Stabilization of Atmospheric Greenhouse Gases: Physical, Biological and Socio-economic Implications the frequency of extreme events, will vary greatly among regions. In order to evaluate the consequences of climate change, one must consider the spatial variability of all factors: climate forcing, climate response, and the vulnerability of regional human and natural resource systems. However, consideration of regional details is outside the scope of this paper. The spatial patterns of some radiative forcing agents, especially aerosols, are very heterogeneous and so add further to the spatial variability of climate change. In this paper, aerosol forcing is presented in terms of global averages so that an impression can be gained of its likely overall magnitude, its effect on global average temperature, and its effect on sea level rise. The effect of aerosol forcing on the detail of climate change, however, is likely to be quite different from the effect of a forcing of similar magnitude, in terms of global average, due to greenhouse gases. In terms of regional climate change and impacts, therefore, the negative forcing or cooling from aerosol forcing must not be considered as a simple offset to that from greenhouse gases. Temperature and sea level projections depend on the assumed climate sensitivity, the target and pathway chosen for CO2 concentration stabilization, and the assumed scenarios for other greenhouse gases and aerosol forcing. The relative importance of these factors depends on the time interval over which they are compared. Out to the year 2050, CO2 concentration pathway differences for any single stabilization target are as important as the choice of target; but on longer time-scales the choice of target is (necessarily) more important. Outweighing all of these factors, however, is the climate sensitivity, uncertainties in which dominate the uncertainties in all projections. 1.2.5 Impacts A great deal is known about the potential sensitivity and vulnerability of particular systems and sectors, and both substantial risks and potential benefits can be identified. Currently however, our ability to integrate this information into an assessment of impacts associated with different stabilization levels or emissions trajectories is relatively limited. While the regional patterns of future climate change are poorly known, it is clear that the altered patterns of radiative forcing associated with anthropogenic emissions will alter regional climates noticeably, and will have different effects on climate conditions in different regions. These local and regional changes will necessarily include changes in the lengths of growing seasons, the availability of water, and the incidence of disturbance regimes (extreme high temperature events, floods, droughts, fires, and pest outbreaks), which, in turn, will have important impacts on the structure and function of both natural and human-made environments. Systems and activities that are particularly sensitive to climate change and related changes in sea level include: forests; mountain, aquatic and coastal ecosys tems; hydrology and water resource management (including the 10 1.2.7 Stabilization of Atmospheric Greenhouse Gases: Physical, Biological and Socio-economic Implications Integrating Information on Impacts and Mitigation Costs This reports provides a framework for integrating information on the costs, benefits and impacts of climate change. The points below must be prefaced with the critical observation that concentration stabilization profiles that follow "business-as-usual" emissions for periods of a few to several decades should not be construed as a suggestion that no action is required for those periods. In fact, studies suggest that even in those cases of business-as-usual emissions for some period of time, actions must be taken during that time to cause emissions to decline subsequently. The strategies for developing portfolios of actions leading to immediate or eventual reductions below business-as-usual are discussed below. This paper is designed to demonstrate how information can be assembled on the costs, impacts and benefits of stabilizing atmospheric greenhouse gases. This analysis, which supports many decision making formats, has two "branches". The first branch, "impacts", assembles information beginning with assumed concentration changes, and then evaluates potential climate change, and its consequences. The second branch, "mitigation", assembles information on emissions and mitigation costs associated with achieving a range of stabilization pathways and levels. The two branches must be combined to produce an integrated assessment of climate change and stabilization (Figure 3). If expressed in terms of CO2 equivalent or total radiative forcing. a given stabilization level can be met through various combinations of reductions in the emissions of different gases and by enhancing sinks of greenhouse gases. Considering all such options, and selecting the least expensive ones while taking account of different sources and sinks, should lower the costs of mitigation. Approaching an optimum mix requires information about the concentration and climate implications of different emissions strategies, the mitigation costs and other characteristics of the different options, and decisions about the appropriate timescales and indices of impacts (climate and non-climate) to be used in comparing the different gases. Because of high uncertainty, as improved information becomes available, these mixes of options must be re-evaluated and modified in an evolving process. In order to implement a portfolio of actions to address climate change, governments must decide both the amount of resources to devote to this issue and the mix of measures they believe will be most effective. Because no-regrets policies are currently beneficial, the issues facing governments are how to implement the full range of no-regrets measures and whether, and if so, when and how far to proceed beyond purely no-regrets options. The risk of aggregate net impacts due to climate change, consideration of risk aversion, and the application of the precautionary principle provide rationales for action beyond no-regrets (SAR WGM). Numerous policy measures are available to facilitate adaptation to climate change, to reduce emissions of greenhouse gases, and to create technologies that will reduce emissions in the future. These include immediate reductions in