2.9.4 Future Climate Impact of Current Emissions

The changes in concentrations since pre-industrial time of the long-lived components causing the RF shown in Figure 2.20 are strongly influenced by the past history of emissions. A different perspective is obtained by integrating RF over a future time horizon for a one-year ‘pulse’ of global emissions (e.g., Jacobson (2002) used this approach to compare fossil fuel organic and BC aerosols to CO₂). Comparing the contribution from each forcing agent as shown in Figure 2.22 gives an indication of the future climate impact for current (year 2000) emissions of the different forcing agents. For the aerosols, the integrated RF is obtained based on the lifetimes, burdens and RFs from the AeroCom experiments, as summarised in Tables 2.4 and 2.5. For ozone precursors (CO, NO_x and NMVOCs), data are taken from Derwent et al. (2001), Collins et al. (2002), Stevenson et al. (2004) and Berntsen et al. (2005), while for the long-lived species the radiative efficiencies and lifetimes are used, as well as a response function for CO₂ (see Section 2.10.2, Table 2.14). Uncertainties in the estimates of the integrated RF originate from uncertainties in lifetimes, optical properties and current global emissions.

Figure 2.22 shows the integrated RF for both a 20- and 100-year time horizon. Choosing the longer time horizon of 100 years, as was done in the GWPs for the long-lived species included in the Kyoto Protocol, reduces the apparent importance of the shorter-lived species. It should be noted that the compounds with long lifetimes and short emission histories will tend to contribute more to the total with this ‘forward looking’ perspective than in the standard ‘IPCC RF bar chart diagram’ (Figure 2.20).

Figure 2.22. Integrated RF of year 2000 emissions over two time horizons (20 and 100 years). The figure gives an indication of the future climate impact of current emissions. The values for aerosols and aerosol precursors are essentially equal for the two time horizons. It should be noted that the RFs of short-lived gases and aerosol depend critically on both when and where they are emitted; the values given in the figure apply only to total global annual emissions. For organic carbon and BC, both fossil fuel (FF) and biomass burning emissions are included. The uncertainty estimates are based on the uncertainties in emission sources, lifetime and radiative efficiency estimates.