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Stabilization of Atmospheric Greenhouse Gases: Physical, Biological and Socio-economic Implications

0 Discuss the potential cavironmental consequences of the from 1S92a, and the "WRE" pathways that follow 1890 derived changes in temperature and sea level;

initially. A single pathway that stabilizes a 1 000 ppmv is also

considered. The WRE pathways imply higher emissions in the (3) Discuss the factors that influence mitigation costs; and short-term, but an earlier and more rapid change from increas

ing to decreasing emissions, and lower emissions later. (h) Review the metodology for integrating climate and sea

level change effects and mitigation costs to produce a more Ecosystem and oceanic feedbacks may reduce terrestrial and complete view of the consequences of changing atmos- oceanic carbon storage to levels somewhat below those pheric composition.

assumed in the simplified global carbon cycle models used bere and in the Second Assessment Report. Vocertainties resulting

from the omission of poteotially critical oceanic and biospberic 1.2 Key Points

processes during transient climate change could have a signifi

cant effect on the conclusions regarding emissions associated 1.2.1 Some Fundamentals Regarding Greenhouse Gases with stabilization.

and Tropospheric Aerosols (see SAR WGI for more
details)

1.2.3 Taking the Climatic Effects of Other Greenhouse Of the greenhouse gases, this paper focuses on CO2 because it Gases and Acrosols into account: the Concept of has bad, and is projected to have, the largest effect on radiative Equivalent CO2 forcing (in 1990, 1.56 W m-2 for CO2 versus 0.47 W m-2 for CHA. 0.14 W m-2 for N20 and 0.27 W m-2 for the halocarbons). Subject to uncertainties concerning the climate sensitivity (see For a discussion of the utility of radiative forcing in climate below), future anthropogenic climate change is determined by change studies see IPCC94 (Chapter 4) and PCC TP SCM the sum of all positive and negative forcings arising from all (1997). This paper also considers the effects that arise wben a anthropogenic greenbouse gases and aerosols, oot by the level series of assumptions are made about potential future emissions of CO2 alone. The forcing scenarios used in many of the model of other greenhouse gases and SO2. a primary aerosol precursor runs are the sum of the radiative forcings of all the trace gases (aerosols may act to cool the planet).

(CO2, CH4, O3, etc.) and aerosols. The total forcing may be

treated as if it came from an "equivalent” concentration of CO2. Tropospheric aerosols (microscopic airborne particles) result- · Therefore, the "equivalent CO," concentration is the conceoing from combustion of fossil fuels, biomass burning and other tration of Coz that would cause the same amount of global anthropogenic sources bave led to a highly uncertain estimate mean radiative forcing as the given mixture of cos, other of direct forcing of 0.5 W m2 (range: -0.25 to -1.0 Wm-2) over greenhouse gases, and aerosols. the past century as a global average. There is possibly also a Degative indirect forcing - via modifications of clouds - that The difference between the equivalent CO, level and the true remains very difficult to quantify (SAR WGI: Chapter 2). Co level depends on the levels at which the concentrations of Because aerosols have short lifetimes in the atmosphere, their other radiatively active gases and aerosols are stabilized. The distribution and bence immediate radiative effects are very stabilization levels chosen for CHA. N20 and SO2 can signifiregional in character. Locally, the acrosol forcing can be large caotly affect equivalent Con. If the emissions of these gases enough to more than offset the positive forcing due to green- were beld constant at today's levels, equivaleat CO, would bouse gases. However, although the negative forcing is focused stabilize at approximately 26 ppmv (S350) to 74 ppmv in particular regions and subcontinental areas, it has continental (WRE1000) ppmv above the level for CO2 alone. Because the to hemispheric scale effects on climate because of couplings effects of greenhouse gases are additive, stabilization of CO2 through atmospberic circulation.

concentrations at any level above about 500 ppmy is likely to result in atmospberic changes equivalent to at least a doubling

of the pre-industrial CO2 level. 1.2.2 Stabilization of CO, Concentrations (see SAR WGI for more details)

1.2.4 The Global Temperature and Sea Level Among the range of stabilization cases studied, accumulated Implications of Stabilizing Greenhouse Gases anthropogenic emissions from 1991 to 2100 fall between 630 and 1410 GC, for stabilization levels between 450 and 1000 This report considers two simple indices of climate change. ppmv. For comparison, the correspooding accumulated emis- global mean temperature and sea level rise. The change in sions for the IPCC IS92 emissions scenarios range from 770 to global mean temperature is the main factor determining the rise 2190 GC.

in sea level; it is also a useful proxy for overall climate change.

