This difference can be accounted for only by absorption in the oceans and biosphere.

Carbon dioxide produced by industrial activity from 1860 to 1973 inclusive was equivalent to 128 Gt of carbon (Keeling, 1973; Chapter 4). If the proportion between carbon remaining in the air and that produced by fossil-fuel combustion during this 113-year period was the same as during 1959–1973, the carbon dioxide content of the air should have increased by 34 parts per million, from 296 to 330 parts per million, or 11.5 percent of the initial value, corresponding to 72 Gt of carbon. The actual increase may have been significantly greater. Although nineteenth-century data on the atmospheric carbon dioxide content are much less accurate than modern measurements, a best value for the middle of the nineteenth century is 290 parts per million (Chapter 4). The increase in atmospheric carbon dioxide up to 1973 is then 40 parts per million, corresponding to 85 Gt of carbon. This is 66 percent of the carbon added by fossil-fuel combustion and 13.8 percent of the carbon in the atmosphere in 1860. Industrial carbon dioxide production during 1860–1973 corresponded to 60 parts per million, or about 20.7 percent of the nineteenth-century atmospheric carbon dioxide.

As we shall see, carbon dioxide has also been released from part of the biosphere, primarily by clearing of forest lands for agriculture. The total quantity produced between 1860 and 1973 may have been between 70 and 80 Gt of carbon. Thus around 205 Gt of carbon (one third of the original atmospheric content) must be accounted for by partitioning among the atmosphere, the ocean, and the biosphere. In this chapter we present models of the atmosphere-ocean-biosphere system that are consistent with these data and provide a range of projections of future atmospheric carbon dioxide content.

The portion of organic matter in the terrestrial biosphere that exchanges carbon with the atmosphere consists of two components: (1) the biomass of living plants and animals, mostly the trunks, branches, roots, and leaves of trees, and (2) litter, detritus, and soil organic matter (humus). Whittaker and Likens (1975) have compiled and evaluated data on the biomass (see Table 10.1). They conclude that 90 percent of the total of around 830 Gt is in the world’s forests, which cover nearly 50 million km², Tropical forests, with an area of 24.5 km², contain more than half of the total. Woodland and shrubland, savannas, grasslands, desert and semi-desert scrub, swamps and marshes, and cultivated land together contain only 84 Gt, or 10 percent of the total biomass, although they cover nearly 75 million km². Net primary production of organic matter (photosynthesis minus plant respiration) is more evenly divided: forests produce 33 Gt of carbon per year, and all other vegetation produces nearly 20 Gt. These estimates correspond to an average efficiency of photosynthetic conversion of solar