean will continue to absorb the extra CO鈧� in the atmosphere and under current projections oceanic pH will reach 7.7 by the end of this century, a decline of 0.5. This doesn鈥檛 sound like all that much, but the pH scale is logarithmic, so what looks like small changes in pH represents large changes in the concentration of hydrogen ions. The change in pH of 0.1 pH units since 1750 translates into an increase in hydrogen ions of around 26 per cent.
What do acidic substances do? They dissolve stuff. And acid is pretty good at dissolving calcium carbonate鈥攖he stuff a huge range of sea creatures use to build their shells or skeletons. So this is a serious problem for a huge range of sea creatures, from corals to crustaceans to shellfish to pteropods鈥攁 graceful creature sometimes known as a 鈥榮ea butterfly鈥�.
But the underlying chemistry isn鈥檛 quite as simple as 鈥榓cid ocean water dissolves shells鈥�. Even more of a problem is that a more acidic ocean also means that it鈥檚 harder for these creatures to build their shells and skeletons in the first place. To build their shells and skeletons, they use calcium (Ca虏鈦�) and carbonate (CO鈧兟测伝) from the ocean. The amount of available carbonate is called the carbonate 鈥榮aturation state鈥� and it varies in different regions of the ocean. In a more acidic ocean, the extra hydrogen ions floating around mean that instead of carbonate (CO鈧兟测伝), bicarbonate (HCO鈧冣伝) is more likely to form. This means there鈥檚 less carbonate鈥攖he carbonate saturation state is lower. There are fewer carbonate ions available for these ocean dwellers to build their shells and skeletons.
What鈥檚 more, calcium carbonate occurs in two forms鈥攃alcite and aragonite. These have the same chemical composition, but the atoms are arranged in a different structure, and have different stabilities. Aragonite is less stable than calcite, which means that it not only dissolves more readily, but its formation also requires a higher carbonate saturation state. This means that the critters that use aragonite for their shells and skeletons鈥攕ome types of coral, and some shellfish鈥攚ill be in even more trouble in a more acidic ocean.
The acidification of the ocean is not a short-term phenomenon; the oceans have a huge capacity to absorb more CO鈧� and they will continue to do so. Over a thousand-year timescale, around 80 per cent of human-produced CO鈧� emissions will end up in the oceans. The reversal of this process will only occur as the chemical weathering of rocks neutralises the acidity in the ocean, another process that will take tens of thousands of years.