Stabilization of Atmospheric Greenhouse Gases Physical. Biological and Socio-economic Implications 450 ppm Year 14 12 10 Anthropogenic emissions (GC/yr) 2140 Year 16 14 121 $750 2 Year 10622 750 ppm ogenic emissions (CIC/yr) Year 1592 $50 ppmv Anthropogenic emissions (CIC/yr) Year 16 14 (d) 450 $450 Year 14 12 10 Anthropogenic emissions (CIC/yr) (b) Anthropogenic emissions (CIC/yr) $650 Year 2200 Figure 5. Implied anthropogenic (fossil fuel, cement and land-use) emissions of CO2 from 1990 to 2300 that achieve stable CO2 concentrations via the profiles shown in Figure 4 computed using the Bern carbon cycle model. The period from 1990 to 2050 is shown in more detail in the expanded panel with the CO2 emissions from IS92a, c and e for comparison. The WRE results, which allow CO2 emissions to follow IS92a initially, have higher maximum emissions than results for the S profiles, but have a more rapid and earlier transition from increasing to decreasing emissions. The analyses in IPCC94 and SAR WGI (Section 2.1) show that results from other models may differ from these results by ±15 per cent. Further uncertainty arises from inadequacies in our understanding, and exclusion from the carbon cycle models used in SAR WGI (Section 2.1), of critical biospheric processes and their responses to climate change (see Section 2.2 10 650 ppm Year 5430 WREAST 15 Stabilization of Atmospheric Greenhouse Gases: Physical, Biological and Socio-economic Implications 2050 2080 2110 2140 2170 2200 2230 2260 2290 Figure 6. Anthropogenic CO2 emissions accumulated over time from 1990. Initially, the cumulative emissions are lower for the S profiles than the WRE profiles, but as the emissions are accumulated over progressively longer periods the results for the two profiles converge for any given stabilization level. Note that the higher the eventual stabilization level, the greater the cumulative emissions (referred to as "the carbon budget in the text) and the later the convergence of the two different profiles. These results were produced using the Bem carbon cycle model (see Section 2.3.3 and the caption of Figure 5 for a discussion of uncertainties). 2.2.1.3 Critical Carbon Cycle Uncertainties For calculations of emissions consistent with a range of stabilization levels and pathways, SAR WGI (Section 2.1) used models and input data which were generally accepted at that time. In this Technical Paper, we review and synthesise material from the SAR and so rely on the models as presented in that document. However, work reviewed in SAR WGI (Section 2.1 and Chapters 9 and 10), suggested that the mechanisms not included in simplified global carbon cycle models could affect the results significantly. Uncertainties resulting from the omission of some potentially critical oceanic and biospheric processes, and their response during transient climate changes could have a significant effect on the conclusions regarding impacts. tion from nitrogen deposition; climate change (Dai and Fung. 1993); and land-use change (SAR WGI: Section 2.1 and Chapter 9). Some of these mechanisms, such as nitrogen deposition, may "saturate" in their effects and even cause forest die back in the future. Although sensitivity to these interactions has been explored (e.g.. VEMAP, 1995), no consensus yet exists on how best to incorporate them into simplified models. Synthesising the results in SAR WGI (Section 2.1 and Chapter 9) and IPCC94 (Chapter 1). biospheric exchange could modify the cumulative emissions from fossil fuels during stabilization by ±100 GtC from the cases discussed. The impact of this on mitigation costs will be discussed in Section 3.2. In addition, no climate feedback to ocean circulation and biogeochemistry or terrestrial ecosystems has been included in the carbon cycle model calculations of emissions from concentrations. There is theoretical (Townsend, et al., 1992; IPCC94: Chapter 1) and observational evidence to support a significant sensitivity of biospheric CO2 emissions to temperature (Keeling, et al., 1995). However, any such temperature sensitivity probably varies geographically (IPCC94: Chapter 1) and its overall effects are thus sensitive to regional climate changes. rather than changes in the global mean (see Section 3.1). Warming and changes in precipitation could cause short-term effluxes of carbon from ecosystems (Smith and Shugart, 1993; Townsend, et al. 1992; Schimel, et al.. 1994; Keeling, et al.. 1995; SAR WGI: Chapter 9) but could also cause long-term accumulation (VEMAP, 1995). Climate feedbacks could significantly affect the oceanic carbon cycle as well. In IPCC94 (Chapter 1), a long-term range of uncertainty for future ocean uptake was estimated