There is estimated to be over 5 x 10 22 g of carbon on earth. One tonne is 10 6 g, so this is 5 x 10 16 tonnes, or to put it another way: 50,000,000,000,000,000 tonnes of carbon on earth About 99% of this is locked away in sedimentary rocks such as limestone and chalk (both of these are made of calcium carbonate CaCO 3 ), the remainder can pass through the various carbon sinks and reservoirs as part of the carbon cycle and so is in some manner "available". This remaining carbon is distributed between a number of reservoirs, some living - for instance you represent a carbon reservoir approximately equivalent in size to a small bag of barbeque charcoal. Some is atmospheric, some is dissolved in water and some is buried beneath the earth. Carbon reservoir Mass in tonnes of carbon x 10 9 (billions of tonnes of carbon) % of total Atmosphere 740.0 1.43 Land plants 550.0 1.06 Land animals 0.5 0.001 Marine plants (algae) 1.5 0.003 Marine animals 1.5 0.003 Dead organic matter (in soil and peat) 1600.0 3.08 Coal 4500.0 8.67 Oil and gas 500.0 0.96 Dissolved in the oceans 38 000.0 73.2 Marine sediments 6000.0 11.6 Totals 51 893.5 100.007 This distribution is often rather surprising at first sight, notably because there is so much carbon dioxide contained dissolved in the oceans and relatively little is contained in the atmosphere. The atmosphere however is exceptionally important in that it is the transfer medium