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Estimates of the Carbon Cycle Flows for the Earth

Chapter 3 of Climate Change 2001, the Assessment Report of the Intergovernmental Panel on Climate Change (IPCC) gives summary estimates of the components of the Earth's carbon cycle. The unit used for these carbon flows is billions of metric tons (109) of carbon. This unit is called a gigaton, or alternatively a petagram (1015 grams). The flows can be expressed in terms of the mass of carbon per se, or the mass of carbon dioxide. The atomic weight of carbon is 12 and the molecular weight of carbon dioxide is 12+2(16)=44. Therefore on ton of carbon dioxide equals 12/44=0.273 tons of carbon. Conversely one ton of carbon equals 3.67 tons of carbon dioxide.

Since the focus is on the carbon flows the figures are reported below in terms of carbon rather than carbon dioxide.

Annual Flows of Carbon
to and from
the Earth's Atmosphere
in Gigatons of Carbon
 Period
 1980-891990-1999
FlowMeanRangeMeanRange
Fossil Fuel
burning plus
cement production
5.45.1 to 5.76.35.9 to 6.7
Atmosphere
to Ocean
1.91.3 to 2.51.71.2 to 2.2
Atmosphere
to Land
0.2−0.5 to 0.91.40.7 to 2.1
Land-use
Change
1.70.6 to 2.5N.A.N.A.
Net Increase
in Atmospheric CO2
3.33.2 to 3.43.23.1 to 3.3

An alternate measure, and one that makes more sense chemically and physically, is moles. This is in terms of the number of molecules. A mole is 6×1023 molecules. The number of moles is found by dividing the mass of the substance in grams by its molecular weight. A metric ton is 1000 kilograms or 1 million grams (106). Thus 1 gigaton or a petragram is 1015 grams. The average annual net increase in CO2 to the atmosphere was 3.2 gigatons of carbon or 11.744 gigatons of carbon dioxide. This is (11.744/44)×1015 moles or 2.67×1014 moles of carbon dioxide or about one quarter of a petramole.


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