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“Those who blithely factor oceanic uptake into the equations of
what people can get away with when it comes to greenhouse-
gas pollution should, perhaps, have second thoughts.”
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The Economist
Feb 21st 2008

Climate change

Sour times

The sea is becoming more acidic. That is not good news if you live in it

EVERY silver lining has its cloud. At the moment,
the world’s oceans absorb a million tonnes of
carbon dioxide an hour. Admittedly that is only a
third of the rate at which humanity dumps the
stuff into the atmosphere by burning fossil
fuels, but it certainly helps to slow down global
warming. However, what is a blessing for the
atmosphere turns out to be a curse for the
oceans. When carbon dioxide dissolves in water it
forms carbonic acid. At the moment, seawater is
naturally alkaline-but it is becoming less so all
the time.

The biological significance of this acidification
was a topic of debate at the American Association
for the Advancement of Science meeting in Boston.
Many species of invertebrate have shells or
skeletons made of calcium carbonate. It is these,
fossilised, that form rocks such as chalk and
limestone. And, as anyone who has studied
chemistry at school knows, if you drop chalk into
acid it fizzes away to nothing. Many marine
biologists therefore worry that some species will
soon be unable to make their protective homes.
According to Andrew Knoll, of Harvard University,
many of the species most at risk are corals.

The acid test

Dr Knoll drew this conclusion by studying the
fossil record. The end of the Permian period,
252m years ago, was marked by the biggest
extinction of life known to have happened on
Earth. At least part of the cause of this
extinction seems to have been huge volcanic
eruptions that poured carbon dioxide into the
atmosphere. But some groups of animals became
more extinct than others. Sponges, corals and
brachiopods (a once-widespread group that look a
bit like bivalve molluscs) were particularly
badly hit.

Rather than counting individual species of
fossils, which vary over time, palaeontologists
who study extinction usually count entire groups
of related species, called genera. More than 90%
of Permian genera of sponges, corals and
brachiopods vanished in the extinction. By
contrast, only half of the genera of molluscs
(the real ones) and arthropods disappeared.

Dr Knoll reckons this is because molluscs and
arthropods are able to buffer the chemistry of
the internal fluids from which they create their
shells. This keeps the acidity of those fluids
constant. Sponges, corals and brachiopods,
however, cannot do this.

The situation at the moment is not as bad as it
was at the end of the Permian. Nevertheless,
calculations suggest that if today’s trends
continue, the alkalinity of the ocean will have
fallen by half a pH unit by 2100. That would make
some places, such as the Southern Ocean,
uninhabitable for corals. Since corals provide
habitat and food sources for many other denizens
of the deep, this could have a profound effect on
the marine food web.

Gretchen Hofmann of the University of California,
Santa Barbara, has brought some experimental
evidence to bear on the question. She is
investigating the effects of changing acidity and
temperature in the sea on a creature called the
purple sea urchin. This animal is a scientists’
favourite for embryological experiments, and has
thus had its genome sequenced (in part by Dr
Hofmann, as it happens), so it is well
understood. Dr Hofmann’s work suggests that a
combination of heat and acidity is more deadly
than either alone. When she and her team
reproduced the conditions which are predicted to
prevail in 2100 if carbon-dioxide emissions are
not curbed, they found that the genes of larval
sea urchins had to work up to three times harder
than normal to form the animals’ skeletons. On
top of that, those skeletons were often deformed.

No corals, no sea urchins and no
who-knows-what-else would be bad news indeed for
the sea. Those who blithely factor oceanic uptake
into the equations of what people can get away
with when it comes to greenhouse-gas pollution
should, perhaps, have second thoughts.
Copyright © 2008 The Economist Newspaper and The
Economist Group. All rights reserved.

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