Through the combustion of carbon-based fuels, humans actively participate in the global carbon cycle, which is the movement of carbon, in its many forms, between the biosphere, atmosphere, oceans, geosphere, and society.  Storage pools (sometimes called stocks or reservoirs) are the places that carbon is found. The movement of carbon from one pool to another is called a flow or flux of carbon. Carbon is found in six major storage pools; the atmosphere, biota, soils, oceans, sediments, and fossil fuels.  These storages are connected by eight flows; photosynthesis, respiration, deforestation, fossil fuel combustion, solution and dissolution in the ocean, sedimentation, and weathering.

Measuring carbon

Before we explore the carbon cycle in more detail, a word about units and measurements. Because the quantities of carbon in the Earth’s major carbon pools can be quite large, it is inconvenient to use familiar units such as pounds or kilograms.  Instead, scientists use other units that are better suited for expressing large numbers. For example, a Petagram of carbon (Pg), also known as a Gigaton (Gt), is equal to 10¹⁵ grams or one billion tonnes.  A tonne, also known as a metric ton, is equal to one thousand kilograms (1,000 kg).  Because one kilogram is equal to 2.205 pounds, one metric tonne is the same as 2205 pounds.  Taking this further, we can see that one Petagram is equal to just about 2,200,000,000,000 (or 2.2 trillion) pounds!  Expressing this as 1 Pg is much simpler than working with that many zeros. 

Carbon storage pools

The total carbon on the Earth is distributed among the six major pools described earlier.  Here we focus on the storage pools most relevant to our discussion of energy and climate. On relatively short time scales (years to centuries), the four major storage pools are the atmosphere, oceans, fossil fuels, and terrestrial ecosystems, including vegetation and soils.

Fossil fuels

Coal, oil, and natural gas were formed from prehistoric plants and animals that lived and died hundreds of millions of years ago, and then were transformed deep underground into their present forms. Thus, the carbon in fossil fuels is the product of past photosynthesis and is often called "fossil carbon." The amount of carbon stored in deposits of coal, oil, and gas is estimated to be 5000–10,000 PgC, larger than any other reservoir except the deep sea, and about ten times the carbon content of the atmosphere.

The atmosphere

In 2009, the globally averaged concentration of CO₂ in the atmosphere was about 0.0390%, or 390 ppmv (parts per million by volume), equivalent to more than 800 PgC.  Although this is considerably less carbon than that contained in the oceans or crust, carbon in the atmosphere is of vital importance because of its influence on climate.  The relatively small size of the atmospheric carbon pool also makes it more sensitive to disruptions caused by an increase in sources or sinks of carbon from the Earth’s other pools.  In fact, the present-day value of 800 PgC is substantially higher than that which occurred before the onset of fossil fuel combustion and deforestation.  Before these activities began, the atmosphere contained approximately 560 PgC.  In the context of global pools and fluxes, the increase that has occurred in the past several centuries is the result of carbon fluxes to the atmosphere from the crust (fossil fuels) and terrestrial ecosystems (via deforestation and other forms of land clearing).

The oceans

The total amount of carbon in the world's oceans is approximately 38,000 PgC, nearly 50 times more carbon than in the atmosphere. Most of this oceanic carbon is in intermediate and deep waters; only 700–1000 PgC are in the surface ocean in direct contact with the atmosphere. There are also 6000 PgC of reactive carbon within ocean sediments, which, although important in determining the long-term concentration of CO₂ in the atmosphere and oceans, are less important as a part of the short-term carbon cycle.

Terrestrial ecosystems: vegetation and soils

The amount of carbon contained in terrestrial vegetation (550 ± 100 Pg) is on the order of the amount in the atmosphere (800 Pg). The organic matter in soils is two to three times this amount [1500–2000 PgC in the top meter and as much as 2300 Pg in the top 3 meters. Forests are particularly important as a carbon reservoir because trees hold much more carbon per unit area than other types of vegetation. 

Carbon fluxes

The movement of any material from one place to another is called a flux and we typically think of a carbon flux as a transfer of carbon from one pool to another. Fluxes are usually expressed as a rate with units of an amount of some substance being transferred over a certain period of time (e.g. PgC yr^-1). A single carbon pool can have several fluxes that simultaneously add and remove carbon.  For example, the atmosphere has inflows from decomposition (CO₂ released by the breakdown of organic matter), forest fires and fossil fuel combustion, and outflows from plant growth and uptake by the oceans.

The single largest flux of carbon associated with human activity is the release of CO₂ by the combustion of fossil fuels, which in 2006 represented a flux to the atmosphere of 8.2 PgC/year.  In geological terms, this represents a new and relatively rapid flux to the atmosphere of large amounts of carbon. The redistribution of fossil carbon from the fossil fuel storage pool to the atmosphere, oceans and lands is currently the dominant flux in the global carbon cycle. Natural flows

of carbon can no longer be distinguished because the pools and fluxes in the active carbon cycle are so altered as a result of this redistribution of fossil carbon over the past few centuries. Even obviously natural processes, such as photosynthesis, which may be readily distinguished from human-induced processes, are impacted by carbon fluxes resulting from the combustion of fossil fuels.

What happens to CO₂ released by the combustion of carbon-based fuels and deforestation? Of the approximately 8.2 PgC emitted each year, about 40 percent accumulates in the atmosphere and about 30 percent is absorbed by the oceans. Scientists believe that terrestrial ecosystems, especially trees, take up the remainder. The annual release of CO₂ from fossil fuel use and other human activities such as cement production and land use change over the past 200 years has produced a large increase in the storage pool of carbon in the atmosphere, which in turn has had a substantial impact on the Earth's climate. We turn to this impact in the next sections of this primer.

Sources

• Boden, T.A., G. Marland, and R.J. Andres. 2009. Global, Regional, and National Fossil-Fuel CO₂ Emissions. Carbon Dioxide Information Analysis Center, Oak Ridge National Laboratory, U.S. Department of Energy, Oak Ridge, Tenn., U.S.A. doi 10.3334/CDIAC/00001
• Houghton, R.A. 2007. Balancing the Global Carbon Budget, Annual Review of Earth and Planetary Sciences, Vol. 35: 313-347. doi:10.1146/annurev.earth.35.031306.140057.
• Mackenzie, Fred  and Abraham Lerman. 2006. Brief Overview of Carbon on Earth, (New York, Springer), ISBN 978-1-4020-4238-6.
• University of New Hampshire, GLOBE Carbon Cycle Program, Accessed 14 May 2009.
• Woods Hole Research Center, Understanding the global carbon cycle, Accessed 14 May 2009.