Part I

2. Global Carbon Cycle Overview

5. The dynamics of terrestrial ecosystems depend on interactions between a number of biogeochemical cycles, particularly the carbon cycle, nutrient cycles, and the hydrological cycle, all of which may be modified by human actions. Terrestrial ecological systems, in which carbon is retained in live biomass, decomposing organic matter, and soil, play an important role in the global carbon cycle. Carbon is exchanged naturally between these systems and the atmosphere through photosynthesis, respiration, decomposition, and combustion. Human activities change carbon stocks in these pools and exchanges between them and the atmosphere through land use, land-use change, and forestry, among other activities. Substantial amounts of carbon have been released from forest clearing at high and middle latitudes over the last several centuries, and in the tropics during the latter part of the 20th century. [1.1.1.2] ¹

6. There is carbon uptake into both vegetation and soils in terrestrial ecosystems. Current carbon stocks are much larger in soils than in vegetation, particularly in non-forested ecosystems in middle and high latitudes (see Table 1). [1.3.1]

7. From 1850 to 1998, approximately 270 (± 30) Gt C has been emitted as carbon dioxide (CO2) into the atmosphere from fossil fuel burning and cement production. About 136 (+ 55) Gt C has been emitted as a result of land-use change, predominantly from forest ecosystems. This has led to an increase in the atmospheric content of carbon dioxide of 176 (± 10) Gt C. Atmospheric concentrations increased from about 285 to 366 ppm (i.e., by ~28%), and about 43% of the total emissions over this time have been retained in the atmosphere. The remainder, about 230 (± 60) Gt C, is estimated to have been taken up in approximately equal amounts in the oceans and the terrestrial ecosystems. Thus, on balance, the terrestrial ecosystems appear to have been a comparatively small net source of carbon dioxide during this period. [1.2.1]

8. The average annual global carbon budgets for 1980-1989 and 1989-1998 are shown in Table 2. This table shows that the rates and trends of carbon uptake in terrestrial ecosystems are quite uncertain. However, during these two decades, terrestrial ecosystems may have served as a small net sink for carbon dioxide. This terrestrial sink seems to have occurred in spite of net emissions into the atmosphere from land-use change, primarily in the tropics, having been 1.7 ± 0.8 Gt C yr^-1 and 1.6 ± 0.8 Gt C yr^-1 during these two decades, respectively. The net terrestrial carbon uptake, that approximately balances the emissions from land-use change in the tropics, results from land-use practices and natural regrowth in middle and high latitudes, the indirect effects of human activities (e.g., atmospheric CO2 fertilization and nutrient deposition), and changing climate (both natural and anthropogenic). It is presently not possible to determine the relative importance of these different processes, which also vary from region to region. [1.2.1 and Figure 1-1]

9. Ecosystem models indicate that the additional terrestrial uptake of atmospheric carbon dioxide arising from the indirect effects of human activities (e.g., CO2 fertilization and nutrient deposition) on a global scale is likely to be maintained for a number of decades in forest ecosystems, but may gradually diminish and forest ecosystems could even become a source. One reason for this is that the capacity of ecosystems for additional carbon uptake may be limited by nutrients and other biophysical factors. A second reason is that the rate of photosynthesis in some types of plants may no longer increase as carbon dioxide concentration continues to rise, whereas heterotrophic respiration is expected to rise with increasing temperatures. A third reason is that ecosystem degradation may result from climate change. These conclusions consider the effect of future CO2 and climate change on the present sink only and do not take into account future deforestation or actions to enhance the terrestrial sinks for which no comparable analyses have been made. Because of current uncertainties in our understanding with respect to acclimation of the physiological processes and climatic constraints and feedbacks amongst the processes, projections beyond a few decades are highly uncertain. [1.3.3]

10. Newly planted or regenerating forests, in the absence of major disturbances, will continue to uptake carbon for 20 to 50 years or more after establishment, depending on species and site conditions, though quantitative projections beyond a few decades are uncertain. [1.3.2.2]


11. Emissions of methane (CH4) and nitrous oxide (N2O) are influenced by land use, land-use change, and forestry activities (e.g., restoration of wetlands, biomass burning, and fertilization of forests). Hence, to assess the greenhouse gas implications of LULUCF activities, changes in CH4 and N2O emissions and removals-the magnitude of which is highly uncertain-would have to be considered explicitly. There are currently no reliable global estimates of these emissions and removals for LULUCF activities. [1.2.2, 1.2.3, 3.3.2]

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