It is important to realize, however, that climate change will not For each stabilization level from 350 to 750 ppmv, two path- occur uniformly over the globe; the changes in temperature and ways are considered: the “S” pathways, that depart immediately in other climate variables such as precipitation, cloudiness, and Stabilization of Atmospheric Greenhouse Gases: Physical, Biological and Socio-economic Implications

the frequency of extreme events, will vary greatly among cryosphere); food and fibre production; human infrastructure regions. Le order to evaluate the consequences of climate and human bealth. Most existing impacts studies are analyses of change, one must consider the spatial variability of all factors: what may result from the equilibrium climate changes associclimate forcing, climate response, and the vulnerability of ated with a doubled equivalent Co, level; few studies have regional buman and natural resource systems. However, consid- considered responses over time to more realistic conditions eration of regional details is outside the scope of this paper. involving increasing concentrations of greenhouse gases. The spatial patteras of some radiative forcing agents, especially Impacts are not a linear function of the magnitude and rate of aerosols, are very beterogeneous and so add further to the climate change. For some species and bence systems), thresbspatial variability of climate change. In this paper, aerosol olds of change in temperature, precipitation, or other factors forcing is presented in terms of global averages so that an may exist, which, once exceeded, may lead to discontinuous impression can be gained of its likely overall magnitude, its changes in viability, structure, or function. effect on global average temperature, and its effect on sea level rise. The effect of aerosol forcing on the detail of climate Aggregation of impacts to produce a global assessment is not change, bowever, is likely to be quite different from the effect currently possible because of our lack of knowledge of regional of a forcing of similar magnitude, in terms of global average, climate changes and regional responses, because of the diffidue to greenhouse gases. In terms of regional climate change culty of valuing impacts of natural systems and buman health, and impacts, therefore, the negative forcing

cooling from

and because of issues related to both interregio and interaerosol forcing must not be considered as a simple offset to that generational equity. from greenhouse gases.

The ultimate concentration of greenbouse gases reached in the Temperature and sea level projections depend on the assumed atmosphere, as well as the speed at which concentrations climate seasitivity, the target and pathway chosen for CO2 increase, is likely to influence impacts, because a slower rate of concentration stabilization, and the assumed scenarios for other climate change will allow more time for systems to adapt. greenhouse gases and aerosol forcing. The relative importance However, knowledge is not currently sufficient to identify clear of these factors depends on the time interval over which they threshold rates and magnitudes of change. are compared. Out to the year 2050, CO, concentration pathway differences for any single stabilization target are as important as the choice of target; but on longer time-scales cbe 1.2.6 Mitigation Costs of Stabilizing Co, Concentrations choice of target is (necessarily) more important. Outweighing all of these factors, however, is the climate sensitivity, uncer- Factors that affect CO, mitigation costs include: tainties in which dominate the uncertainties in all projections.

(a) Future emissions in the absence of policy intervention

("baselines"); 1.2.5 Impacts

(b) The concentration target and route to stabilization, whicb A great deal is known about the potential sensitivity and vulner- determine the carbon budget available for emissions; ability of particular systems and sectors, and both substantial risks and potential benefits can be identified. Currendy (c) The behaviour of the natural carbon cycle, which influbowever, our ability to integrate this information into an assess- ences the emissions carbon budget available for any

chosen ment of impacts associated with different stabilization levels or concentration target and pathway; emissions trajectories is relatively limited.

