Page 447 - Environment: The Science Behind the Stories

the SCIeNCe behINd the StORy

Will Climate Change This carbonic acid molecule soon
Rob Us of Coral dissociates 3 into a bicarbonate ion
–
(HCO ) and a proton, or hydrogen ion:
Reefs? 3 H CO S HCO 1 H 1
2
2 3 3
The oceans have helped delay the The free proton then combines
advance of global climate change by with a carbonate ion (CO ) 4 to form
2–
3
soaking up a portion of the excess a second bicarbonate ion:
carbon dioxide that we pump into 1 22 2
the atmosphere when we burn fossil H 1 CO 3 S HCO 3
fuels and clear forests. But when we This is bad news for marine organ-
alter a flux in a biogeochemical cycle, isms such as corals, which depend on
there are usually consequences. What carbonate ions in order to form the cal-
consequences might there be for this Tiny corals, the builders of reefs cium carbonate (CaCO ) that comprises
3
increase in the flux of carbon from the their hard shells. As the availability of
atmosphere to the oceans? Hundreds face with water (H O) to form carbonic carbonate ions in the water declines,
of scientists in recent years have been acid (H CO ) 2 . This is what happens it gets more difficult to build calcium
2
working to find out. in soft drinks when carbon dioxide is carbonate shells by the process known
2
3
Let’s start with the chemistry added to give them fizz: as calcification. In fact, as carbonic
(Figure 1). Carbon dioxide (CO ) from acid and bicarbonate ions become
2
the air 1 can react on the ocean’s sur- CO 1 H O S H CO more common in the water, the calcium
2 2 2 3

1 Human activity increases CO
carbon dioxide in air 2

2 Carbon dioxide reacts with ocean
2
2
water, forming carbonic acid CO + H O H CO 3
2

3 Carbonic acid dissociates into – +
2
3
3
a bicarbonate ion and a proton H CO HCO + H
4 Proton combines with carbonate + 2– –
ion to form another bicarbonate ion H + CO 3 HCO 3

2+
5 As carbonate ions are pulled into solution, CaCO Ca + CO 3 2–
3
acidity dissolves calcium carbonate in coral reefs (coral)

Figure 1 excess carbon dioxide from the atmosphere changes the ocean’s chemistry. Carbon dioxide reacts
with seawater to create carbonic acid, eventually diminishing the number of carbonate ions available for corals
to use to build their shells. Adapted from Hoegh-Guldberg, O., et al., 2007. Coral reefs under rapid climate change and ocean
acidification. Science 318: 1737–1742. Fig 1a. Reprinted with permission from AAAS.
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