The Carbon Cycle
Carbon is an essential part of life on Earth. About half the dry weight of most living organisms is
carbon. It plays an important role in the structure, biochemistry, and nutrition of all living cells.
The process by which carbon moves through an ecosystem is called the carbon cycle. This cycle
is usually thought of as including the following major reservoirs of carbon interconnected by
pathways of exchange:
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the atmosphere
the landmass of Earth (including the interior)
all of Earth’s water
all living organisms
Carbon exists in the Earth's atmosphere primarily as the gas carbon dioxide. Although it is a
small percentage of the atmosphere (0.04%), it plays a vital role in supporting life. The
atmosphere contains about 750 billion tons of carbon.
Earth’s landmass is the largest reservoir of carbon. The sedimentary rocks that make up Earth’s
crust contain about 80 quadrillion tons of carbon. Most of this carbon is inaccessible, as it will
remain trapped within the rock for millions of years. The soil near the surface of Earth, however,
contains about 1500 billion tons of carbon that moves more regularly through the carbon cycle.
The oceans contain around 36 trillion tons of carbon, mostly in the form of bicarbonate ion (over
90%, with most of the remainder being carbonate ion).
Living biomass contains about 575 billion tons of carbon, most of which is contained in living
vegetation. Most of this carbon is in the form of organic compounds (such as carbohydrates –
compounds consisting of carbon, hydrogen, and oxygen).
Carbon is cycled through the biosphere primarily by two processes: photosynthesis and cellular
respiration. Photosynthesis is a process by which plants use the Sun’s energy to convert carbon
dioxide into glucose, releasing oxygen in the process.
carbon dioxide + water + energy → glucose + oxygen gas
This process removes carbon (in the form of carbon dioxide) from the atmosphere and stores it in
biomass (plants) in the form of glucose. The carbon in glucose then moves through the biosphere
by way of the food chain.
Cellular respiration is essentially the reverse of photosynthesis: it combines glucose and
oxygen and converts it into carbon dioxide, water, and energy.
glucose + oxygen gas → carbon dioxide + water + energy
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The energy is used by organisms for growth, movement, reproduction, etc. The carbon dioxide is
returned to the atmosphere, and the cycle continues.
Carbon is also returned to the atmosphere in several other ways:
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the decay of animal and plant matter releases methane gas ( CH 4 )
burning fossil fuels (gasoline, coal, etc.) releases carbon dioxide gas
as sea water warms, it releases dissolved carbon dioxide gas
volcanic eruptions release carbon dioxide gas
The diagram below illustrates the carbon cycle.
Under normal conditions, there is a balance to the exchanges of carbon between the various
reservoirs. This balance means that the amount of carbon in each reservoir remains
approximately the same all the time, since the amount being removed and the amount being
returned are equivalent.
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Disturbing the Carbon Cycle
When the carbon cycle is disturbed, it results in a change in either (a) the amount of carbon being
removed from a particular reservoir, or (b) the amount of carbon being returned to the same
reservoir. Because of this change, the amount of carbon contained in the reservoir begins to
either increase or decrease, both of which can have grave consequences for the biosphere.
The most well known example of the disruption of the carbon cycle is global warming. Global
warming is the increase in the average temperature of Earth's near-surface air and oceans since
the mid-20th century and its projected continuation. Most of the observed temperature increase
since the middle of the 20th century has been caused by increasing concentrations of greenhouse
gases (such as carbon dioxide).
It is widely believed that the increase in carbon dioxide levels in the atmosphere is a result of
increased burning of fossil fuels by humans, resulting in a disruption of the carbon cycle.
The carbon cycle can be disturbed by both natural events and human activities.
Natural Events
Forest fires are an example of a natural event that may disrupt the carbon cycle. The combustion
or burning of plant material (wood) releases large amounts of carbon dioxide into the atmosphere.
This creates an increase in the amount of carbon being returned to the atmosphere, resulting in a
net increase in the amount of carbon in the atmosphere.
Volcanoes can also disrupt the carbon cycle. Volcanic activity breaks down rocks containing
carbon and releases carbon dioxide into the atmosphere. The ash generated from a volcano can
also block sunlight from reaching Earth’s surface, resulting in a decrease in the amount of
photosynthesis taking place. These two effects combined will both decrease the amount of
carbon being removed from the atmosphere and increase the amount being returned, resulting in
a net increase in the amount of carbon in the atmosphere.
Human Activities
Deforestation, or the cutting down of forests, is a human activity that impacts the carbon cycle.
The removal of a large number of trees decreases the amount of plants available for
photosynthesis. This in turn decreases the amount of carbon being removed from the atmosphere,
resulting in a net increase in the amount of carbon in the atmosphere.
The human activity that is believed to have the biggest impact on the carbon cycle is the burning
of fossil fuels. When fossil fuels like gasoline, natural gas, coal, or oil are burned they release
carbon dioxide into the atmosphere. In industrialized countries, the amount of fossil fuels burned
is considerable. This results in a large increase in the amount of carbon being returned to the
atmosphere, and a net increase in the amount of carbon in the atmosphere.
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Worksheet
1. How are photosynthesis and cellular respiration similar?
2. How are photosynthesis and cellular respiration different?
3. How might the carbon in a living plant become part of the body of a living bear?
4. Explain the importance of decomposers in the carbon cycle.
5. What might happen if living things did not die?
6. What three elements are carbohydrates made from?
7. Explain how deforestation, fire, and combustion of fossil fuels disrupt the balance between
photosynthesis and cellular respiration.
8. The carbon dioxide levels in Biosphere II decreased each day and increased each night.
Suggest an explanation for this pattern. Why do carbon dioxide levels not fluctuate in the
same way in Earth’s atmosphere?
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9. What are some of the obstacles that stand in the way of reducing human disruption of the
carbon cycle?
10. Some Canadians are not concerned about global warming. How would you respond to
someone who thinks global warming would be great because it would make our winters
shorter and not as cold?
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