(d) The cost differential between fossil fuels and carbon-free While the regional patterns of future climate change are poorly alternatives and between different fossil fuels; koown, it is clear that the altered patterns of radiative forcing associmed with anthropogenic emissions will aker regional (e) Technological progress and the rate of adoption of techclimates noticeably, and will bave different effects on climate dologies that emit less carbon per unit of energy produced; conditions in different regions. These local and regional, changes will necessarily include changes in the lengths of Transitional costs associated with capital stock turnover, growing seasons, the availability of water, and the incidence of which increase if carried out prematurely; disturbance regimes (extreme high temperature events, floods, droughts, fires, and pest outbreaks), which, in turn, will have (8) The degree of international cooperation, which determines important impacts on the structure and function of both natural the extent to which low cost mitigation options in different and buman-made cavironments. Systems and activities that are parts of the world are implemented; and particularly sensitive to climate change and related changes in sea level include: forests; mountain, aquatic and coastal ecosys () Assumptions about the discount rate used to compare costs tems; hydrology and water resource management (including the at different points in time.

10

Stabilization of Amospheric Greenhouse Gases: Physical, Biological and Socio-economic Implications

1.2.7 Integrating Information on Impacts and Mitigation to create technologies that will reduce emissions in the future. Costs

These include immediate reductions in emissions to slow

climate change; research and development on new supply and This reports provides a framework for integrating information on conservation technologies to reduce future abatement costs; the costs, benefits and impacts of climate change. The points continued research to reduce critical scientific uncertainties, below must be prefaced with the critical observation that concen- and iovestments in actions to help bumao and natural systems tration stabilization profiles that follow 'business-as-usual" adapt to climate change through mitigation of negative impacts emissions for periods of a few to several decades should not be and through advantages resulting from increasing CO2 (4.8construed as a suggestion that no action is required for those increased water or gutrient use efficiency of some crops with periods. In fact, studies suggest that even in those cases of busi- elevated CO2). The issue is not one of “either-or" but one of Dess-as-usual emissions for some period of time, actions must be finding the right mix (i.c., portfolio) of options, taken together taken during that time to cause emissions to decline subsequently. and sequentially. The mix at any point in time will vary and The strategies for developing portfolios of actions leading to depend upon the concentration objective, which may itself be immediate or eventual reductions below business-as-usual are adjusted with advances in the scientific and economic knowldiscussed below.

edge base. The appropriate portfolio also varies among

countries and depends upon energy markets, economic considThis paper is designed to demonstrate bow information can be erations, political structure, and societal receptiveness. assembled on the costs, impacts and benefits of stabilizing atmos pberic greenhouse gases. This analysis, which supports many decision making formats, has two "branches". The first branch,

1.3

A "Road map to this Report "impacts”, assembles information beginning with assumed concentration changes, and then evaluates potential climate 1.3.1 Report Strategy change, and its consequences. The second branch, “mitigation", assembles information on emissions and mitigation costs associ- The organization of this report is illustrated in Figure 3. This ated with achieving a range of stabilizacion pathways and levels organization is designed to assemble important information The two branches must be combined to produce an integrated relevant to a wide variety of policy makers concerned with assessment of climate change and stabilization (Figure 3). implementing the goal of the UNFCCC. Information falls into

two general categories needed to understand the costs and beneIf expressed in terms of CO2 equivalent or total radiative forcing.fits associated with atmospheric stabilization. The first category a given stabilization level can be met through various combina- (or branch") assembles information about climate change, and tions of reductions in the emissions of different gases and by its consequences, whereas the second category assembles infor enbancing sinks of greenhouse gases. Considering all such mation about emissions and mitigation costs. This approach options, and selecting the least expensive ones while taking organizes informacion from SAR WGI, WGII and WGM releaccount of different sources and sinks, should lower the costs of vant to the issue of greenhouse gas stabilization for use in a mitigation. Approaching an optimum mix requires information more integrated analysis. about the concentration and climate implications of different emissions strategies, the mitigation costs and other characteristics The strategy chosen flows forward from SAR WGI, which of the different options, and decisions about the appropriate time considers a series of concestration profiles as a basis for deducscales and indices of impacts (climate and non-climate) to be used ing anthropogenic emissions consistent with the underlying 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 currendy beneficial, the issues facing governments are bow to implement the full range of no-regrets measures and whether, and if so, when and bow fæ to proceed beyond purely no-regrets options. The

toring 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).

[graphic]

Numerous policy measures are available to facilitate adaptation to climate change, to reduce emissions of greenhouse gases, and

Prigure 3. As overview of the structure and logic of this Technical Paper.

