A. Biospheric Carbon Fluxes

Most global studies of the transport and distribution of the earth’s carbon eventually lead many analysts to conclude that the continuous exchange of carbon with the atmosphere and the assumptions and extrapolations that must be employed make it next to impossible to eliminate large errors in the results and uncertainty in the conclusions. Only a very small fraction of the immense mass of carbon at or near the earth’s surface is in relatively rapid circulation in the earth’s biosphere, which includes the upper portions of the earth’s crust, the hydrosphere, and biomass. There is a continuous flow of carbon between the various sources and sinks. The atmosphere is the conduit for most of this flux, which occurs primarily as C02.

Some of the difficulties encountered in analyzing this flux are illustrated by estimating the C02 exchanges with the atmosphere (Table 2.1). Despite the possibilities for errors in this tabulation, especially regarding absolute values, several important trends and observations are apparent and should be valid for many years. The first observation is that fossil fuel combustion and industrial operations such as cement manufacture emit much smaller amounts of C02 to the atmosphere than biomass respiration and decay, and the physical exchanges between the oceans and the atmosphere. The total amount of C02 emissions from coal, oil, and natural gas combustion is also less than 3% of that emitted by all sources. This is perhaps unexpected because most of the climate change literature indicates that the largest source of C02 emissions is fossil fuel combustion. Note that human and animal respiration are projected to emit more than five times the C02 emissions of all industry exclusive of energy-related emissions. Note also that biomass burning appears to emit almost as much C02 as oil and natural gas consumption together.

One of the C02 sources not listed in Table 2.1 that can result in significant net C02 fluxes to the atmosphere is land cover changes such as those that result from urbanization, highway construction, and the clear-cutting of forestland for agricultural purposes. It has been estimated that the net flux of C02 to the atmosphere in 1980, for example, was 5.13 Gt, or 1.40 Gt of carbon, because of land cover changes (Houghton and Hackler, 1995). Land cover changes are usually permanent, so the loss in atmospheric carbon-fixing capacity and annual biomass growth are essentially permanent also. It has been estimated from the world’s biomass production data that losses of only 1% of standing forest biomass and annual forest biomass productivity correspond to the ulti­mate return of approximately 27 Gt of C02 to the atmosphere, and an annual loss of about 1.22 Gt in atmospheric C02 removal capacity (cf. Klass, 1993).

Overall, the importance of the two primary sinks for atmospheric C02— terrestrial biota and the oceans—is obvious. No other large sinks have been identified. It is evident that only small changes in the estimated C02 uptake and release rates of these sinks determine whether there is a net positive or negative exchange of C02 with the atmosphere. A small change in either or both carbon fixation in biomass by photosynthesis or biomass respiration estimates tends to cause a large percentage change in the arithmetic difference

“The fossil fuel, human, and animal emissions were estimated by the author (Appendix C). Most of the other exchanges are derived from exchanges that have been reported in the liter­ature (с/. Boden, Marland, and Andres, 1995) or they are based on assumptions that have generally been used by climatologists. It was assumed that 50% of the terrestrial biomass car­bon fixed by photosynthesis is respired and that an equal amount is emitted by the soil. The total uptake and emission of carbon dioxide by the oceans were assumed to be 104 and 100 Gt C/year (Houghton and Woodwell, 1989), and biomass respiration was assumed to emit 50% of the carbon fixed by photosynthesis. The carbon dioxide emissions from cement pro­duction and other industrial processes are process emissions that exclude energy-related emis­sions; they are included in the fossil fuel consumption figures.

between them. And the impact of the assumptions is very large. The assumption that live biomass respires about 50% per year of the total carbon that is photochemically fixed results in a substantial calculated addition of CO2 to the atmosphere, far more than that from fossil fuel combustion. The other assumption incorporated in most biospheric carbon budgets concerns the annual emission of CO2 from soils by microbial action and the oxidation of dead biomass, namely that the emission of C02 occurs at an annual rate approximately equal to 50% of the gross annual photosynthetic carbon uptake. This assumption has little experimental support. The end result of the use of these assumptions with respect to terrestrial biomass, the soils, and the oceans is that they are almost neutral factors in the scenarios generally published on carbon exchanges with the atmosphere and the buildup of atmospheric C02; that is, about the same amount of C02 is emitted as is taken up each year, as shown in the tabulation. This conclusion can be subject to major error when attempting to quantify carbon exchanges with the atmosphere. The largest reservoir of biomass carbon resides in live forest biomass, as will be shown later, and unless this biomass is removed or killed, it fixes atmospheric C02 with the passage of time during most of its life cycle. Tо sustain the environmen­tal benefits of biomass growth as a sink for the removal of C02 from the atmosphere, it is evident that biomass growth should be sustained and ex­panded. The large-scale use of virgin biomass for energy will not adversely affect these benefits if it is replaced at the same or a greater rate than the rate of consumption.