18 Stabilization of Atmospheric Greenhouse Gases: Physical, Biological and Socio-economic Implications Table 1. Relative contributions to total global radiative forcing change over 1990-2100 of different gases under the IS92a, c and e emissions scenarios. The forcing values here are those used in SAR WGI (Section 6.3). The low, mid and high sulphate aerosol forcing values are based on 1990 forcings of: direct aerosol forcing: -0.2, -0.3, -0.4 W m2; indirect aerosol forcing: -0.4, -0.8, -1.2 W m2 (the full range of aerosol forcing uncertainty is larger than this; see SAR WGL, pp. 113-115). Only the mid-aerosol forcing values were used in SAR WGI (Section 6.3). Forcing values are given in W m2, non-CO2 gas forcing values are also given as percentages of the CO2 value. CH, forcing includes the related effects of tropospheric ozone and stratospheric water vapour changes. Halocarbon forcing includes the effects of stratospheric ozone changes. Given the very large uncertainties in the roles of the non-CO2 gases relative to CO2 under an "existing policies" assumption, and given that no comprehensive studies have been carried out to examine their roles under the assumption of concentration stabilization, we can only consider them in a sensitivity study context. We, therefore, begin with a set of reference cases in which the emissions of CO2 follow a range of stabilization pathways, the emissions of CH4, N2O and SO2 are assumed to remain constant at their 1990 levels, and halocarbons follow the Montreal Protocol scenario used in the SAR WGI (Section 6.3) global mean temperature and sea level calculations. For halocarbons in the reference scenarios we assume that the Montreal Protocol applies strictly (see SAR WGI: Chapters 2 and 6) so that there is only a single future scenario for these gases. Because the total forcing for these gases over 1990-2100 (accounting for the effects of stratospheric ozone changes) is relatively small (0.3 W m-2), uncertainties due to incomplete compliance with the Protocol and/or future emissions of substitute (hydrofluorocarbon, HFC) or non-controlled gases may be even smaller. In the context of global climate change, therefore, and given that they are not addressed by SAR WGI, we have chosen not to include these uncertainties. However, should a comprehensive (multi-gas) framework for stabilization be adopted, a more detailed gas-by-gas assessment of halocarbon forcing may be required at a specific country level. Because the calculations performed here run beyond 2100, some assumption must be made regarding halocarbon emissions after this date. If these emissions remain constant at their 2100 level, the forcing level would remain close to 0.3 W m2. This would stabilize halocarbon (primarily HFCs) concentrations at relatively high levels. For the reference cases we assume that halocarbon emissions remain constant at their 2100 levels. Hence, eventually, concentrations will remain constant in accordance with Article 2. We note, however, that the constant-2100 emissions assumption leads to a potential global mean forcing overestimate after 2100 of, eventually, up to 0.4 W m-2. For tropospheric ozone, in the absence of any projections, and again following SAR WGI (Section 6.3), we assume that the only forcing changes are those that arise from the ozone that is produced by methane induced changes in tropospheric chemistry. This term amounts to around 0.15 W m2 by 2100 under IS92a, but is much less for the reference case of constant CH emissions. Our assumption here may be unrealistic if nitrogen, hydrocarbon, or other ozone precursors associated with ozone concentrations increase due to a rise in anthropogenic pollution. It should be noted that we are not suggesting that the reference cases in any way reflect predictions of the future, especially with regard to future SO2 emissions, nor that they should be a target for policy. The point of the reference cases is to help assess the relative importance of CH4, N2O and SO2 emissions in determining future global mean temperature and sea level change. To quantify the sensitivity of equivalent CO2 to other gases we consider perturbations from the reference cases in which annual CH, emissions increase or decrease linearly over 1990-2100 by a total of ±100 Tg(CH4) (ie., £75 TgC) relative to 1990 and remain constant thereafter, annual N2O emissions increase or decrease linearly over 1990-2100 by a total of 12 Tg(N) relative to 1990 and remain constant thereafter, and annual SO2 emissions increase or decrease linearly over 1990-2100 by Stabilization of Atmospheric Greenhouse Gases: Physical, Biological and Socio-economic Implications £50 per cent (i.e.. 37.5 TgS) relative to their 1990 level and remain constant thereafter. For all three gases, these scenarios lead to concentration stabilization, effectively instantly for SO2, over a few decades for CH, and over a period of centuries for NO. To put these perturbations into a wider context, they are compared with IS92a, c and e in Table 2. Note again that these perturbations should not be construed as representing particular future outcomes or policy targets. 