Ischia, Campania, Italy. February 2019.
The Ischia volcanic complex forms a rectangular, 6 km x 9 km island immediately southwest of Campi Flegrei (EN: Phlegraean Fields). Specifically, Castello Aragonese is a tiny island that is a product of the northward drift of Africa. Along a complicated set of folds, the African plate is pressing into Eurasia, bringing Tripoli 3 cm or so closer to Rome every year. Occasionally, this process results in violent volcanic eruptions such as the one in 1302, which led the entire population of Ischia to take refuge on Castello Aragonese. On a more regular basis, it sends streams of carbon dioxide bubbling out of vents in the sea floor, as close as 20 m from the rocky cliffs.
Due to all the extra carbon dioxide pumped into the atmosphere, the pH of the oceans’ surface waters has already dropped, from an average of around 8.2 to an average of around 8.1, i.e., the oceans are now 30% more acidic than they were in 1800. Assuming that humans continue to burn fossil fuels, the oceans will continue to absorb carbon dioxide and will become increasingly acidified. Under the “business as usual” emissions model, surface pH will fall to 8.0 by 2050, and to 7.8 by 2100, at which point, the oceans will be 150% more acidic than they were at the start of the industrial revolution.
At Castello Aragonese, the vents produce a pH gradient. On the eastern edge of the island, the waters are more or less unaffected. As you move closer to the vents, the acidity of the water increases. In the waters far from the vents one finds 69 species of animals and 51 species of plants. Closer to the vents, one-third of the species are absent in the pH 7.8 zone, and three-quarters of the missing species are calcifiers (any organism that builds a shell or external skeleton or, in the case of plants, a kind of internal scaffolding out of the mineral calcium carbonate). Calcifying seaweed is completely absent.
The vents at Castello Aragonese have been spewing carbon dioxide for at least several hundred years, maybe longer. Any mussel, barnacle or keel worm that can adapt to lower pH in a time frame of centuries would presumably already have done so. The lower the pH drops, the worse it is for calcifiers. Right up near the vents, where the bubbles of carbon dioxide stream up in thick ribbons, they are entirely absent. In fact, all that remains in this area are a few hardy species of native algae, some species of invasive algae, one kind of shrimp, a sponge, and two kinds of sea slugs.
Owing to the carbon dioxide pouring out of the vents, the waters around Castello Aragonese provide a near-perfect preview of what lies ahead for the oceans.
Sources: E. Kolbert,
The Sixth Extinction: An Unnatural History, 2014.
Smithsonian Institution Global Volcanism Program.
Eyewitness Naples and the Amalfi Coast (2013).
