An empirical approach toward the SLCP reduction targets in Asia for the mid-term climate change mitigation

Although importance of co-control of SLCPs together with the emission reduction of CO2 has attracted much attention for the mid-term climate change mitigation, the contribution to radiative forcing (RF) is rather complex, and chemistry-climate model analysis for the future scenario tends to give a “black box” for the contribution of each species. In order to deliver a more straightforward message on the effect of the reduction of SLCPs to policymakers, we propose “top-down” reduction targets of CH4 and tropospheric O3 in reference to the historical levels of their RF. Although the RF increase due to the increasing CO2 concentration is inevitable in mid-term future (ca. 0.80 W m−2 in 2040), the RF of CH4 and O3 is expected to decrease from 0.48 to 0.41, 0.34, 0.27, and 0.22 W m−2, and from 0.40 to 0.29, 0.23, 0.19, and 0.15 W m−2, respectively, if their atmospheric concentrations decrease from the level of 2010 to those of 1980, 1970, 1960, and 1950, according to the IPCC 2013 database. Consequently, the sum of ΔRFx(CH4) and ΔRFx(O3) (the difference of RF between the target year of x and 2010 as the base year) are 0.18, 0.31, 0.42, and 0.51 W m−2 in 1980, 1970, 1960, and 1950, indicating that the increase of ΔRF2040(CO2) can be compensated by 23, 39, 53, and 64%, respectively. The policy target can be selected from the combination of different target years each for CH4 and O3. With this global reduction ratio, the necessary reductions in CH4, NOx, and NMVOC in Asia were estimated and compared with the GAINS model-based cost-beneficial reduction amount proposed by the Solution Report prepared under UN Environment Asia and the Pacific Office. In order to attain the targeted reduced emission level of CH4 and NOx, new technology/practice for the reduction of livestock emission of CH4 and energy transformation from fossil fuel to renewable energy is highly advantageous for NOx reduction from industrial/power plant sources.