An Introduction to the Global
Carbon Cycle
Carbon: the building block of life. You may have heard this
phrase, but have you fully considered what it really means? All
living things are made of elements, the most abundant of which are,
oxygen, carbon, hydrogen, nitrogen, calcium, and phosphorous. Of
these, carbon is the best at joining with other elements to form
compounds necessary for life, such as sugars, starches, fats, and
proteins. Together, all these forms of carbon account for
approximately half of the total dry mass of living things.
Carbon is also present in the Earth's atmosphere, soils, oceans,
and
crust. When viewing the Earth as a system, these components can
be referred to as carbon pools (sometimes also called stocks or
reservoirs) because they act as storage houses for large amounts of
carbon. Any movement of carbon between these reservoirs is called
a flux. In any integrated system, fluxes connect reservoirs
together to create cycles and feedbacks. An example of such a
cycle is seen in Figure 1 where, carbon in the atmosphere is used in
photosynthesis to create new plant material. On a global basis, this
processes transfers large amounts of carbon from one pool (the
atmosphere) to another (plants). Over time, these plants die and
decay, are harvested by humans, or are burned either for energy or in
wildfires. All of these processes are fluxes that can cycle
carbon among various pools within ecosystems and eventually releases it
back to the atmosphere. Viewing the Earth as a whole, individual
cycles like this are linked to others involving oceans, rocks, etc. on
a range of spatial and temporal scales to form an integrated global
carbon cycle (Figure 2).
On the shortest time scales, of seconds to minutes, plants take carbon
out of the atmosphere through photosynthesis and release it back
into the atmosphere via respiration. On longer time scales,
carbon from dead plant material can be incorporated into soils, where
it might reside for years, decades or centuries before being broken
down by soil microbes and released back to the atmosphere. On
still longer time scales, organic matter1 that became buried in deep
sediments (and protected from decay) was slowly transformed into
deposits of coal, oil and natural gas, the fossil fuels we use
today. When we burn these substances, carbon that has been stored
for millions of years is released once again to the atmosphere in the
form of carbon dioxide (CO2).
The carbon cycle has a large effect on the function and well being of
our planet. Globally, the carbon cycle plays a key role in regulating
the Earth’s climate by controlling the concentration of carbon dioxide
in the atmosphere. Carbon dioxide (CO2) is important because it
contributes to the greenhouse effect, in which heat generated from
sunlight at the Earth’s surface is trapped by certain gasses and
prevented from escaping through the atmosphere. The greenhouse
effect itself is a perfectly natural phenomenon and, without it, the
Earth would be a much colder place. But as is often the case, too
much of a good thing can have negative consequences, and a unnatural
buildup of greenhouse gasses can lead to a planet that gets unnaturally
hot.
In recent years CO2 has received much attention because its
concentration in the atmosphere has risen to approximately 30% above
natural background levels and will continue to rise into the near
future. Scientists have shown that this increase is a result of
human activities that have occurred over the last 150 years, including
the burning of fossil fuels and deforestation. Because CO2 is a
greenhouse gas, this increase is believed to be causing a rise in
global temperatures. This is the primary cause of climate change
and is the main reason for increasing interest in the carbon
cycle.
The Earth’s carbon reservoirs naturally act as both sources, adding
carbon to the atmosphere, and sinks, removing carbon from the
atmosphere. If all sources are equal to all sinks, the carbon
cycle can be said to be in balance and there is no change in the size
of the pools over time. Maintaining a steady amount of CO2 in the
atmosphere helps maintain stable average temperatures at the global
scale. However, because fossil fuel combustion and deforestation
have increased CO2 inputs to the atmosphere without matching increases
in the natural sinks that draw CO2 out of the atmosphere (oceans,
forests, etc.), these activities have caused the size of the
atmospheric carbon pool to increase. This is what has been
responsible for the present buildup of CO2 and is believed to cause the
observed trend of increasing global temperatures. How far will
CO2 levels rise in the future? The answer depends both on how
much CO2 humans continue to release and on the future amount of carbon
uptake and storage by the Earth's natural sinks and reservoirs.
In short, it depends on the carbon cycle.
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1. We often refer to carbon occurring in “organic” versus “inorganic”
forms. This is a simple way of grouping different forms of carbon
into biologically derived compounds (complex substances produced only
by the growth of living organisms) and mineral compounds that can be
formed in the absence of biological activity (but can sometimes be
formed with the assistance of living things, as in the case of sea
shells). Organic compounds includes such things as sugars, fats,
proteins and starches and are contained in both living organisms and
the material that remains after their death and partial decomposition
(including the organic matter in soils as well as the deposits of coal
and oils we refer to as fossil fuels). Note that complete
decomposition of organic matter results in a return to mineral forms,
often as CO2. Mineral forms of carbon include carbonates
contained in rock and seawater as well as CO2 itself.