2.2.4 Stabilizing Equivalent CO2 Concentration Stabilizing the atmospheric concentrations of greenhouse gases, an explicit goal of Article 2, would not necessarily result in stabilizing the human caused perturbation in radiative forcing. This is because aerosols, which are not explicitly addressed by Article 2, also have radiative effects. If concentrations of both greenhouse gases and aerosols are stabilized, this would stabilize the human perturbation in global mean radiative forcing. Note also that because aerosols are not uniformly mixed gases, the geographical distribution of emissions of aerosols and their precursors can have important effects on regional climate. Stabilizing the human perturbation in global mean radiative forcing is clearly different from stabilizing CO2 concentration alone. Thus, while mitigation efforts may target members of a suite of greenhouse gases, impact studies must consider climates influenced by multiple gases and aerosols. “Equivalent CO2" is a technique for considering multiple radiative forcing components in the aggregate. In the calculations of future global mean temperature and sea level change given in SAR WGI (Chapters 6 and 7), the models were driven by the total radiative forcing, which "This will not eliminate climate variability because the climate system exhibits considerable natural variability, beyond anthropogenic influences. 19 was obtained by summing the forcings due to all anthropogenic trace gases (see Table 1). In global mean terms, this total forcing can be treated as if it came solely from changes in CO2; i.e., from an "equivalent CO2 concentration". The equivalent CO, concentration, Ceq. can be defined, therefore, from the relationship between actual CO2 concentration and radiative forcing. In SAR WGI, the relationship used was that from the First IPCC Assessment Report (IPCC, 1990). The uncertainty in this relationship may be up to approximately 120 per cent (see IPCC TP SCM, 1997). Although the equivalent CO, concept is pedagogically useful and provides a means to compare the effects of CO2 with other gases, it does have disadvantages. An important disadvantage arises from the non-linear relationship between radiative forcing and CO2 concentration. This nonlinear relationship means that, at higher CO2 levels, it requires a larger CO, change to increase radiative forcing by the same amount. Because of this, radiative forcing changes can be added, but CO, equivalents can not be. We have therefore retained the use of radiative forcing as our primary variable. A further disadvantage of the equivalent CO2 concept is that, in the context of impact assessments, it addresses only the climate change aspect. Other impacts of increasing CO2 (e.g., fertilization), sulphate aerosol (acidification), and ozone may also be important. Also with the equivalent CO2 concept, as with radiative forcing, a global aggregate measure subsumes information about regional aspects of climate change that are critical in assessing impacts. It would be possible, for example, to impose a forcing pattern on the climate system that had zero global mean forcing, but which would lead to large changes in regional climate. We now give equivalent CO2 results for different concentration stabilization levels. We consider S350, $450, S550, $650 $750. and WRE1000, together with the constant 1990-level emissions reference cases for CH4, N2O and SO2, and halocarbon emissions following the Montreal Protocol (see Section 2.2.2). To illustrate the dependence of equivalent CO2 level on the pathway to CO2 stabilization, we also consider WRE550. These reference case results are given in Figure 7, where the forcing values are given relative to 1990 (some 1.3 W m2 above the pre-industrial level). In the year 2500, close to the point of equivalent CO2 stabilization, the equivalent CO, concentrations vary from 26 ppmv (S350) to 74 ppmv (WRE1000) above the actual CO2 level. In all cases, the forcing difference due to gases other than CO2 is the same: 0.66 W m2 over 1990 to 2500. As noted above, this is equivalent to differing amounts of CO2 at different concentration levels because of the non-linearity of the equivalent CO/radiative forcing function. Note that here the mid-1990 equivalent CO2 level is 342 ppmv, slightly below the actual CO2 level (354 ppmv). This is Stabilization of Atmospheric Greenhouse Gases: Physical, Biological and Socio-economic Implications Radiative forcing (W m2) -2 $350 400 1992 342 Equivalent CO, (ppmv) Figure 7. Radiative forcing from 1990 to 2100 (relative to 1990) for CO2 concentrations following the $350, $450, S550, WRES50, $650, $750 and WRE1000 profiles (see Figure 