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.

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.