Essay code: B10/C4
Carbon Dioxide, a Simple but Miracle Molecule
Carbon dioxide is an essential constituent of the Earth atmosphere. If oxygen is
considered to be the most important element required for the sustainability of life, then its
carbonic form (carbon dioxide) is considered to be a miracle molecule to the biosphere at the
planetary level, despite its tiny percent composition to the atmosphere (0.04 percent) (1).
Regarding carbon dioxide, most people are concerned about its current level in the atmosphere
due to the current scare over global warming. However, Earth would not be able to maintain
the appropriate temperatures to develop and support life without carbon dioxide. In history,
carbon dioxide played different roles in regulating the Earth’s climate though different
methods. Also, photosynthesis requires carbon dioxide in order to convert light energy into
useful chemical energy. Therefore, it is important to learn about the contributions atmospheric
carbon dioxide provides in order to have better understandings about the Earth’s climate and
how to protect it.
On June 11th, 1754, a Scottish chemist named Joseph Black accidentally discovered
carbon dioxide while working with limestone (6). He observed a gas, which he dubbed “fixed
air”, and it turned out to be carbon dioxide. Carbon dioxide is a naturally occurring molecule in
the C2v point group, so one carbon is covalently bonded linearly with two oxygen atoms (as
shown in the picture below (2)). This molecule has polar property with zero dipole moment.
Although the structure of carbon dioxide may be simple, but the gas contributes
significantly to the Earth’s circulation. First of all, carbon dioxide dissolves minerals on Earth by
the following reactions. Carbon dioxide gas dissolves in water to produce carbonic acid.
H2O + CO2  H2CO3
This weak carbonic acid can lead to two different outcomes. In the first path, it deprotonates
itself to provide free H+ ions, which would reduce the pH of the environment. The newly acidic
environment allows many subsequent reactions to occur.
H2CO3  H+ + HCO3Interestingly, the carbonic acid can dissolve minerals on the Earth’s crust. For instance, it can
reacts with calcium carbonate in limestone to break it down into bicarbonates and calcium
cations. This reaction helped reduce the amount of carbon dioxide in the climate of the early
Earth.
CaCO3 + H2CO3 -> Ca2+ + 2 HCO3Secondly, through the redox reaction of carbon dioxide and water, a new molecule was
introduced to Earth’s early atmosphere. This process is photosynthesis and it allowed complex
organisms to develop and live on the planet. About two billion years ago, Cyanobacteria
appeared on Earth and started to do photosynthesis. This organism took the abundant carbon
dioxide in the atmosphere and reacted it with water in the presence of light to produce oxygen.
2nH2O + 2nCO2 + light -> 2(CH2O)n + 2nO2
Another important reaction is methanogenesis, a process that produces methane and energy
by reducing carbon dioxide with hydrogen gas (7). Methane is one the most significant
greenhouse gases because it provides protection for the Earth from the sun with an ultraviolet
shield and also regulates the earth climate (7).
CO2 + 4H2 -> CH4 + 2H2O
Carbon dioxide is a very important greenhouse gas of the Earth atmosphere. These
greenhouse gases act as a bubble shield to protect the Earth from the incoming solar radiation
(5).
The greenhouse gases, like carbon dioxide, methane, and water vapor, absorb the
ultraviolet radiation in rays of sunlight. However, some of solar radiations are still able to pass
through the greenhouse gases shield and warm up the Earth surface (5). The surface of the
Earth then continually emits infrared radiation to the atmosphere to produce a cooling effect.
Some of the infrared radiation trapped by the greenhouse gases is then reflected or re-emitted
back to Earth, thus resulting in a warming effect. This helps keep the certain parts of the Earth’s
surface warm at times when they are not exposed to incoming solar radiation (5).
Understanding the relationship between carbon dioxide and the ocean is critical to
having a complete understanding of the carbon cycle. Phytoplankton photosynthesize in the
ocean to react with aqueous carbon dioxide near the surface of the ocean with water molecules
in the presence of sunlight. This process removes carbon dioxide in the atmosphere by
converting it into organic carbons and stores them in the deep ocean. This is able to take place
because the maximum density of sea water is at about 4 degree Celsius. Hence, water at this
temperature will sink underneath the warmer water and take carbon dioxide with it. Due to
these processes, the carbon dioxide gets stored in deep ocean until upwelling occurs, which will
then resurfaces carbon dioxide from the deeper waters. The carbon dioxide in the atmosphere
also interacts with the lithosphere. The silicate weathering occurs when carbon dioxide in the
atmosphere is dissolved in rain water, producing carbonic acid with the same mechanism
showed above. Carbonic acid will once again interact with limestone on rocks to produce
calcium and bicarbonate ions that run off into river and then back to the ocean. In the ocean,
the calcium and bicarbonate ions will be further precipitated by sea organisms to create shells
and skeletons. Therefore, carbon dioxide will always be maintained and circulated within the
atmosphere by the process of this carbon cycle.
