9.4.4 Global summation and comparison

An overview of estimates derived in the regional bottom-up estimates as given in Section 9.4.3.1 are presented in Table 9.6. Based on indications in literature and carbon supply curves, the fraction of the mitigation potential in the cost class < 20 US$/tCO₂ was estimated.

Assuming a linear implementation rate of the measures, the values in Table 9.4 were adjusted to 2030 values (the values required in the cross sector summation in Chapter 11, Table 11.3). The 2030 values are presented in Table 9.7 against the values derived from global forest sector models, and from global integrated models for three world regions. The mitigation effect of biomass for bioenergy (see text, Box 9.2) was excluded.

The range of estimates in the literature and presented in Table 9.7 help in understanding the uncertainty surrounding forestry mitigation potential. Bottom-up estimates of mitigation generally include numerous activities in one or more regions represented in detail. Top-down global modelling of sectors and of long-term climate stabilization scenario pathways generally includes fewer, simplified forest options, but allows competition across all sectors of the economy to generate a portfolio of least-cost mitigation strategies. Comparison of top-down and bottom-up modelling estimates (Figure 9.13) is difficult at present. This stems from differences in how the two approaches represent mitigation options and costs, market dynamics, and the effects of market prices on model and sectoral inputs and outputs such as labour, capital, and land. One important reason that bottom-up results yield a lower potential consistently for every region (Figure 9.13) is that this type of study takes into account (to some degree) barriers to implementation. The bottom-up estimate has, therefore, characteristics of a market potential study, but the degree is unknown.

Errata Figure 9.13: Comparison of estimates of economic mitigation potential in the forestry sector (up to 100 US$/tCO₂ in 2030) as based on global forest sector models (top-down) versus regional modelling results (bottom-up).

Note: Excluding bioenergy; data from Table 9.3 and Table 9.6.

The uncertainty and differences behind the studies referred to, and the lack of baselines are reasons to be rather conservative with the final estimate for the forestry mitigation potential. Therefore, mostly the bottom-up estimates are used in the final estimate. This stands apart from any preference for a certain type of study. Thus synthesizing the literature, we estimate that forestry mitigation options have the economic potential (at carbon prices up to 100 US$/tCO₂) to contribute between 1270 and 4230 MtCO₂/yr in 2030 (medium confidence, medium agreement). About 50% of the medium estimate can be achieved at a cost under 20 US$/tCO₂ (= 1550 MtCO₂/yr: see Figure 9.14). The combined effects of reduced deforestation and degradation, afforestation, forest management, agro-forestry and bioenergy have the potential to increase gradually from the present to 2030 and beyond. For comparison with other sectors in Chapter 11, Table 11.2, data on cost categories <0 US$/tCO₂ and 20-50 US$100/tCO₂ have been derived from Tables 9.3 and 9.6, using cost information derived from regional bottom-up studies and global top- down modelling. The cost classes assessed should be seen as rough cost-class indications, as the information in the literature varies a lot. These analyses assume gradual implementation of mitigation activities starting at present.

This sink enhancement/emission avoidance will be located for 65% in the tropics (high confidence, high agreement; Figure 9.14); be found mainly in above-ground biomass; and for 10% achieved through bioenergy (medium confidence, medium agreement). In the short term, this potential is much smaller, with 1180 MtCO₂/yr in 2010 (high confidence, medium agreement). Uncertainty from this estimate arises from the variety of studies used, the different assumptions, the different measures taken into account, and not taking into account possible leakage between continents.

Figure 9.14: Annual economic mitigation potential in the forestry sector by world region and cost class in 2030.

Note: EECCA=Countries of Eastern Europe, the Caucasus and Central Asia.

These final results allow comparison with earlier IPCC estimates for forestry mitigation potential (Figure 9.15). The estimates for Second Assessment Report (SAR), Third Assessment Report (TAR) and Special Report have to be seen as estimates for a technical potential, and are comparable to our Fourth Assessment Report (AR4) estimates for a carbon dioxide price < 100 US$/tCO₂ (as displayed). As the bars in this figure are lined by the year to which they apply, one would expect an increasing trend towards the right-hand columns. This is not the case. Instead a large variety is displayed. There is a trend visible through the consecutive IPCC reports, and not so much through the years to which the estimate applies. When ignoring the TAR synthesis, we start with the highest estimate in SAR (just over 8000 MtCO₂/yr), then follows SR LULUCF with 5500 MtCO₂, and TAR with 5300. Finally, the present report follows with a conservative estimate of 3140 (including bioenergy).

Figure 9.15: Comparison of estimates of mitigation potential in previous IPCC reports (blue) and the current report (in red).

Note the difference in years to which the estimate applies, in applied costs, and between forest sector only versus whole LULUCF estimates.