The Role of Carbon Dioxide
Introduction
We have already seen that carbon dioxide plays an important role in producing the greenhouse
effect, which makes Earth a comfortable place to live, and that it does so by interfering with the return
of energy to space from the Earth's surface by infrared radiation. As with most things in life that are
necessary, useful and pleasant, we can have too much of a good thing. Carbon dioxide is now 30 percent
higher than it was 150 years ago. Carbon dioxide has never been as high as it is now for the last 400,000
years, as far as we know from the results of drilling polar ice in Antarctica. Also, the climate has never
been as unusually warm as it has been in the last 20 years and, as far as we know, for even the last
several thousand years. Only extreme skeptics, or the ignorant, would seriously question a connection
between these two facts.
CO2 in the Oceans
What exactly happens to the carbon dioxide introduced by burning wood, coal and
hydrocarbons? This question is easy to ask but actually difficult to answer. At some point, some fifty
years ago, it was thought that the buildup of carbon dioxide in the atmosphere would be rather slow
because there is 50 times more carbon dioxide in the ocean than in the air. Scientists expected that the
ocean and the atmosphere would share the newly introduced carbon dioxide, and that by far the
greater proportion would be accommodated by the ocean, with its much larger capacity. Although this is
not an unreasonable expectation, the fact to keep in mind is that carbon dioxide is first introduced into
the atmosphere and from there it has to be passed along to the ocean. Carbon dioxide can only enter
the ocean at its surface since the deep ocean is not immediately accessible. To fully reach the deep
ocean takes a molecule of carbon dioxide about a thousand years. On the other hand, the uppermost
ocean layer, called the mixed layer, can be accessed on a 10-year time-scale.
These facts were first established by measuring the radiocarbon content of the ocean, from
surface waters to great depths, by Hans Suess (1909-1989) at UCSD. Radiocarbon (14C) is made in the
atmosphere, from bombardment of nitrogen atoms with cosmic rays. This radiocarbon reacts to make
up a certain percentage of the carbon dioxide in our atmosphere. Half of a given number of radiocarbon
atoms decay back to nitrogen within 6000 years. Radiocarbon in the ocean can only come from the
atmosphere. Since it constantly disappears by decay, the amount of radiocarbon within the ocean
depends on how fast the carbon dioxide from the atmosphere penetrates into the ocean and
replenishes the store of radiocarbon there.
The fact that much of the ocean is not readily available to take up the additional carbon dioxide
introduced through human activities raised a warning flag in the scientific community. However,
another factor of equal importance emerged when considering how carbon dioxide enters the ocean.
Seawater is somewhat reluctant to take on more carbon dioxide than it already has. The reason is that
carbon dioxide reacts with water to make carbonic acid. In turn, the presence of this acid tends to
discourage acceptance of additional carbon dioxide. This back-pressure effect has the consequence that
the ability of the water to hold carbon dioxide increases by only about 1% for each 10% increase of the
gas in the atmosphere. This ratio is widely known as the Revelle buffer factor, after Dr. Roger Revelle
(1909-1991) who drew attention to this complication in the mid-1950.
There is one way to decrease this carbon dioxide back-pressure and that is to change the ocean’s
chemistry by dissolving calcium carbonate. Carbonate is plentiful in the shells of marine organisms,
including coral reef materials. This has led a number of scientists to raise the yet unanswered question
of whether the continued addition of carbon dioxide to the atmosphere (which adds acid to the upper
ocean) will eventually result in damage to coral reefs.
Besides radiocarbon (14C), the distribution of the stable isotopes of carbon atoms (13C and 12C)
has been very useful for investigating the rate at which carbon dioxide invades the ocean. The materials
burned to produce energy (such wood, coal, and hydrocarbons) have more of the carbon-12 isotope
than the CO2 normally dissolved in the ocean. Thus, a change in the ratios of carbon these two stable
isotopes (that is, 13C /12C ) in seawater contains clues to the progressive invasion of the industrial
carbon dioxide from the atmosphere into the upper ocean. The isotopic change of seawater is recorded
in the isotopic composition of the skeletons of marine organisms, seashells, corals and certain sponges.
We will learn more about the way carbon isotopes have helped unravel climate change in later lessons.
(See the Glossary for more information on Stable Isotopes and 14C.)