The role of carbon dioxide in the atmosphere can be better understood by studying
those of Venus and Mars. Venus and Mars began with the having of water, like Earth, but these
three planets evolved divergently, resulting in the three different environments we know today.
Venus is about the size of the Earth, but it is closer to the Sun (4). Therefore, the temperature
on Venus is extremely higher than Earth (462 °C; 863 °F). Consequently, any water present on
Venus immediately evaporated so the water vapor was concentrated in the atmosphere (4).
The temperature on Venus continued to increase because the heat on Venus could not escape.
As a result, all of the water on Venus disappeared. Carbon dioxide in the atmosphere of Venus
does not have water to dissolve in, so the amount of carbon dioxide in Venus atmosphere
rocketed (4). Venus has too much carbon dioxide, no water, and the temperature is too hot to
be able to support life.
The size of Mars is roughly 1/3 of the Earth and further away from the Sun than the
Earth. Therefore, the average temperature of Mars is expectedly lower than that of Earth (3).
Also, levels of volcanic activity continue to decrease, which leads to less carbon dioxide being
supplied into the atmosphere. The temperature on Mars continues to decrease until the core of
Mars loses its ability to create magnetic field (3). Without the magnetic field, Mars loses most
of its atmosphere to the solar wind and this consequently leads to thinner atmosphere on
Mars. The atmosphere on Mars extends further than that on Earth because of the low
gravitational force of Mars. Mars has a thinner atmosphere than Earth (3). Due to the overall
low temperatures, carbon dioxide is frozen and Mars is unable to sustain life.
Unlike Venus and Mars, the Earth has just the right amount of carbon dioxide and the
temperature is appropriate to sustain life. In the early history of Earth, dense iron sank to the
core of the Earth with liquid outer core, which was known as the Iron Catastrophe. The
convicting of the liquid core had created a dynamo that generated the Earth’s magnetic field.
The magnetic field protected the Earth atmosphere from being blown away by the solar wind.
The carbon dioxide gas was formed from volcano eruptions and was emitted into the
atmosphere. Then, it dissolved in the water to dissolve minerals on the Earth’s crust. This
process helped reduce the amount of carbon dioxide gas in the atmosphere and kept the
temperature of the Earth from getting too hot. The melted iron core led to volcano eruption
that emits the sunken carbon dioxide gas which had been trapped in the mineral phases back
into the atmosphere and helped keep the Earth from getting too cold.
For all aforementioned reasons, carbon dioxide can be considered to be one of important
molecules on Earth. People need to fully understand the roles of carbon dioxide in regulating
the atmosphere. Due to the recent widespread exposure to the concept of global warming,
many people have had bad impressions about this molecule because they do not fully
understand its contribution to help sustain life on Earth. If the all carbon dioxide was removed
from the atmosphere, the overall temperature of Earth would drop and may be able to
compensate for the effects of global warming. However, people would be challenged with a
new problem: the temperature will be too low to survive. On the other hand, if humans keep
increasing the rate at which carbon dioxide is emitted into the atmosphere, the overall
temperature of this planet will continue to increase. Earth may face the same fate as Venus,
where all the water would be in the gaseous. Thus, humans would not be able to survive due to
the lack of water. Therefore, it is crucial for us as a society to minimize the amount of carbon
dioxide produced, but also maintain the proper amount of this gas so that we do not have to
deal with another Ice Age.
REFERENCES
(1) "Causes of Climate Change." Climate Science Investigations South Florida -. Nasa, n.d.
Web. 13 Feb. 2014.
(2) Craig H. 1957. Isotopic standards for carbon and oxygen and correction factors for massspectrometric analysis of carbon dioxide. Geochim. Cosmochim. Acta. 12: 133-49
(3) Coffey, Jerry. "Atmosphere of Mars." Universe Today. N.p., 19 Dec. 2008. Web. 13 Feb.
2014.
(4) Coffey, Jerry. "Atmosphere of Venus." Universe Today. N.p., 7 May 2008. Web. 13 Feb.
2014.
(5) "Global Climate Change." Forest Forever. N.p., n.d. Web. 13 Feb. 2014.
(6) "On This Day – June 11 : The Scottish Chemist Joseph Black Discovered Carbon Dioxide
on This Day in 1754- Learn Chemistry." Learn Chemistry Enhancing Learning and
Teaching. Royal Society of Chemistry 2014, n.d. Web. 13 Feb. 2014.
(7) Thauer, R. K., “Biochemistry of Methanogenesis: a Tribute to Marjory Stephenson”,
Microbiology, 1998, Volume 144, pages 2377-2406