Carbon Dioxide

Carbon dioxide has been emitted to the extent of about 5 x 104g/cm2 of the earth's surface over all time. Most of this C02 has dissolved in the oceans and much has precipitated from the oceans as calcium carbonate. 0nly about 0.45 g/cm2 CO2 remains in the atmosphere, while about 27 g/cm2 averaged over all the earth's surface is dissolved in the oceans. There is 60 times as much C02 dissolved in the oceans as there is free in the atmosphere, but most of the carbon dioxide released by the degassing of the earth is now tied up in geologic deposits (i.e., carbonate rocks).

There are complex equilibria, discussed in Chapter 9, between the atmospheric C02 and that dissolved in the oceans. Recent experiments have shown that it takes about 470 days to dissolve half the C02 placed in contact with turbulent seawater. Complete equilibrium takes much longer, with estimates ranging from 5 to 500 years, but, given sufficient time, the oceans and rocks act as an enormous sink for any C02 that is added to the atmosphere. That is, most of the C02 added to the atmosphere eventually ends up in the oceans and in geologic deposits. However, we are now adding C02 to the atmosphere by burning fossil fuels in the amount of about 1.9 x 1010 metric tons per year (see Section 2.2); this adds to the 2.6 x 1012 metric tons of C02 already present in the atmosphere. Some of the added C02 will remain in the atmosphere even when atmosphere-ocean equilibrium has been achieved. The World Energy Council estimated that the amount of all recoverable fossil fuels was about 4.6 x 1012 metric tons4 in 1993. This would allow 1.9 x 1010 metric tons to be burned each year for over 300 years. However, the provisions of the Kyoto Protocol may limit the amount of fossil fuels burned in the future so that the earth's supplies could last much longer than 300 years. The concentration of carbon dioxide in the atmosphere in the nineteenth century was about 290 ppm, while in 2000, as mentioned in Section 2.2, it was about 370 ppm. The increased concentration in this century is well documented and will be discussed later, together with its possible effects on climate. Much carbon dioxide is used by green plants in photosynthesis, but this carbon dioxide is returned to the atmosphere by decay and oxidation in living things.

Production of C02 from fossil fuels uses atmospheric oxygen, but this has little effect on the oxygen content of the atmosphere. For example, if all easily recoverable fossil fuels were burned at once, calculations indicate that only 10% of the atmospheric oxygen (a total of 5 x 1014 metric tons) would be used up.

4This is given in terms of the equivalent mass of petroleum, since fossil fuels include soft and hard coals as well as petroleum.

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