4) and constant 1990 emissions of CH4. N2O and SO2. For halocarbons, a single emissions scenario consistent with compliance with the Montreal Protocol is assumed. These assumptions are referred to in the text and in later captions as the "reference case". Equivalent CO2 levels are shown by the dots on the right-hand axis. For the S450 (S650) profile for example, the CO2 concentration in 2100 is 450 (575) ppmv (from Figure 4), but the additional effect of other greenhouse gases and SO, gives an equivalent CO2 concentration of 473 (604) ppmv. These results were produced using the Wigley and Raper simple climate model (see IPCC TP SCM, 1997), and the radiative forcing/concentration relationships given in IPCC (1990) and subsequent updates. Radiative forcing (W m1) 1990 2000 2010 2020 2030 2040 2050 2060 2070 2080 2090 2100 Year Equivalent CO, (ppmv) Figure 8. The effect of different non-CO2 gas emission profiles on radiative forcing (and equivalent CO2) for the $450 and $650 concentration profiles (see Figure 4). The short dashed lines give the "CO2-alone" results; the solid lines the "reference case" (see Figure 7) and the long dashed lines give results where CH,. N2O and SO, emissions increase according to IS92a to 2100 and then stabilize (the "IS92a case"). Note that, initially, the radiative forcing for the reference case is less than for the "IS92a case". This is due to the negative forcing effect of aerosols. Note also that, for the CO-alone cases, the equivalent CO, levels are less than the actual CO, levels because of differences in their 1990 values. because, in 1990, the negative forcing due to aerosols more than offsets the positive forcing due to non-CO2 greenhouse gases. This value is, however, quite uncertain due mainly to uncertainties in the magnitude of aerosol forcing. For aerosol forcing uncertainties of ±0.5 W m2 in 1990, the 1990 equivalent CO2 level varies between 316 and 370 ppmv. The overall sensitivity to the assumptions regarding the emissions of non-CO2 gases is shown in Figure 8, for S450 and S650. Here, the same reference cases (Figure 7) are shown together with cases where IS92a emissions are used for CH4. N2O and SO2 out to 2100 with constant emissions thereafter. In this second case, the eventual forcing increment from 1990 due to non-CO2 gases is 1.13 W m2 (compared with 0.66 W m2 for the reference case). The equivalent CO2 levels in 2100 are 491 ppmv (S450) and 627 ppmv (S650) compared with 473 ppmv (S450) and 604 ppmv (S650) for the reference cases. Figure 8 also shows the forcing due to CO2 alone. The results presented in Figures 7 and 8 are characterized and summarized in Table 3. This shows radiative forcing changes from 1765 and equivalent CO2 levels for CO2 stabilization levels of 350 ppmv to 1 000 ppmv under three different assumptions regarding the forcing effects of other gases: no other-gas effects (i.e., CO2 changes alone), the reference case (constant CH4. N2O, and SO2 emissions), and the extended IS92a case. Results are shown at the date of CO2 stabilization (which varies according to stabilization level). The above calculations are presented to illustrate the importance of other gases in determining the equivalent CO2 level, and the overall level of uncertainty involved in determining their contribution. None of the cases studied (CO2 alone, constant 1990 emissions, or IS92a based emissions for CH. NO and SO2) should be taken as a particular future scenario, nor as a policy recommendation. The results show that the concentration stabilization levels chosen for CH4. N2O and SO2 may have a significant influence on future equivalent CO2 changes and on the equivalent CO2 stabilization level. Individual sensitivities are addressed in the next section. As a final point in this section, we note that equivalent CO2 levels do not stabilize in our examples, even by 2500. Small but noticeable forcing changes (of order 0.1-0.3 W m2) occur after the point of CO2 stabilization (viz. 2100 in S450, 2200 in S650), due mainly to the long lifetime of N2O, which leads to significant concentration changes for this gas after emissions stabilize in 2100. Changes after 2500, however, are very small. Stabilization of Atmospheric Greenhouse Gases: Physical, Biological and Socio-economic Implications 21 Table 3. Equivalent CO2 (ppmv) and radiative forcing (from 1765) (AF) at the point of CO, stabilization, for various assumptions about nonCO2 greenhouse gases and aerosols. The reference case assumes constant emissions for SO,, NO and CH, after 1990. The "CO2 only" column assumes changes after 1990 are in CO2 only (as in SAR WGI). Note that the equivalent CO2 level at CO2 stabilization in these cases differs from the CO2 stabilization level because of differences between the 1990 CO, and equivalent CO, levels. For CH4 (Figure 9a), a perturbation in annual emissions from 1990 to 2100 of ±75 TgC (±100 Tg(CH)) changes