The global emissions scenarios in Figures 3 and 4 are interpreted in terms of future CO2 concentrations by making use of a carbon cycle model. These calculations have been carried out using the three models employed previously in the Working Group I volume of the IPCC Second Assessment Report13 (Schimel, et al., 1996) and in TP3:

⚫ Jain, et al. (1995);

Siegenthaler and Joos (1992; see also Joos, et al., 1996); and Wigley (1993).

All three models give similar results. Only results from the Siegenthaler and Joos model (referred to in SAR WGI and TP3 as the Bern model) are shown here. To carry out these calculations it is necessary to specify, not only the fossil CO2 emissions that the various emissions limitation cases define, but also emissions from land-use changes. The appropriate IS92a, c or e land-use change emissions scenarios from IPCC92 are used here (Leggett, et al., 1992).

We consider concentration effects (a) up to 2030, using the full set of emissions limitation proposals, and (b) up to 2100 using the two sets of proposals that allow such an extended analysis (i.e., the FR and NL proposals):

(a) Concentration effects up to 2030. Figure 5 shows CO2 concentrations for the full set of proposals for Annex I countries, where the proposed emissions limitations are combined with IS92a emissions for non-Annex I countries (see Figure 3 for emissions). In this case, the baseline for Annex I countries, which determines the magnitude of the emissions reductions, is also IS92a. CO2 concentrations for the original (no-limitation) IS92a emissions scenario are shown for comparison (i.e., where both Annex I and nonAnnex I country emissions follow the IS92a scenario). The relatively wide range of emissions differences in the years up to 2030 (Figure 3) result in only small concentration differences (Figure 5). In the year 2010, the emissions limitation cases differ by less than 3 ppmv, ranging from 3.7 to 6.2 ppmv below the no-limitation (IS92a) case. The concentration differences are small because, even though the endpoint emissions (in 2010-2030) differ markedly between the various cases, the cumulative global emissions differences are small relative to the total cumulative emissions in any particular case.

For the four emissions limitation cases that extend to 2030, the range of concentrations is 14-25 ppmy below the IS92a no-limitation case. The lowest concentration is for the DK proposal (see Tables 2 and 3); this is less than 3 ppmv below the next lowest (viz. NL-2%). Thus, the full concentration range is well represented by the FR and NL proposals.

(b) Concentration effects up to 2100. Figure 6 shows concentration results out to 2100 for the emissions shown in Figure 4 (i.e., for the FR and NL emissions limitation

proposals for Annex I countries, combined with IS92a, cor

e emissions for non-Annex I countries). This figure shows the long-term effect of the different limitation proposals in reducing future CO2 concentrations.

For the limitation cases where the baseline for the Annex I emissions reductions is IS92a, the concentration reductions are substantial. It should be noted, however, that, in all these cases, CO2 concentrations in 2100 exceed double the preindustrial level (ie., above 2 × 278 = 556 ppmv), and are rising rapidly at this time (at a rate in the year 2100 of more than 3 ppmv/yr, compared with the 1980-1989 long-term rate of around 1.5 ppmv/yr - see SAR WGI, Figure 2.2); there is no indication that CO2 concentrations are beginning to stabilize.

The situation using IS92e emissions as the baseline for Annex I country emissions is qualitatively the same as for IS92a. Since the baseline is higher, the emissions reductions under the limitation proposals are larger, so the CO2 concentration reductions are also larger. Nevertheless, concentrations still attain high levels by 2100 (2.6-2.9 times the pre-industrial level) and are increasing very rapidly at this time (at 7-9 ppmv/yr, about five times the current rate of increase). There is no indication of any tendency towards stabilization.

When the IS92c scenario is used as the baseline, the situation is markedly different from the IS92a and e cases. In this case, the concentration reductions resulting from the limitation proposals are much more modest (8-33 ppmv in 2100). This is because emissions for Annex I countries under IS92c are quite similar to those under the limitation proposals (see Figure 1) and because emissions for non-Annex 1 countries under IS92c, the lowest of the IPCC emissions projections, never exceed 4 GtC/yr (Figure 2). With the limitation proposals, there is a clear tendency towards CO2 concentration stabilization (eventually at around 500 ppmv if the emissions trends in 2100 were extrapolated beyond 2100). By 2100 the rate of increase in concentration in all cases where IS92c is used as the baseline for emissions is much less than the current rate of increase (1.5 ppmv/yr). This is also the case for the original IS92c global emissions scenario.

Implications of Proposed CO, Emissions Limitations

the carbon cycle model calculations in a way that ensures a balanced carbon budget over the 1980s (see Enting, et al., 1994, and TP3 for further details of the procedure). Changing Dn80s in turn changes the magnitude of the terrestrial CO2 fertilization sink used in balancing the 1980s-mean carbon budget. If the implied fertilization effect is constrained to lie within a priori defined realistic limits, this method also accounts for uncertainties in the atmosphere-to-ocean CO2 flux (Wigley, 1993; Enting, et al., 1994). A reasonable estimate of this uncertainty range may be obtained by using Dn80s = 0.4–1.8 GtC/yr (compared with the standard central value of 1.1 GtC/yr). Low values of Dn80s lead to a reduced-magnitude fertilization sink and, hence, to higher concentrations, and vice versa. In the IS92a (no-limitation) case, for example, the 2100 concentration uncertainty is approximately ±50 ppmv (see Table 5). As noted in SAR WGI (Schimel, et al., 1996) and in TP3, there are other uncertainties associated with possible climate-related changes in the terrestrial biosphere and the ocean that could inflate this uncertainty range appreciably.

While uncertainties in concentration levels for any given case are substantial, uncertainties in the concentration reductions resulting from the various emissions limitation proposals are much less - approximately ±10 ppmy for the cases where nonAnnex I country emissions follow IS92a (see the bracketed values in Table 5). This is because all emissions cases are subject to similar concentration uncertainties associated with the baseline upon which the limitations are imposed. The limitation proposals modify the baseline cumulative emissions by no more than 20 per cent, so the concentration uncertainty

450

400

350

1990 2000 2010 2020 2030 2040 2050 2060 2070 2000 2090 2100 Year

Figure 6. Global CO2 concentrations (ppmv) calculated using the Bern model for the IS92a, cand e emissions scenarios compared with their modifications where Annex I country emissions follow the French (FR-Low, FR-Central, FR-High) or Netherlands (NL-1%. NL2) emissions limitation proposals and non-Annex I countries follow

Table 5. Global CO2 concentrations (ppmv) in the year 2100 and (in brackets) concentration reductions for the emissions limitations proposals (NL-1%, NL-2% and FR-Central) when Annex I country emissions under these proposals are combined with IS92a emissions for non-Annex I countries. Concentration values are mid-year values. Reductions are relative to the "no emissions limitation" (IS92a) case. Estimates are given for three different values of the average net landuse change emissions (GtC/yr) during the 1980s (Dn80s) in order to reflect uncertainties in modelling the carbon cycle. Lower Dn80s leads to lower CO, fertilization and higher concentrations. Note that the uncertainty range for IS92a is at least ±50 ppmv, while the uncertainty range for the concentration reductions is only around ±10 ppmv.