Human Influences on the Global Carbon Cycle
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Human Influences on the Global Carbon Cycle
Processes of Carbon Flow in the Human Realm
Humans have exerted an enormous influence on the global carbon cycle, largely through
deforestation and fossil fuel burning. In this section, we explore how these processes have
led to changes in the dynamics of carbon in the atmosphere.
Fossil Fuel Burning
Another pathway for carbon to move from the sedimentary rock reservoir to the
atmosphere is through the burning of fossil fuels by humans. Fossil fuels include petroleum,
natural gas, and coal, all of which are produced by slow transformation of organic carbon
deposited in sedimentary rocks — essentially the fossilized remains of marine and land
plants. In general, this transformation takes many millions of years; most of the oil and gas
we now extract from sedimentary rocks is on the order of 70-100 million years old. New
fossil fuels take a very long time to form, and we are using them up much, much faster than
they are being formed, meaning that if we keep using fossil fuels at the rate we are today,
we will run out! This run out date depends on new discoveries and our ability to extract
fuels more efficiently by processes like fracking, but we will be close to running out late this
century.
These fossil fuels are primarily composed of carbon and hydrogen. For instance, methane,
the main component of natural gas, has a chemical formula of CH 4; petroleum is a more
complex compound, but it, too, involves carbon and hydrogen (along with nitrogen, sulfur,
and other impurities). The combustion of fossils fuels involves the use of oxygen and the
release of carbon dioxide and water, as represented by the following description of burning
natural gas:
CH4 + 2O2 => CO2 + 2H2O
Beginning with the onset of the industrial revolution at the end of the last century, humans
have been burning increasing quantities of fossil fuels as our primary energy source.
Pollution (including greenhouse gas emission from a factory in China)
, Credit: Wikipedia(link is external) / CC BY-SA 3.0(link is external) (Creative Commons)
Smog hanging over Los Angeles.
Credit: Wikipedia(link is external) / CC BY-SA 3.0(link is external) (Creative Commons)
As a consequence, the amount of CO2 emitted from this burning has undergone an
exponential rise that follows the exponential rise in the human population. The magnitude
of this flow is currently about 9 Gt C/yr. This number also includes the CO 2 generated in the
production of cement, where limestone is burned, liberating CO2.
Increase in the emission of carbon from fossil fuels since 1800
Credit: Wikipedia(link is external) / CC BY-SA 3.0(link is external) (Creative Commons)
As you can see in the graph above, this flow has changed considerably over time, as human
population has increased and as our economies have become more industrialized with a big
thirst for the energy provided from the combustion of fossil fuels. The model we will work
within the lab activity for this module includes this history, beginning in 1880 and going up
to 2010; beyond 2010 is the realm of future projections, which can be altered to explore the
consequences of choices we might make or not make in the future. Part of the new energy
, economy that is key to our future is the use of so-called renewable energy sources,
including wind, solar and geothermal energy, that emit little or no carbon.
Land-Use Changes - Forest Burning and Soil Disruption
The other form of human alteration of the global carbon cycle is through forest cutting and
burning and the disruption of soils associated with agriculture. When deforestation occurs,
most of the plant matter is either left to decompose on the ground, or it is burned, the latter
being the more common occurrence. This process reduces the size (the mass) of the land
biota reservoir, and the burning adds carbon to the atmosphere. Land-use changes other
than deforestation can also add carbon to the atmosphere. Agriculture, for instance,
involves tilling the soil, which leads to very rapid decomposition and oxidation of soil organic
matter. This means that in terms of a system, we are talking about two separate flows here
— one draining the land biota reservoir, the other draining the soil reservoir; both flows
transfer carbon to the atmosphere. Current estimates place the total addition to the
atmosphere from forest burning and soil disruption at around 2-3 Gt C/yr; estimates divide
this into 70% to 50% forest burning, with soil disruption making up the remainder.
Deforestation is a particular problem in the Amazon as we will see in Module 9.
A carbon cycle disrupted by human activities
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All living beings are built from carbon atoms. These are extracted from atmospheric CO2 by
plants, algae and certain bacteria, using solar energy: this is photosynthesis. The respiration
and decomposition of living beings release this CO2 back into the atmosphere. In addition to
this short life cycle, there is a slow geological cycle that stores carbon in the form of
limestone and fossil hydrocarbons. Limestone comes from the shells of marine organisms
while hydrocarbons are formed by burial of organic sediments. The combustion of fossil
resources currently represents a short circuit from this slow cycle to the short cycle that
largely dominates natural regeneration processes. This leads to a rapid accumulation of
CO2 in the atmosphere, causing global warming, and ocean acidification that can disrupt
marine life.