radiative forcing by approximately ±0.20 W m2 at concentration stabilization. This translates to equivalent CO2 differentials of approximately 15 ppmv for S450 and +22 ppmv for $650. For annual N2O emissions, a perturbation of ±2 Tg(N) from 1990 to 2100 changes forcing by 10.16 W m2 at concentration stabilization, and gives concentration differentials of 12 ppmv for $450 and ±18 ppmv for S650 (see Figure 9b). Sulphur dioxide sensitivities occur in two ways. First, there is the basic sensitivity to emissions uncertainties (Figure 10a). At concentration stabilization, perturbations of £50 per cent relative to 1990 in annual SO2 emissions (i.e., ±37.5 TgS) lead to forcing differentials of -0.37/+0.45 W m2, which translates to equivalent CO, concentration differentials of -27/+36 ppmv for S450 and -40/+52 ppmv for S650 (note that the sign of the forcing or concentration differential is opposite to the sign of the emissions perturbation). In addition to the influence of emissions uncertainties, the effect of SO, on equivalent CO2 concentrations is sensitive to the highly uncertain relationships between SO2 emissions and radiative forcing. SO2-derived sulphate aerosol affects radiative forcing both directly, under clear-sky conditions, and indirectly, through changes in cloud albedo. The central estimate of direct sulphate aerosol forcing for 1990 was calculated in SAR WGI as -0.4 W m2, an estimate of -0.8 W m2 was used in Section 6.3 of SAR WGI for the indirect forcing. When combined with a carbonaceous (soot) aerosol forcing of +0.1 W m2, this gives a total sulphate aerosol forcing of -1.1 W m2. To assess the sensitivity to uncertainties in this quantity, we use the range of ±0.1 W m2 for direct forcing and ±0.4 W m2 for indirect forcing (giving a total sulphate (plus soot) aerosol forcing range of -1.1 ± 0.5 W m2). 22 Radiative forcing (W m2) Radiative forcing (W m2) Stabilization of Atmospheric Greenhouse Gases: Physical. Biological and Socio-economic Implications Figure 9. (a) The sensitivity of radiative forcing (and equivalent CO2 concentration) to CH, emissions for the $450 and $650 concentration profiles (see Figure 4). The "CH, low"/"CH, high" curves assume annual CH, emissions decrease/increase linearly by 100 Tg(CH) over 1990 to 2100 (see Table 4); (b) The sensitivity of radiative forcing (and equivalent CO2 concentration) to N2O emissions for the $450 and $650 concentration profiles (see Figure 4). The "N2O low"/"NO high" curves assume annual N2O emissions decrease/increase linearly by 2 Tg(N) over 1990 to 2100 (see Table 4). The way this emissions/forcing uncertainty manifests itself initially in our calculations is in the 1990 equivalent CO2 level. As noted earlier, whereas the "best guess" value of Ceq(1990) is 342 ppmv, the range corresponding to ±0.5 W m2 in the 1990 aerosol forcing level is 316-370 ppmv. For future forcing, if we use the reference case of no change in SO2 emissions, then the emissions/forcing uncertainty has no effect-zero emissions change means zero forcing no matter what the emissions/forcing relationship is. The 1990 forcing uncertainty is simply propagated "as is" into the future (Figure 106). Radiative forcing (W m2) ▲ Equivalent CO, (ppm) Radiative forcing from 1765 (W m1) If, however, future SO2 emissions increase or decrease from their 1990 level (as in the emissions perturbation cases considered in Figure 10a), then the emissions/forcing uncertainty does affect future aerosol forcing. This is illustrated in Figure 10c, where (for the S650 case only) we show the uncertainties associated with both emissions and forcing together. Radiative forcing (W m1) 0 342 1990 2000 2010 2020 2030 2040 2050 2060 2070 2080 2090 2100 Year Equivalent CO, (ppm) 0 278 1990 2000 2010 2020 2030 2040 2050 2060 2070 2080 2090 2100 Year 0 1990 2000 2010 2020 2030 2040 2050 2060 2070 2080 2090 2100 Year Figure 10. (a) Sensitivity of radiative forcing (and equivalent CO2 concentration) to SO2 emissions for the $450 and S650 concentration profiles. The solid lines give the "reference" cases; the short/long dashed lines show the "high SO2/low SO2" cases where emissions increase/decrease linearly by 50 per cent over 1990-2100; (b) Sensitivity of radiative forcing (and equivalent CO2 concentration) to sulphate aerosol forcing in 1990 (relative to pre-industrial times) of -0.6, -1.1 and -1.6 W m2. respectively. Note that the radiative forcing values in this Figure are relative to pre-industrial, (c) The combined effects on radiative forcing (and equivalent CO2 concentration) of sensitivity to SO2 emissions and 1990 aerosol forcing for the $650 concentration profile only. E high/E low indicates increasing/decreasing emissions of SO2 from 1990 to 2100 (these are the same as the corresponding curves in Figure 10a); Q high/Q low indicates high/low 1990 aerosol forcing (these are the same as the corresponding curves in Figure 10b). |