ChemMatters | FEBRUARY/MARCH 2018 11
ing the building warm. The greenhouse effect
is a natural process that warms the Earth’s
surface. When the sun’s energy reaches the
Earth’s atmosphere, some of it is reflected
back to space and the rest is absorbed by
the land and the oceans, heating the Earth.
The surface of the Earth loses energy back
into space by emitting infrared radiation, but
greenhouse gases absorb some of this infrared light, retaining the energy on Earth and so
keeping it warm enough to sustain life.
Human activities, such as burning fossil fuels, agriculture and land clearing, are
increasing the amount of greenhouse gases,
such as carbon dioxide, released into the
atmosphere. This is trapping extra heat, and
causing the Earth’s temperature to rise. These
rising temperatures cause environmental
changes, such as melting glaciers and ice
Excess heat is not limited to the air. Ocean
temperatures have also increased,
which affects coral reefs around the
world. When water becomes too warm,
corals can become stressed, and they
expel the symbiotic algae living in
their tissues. Without the algae, the coral
loses its major source of food, turns white or
very pale, and is more susceptible to disease.
Carbon dioxide’s effects on the ocean go
beyond warming, though. The ocean has dissolved a lot of carbon dioxide, which keeps
it out of the atmosphere. But as the concentration of carbon dioxide in the atmosphere
increases, even more is added to the ocean,
and it changes the water’s chemistry. As
shown in the ;rst equation above,
gaseous carbon dioxide dissolves in
ocean surface water to produce aqueous carbon dioxide. Carbon dioxide reacts
with the water molecules and releases hydrogen ions, increasing the pH.
Shifting ocean pH
As a result of shifting the carbonate-bicar-bonate equilibrium, the pH of the ocean is also
changing. Texas A&M University oceanographer Kathryn Shamberger says ocean pH has
dropped from 8. 2 to 8. 1. Continuing to release
carbon dioxide at the current rate will mean
the oceans will continue to increase in acidity.
The loss of reef habitats and damage to
1. CO2(g) ; CO2(aq)
species either directly or through disruptions
to the food web could also affect human
populations. In many areas of the world, coral
reefs block and deflect some of the energy of
ocean waves, meaning that water between the
shore and reef is calmer than it would be oth-
erwise. If coral structures weaken, they could
be damaged by storms, severely reducing this
calming effect. In addition, habitat loss for
the 25% of ocean species that call coral reefs
home could have far-reaching effects.
Carbon dioxide gas dissolves in water.
2. CO2(aq) + H2O ; HCO3-(aq) + H+(aq)
Dissolved carbon dioxide reacts with water and dissociates into
bicarbonate ion (HCO3-) and a hydrogen ion (H+). Because the ocean
is basic (pH around 8. 2), the H+ concentration is relatively low. As
a result, this equilibrium is shifted almost completely to the right.
Only 0.5% of CO2 is found in seawater as dissolved CO2. Most
(89.5% is in the form of bicarbonate ion). Adding more CO2 from the
atmosphere increases the amount of H+ present through this reaction and contributes to lowering the pH of the ocean.
3. HCO3–(aq) ; CO32–(aq) + H+(aq)
It is more difficult for the second H+ to split off from the bicarbonate
ion (it is not very acidic) and as a result only 10% of the dissolved
CO2 is CO32– (aq).
4. Ca2+(aq) + 2 HCO3–(aq) ; CaCO3(s) + CO2(aq) + H2O
Corals, plankton, and shellfish use calcium ions (Ca2+) and carbonate ions (CO32-) to build their calcium carbonate skeletons and
shells, a process known as calcification. Equation 4 shows the precipitation reaction of calcium ions and hydrogen carbonate, which
react in the ocean to form calcium carbonate. As the acidity of the
ocean increases, the H+ concentration increases, there is less HCO3-
and more CO2 (aq) available (see equation 2), and this equilibrium
shifts to the left. The minor carbonate species (in equation 3) can
also react with calcium ion to form calcium carbonate, but there is
also relatively less of it too with increasing H+. This makes it more
difficult for calcium carbonate to form, which means shells can’t
form as readily.
says ocean pH has
8. 2 to 8. 1
Bleaching in Kaneohe Bay in Hawaii
Oceans are still in
the basic pH range,
but they are becoming
more acidic (or less