IPCC Fourth Assessment Report: Climate Change 2007
Climate Change 2007: Working Group III: Mitigation of Climate Change Comparison with the Third Assessment Report (TAR)

Table 11.4 compares the estimates in this report (AR4) for 2030 with those from the TAR for 2020, which were evaluated at costs less than 27 US$/tCO2-eq (100 US$/tCO2-eq). The last column shows the AR4 estimates for potentials at costs of less than 20 US$/tCO2-eq, which are more comparable with those from the TAR. Overall, the estimated bottom-up economic potential has been revised downwards compared to that in the TAR, even though this report has a longer time horizon than the TAR. Only the buildings sector has been revised upwards in this cost category. For the forestry sector, the economic potential now is significantly lower compared to TAR. However, the TAR numbers for the forestry potential were not specified in terms of cost levels and are more comparable with the < 100 US$/tCO2-eq potential in this report. Even then, they are much higher because they are based on top-down global forest models. These models generally give much higher values then bottom-up studies, as reflected in Chapter 9 of this report. The industry sector is estimated to have a lower potential at costs below 20 US$/tCO2-eq, partly due to a lack of data available for use in the AR4 analyses. Only electricity savings have been included for light industry. In addition, the potential for CHP was allocated to the industry sector in the TAR and was not covered in this report. The most important difference between the TAR and the current analysis is that, in the TAR, material efficiency in a wide sense has been included in the industry sector. In this report, only some aspects of material efficiency have been included, namely in Chapter 7.

Table 11.4: Comparison of potential global emission reductions for 2030 with the global estimates for 2020 from the Third Assessment Report (TAR) in GtCO2-eq

Sector Options TAR potential emissions reductions by 2020 at costs <27.3 US$/tCO2 a) AR4 potential emissions reductions by 2030 at costs <20 US$/tCO2 b) 
Estimate Low High Low High 
Energy supply and conversion   1.3 2.6 1.2 2.4 
Transport CO2 only 1.1 2.6 1.3 2.1 
Buildings CO2 only 3.7 4.0 4.9 6.1 
Industry       0.70 1.5 
- energy efficiency   2.6 3.3     
- material efficiency   2.2 2.2     
 non-CO2  0.37 0.37     
Agriculturec) C-sinks and non CO2 c) 1.3 2.8 0.30 2.4 
Forestry   (11.7)d) (11.7) 0.55 1.9 
Waste CH4 only 0.73 0.73 0.35 0.85 
Total   13.2e) 18.5e) 9.3 17.1 


a) The TAR range excludes options with costs above 27.3 US$/tCO2 (100 US$/tC) (except for non-CO2 GHGs), and options that will not be adopted through the use of generally accepted policies (p. 264). Differences are due to rounding off.

b) This is the sum of the potential reduction at negative costs and below 20 US$/tCO2. See, however, notes to Table 11.2.

c) Note that TAR estimates are for non-CO2 emissions only. The AR4 estimates also include soil C sequestration (about 90% of the mitigation potential).

d) TAR copied the estimate of Special Report on LULUCF for 2010, which was seen as a technical potential.

e) The 2020 emissions for the SRES B2 baseline was estimated at 49.5 Gton CO2-eq (IPCC, 2000)

The updated estimates might be expected to be higher due to:

  • The greater range of economic potentials, extending up to 100 US$/tCO2, compared to less than 27.3 US$/tCO2 (100 US$/tC) in the TAR;
  • The different time frame: 2030 compared to 2020 in the TAR.

However, the overall estimated bottom-up economic potential has been revised downwards somewhat, compared to that in the TAR, especially considering that the AR4 estimates allow for about five more years of technological change. Part of the difference is caused by the lower coverage of mitigation options up to 2030 in the AR4 literature.