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THE CARBON CYCLE
Summary
All life on Earth is based upon carbon. The carbon cycle describes a set of
processes governing the movement of carbon elements or compounds
through living organisms and the environment and how carbon is stored
and released in various ways. The cycle can be described in terms of
stores of carbon and the processes that govern exchanges between them.
The carbon cycle underpins all life on earth and is important in
understanding the causes and effects of climate change.
1. The natural world
Principles of environmental systems
Ecological systems
Water and the hydrologic cycle
The carbon cycle
Climate and atmospheric systems
Relevance to business
Biodiversity
The carbon cycle is vital to all life on earth and it is important to understand how
it works and what happens when it is modified. Human activities impact upon the
carbon cycle with a variety of far-reaching implications for human populations
and economies. An understanding of the underlying principles of carbon
management is increasingly required when considering action to combat climate
change.
Carbon
Carbon is the fourth most abundant element on Earth and present in all living
organisms. The arrangement of electrons on the carbon atom means that it
easily forms covalent bonds (sharing electrons between neighbouring atoms)
with other carbon atoms and those of other elements, particularly hydrogen,
nitrogen, oxygen and phosphorus. Carbon can bond to itself, forming molecules
in long chains, branches and rings. The complexity of these molecules helps
form the basis of life. On average, living organisms include about 18 per cent
carbon – around one hundred times greater than its concentration in the Earth.
The total quantity of carbon on Earth is fixed and it is cycled and recycled
through a series of geological and biological (collectively biogeochemical)
exchanges known as the carbon cycle. This cycle is exceptionally complex and
still only partially understood, but is becoming clear that human activities are
impacting upon and altering the balance of the free carbon budget.
Carbon stores
Carbon resides in a number of ‘stores’ (Table 1) and exchanges or flows occur
between them. They can be considered as sinks when they sequestrate carbon
or sources when they provide it to another store.
Carbonate rocks: carbon is present as CACO3 (e.g. limestone) formed by
sedimentary geological processes from the shells of dead marine animals that
fixed bicarbonate (from seawater) with calcium to produce calcium carbonate
(CACO3).
Kerogens and fossil stores: carbon is stored as geochemical carbonates and
organic compounds including coal, oil, oil bearing shales (kerogens) and natural
gas formed by geological processes from organic deposits.
Terrestrial biosphere: carbon is present in biomass (living plants and animals)
and dead organic matter, including soil humus and peat.
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Atmosphere: carbon exists in the atmosphere as carbon dioxide (CO2).
Oceans: carbon arises primarily from atmospheric CO2, entering water by
-2
diffusion as dissolved CO2, or converted into carbonate (CO3 ) or bicarbonate
(HCO3 ).
These stores exchange carbon with each other through processes which are not
yet fully understood and are difficult to measure. The timescales of these
exchanges vary significantly. Carbon in rocks, kerogens and fossil fuels is fixed
almost permanently (in human timescales) unless fossil fuels are burnt or
carbonates processed. Carbon in the terrestrial biosphere, atmosphere and
oceans is more freely available and cycles more actively.
Table 1. Major stores of carbon
Carbon store
Size/gigatonnes carbon
Rock carbonates
<60,000,000
Kerogens
15,000,000
Ocean
38,400
Coal
3,510
Soil
1,500
Atmosphere
720
Living biomass
600
Peat
250
Oil
230
Natural gas
140
Source: Smithson et al (2002)
Processes and flows
All carbon stores are interlinked and ultimately interdependent. A number of
processes and flows govern their exchange.
Photosynthesis is the process by which plants convert solar energy into food to
enable growth. Photosynthesis is conducted primarily by photoautotrophs like
plants and algae; organisms capable of synthesizing organic molecules from
carbon dioxide using sunlight as their energy source. Carbon dioxide and
hydrogen from water are combined to form carbohydrates, mainly the sugar
glucose. This primary production forms the basis for food chains supporting most
life on earth and is the route for bringing carbon into the biosphere from the
atmosphere. In the oceans, marine biota use dissolved CO2 for photosynthesis.
As the atmospheric concentration of CO2 rises the biochemical ability of plants to
fix carbon deceases thereby lowering their capacity to act as a carbon sink.
As part of primary production, plants absorb CO2 from the atmosphere during
photosynthesis and release CO2 back into the atmosphere during respiration.