emissions to slow climate change; research and development on new supply and conservation technologies to reduce future abatement costs; continued research to reduce critical scientific uncertainties; and investments in actions to help human and natural systems adapt to climate change through mitigation of negative impacts and through advantages resulting from increasing CO2 (6.8. increased water or nutrient use efficiency of some crops with elevated CO22). The issue is not one of "either-or" but one of finding the right mix (i.e., portfolio) of options, taken together and sequentially. The mix at any point in time will vary and depend upon the concentration objective, which may itself be adjusted with advances in the scientific and economic knowledge base. The appropriate portfolio also varies among countries and depends upon energy markets, economic considerations, political structure, and societal receptiveness. The organization of this report is illustrated in Figure 3. This organization is designed to assemble important information relevant to a wide variety of policy makers concerned with implementing the goal of the UN/FCCC. Information falls into two general categories needed to understand the costs and benefits associated with atmospheric stabilization. The first category (or "branch") assembles information about climate change, and its consequences, whereas the second category assembles information about emissions and mitigation costs. This approach organizes information from SAR WGI, WGII and WGIII relevant to the issue of greenhouse gas stabilization for use in a more integrated analysis. The strategy chosen flows forward from SAR WGI, which considers a series of concentration profiles as a basis for deducing anthropogenic emissions consistent with the underlying Figure 3. An overview of the structure and logic of this Technical Paper. Stabilization of Atmospheric Greenhouse Gases: Physical, Biological and Socio-economic Implications physics and biology of oceanic and terrestrial ecosystems, albeit simplified (see Section 2.2.1.3 on uncertainties). Beginning with concentration profiles, we calculate, using simplified climate models from SAR WGI (Section 6.3), the global mean temperature and sea level consequences of these CO2 concentration profiles (covered in Section 2.3). We also carry out sensitivity analyses showing the effects of other gases and aerosols on these central CO2 analyses. These global mean temperature and sea level changes provide a context for considering the consequences for natural resources, infrastructure, human health, and other sectors affected by the climate (covered in Section 3.1). This completes the “impacts branch" of the analysis (see Figure 3). Note that this analysis provides only a simplified global mean view of consequences. For a more appropriately detailed view, regional climate changes and system vulnerabilities must be considered (see SAR WGI: Chapter 6 and SAR WGII for discussions of regional climate change and vulnerabilities). The "mitigation costs branch" of this analysis also begins with concentration profiles (see Figure 3). The concentration profiles are then used together with carbon cycle models (see SAR WGI: Section 2.1 and IPCC94: Section 1.5) to compute anthropogenic emissions (covered in Section 2.2.1). These deduced emissions can be used in economic models to estimate the "mitigation" costs of following the stabilization profile rather than a business-as-usual trajectory (covered in Section 3.2), given the appropriate assumptions. Mitigation costs can be computed for a wide range of stabilization profiles and with multiple economic models to provide a sense of the range of possible mitigation costs as a function of an eventual stabilization target and pathway. Note that all of these analyses consider the economic costs for mitigation associated with particular specified concentration profiles. They are thus not "optimal" trajectories nor do they represent policy recommendations. Rather, they are illustrative of the links from concentrations to emissions and thence to mitigation costs. The two branches come together, conceptually, in the end in the section on integrating information on impacts and mitigation costs (Section 3.3). Neither branch provides a complete basis for 11 Although it is important to assemble information about the costs and benefits associated with atmospheric stabilization, assemblage is not the same as recommending a simple costbenefit analysis. The cost-benefit paradigm is the most familiar decision related application of the economics of balancing costs and benefits, but it is not the only approach available. Other techniques include cost effectiveness analysis, multi-criteria analysis, and decision analysis (SAR WGIII, p.151). Decisionmaking frameworks must consider uncertainty in projected concentration changes, in consequent climate effects, and in consequences for human and natural systems. A wide range of paradigms for dealing with this uncertainty likewise exist, and are summarized in SAR WGIII. The analysis of biophysical and economic uncertainties presented in this report is only a brief summary of issues. While a more detailed discussion can be found in SAR WGI, WGII, and WGIII, the full dimensions of uncertainty in the analysis linking concentrations to, ultimately, costs and consequences, remains an active area of investigation. Regardless of the method eventually employed in the decision-making process, information about the costs and benefits of emissions mitigation can be used to improve the quality of policy decisions. The present document makes no attempt to judge the practical issues of implementing emissions mitigation strategies, nor does it consider the fairness and equity concerns that surround such deliberations. The global perspective employed here is for methodological and pedagogical convenience: it is not meant to imply that regional issues are less important — clearly, climate policy must be made within the context of a wide array of national and international policy considerations. Such matters add to the rich complexity of issues with which policy makers must grapple. 