|Working Group III: Mitigation|
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4.3 Processes and Practices that Can Contribute to Climate
4.3.1 System Constraints and Considerations
In terrestrial ecosystems the carbon cycle exhibits natural cyclic behaviour on a range of time scales. Most ecosystems, for example, have a diurnal and seasonal cycle. Often this means that the ecosystem functions as a source of C in the winter and a sink for C in the summer, and this shows up in fluctuations at the global scale, as shown by the annual oscillations in the global atmospheric CO2 concentration. Large-scale fluctuations occur at other temporal scales as well, ranging from decades (Braswell et al., 1997; Turner et al., 1997; Karjalainen et al., 1998; Kurz and Apps, 1999; Bhatti et al., 2001) to several centuries (Campbell et al., 2000) and longer (Harden et al., 1992).
The net balance of C flows between the atmosphere and the terrestrial biosphere also undergoes management-induced cycles that occur over long time scales (decades to millennia), and that can cause the transition of terrestrial systems from sink to source and back (Harden et al., 1992). Of relevance for C mitigation are the human-induced changes that occur on an annual to centennial time scale. This would include the harvest cycle of managed, production forests.
The intent of any mitigation option is to reduce atmospheric CO2 relative to that which would occur without implementation of that option. Biological approaches to curb the increase of atmospheric CO2 can occur by one of three strategies (IPCC, 1996):
The benefits of these strategies show contrasting temporal patterns. Conservation offers immediate benefits via prevented emissions. Sequestration impacts often follow an S-curve: accrual rates are often highest after an initial lag phase and then decline towards zero as C stocks approach a maximum (e.g., Figure 4.3). Substitution benefits often occur after an initial period of net emission, but these benefits can continue almost indefinitely into the future (Figure 4.6).
This section deals primarily with carbon conservation and sequestration in the terrestrial biosphere, but acknowledges the complementarity and trade-offs among the three strategies. Carbon sequestration in forest products is included here and the substitution benefits of forest products are treated briefly. The role of energy cropping is treated in greater depth in Chapter 3 (Section 18.104.22.168) and in the IPCC Special Report on LULUCF (IPCC, 2000a). Here the discussion is restricted to the secondary use of biomass products for energy (e.g., waste products) and non-commercial uses (e.g., domestic heating, cooking, etc.).
The general goal of sequestration activities is to maintain ecosystems in the sink phase. However, if the system is disturbed (a forest burns or is harvested, or land is cultivated), a large fraction of previously accumulated C may be released into the atmosphere through combustion or decomposition (Figure 4.2). When the system recovers from the disturbance, it re-enters a phase of active carbon accumulation. Thus, the disturbance history of terrestrial ecosystems involves in large C losses in the past (Houghton et al., 1999; Kurz and Apps, 1999), but opportunities for C sequestration in the present.A comprehensive systems analysis is useful to fully evaluate mitigation options. Factors to be considered may include: ecosystem C stocks and sinks; sustainability, security, resilience, and robustness of the C stock maintained or created; temporal patterns of C accumulation; other land-use goals and related C flows in the energy and materials sector; and effects on other non-CO2 GHGs. For example, one option might have both a high maximum C stock and a high or more sustained rate of sequestration, yet be incompatible with other demands placed on the land. A second option may have a high maximum C stock, but reach that level only very slowly. Still another option may offer high short-term sequestration, but reach maximum C stocks very quickly. Yet another option might manage production systems to maximize the flow of harvested carbon into products, thus maximizing the displacement of alternate, energy-intensive products. Thus, while a wide array of practices may be technically possible, options that meet all criteria may be much fewer, and a combination of complementary options may best accomplish C mitigation goals. Although scientists now recognize the value of system-wide analyses (Cohen et al., 1996; Alig et al., 1997), rarely have mitigation options been subjected to such comprehensive evaluations.
An upper bound for the technical potential for global C mitigation in the terrestrial biosphere, a physical upper limit, can be estimated for conservation, sequestration, and substitution measures. The technical potential for conservation measures would equal the current existing C stock of the worlds ecosystems. This assumes that all ecosystems are threatened, but all could be conserved by implementing protection measures. The technical potential for sequestration would roughly equal the carbon stocks lost in deforestation, desertification, and other human-induced changes in land cover and land use over centuries and millennia. The theoretical upper limit would thus correspond to the full recovery of lost biomass in ecosystems, and to a steady state at the natural carrying capacity for biomass on earth. The technical potential for substitution is related to the sustainable production of harvestable biomass and its substitution for fossil fuels and energy-intensive products. Clearly, each of these upper limits violates in practice the ideals of development, equity, and sustainability. And yet, they help to appreciate that there are bounds on the role that managing the biosphere might play in carbon mitigation.
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