as -120 ppmv to +170 ppmv, based on assumptions regarding the role of biological processes in potential future oceans with two different steady state oceans. The impacts of changing ocean circulation during a climate transition (as in Manabe and Stouffer, 1994), however, have not yet been examined. The potential effects of ocean carbon cycle changes could noticeably modify the fossil fuel emissions consistent with stabilization, and future analyses should take account of these factors. The models of the carbon cycle used in SAR WGI, and relied Stabilization of CH, NO and Other Gases The potential global mean temperature and sea level consequences of the various CO, concentration stabilization profiles are described in Section 2.2.4. To make these calculations, assumptions are needed concerning how the emissions or concentrations of other gases may change in the future, because CO, is not the only anthropogenic climate forcing factor. Although Article 2 of the UN/FCCC has stabilization of greenhouse gas concentrations in general as its goal, it does not specify stabilization levels nor the pathways to stabilization. Furthermore, the UN/FCCC does not cover SO, a major aerosol precursor. nor other aerosols or aerosol precursors. Here, therefore, we consider a range of possibilities of how other trace gases may change in the future. 46-495-26 Stabilization of Atmospheric Greenhouse Gases: Physical, Biological and Socio-economic Implications The greenhouse gases other than CO2 that must be considered are those covered in SAR WGI: CH4, N2O, the halocarbons, and tropospheric ozone. Water vapour, also a greenhouse gas, enters into our analysis as a part of climate feedback (see IPCC TP SCM, 1997). Methane influences climate directly and also through its effects on atmospheric chemistry (generating tropospheric ozone) and as a result of its oxidation. Oxidation of methane affects tropospheric OH concentration and thereby influences the oxidizing capacity of the atmosphere, and, thus, the concentrations of other trace gases, and adds water vapour to the stratosphere. Halocarbon-induced ozone depletion in the lower stratosphere also has climatic consequences that must be accounted for (see SAR WGI: Section 2.4 and IPCC TP SCM, 1997). Finally, the emissions of SO2 (which are oxidized to sulphate species) lead to the production of aerosol which acts to cool the climate by reflecting sunlight (SAR WGI). Sulphate particles may also act as condensation nuclei, thereby changing the radiative properties of some clouds. Assessing the general implications of Article 2, involving the stabilization of all greenhouse gases (i.e., not just CO2) is difficult because we lack clearly defined ranges for likely future emissions of methane, N2O, SO2 and other gases. Thus, one can construct a near-infinite number of factorial combinations for the various gases. We have attempted to choose some illustrative combinations to demonstrate the potential sensitivity of radiative forcing and climate responses to a range of combinations of gases and aerosols. We have not tried to "bound" the problem, as there is no agreement on the likely ranges of future methane and N2O emissions, reflecting uncertainties in the biogeochemistry and in the sensitivity of emissions of these gases to climate. Nor is there agreement on future SO2 emission ranges, which depend upon technology choices, economic activity, and the extent to which "clean air" policies become global. The effects of sulphate aerosol are particularly difficult to evaluate in this regard. Aerosol effects have been important to date (see, e.g., SAR WGI: Chapter 8; Penner, et al., 1994; Mitchell, et al., 1995), and so must be included in any model calculations of future climate change, because the magnitude of these changes depends on the assumed history of past radiative forcing. For future climate change projections, aerosol related uncertainties are of considerable importance. These uncertainties arise for two reasons: through the uncertain relationship between SO2 emissions and radiative forcing; and through uncertainties regarding future SO2 emissions. These uncertainties are addressed here (see below) because they have been considered in the literature described in SAR WGI: Raper, et al. (1996) address the former uncertainty (by assuming different values for the 1990 level of aerosol forcing), whereas Wigley, et al., (1996) consider the latter (by evaluating future scenarios with increasing and constant SO2 emissions). Stabilization calculations in SAR WGI (Section 6.3) assume quite specific but arbitrary scenarios for these other gases (constant emissions for SO2, constant concentrations for non-CO2 