2.1 2. GEOPHYSICAL IMPLICATIONS ASSOCIATED WITH GREENHOUSE GAS General Principles of Stabilization: Stabilization of There has been confusion about the scientific aspects of stabilizing the atmospheric CO2 concentration vis-à-vis the stabilization of the concentrations of other gases, particularly with regard to the concept of "lifetime". The processes that control the lifetimes of the key gases are reviewed in detail in SAR WGI (Chapter 2) and IPCC94, which provides vital background material for this brief review. of methane. Methane can be stabilized on the time-scale of its atmospheric lifetime: decades or less. Nitrous oxide has a long lifetime, 100 to 150 years. N20 is removed from the troposphere (where it acts as a greenhouse gas) by exchange with the stratosphere where it is slowly destroyed by photochemical decomposition. Like methane, its lifetime is controlled by its destruction rate, and, like methane, it is destroyed rather than exchanged with other reservoirs of N2O. Stabilization of the N2O concentration requires reduction of sources, and such reductions would need to extend over lengthy periods to influence concentrations because of the ~120-year lifetime of this gas. On the other hand, atmospheric aerosol concentration adjusts within days to weeks to a change in emissions of aerosols and aerosol precursor gases. Most carbon reservoirs exchange CO2 with the atmosphere: In contrast to CO2, aerosols and non-CO2 greenhouse gases such as the halocarbons, methane and N2O are destroyed (e.g., by oxidation, photochemical decomposition, or, for aerosols, by deposition on the ground). The time such ́a molecule (or particle) spends on average in the atmosphere (ie., its turnover time) is equal or roughly similar to the adjustment time. Description of Concentration Profiles, Other Trace Concentration Profiles Leading to Stabilization In this Technical Paper, we evaluate the 11 illustrative CO2 concentration profiles (stabilizing at 350 to 1000 ppmv, referred to as the "S" and "WRE” profiles) as discussed in SAR WGI. These profiles prescribe paths of concentration with time, leading gradually to stabilization at the prescribed level (Figure 4). The WRE profiles prescribe larger increases in CO2 concentration earlier in time when compared with the S profiles, but lead to the same stabilized levels (Wigley, et al., 1996). The concentration profiles can also be used as input to compute a range of allowed emissions over time. Deduced emissions, in turn, can be used as inputs to economic models to compute the mitigation costs associated with reducing emissions to follow a specified concentration profile. It should be noted that this approach does not allow calculation of, or imply anything about, optimal paths of emissions. Methane is emitted to the atmosphere from a range of sources Emissions Implications of Stabilization of CO2 In this analysis, we again consider the S350-750 profiles and the WRE350-1000 profiles described in IPCC94 (Chapter 1) and SAR WGI (Section 2.1), but more completely than was possible in either of those documents. First, we present graphs showing CO2 concentrations versus time (Figure 4) and the corresponding emissions versus time for all 11 profiles together with, for comparison, the IS92a, c, and e scenarios (Figure 5). Note that CO2 emissions for the IS92a and e scenarios are higher in year 2050 than are emissions for concentration (ppmv) Atmospheric CO, Atmospheric CO, concentration (ppmv) concentration (ppmv) Atmospheric CO, 14 350 ppm Stabilization of Atmospheric Greenhouse Gases: Physical. Biological and Socio-economic Implications 450 ppm 460 Figure 4. Profiles of CO2 leading to stabilization at concentrations from 350 to 1000 ppmv. For comparison the pre-industrial concentration was close to 280 ppmv and the current concentration is approximately 360 ppmv. For stabilization at concentrations from 350 to 750 ppmv, two different routes to stabilization are shown: the S profiles (from IPCC94) and the WRE profiles (from Wigley, et al., 1996) which allow CO2 emissions to follow IS92a until 2000 or later (depending on the stabilization level). A single profile is defined for 1000 ppmv. These two sets of profiles are merely examples from a range of possible routes to stabilization that could be defined. all the S and WRE profiles (except for WRE1000, which was constructed to follow IS92a concentrations to 2050). The IS92c scenario suggests emissions lower in 2050 than for S550, WRE550 and all higher levels of stabilization for either emissions pathway. For further information concerning the assumptions made to derive these results, as well as inter-model differences, see Enting. et al., (1994). For the given stabilization profiles, a period of increasing emissions is generally followed by a rapid decrease to a stabilized level. We note again that this pattern does not apply to the $350 and WRE350 profiles, and that they imply negative emissions for a period of time in these cases, because 350 ppmv is lower than the current atmospheric concentration. It can be seen from Figure 5 that the WRE profiles allow higher emissions initially, but imply a more rapid transition from increasing to decreasing emissions. and lower emissions later, before emissions for both the S and WRE profiles converge. We do not address here what an optimal emissions pathway is, but merely show the emissions consequences of prescribed pathways to concentration stabilization. Figure 6 shows the cumulative CO2 emissions over time for stabilization at 350, 450, 550, 750, and 1 000 ppmv, and the corresponding emissions associated with the IS92a, c and e scenarios. It shows clearly that by 2100, cumulative emissions associated with the IS92a and e scenarios are higher than those for all S and WRE profiles. As in Figure 5, it is clear in Figure 6 that the WRE profiles allow significantly higher emissions in the near-term future, but also that for later times the cumulative emissions in the WRE profiles are very similar to the total amount under the S profiles. This is because, for a given stabilization level, the long-term cumulative emissions are relatively insensitive to the pathway taken to stabilization. The deduced emissions for a given concentration profile leading to stabilization define the "carbon budget” available for anthropogenic emissions from fossil fuel burning, cement manufacture, land-use conversion, and other activities. The larger the cumulative emissions (corresponding to higher stabilization levels) the larger the carbon budget available for anthropogenic activities (see Section 3.2). The size of the carbon budget is also sensitive. especially early on, to the choice of pathway (illustrated by differences between the S and WRE profiles in Figure 6). |