Respiration is the biological equivalent of combustion: it breaks hydrocarbons
and carbohydrates into carbon dioxide and water thereby releasing energy held
in the sugars for use in metabolism. The fluxes of carbon produced by
photosynthesis and respiration are around 1,000 times greater than exchanges
from the geological cycle.
Combustion occurs through natural wildfires that oxidize organic matter,
releasing CO2, and through anthropogenic activity, mainly the burning of fossil
fuels to release energy. Burning coal, oil and gas releases carbon previously
stored in the geosphere into the atmosphere.
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Carbon is also returned to the atmosphere through decaying fungi and bacteria.
These produce CO2 if oxygen is present and methane (CH4) if it is not. Both
these gases are significant in the global radiation balance (see R 1.5 Climate
and atmospheric systems).
Figure 1. Carbon flows
Source: NASA Earth Science Enterprise. GT = Gigagtonnes (1 billion tonnes)
http://earthobservatory.nasa.gov/Library/CarbonCycle/carbon_cycle4.html
Carbon can transfer from geological sinks through volcanic activity, normally in
dramatic but sporadic events and through the weathering of carbon-rich rocks in
slow but continuous processes. The biological fixation of carbon, where
photosynthesis exceeds respiration, leads to the accumulation of organic matter
and supplies the geological processes of coal and oil formation. Through these
processes carbon is fixed in the geological store, resulting in a net removal of
carbon from the atmosphere. However, these processes take millions of years
and hence the current concern over the accelerated changes to the carbon
budget resulting from human activities.
The atmosphere stores about 750 gigatonnes of carbon, but this level is rising
due to human activities. The annual receipt of carbon from fossil fuel combustion
is thought to be about 5.5 gigatonnes and land use changes such as
deforestation are thought to contribute 1.5 gigatonnes comprising an annual
contribution of c. 7 gigatonnes. Around 2 gigatonnes of carbon diffuse into the
oceans as dissolved CO2 or as carbonates. Measurements suggest that at least
3 gigatonnes remain in the atmosphere as a net contribution, but nearly 2
gigatonnes cannot be readily accounted for. It is unclear what is happening to
the ‘missing’ 2 gigatonnes annually but is most likely being carried within the
biosphere perhaps though increased plant and microbial activity. There is much
uncertainty in this area, it is unknown whether this extra carbon can or will be
absorbed in the future or how it is being stored. This lack of knowledge adds
greatly to the uncertainty of planning for and attempting to mitigate climate
change.
Over the past century, the average carbon dioxide content of the atmosphere
has risen from 290 to 340 ppm (parts per million) or higher, a rise of c.17 per
cent. About 65 per cent of the additional carbon dioxide in the atmosphere
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comes from burning fossil fuel (and cement production); the other 35 per cent is
derived from deforestation and the conversion of natural ecosystems into
agricultural systems that results on the loss of organic content from soil. Natural
ecosystems can store from 20-100 times more carbon than agricultural systems
because of their more natural, complex and diverse structures. Thus, activities
such as deforestation have both short and long-term impacts on carbon budgets.
Forest clearance, particularly if burning is used, can result in a short term release
of carbon dioxide that was fixed in the biomass. If the land is transferred to
agricultural production the net capacity to fix carbon in the future is also
diminished. In system terms this is an example of a positive feedback loop where
change results in reinforcing the direction and strength of change (see R 1:1
Principles of environmental systems).
Increasing levels of carbon dioxide in the oceans are also of concern to
scientists. This can lead to increased acidity and it is thought that may impact
upon the productivity of phytoplankton and thus have ‘upstream’ effects for
carbon fixing and primary production in the oceans and thereby potentially
serious implications for marine ecosystems. For example, increased acidity
together with increased average temperatures are thought to be partly
responsible for the recent dramatic worldwide decline in corals.
Climate change
The concentration of carbon dioxide in the atmosphere is recorded in a number
of locations but the longest continuously recorded data come from the Mauna
Loa station in Hawaii (Figure 2). The Mauna Loa record shows a 19.4 per cent
increase in the mean annual concentration, from 316ppmv (parts per million by
volume, dry air) in 1959 to 377 ppmv (parts per million per year) in 2004. Carbon
dioxide is important in the global radiation balance because it acts as a
greenhouse gas.
