Stabilization of Atmospheric Greenhouse Gases: Physical. Biological and Socio-economic Implications The bold curve in the centre is the reference SO, emissions case (no change from 1990), for which there is no emissions/forcing uncertainty band. The upper three curves correspond to the case of decreasing SO2 emissions (by 50 per cent over 1990-2100) and give results for low, mid and high values of the 1990 sulphate aerosol forcing level (-1.1 ± 0.5 W m2). High 1990 forcing leads to a larger departure from the reference case. The lower three curves are for the case where SO2 emissions increase by 50 per cent over 1990-2100. Here, the high 1990 forcing case must again lead to a larger (this time, negative) departure from the reference case. 23 of stabilization. To do so comprehensively requires, at least, that regional-scale changes in temperature and sea level, and changes in other climate variables (such as rainfall or soil moisture) be considered. However, climate models are not yet sufficiently accurate to allow confident prediction of such regional, multivariate influences. The present analysis includes CO2, together with a number of possible combinations of other gas influences, as shown in Table 4. This approach was chosen to give some insights into the sensitivities of temperature and sea level to the assumptions regarding future greenhouse gas and SO, emissions. The approach is not meant to span the full range of possibilities. For each combination we compute four variables: (a) Radiative forcing (W m2); (b) The equivalent CO2 concentration associated with the particular combination of other gases: (c) Global mean temperature changes; The CO2 concentration stabilization profiles described above Low: High: Halocarbons Linear decrease by 50 per cent over 1990-2100, constant emissions after 2100 SAR WGI to 21008, constant emissions after 2100 Tropospheric 03 As SAR WGI: no direct changes after 1990. CH4-induced changes included with CH4 *With emissions adjusted to balance the 1990 budget, as in SAR WGI: Chapter 6. +75 TgS/yr as in the IS92 scenarios. A synthesis of emissions as given in Chapter 2 of SAR WGI, with other minor species as given in Chapter 6. Stratospheric ozone effects accounted for as in Chapter 6. Table 4. Emissions cases considered in the sensitivity studies. 24 Stabilization of Atmospheric Greenhouse Gases: Physical, Biological and Socio-economic Implications The primary calculations use the reference case of constant Results for (a) and (b) have been given in Section 2.2; this section considers the global mean temperature and sea level implications. Rates of change may be estimated graphically from the results provided. In addition, we need to consider uncertainties in the response of the climate system to external forcing, due largely to uncertainties in the climate sensitivity (we consider three cases, following SAR WGI (Section 6.3); viz. AT2 = 1.5, 2.5 and 4.5°C), and sea level rise uncertainties due to uncertainties in modelling ice-melt (SAR WGI: Chapter 7). For the latter, we span the range by considering low (AT2 = 1.5°C. combined with low ice-melt), mid (2.5°C, mid ice-melt), and high sea level rise cases (4.5°C, high ice-melt). This gives three sets of climate/sea level output for each forcing case. The results given use the Wigley and Raper (1992) models (see also Raper, et al., 1996) as employed in SAR WGI (Section 6.3). In SAR WGI a model developed by de Wolde and colleagues (e.g., de Wolde, et al., 1995) was used, but their climate model has a fixed sensitivity for temperature change at doubled CO2 of 2.2°C (AT2x = 2.2), which precludes its use in the present context. For information on model structure and intermodel differences, see IPCC TP SCM (1997). The results presented here provide a more unified view of the issues related to stabilization than is available from any single chapter in SAR WGI. The bulk of these results are for a climate sensitivity (AT) of 2.5°C, a mid-range value. If the true value were lower or higher, the results would scale accordingly, as discussed below. In addition, we emphasize that the results shown are globally averaged: both impacts and mitigative actions are sensitive to regional patterns of climate and sea level change, because regional opportunities and vulnerabilities are highly variable. The temperature and sea level results given here were computed using relatively simple models. As discussed in IPCC TP SCM (1997), these models are designed to reproduce, with reasonable fidelity, the globally averaged behaviour of complex models. They have also been compared to historical and/or present day observations. They, in common with more