greenhouse gases after 1990). In the climate 17 calculations for the IS92 emissions scenarios, SAR WGI considers a wider range of possible future scenarios for aerosols and non-CO2 greenhouse gases. In particular, for sulphate aerosol, SAR WGI considers both changing SO2 emissions (as prescribed by the IS92 scenarios) and constant post-1990 SO2 emissions. The approach we take here is directed towards estimating both overall and individual gas sensitivities. It is based on data given by SAR WGI regarding future non-CO2 greenhouse gas concentrations and the models used to derive these concentrations, and on the simplified climate model used in SAR WGI (Section 6.3), which (as we do here) uses individual gas forcing data as its primary input. Some insight into the importance of non-CO2 gases can be obtained by looking at the relative contributions of different gases to forcing under the IS92 scenarios (see Table 1). This shows that, under a range of “existing policies" scenarios, CO2 is by far the dominant gas. Cumulatively, however, the effects of the non-CO2 greenhouse gases may be quite appreciable: over 1990-2100 their contribution ranges from 0.7 W m2 (IS92c) to 1.8 W m22 (IS92ƒ, not shown in Table 1). As percentages of the CO2 forcing, non-CO2 greenhouse gas forcing ranges from 28 per cent (IS92e) to 40 per cent (IS92c). This contribution is noticeably offset by negative aerosol forcing in IS92a, b, e, and f, but in IS92c and d, changes in aerosol forcing add to the forcing from other gases because SO2 emissions in 2100 are less than in 1990. When aerosol and non-CO2 greenhouse gas forcings are combined, their total over 1990-2100 in the IS92 scenarios ranges from 0.4 W m22 (IS92e) to 1.0 W m-2 (IS92c and f). When expressed as percentages of CO2 forcing, the values for non-CO2 gases range from 9 per cent (IS92e) to 53 per cent (IS92c). The figures given here are those used in SAR WGI (Section 6.3). For aerosol, SAR WGI (Section 6.3) uses only a central estimate for the relationship between SO2 emissions and aerosol forcing (which has a total sulphate aerosol forcing contribution to 1990 of -1.1 W m22, compared to the total greenbouse gas contribution of 2.6 W m-2). Changing the aerosol forcing would decrease or increase its relative importance; but this clearly would not affect the undoubted significance of nonCO2 greenhouse gases. It should be noted that aerosol forcing uncertainties are exacerbated by uncertainties in future SO2 emissions, and by the uncertain influences of other aerosols (due to biomass burning. mineral dust, nitrates, etc.). With regard to future emissions, recent studies (IIASA/WEC, 1995) suggest that SO2 emissions may be lower in the future than assumed in the IS92a and e scenarios. If so, the global offsetting effect in Table 1 may be overestimated, but SAR WGI accounts for this possibility by considering cases in which future SO2 emissions are beld constant at their 1990 level (see SAR WGI: Section 6.3). Future SO2 emissions are the subject of some controversy, with strong arguments being presented for the likelihood of both increasing and decreasing emissions. 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 m22; indirect aerosol forcing: -0.4, -0.8. -1.2 W m2 (the full range of aerosol forcing uncertainty is larger than this; see SAR WGI, 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. 2.2.3 Reference Stabilization Scenarios 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 SO, 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 m2), 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 CH4 emissions increase or decrease linearly over 1990–2100 by a total of £100 Tg(CH1) (i.e., ±75 TgC) relative to 1990 and remain constant thereafter; annual N2O emissions increase or decrease linearly over 1990-2100 by a total of ±2 Tg(N) relative to 1990 and remain constant thereafter, and annual SO2 emissions increase or decrease linearly over 1990-2100 by 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, cand 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 Wm2 (the full range of aerosol forcing uncertainty is larger than this; see SAR WGI, 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. 2.2.3 Reference Stabilization Scenarios 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 m2), 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 m22. 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) (i.e., £75 TgC) relative to 1990 and remain constant thereafter, annual N2O emissions increase or decrease linearly over 1990-2100 by a total of ±2 Tg(N) relative to 1990 and remain constant thereafter, and annual SO2 emissions increase or decrease linearly over 1990-2100 by |