Detailed estimation of the amounts of biomass carbon on the earth’s surface is the ultimate problem in global statistical analysis. Yet what appear to be reasonable projections have been made using available data, maps, and surveys. The validity of the conclusions in their entirety is difficult to support with hard data because of the nature of the problem. But such analyses must be performed to assess the practical feasibility of biomass energy systems and the gross types of biomass that might be available for energy applications.

The results of one such study are summarized in Table 2.2. Ignoring the changes in agricultural practice and the deforestation that have taken place over the last few decades, this is perhaps one of the better attempts to con­duct an analysis of the earth’s biomass carbon distribution (Whittaker and Likens, 1975). Each ecosystem on the earth is considered in terms of area, mean net carbon production per year, and standing biomass carbon. Standing biomass carbon is that contained in biomass on the earth’s surface and does not include the carbon stored in biomass underground. A condensation of this data (Table 2.3) facilitates interpretation. Of the total net carbon fixed on the earth each year, forest biomass, which is produced on only 9.5% of the earth’s surface, contributes more than any other source. Marine sources of net fixed carbon are also high, as might be expected because of the large area of the earth occupied by water. But the high turnover rates of carbon in a marine environ­ment result in relatively small steady-state quantities of standing carbon. In contrast, the low turnover rates of forest biomass make it the largest contributor

to standing carbon reserves. According to this assessment, the forests produce about 43% of the net carbon fixed each year and contain over 89% of the standing biomass carbon of the earth. Tropical forests are the largest sources of these carbon reserves. Temperate deciduous and evergreen forests are also major sources of biomass carbon. Next in order of biomass carbon supply would probably be the savanna and grasslands. Note that cultivated land is one of the smaller producers of fixed carbon and is only about 9% of the total terrestrial area of the earth.

It is necessary to emphasize that anthropological activities and the increasing population, particularly in developing and Third World countries, continue to make it more difficult to sustain the world’s biomass growth areas. It has been estimated that tropical forests are disappearing at a rate of tens of thousands of square miles per year. Satellite imaging and field surveys show that Brazil alone has a deforestation rate of about 8 X 106 ha/year (19.8 X 106 ac/year; 30,888 mi. Vyear) (Repetto, 1990). At mean net biomass carbon yields of 9.90 t/ha-year for tropical rain forests (Table 2.2), this rate of deforestation corresponds to a loss of 79.2 x 106 t/year of net biomass carbon productivity.

The remaining carbon transport mechanisms on earth are primarily physical mechanisms, such as the solution of carbonate sediments in the sea and the release of dissolved C02 to the atmosphere by the hydrosphere. Because of the relatively short lifetimes of live biomass (phytoplankton and zooplankton) in the oceans compared to those of land biomass, there is a much larger amount of carbon in viable land biomass at any given time. The great bulk of carbon, however, is contained in the lithosphere as carbonates in rock. The carbon deposits that contain little or no stored chemical energy, although some high — temperature deposits can provide considerable thermal energy, consist of lith­ospheric sediments and atmospheric and hydrospheric C02. Together, these carbon sources comprise 99.96% of the total carbon estimated to exist on the earth (Table 2.4). The carbon in fossil fuel deposits is only about 0.02% of the total, and live and dead biomass carbon makes up the remainder, about 0.02%. Biomass carbon is thus a very small fraction of the total carbon inventory of the earth, but it is an extremely important fraction. It helps to maintain the delicate balance among the atmosphere, hydrosphere, and biosphere necessary to support all life forms, and is essential to maintain the diversity of species that inhabit the earth and to sustain their gene pools. Any large-scale utilization of biomass carbon, especially virgin material, therefore requires that it be replaced, preferably as it is consumed so that the biomass reservoirs are not

reduced. Indeed, enlargement of these reservoirs may become necessary as the world’s population expands and climate changes occur.