complex models, do not include all possible interactions and climate feedbacks, but they do reflect our We have noted above that the future emissions trajectories of Global mean temperature and sea level change results for 1990 It should be noted that global mean quantities are only indica- Figures 11a and b show temperature and sea level changes Stabilization of Atmospheric Greenhouse Gases: Physical, Biological and Socio-economic Implications Figure 11. (a) Projected global mean temperature when the concentration of CO2 is stabilized following the S profiles and the WRESSO and 1 000 profiles shown in Figure 4. CH4. N2O and SO2 emissions are assumed to remain constant at their 1990 levels and halocarbons follow an emissions scenario consistent with compliance with the Montreal Protocol (i.e., the reference case). The radiative forcing (and equivalent CO2) from which the global temperatures were derived were shown earlier in Figure 7. The climate sensitivity is assumed to be the mid-range value of 2.5°C. For comparison, results for the IS92a, cand e emissions scenarios are shown for the year 2100. To obtain the anthropogenic change in global mean temperature from 1880, based on the central estimate of historical forcing used in SAR WGI, 0.2-0.5°C should be added. To obtain the change from preindustrial times, a further 0.1-0.2°C should be added; (b) As for (a). but for global sea level change using central ice-melt parameters. All results were produced using the Wigley and Raper simple climate/sea level model (see IPCC TP SCM, 1997). Figures 12a and b illustrate how the emissions of non-CO2 gases might influence future global mean temperature and sea level change (for CO2 stabilization levels of 450 ppmv and 650 ppmv). The cases shown are the reference case used in Figure 12; the case where all emissions (other than CO2) follow IS92a to 2100; and the case where only CO2 changes are considered from 1990-i.e., where the radiative forcings for all other gases remain at their 1990 levels. Only the last case was considered in SAR WGI (see Figures 6.26 and 7.12). The 1990 2000 2010 2020 2030 2040 2050 2060 2070 2080 2090 2100 Year Figure 12. (a) The effect of different non-CO2 gas emission profiles on global temperature change for the $450 and S650 concentration profiles (see Figure 4). The solid lines give the "reference" results; the short dashed lines the "CO2 alone" results and the long dashed lines give results where CH4. N2O and SO2 emissions increase according to IS92a to 2100 (the "IS92a case"). The climate sensitivity is assumed to be the mid-range value of 2.5°C; (b) As for (a), but for global sea level change. Central values of the ice-melt parameters are importance of other gases is clearly seen from this Figure. Differences between the reference case and the case with IS92a emissions for other gases exceed the differences between $450 and S650 out to around 2050. The IS92a results are (to around 2050) lower than the others due to the global mean offsetting effect of increasing SO2 emissions in this scenario: but this hides important regional details and it does not necessarily mean that the severity of climate changes associated with this case (in the sense of their impacts) would be less. The results in Figures 11 and 12 are for "best guess" climate and ice-melt model parameters only. Figure 13 shows 450 ppmv and 650 ppmv results for different climate sensitivities (1.5, 2.5 and 4.5°C) coupled (for sea level rise) with low, mid and high icemelt model parameters respectively. Uncertainties related to model parameter uncertainties for any given stabilization level are much larger than the differences between the 450 ppmv and 650 ppmv stabilization level results, particularly for sea level. 25 Stabilization of Atmospheric Greenhouse Gases: Physical, Biological and Socio-economic Implications 1990 2000 2010 2020 2030 2040 2050 2060 2070 2080 2090 2100 Year Figure 13. (a) The effect of climate sensitivity uncertainties on global mean temperature for the $450 and $650 CO2 concentration profiles and the reference case for non-CO2 gases. The range of climate sensitivity (AT) is 1.5 to 4.5°C with a mid-range value of 2.5°C. For the same range in climate sensitivity, the global mean temperature change from 1990 to 2100 for the IS92a emissions scenario is between 1.4 and 2.9°C with a mid-range value of 2.0°C; (b) As for (a), but for global mean sea level change. The low, mid and high values of climate sensitivity are combined with low, mid and high ice-melt parameters, respectively, to give extreme ranges. For the same range in climate sensitivity and ice-melt parameteres, the global mean sea level rise from 