Carbon in the Atmosphere

A brief account of the key features of the exchange of carbon between the atmosphere, the living and dead organic matter on land (the terrestrial biosphere), and the oceans is essential as a basis for the discussion that follows. The intermediate layers (100–1000 m) of the oceans also play a central role both as a sink for excess atmospheric CO2 and for heat. For these reasons some basic features of the carbon cycle will be outlined, based primarily on the recently published review by the Scientific Committee on Problems of the Environment (SCOPE) of the International Council of Scientific Unions (Bolin et al., 1979).

The CO2 concentration in the atmosphere has risen from about 314 ppm (parts per million, volume) in 1958 to about 334 ppm in 1979, i.e., an increase of 20 ppm, which is equivalent to 42×109 tons of carbon. During this same period, about 78×109 tons of carbon have been emitted to the atmosphere by fossil-fuel combustion. It has further been estimated that more than 150×109 tons of carbon have been released to the atmosphere since the middle of the nineteenth century, at which time the CO2 concentration in the atmosphere most likely was less than 300 ppm, probably about 290 ppm.

By reducing the extent of the world forests (at present about 30 percent of the land surface) and increasing the area of farmland (at present about 10 percent of the land surface) man has also transformed carbon in trees and in organic matter in the soil into CO2. The magnitude of this additional emission into the atmosphere is poorly known. Estimates range between 40×109 tons and more than 200×109 tons for the period since early last century.

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