Working Group III: Mitigation

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4.3 Processes and Practices that Can Contribute to Climate Mitigation 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):

  • conservation: conserving an existing C pool, thereby preventing emissions to the atmosphere;
  • sequestration: increasing the size of existing carbon pools, thereby extracting CO2 from the atmosphere; and
  • substitution: substituting biological products for fossil fuels or energy-intensive products, thereby reducing CO2 emissions.

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).

Figure 4.7: Indications of the magnitude of the carbon sink in case study countries for a set of forest management measures (MtCO2eq, adapted after Nabuurs et al. 2000). The values for the three bars for Iceland are 2.6, 2.8, and 2.9, respectively. The figure is based on the forest part of the model “Access to Country Specific Data” (ACSD). It was designed to provide insight into the potential magnitude of carbon sequestration that may be achieved when alternative sets of management measures are adopted. Therefore, the exact numbers provided in this figure result from the assumptions chosen for a certain set of measures. The estimates in this figure are tentative and only illustrative. In these studies all forestry activities under discussion were included, but applied on average on some 10% of mostly the exploitable forest area.

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 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 world’s 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.

Table 4.3: Mitigation options, mitigation potential, and investment cost per tonne of carbon (US$/tC) abated in selected countries (Sathaye and Ravindranath, 1998)
Mitigation option
Mitigation potential (tC/ha) Investment cost1 (US$/tC)
Mitigation option
Mitigation potential (tC/ha) Investment cost1 (US$/tC)  
North & North West
Assisted natural regeneration2
13.0 1.3  
Natural regeneration2
62.0 1.5
55.0 1.3  
Enhanced natural regeneration2
87.5 2.5
15.0 16.3  
25.4 1.6
South, South West & North East
Community woodlot
75.8 5.6
Assisted natural regeneration
13.9 3.5  
Softwood forestry
80.1 7.3
71.0 5.0  
Timber forestry
120.6 3.3
6.0 9.8  
South Korea
Timber estate 165.0 1.9   Improved management of natural forest 99.4 6.0
Social forestry 94.0 1.1   Urban forestry 299.0 9.2
Reforestation4 214.0 0.9   Enhanced regeneration of
L. leptolepsis
123.0 13.8
Private forests 99.0 2.1   P. koraiensis 85.0 21.0
Afforestation 106.0 0.6        
Mongolia       Pakistan    
Private forests 99.2 0.8   Intensified forest management    
Natural regeneration 67.5 0.6   - Conifer forest – protection 41.6 0.1
Agroforestry 9.8 0.8   - Conifer forest – natural regeneration 33.8 8.8
Bioenergy 80 -   Reforestation4 39.1 19.3
Shelter belt 101.7 0.9   Riverain forest plantation 32.9 40.6
        Commercial forest plantation 54.6 40.6
        Watershed management 26.7 34.8
        Agroforestry 29.7 1.6
        Plantation on agricultural land2 7.5 0.7
        Rangeland management. 20.0 17.4
Philippines       Thailand    
Forest protection plus sustainable 215.0 1.3   Short rotation in:    
management       - Managed forests 185.5 2.5
Forest protection – total log ban 215.0 0.5   - Non protected areas 158.9 2.9
Long rotation forestry 236.0 2.1   Long rotation in community managed forests 169.0 3.2
Urban forestry 90.0 5.3   Medium rotation in non protected areas 112.5 4.3
        Forest protection and rotation forestry
for conservation in
        - Protected area 38.6 7.5
        - Community managed forests 38.1 10.7
Vietnam       Myanmar    
Forest protection 106.9 0.1   Natural regeneration 33.0 0.1
Degraded forest protection 64.3 0.2   Reforestation long4 155.0 0.8
Natural regeneration (enhanced) 57.1 0.8   Forest protection 47.0 1.6
Scattered trees 64.0 0.9   Reforestation short4 55.0 3.8
Reforestation short4 43.0 2.2   Bio electricity 78.0 21.4
Reforestation long 68.2 1.7        
Ghana       Cameroon    
Evergreen forest       Evergreen forest    
- agroforestry 13-88 1-6   - agroforestry 16-58 1-5
- slowing deforestation 35-140 1-2   - slowing deforestation. 40-160 1-2
Deciduous forest       - forestation5 73-195 1-19
-slowing deforestation 35-140 1-2   Deciduous forest    
- forestation5 31-154 1-27   - forestation5 27-169 21-19
Savannah       Savannah    
- agroforestry 29-61 4-12   - forestation5 36-170 1-31
1 Investment costs (US$/tC): This largely includes forest or plantation establishment costs incurred during the initial 2-3 years;
discounted for only the initial 2-3 year period. For forest protection, the costs include expenditure on erecting barriers for protection, training, and other organizational costs incurred during the initial 2-3 year period. Mitigation potential is in pertuity, assuming one full cycle; rotation length for mitigation option subject to harvesting (such as short and long rotation) and for others 40 years.
2 Natural regeneration of forest is increasing the biomass density to that of closed forests on partially degraded open forest areas; assisted or enhanced natural regeneration would involve planting a (few) trees and/soil and water conservation activity to assist or enhance natural regeneration.
3 Plantations involve planting of one or more species at high densities.
4 Reforestation in a short rotation has a 5 to 15 year harvest cycle, reforestation in a long rotation has a 30-100 year harvest cycle.
5 Forestation includes both afforestation and reforestation.

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