1990 to 2100 for the IS92a emissions scenario is between 19 and 86 cm with a mid-range value of 49 cm. For planning purposes, reducing model parameter uncertainties would clearly be advantageous. These are uncontrollable aspects of the climate/sea level system, however, while the stabilization level is potentially controllable. The comparison in Figure 13. therefore, provides a graphic illustration of the extent of potential control relative to overall uncertainties in the climate and sea level projections. Figures 14 and 15 show the sensitivity of the 450 ppmy and 650 ppmv results to gas-specific uncertainties in future emissions: a change over 1990-2100 of ±100 Tg(CH4) about the reference CH, emissions case in Figure 14, and a change over 1990-2100 of ±50 per cent (i.e., 37.5 TgS) about the reference 1990 2000 2010 2020 2030 2040 2050 2060 2070 2080 2090 2100 Year Figure 14. (a) Sensitivity of global mean temperature change to CH4 emissions for the S450 and $650 concentration profiles (see Figure 4). The solid lines give the "reference" results; the "CH4 low"/"CH high" curves assume annual CH4 emissions decrease/increase linearly by 100 Tg(CH4) over 1990 to 2100 (see Table 4). The radiative forcing (and equivalent CO2) from which the global temperatures were derived were shown earlier in Figure 9a; (b) As for (a), but for global sea level change. Central values of the ice-melt parameters are assumed. SO2 emissions case in Figure 15. (The same sensitivity cases were considered in the assessment of forcing and equivalent CO2 uncertainties in Section 2.3.1.) N2O sensitivity is not shown because, for the ±2 Tg(N) perturbations considered previously, this is appreciably less in the near-term than for CH4 due to the long lifetime of N2O relative to CH4 (compare Figures 9a and 96). In the context of this sensitivity analysis, the long-term effects of CH, and SO, for the considered perturbations are relatively small compared with the differences between the results for different stabilization levels (see Figures A4 and A5 in Appendix 1). However, the short-term effects are, relatively, 5450 Global sea level change (cm) Global temperature change (°C) Stabilization of Atmospheric Greenhouse Gases: Physical, Biological and Socio-economic Implications Figure 15. (a) Sensitivity of global mean temperature change to SO2 emissions for the S450 and $650 concentration profiles (see Figure 4). As in Figure 10a, the solid lines give the "reference" cases; the short dashed lines show the "high SO2" cases where emissions increase linearly from 75 TgS/yr in 1990 to 112.5 TgS/yr in 2100 and the long dashed lines show the "low SO2" cases where emissions decrease linearly to 37.5 TgS/yr in 2100. The radiative forcing (and equivalent CO2) from which the global temperatures were derived were shown earlier in Figure 10a. (b) As for (a), but for global sea level change. Central values of the ice-melt parameters are assumed. 27 much larger (compare Figures 11 and A2). This is because both CH4 and SO-derived aerosol have much shorter response times than CO,. The full differential effects on climate related to different CO, stabilization targets therefore take much longer to manifest themselves compared with the more rapid responses to CH4 and SO, emissions changes. Although we cannot yet characterize the differences among stabilization levels and pathways in terms of their degree of risk, it is clear, as noted in SAR WGI (Section 6.3) and in Wigley, et al. (1996) that the choice of both stabilization level and pathway affects the magnitudes and rates of future climate and sea level change. Future emissions of other greenhouse gases also influence future climate and sea level appreciably, generally leading to larger changes than from CO2 emissions alone. Thus, mitigation of these other-gas emissions is a valuable component of a programme designed to prevent dangerous interference with the climate system. In the long-term (beyond 2100), uncertainties in the future emissions of CH4, N2O and SO, have effects that are generally less than those associated with the differences between different CO2 stabilization levels. In the short-term (to around 2050), however, the importance of other-gas emissions is, relatively, much larger. Uncertainties in future CH4 and SO2 emissions lead to climate change uncertainties that exceed those due to different CO, concentration profiles. The situation with regard to SO, emissions is more complex than that for greenhouse gas emissions because of their extreme spatial heterogeneity. The cooling effect of SO2 emissions cannot be considered as merely offsetting the warming effect of greenhouse gas emissions. |