Oceans in Crisis - Ocean Worlds: The Story of Seas on Earth and Other Planets - page 189

of ecologically important reef fish). In short, coral reef systems
worldwide today are struggling (some are already dead), and the
growing acidification threatens to be the coup de grâce (see Plate 4).
The latest studies suggest that at atmospheric carbon dioxide lev-
els of somewhere near 550 ppm the coral animals will not be able to
maintain a positive balance of calcium carbonate formation. From
around that point (and it will vary between different species, and in
different settings), the coral reefs will stop growing and start to
shrink back. At current rates of carbon emissions, this particular
tipping point for reefs will occur around the middle part of this cen-
tury. Small wonder that some marine biologists have referred to
coral reefs as zombie ecosystems: still alive, if increasingly in poor
health, but doomed. The Earth, then, would go through another of
the events that geologists term a 'reef gap', when these magnificent,
diverse structures disappear from the world. The last reef gap was 55
million years ago—and was also associated with an ancient global
warming and marine acidification event, termed the Paleocene-Eocene
Thermal Maximum. It took millions of years for the reef systems
to recover.
It is not just the corals. Other creatures are already showing signs
of being eaten away, just as a result of the 0.1 pH increase so far.
Pteropods—otherwise known as sea butterflies—are planktonic mol-
luscs that secrete delicate and rather beautiful shells of calcium car-
bonate. They are also rock builders, their shells falling on to some
areas of the deep-sea floor in such amounts that they build sedimen-
tary layers. Pteropod skeletons have lost weight in recent years. 110 They
too look to be in danger of declining, perhaps vanishing, this century.
Acidification does not only affect skeleton building. If the pH of
seawater drops to the kind of levels envisaged, then that interferes
with physiological processes such as respiration in fish, too.