Figure 2. C02 concentrations monitored at Mauna Loa observatory 1958 – 2004
CO2 concentration PPM
380
370
360
350
340
330
320
310
1958 1962 1966 1970 1974 1978 1982 1986 1990 1994 1998 2002
Year
Source: Keeling & Whorf (2005) http://cdiac.ornl.gov/ftp/trends/co2/maunaloa.co2
There are other greenhouse gases both natural and human-made but carbon
dioxide is produced in the largest quantities. Greenhouse gases allow solar
radiation to pass through them to the earth’s surface. Some is reflected back as
infrared radiation but greenhouse gases absorb this and retain heat within the
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atmosphere. While this phenomena is essential to maintain hospitable
temperatures on earth, the rise in concentration of greenhouse gases such as
carbon dioxide is changing the overall atmospheric equilibrium and leading to
overall warming and impacts upon the climate. Climate change is difficult to
predict but most scientists agree that warming will result in sea-level rises, more
climatic variability leading to more frequent and severe weather events and
impacts such as drought, flooding and decreased food supply security. Low-lying
land is likely to be inundated by sea-level rises (see R 5:1 Climate Change).
Carbon sequestration and carbon sinks
The concept of a carbon sinks has emerged as one of the ways of managing the
global carbon balance and mitigating climate change. The concept of stores and
sinks is somewhat variable and the terms are used interchangeably by some
commentators. Sinks generally act to retain or sequestrate carbon and sources
store carbon and emit it under certain circumstances. Stores can act as both
sinks and sources.
Carbon sinks are based upon the ability of plants, in particular trees, and soil to
tie-up and temporarily store carbon. This process is scientifically valid but its use
for carbon management as carbon sink ‘credits’ ignores other related issues. The
Kyoto Protocol allows the use of carbon sinks to offset carbon emissions from
fossil fuels. However, there is a fundamental imbalance in this equation because
carbon released from burning fossil fuels moves from a permanent store into a
dynamic (and accessible) role in the carbon cycle. The carbon offset within such
a carbon sink (i.e. a forest) is only temporarily sequestered and is still available
to the active carbon cycle in the biosphere. When the trees die and decompose
they will release their carbon and increase atmospheric concentrations.
There is three times the quantity of carbon in terrestrial biomass and soils than in
the atmosphere and perhaps ten times the quantity in fossil fuels compared to
the atmosphere. As the concentration of carbon dioxide in the atmosphere is
important for the global radiation balance and climate change, the net transfer of
carbon from these stores into the atmosphere has the potential to significantly
change the carbon balance of the atmosphere. Once carbon is active in the
atmospheric cycle it is difficult to lock it up again in less than geological time
spans. Temporary carbon sequestration is performed by forests but there are
limitations to the effectiveness of these to mitigate climate change for a number
of other reasons. Increased temperatures and quantities of dead organic matter
increase the rate of activity and hence respiration of soil microbes. Boreal
(northern temperate) forests are relatively secure carbon sinks but global
warming may cause a shift of their geographical location northwards. While at
first this may appear positive for carbon sequestration, there are likely to be other
knock-on effects. If they move north into areas that were previously tundra there
will be other effects such as reduction in ground albedo (the reflectiveness of the
earth). Tundra reflects a large quantity of insolation (solar radiation) whereas
forests absorb much incoming radiation - thereby retaining the Sun’s energy in
the form of heat.
Conclusion
The issues of carbon capture and release are complex and imperfectly
understood. However, it is clear that the continued transfer of carbon from longterm fossil stores to the atmosphere (and to a lesser extent oceans) is
significantly changing the planet’s carbon balance. The impact of these changes
is uncertain and hence controversial. The full extent of the impact upon
environmental processes and systems may not be evident until well into the
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future. Meanwhile, there is every reason for prudence. Business has a key role
to play in helping to minimize further disruption to the natural carbon cycle.
References
Keeling, C D and Whorf T P. 2005. Atmospheric CO2 records from sites in the
SIO air sampling network. In: Trends: A Compendium of Data on Global Change.
Carbon Dioxide Information Analysis Center, Oak Ridge National Laboratory, US
Department of Energy, Oak Ridge, Tenn., USA.
http://cdiac.ornl.gov/trends/co2/sio-mlo.html
Smithson, P, Addison, K and K Atkinson. 2002. Fundamentals of the Physical
Environment. Routledge, London, UK.
Sources of further information
Safe climate for business, Carbon Footprint calculator.
http://www.safeclimate.net/business/understanding/carboncycle.php
NASA Earth Observatory.
http://earthobservatory.nasa.gov/Library/CarbonCycle/carbon_cycle4.html
The opinions expressed in this reading are not necessarily those of WWF.
© 2008/9 All rights reserved WWF, World Wide Fund for Nature.
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