8.4.3 Global and regional estimates of agricultural GHG mitigation potential

8.4.3.1 Technical potential for GHG mitigation in agriculture

There have been numerous attempts to assess the technical potenttial for GHG mitigation in agriculture. Most of these have focused on soil carbon sequestration. Estimates in the IPCC Second Assessment Report (SAR; IPCC, 1996) suggested that 400-800 MtC/yr (equivalent to about 1400-2900 MtCO₂-eq/yr) could be sequestered in global agricultural soils with a finite capacity saturating after 50 to100 years. In addition, SAR concluded that 300-1300 MtC (equivalent to about 1100-4800 MtCO₂-eq/yr) from fossil fuels could be offset by using 10 to15% of agricultural land to grow energy crops; with crop residues potentially contributing 100-200 MtC (equivalent to about 400-700 MtCO₂-eq/yr) to fossil fuel offsets if recovered and burned. Burning residues for bio-energy might increase N₂O emissions but this effect was not quantified.

SAR (IPCC, 1996) estimated that CH₄ emissions from agriculture could be reduced by 15 to 56%, mainly through improved nutrition of ruminants and better management of paddy rice, and that improved management could reduce N₂O emissions by 9-26%. The document also stated that GHG mitigation techniques will not be adopted by land managers unless they improve profitability but some measures are adopted for reasons other than climate mitigation. Options that both reduce GHG emissions and increase productivity are more likely to be adopted than those which only reduce emissions.

Of published estimates of technical potential, only Caldeira et al. (2004) and Smith et al. (2007a) provide global estimates considering all GHGs together, and Boehm et al. (2004) consider all GHGs for Canada only for 2008. Smith et al. (2007a) used per-area or per-animal estimates of mitigation potential for each GHG and multiplied this by the area available for that practice in each region. It was not necessary to use baseline emissions in calculating mitigation potential. US-EPA (2006b) estimated baseline emissions for 2020 for non-CO₂ GHGs as 7250 MtCO₂-eq in 2020 (see Chapter 11; Table 11.4). Non-CO₂ GHG emissions in agriculture are projected to increase by about 13% from 2000 to 2010 and by 13% from 2010 to 2020 (US-EPA, 2006b). Assuming a similar rate of increase as in the period from 2000 to 2020, global agricultural non-CO₂ GHG emissions would be around 8200 MtCO₂-eq in 2030.

The global technical potential for mitigation options in agriculture by 2030, considering all gases, was estimated to be ~4500 by Caldeira et al. (2004) and ~5500-6000 MtCO₂-eq/yr by Smith et al. (2007a) if considering no economic or other barriers. Economic potentials are considerably lower (see Section 8.4.3.2). Figure 8.3 presents global and regional estimates of agricultural mitigation potential. Of the technical potentials estimated by Smith et al. (2007a), about 89% is from soil carbon sequestration, about 9% from mitigation of methane and about 2% from mitigation of soil N₂O emissions (Figure 8.4). The total mitigation potential per region is presented in Figure 8.5.

Figure 8.3: Global (A) and regional (B) estimates of technical mitigation potential by 2030

Note: Equivalent values for Smith et al. (2007a) are taken from Table 7 of Smith et al., 2007a.

Figure 8.4: Global technical mitigation potential by 2030 of each agricultural management practice showing the impacts of each practice on each GHG.

Note: based on the B2 scenario though the pattern is similar for all SRES scenarios.

Source: Drawn from data in Smith et al., 2007a.

Figure 8.5: Total technical mitigation potentials (all practices, all GHGs: MtCO2-eq/yr) for each region by 2030, showing mean estimates.

Note: based on the B2 scenario though the pattern is similar for all SRES scenarios.

Source: Drawn from data in Smith et al., 2007a.

The uncertainty in the estimates of the technical potential is given in Figure 8.6, which shows one standard deviation either side of the mean estimate (box), and the 95% confidence interval about the mean (line). The range of the standard deviation, and the 95% confidence interval about the mean of 5800 MtCO₂-eq/yr, are 3000-8700, and 300-11400 MtCO₂-eq/yr, respectively, and are largely determined by uncertainty in the per-area estimate for the mitigation measure. For soil carbon sequestration (89% of the total potential), this arises from the mixed linear effects model used to derive the mitigation potentials. The most appropriate mitigation response will vary among regions, and different portfolios of strategies will be developed in different regions, and in countries within a region.

Figure 8.6: Total technical mitigation potentials (all practices, all GHGs) for each region by 2030

Note: Boxes show one standard deviation above and below the mean estimate for per-area mitigation potential, and the bars show the 95% confidence interval about the mean. Based on the B2 scenario, although the pattern is similar for all SRES scenarios.

Source: Drawn from data in Smith et al., 2007a.