Seabilization of Abnospheric Greenhouse Gases: Physical, Biological and Socio-economic Implications

11

pbysics and biology of oceanic and terrestrial ecosystems, decision making. This general type of problem supports a wide albeit simplified (see Section 2.2.1.3 on uocertainties). range of decision-making frameworks, which may integrate this Beginning with concentration profiles, we calculate, using information in a variety of ways (see SAR WGOI: Chapter 4). simplified climate models from SAR WGI (Section 6.3), the global mean temperature and sea level consequences of these co, concentration profiles (covered in Section 2.3). We also 1.3.2 Decision-making Frameworks carry out seositivity analyses showing the effects of other gases and aerosols on these central Co, analyses. These global mean Although it is important to assemble information about the temperature and sea level changes provide a context for consid- costs and benefits associated with atmospheric stabilization, ering the consequences for natural resources, infrastructure, assemblage is not the same as recommending a simple costhuman health and other sectors affected by the climate beoefit analysis. The cost-benefit paradigm is the most familiar (covered in Section 3.1). This completes the “impacts brancb" decision related application of the economics of balancing costs of the analysis (see Figure 3). Note that this analysis provides and benefits, but it is not the only approach available. Other only a simplified global mean view of consequences. For a techniques include cost effectiveness analysis, multi-criteria more appropriately detailed view, regional climate changes and analysis, and decision analysis (SAR WGIII, p.151). Decisionsystem vulnerabilities must be considered (see SAR WGI: making frameworks must consider uncertainty in projected Chapter 6 and SAR WGII for discussions of regional climate concentration changes, in consequent climate effects, and in change and vulnerabilities).

consequences for human and natural systems. A wide range of

paradigms for dealing with this uncertainty likewise exist, and The "mitigation costs brasch“ of this analysis also begios with are summarized in SAR WGIII. concentration profiles (see Figure 3). The concentration profiles are then used together with carbon cycle models (see SAR The analysis of biophysical and economic uncertainties WGI: Section 2.1 and IPCC94: Section 1.5) to compute an:hro presented in this report is oaly a brief summary of issues. While pogenic emissions (covered in Section 2.2.1). These deduced a more detailed discussion can be found in SAR WGI, WGI. emissions can be used in economic models to estimate the and WGII, che full dimensions of uncertainty in the analysis "mitigation" costs of following the stabilization profile rather linking concentrations to, ultimately, costs and consequences, than a business-as-usual trajectory (covered in Section 3.2), remains an active area of investigation. Regardless of the given the appropriate assumptions. Mitigation costs can be method eventually employed in the decision-making process, computed for a wide range of stabilization profiles and with information about the costs and benefits of emissions mitigation multiple economic models to provide a sense of the range of can be used to improve the quality of policy decisions. possible mitigation costs as a function of an eventual stabiliza tion target and pathway. Note that all of these analyses consider The present document makes no attempt to judge the practical the economic costs for mitigation associated with particular issues of implementing emissions mitigation strategies, por specified concentration profiles. They are thus not "optimal" does it consider the faimess and equity concerns that surround trajectories por do they represent policy recommendations. such deliberations. The global perspective employed here is for Rather, they are illustrative of the links from concentrations to methodological and pedagogical convenience: it is not meant to emissions and thence to mitigation costs.

imply that regional issues are less important -- clearly, climate

policy must be made within the context of a wide array of The two branches come together, conceptually, in the end in the national and international policy considerations. Such matters section on integrating information on impacts and mitigation add to the rich complexity of issues with which policy makers costs (Section 3.3). Neither branch provides a complete basis for must grapple.

2. GEOPHYSICAL IMPLICATIONS ASSOCIATED WITH GREENHOUSE GAS

STABILIZATION

2.1

General Principles of Stabilization: Stabilization of
Carbon Dioxide and Other Gases

of methane. Methane can be stabilized on the time-scale of its atmospberic lifetime: decades or less.

There has been confusion about the scientific aspects of stabi- Nitrous oxide has a long lifetime, 100 to 150 years. N20 is lizing tbe atmospheric CO2 concentration vis--vis the removed from the troposphere (where it acts as a greenhouse stabilization of the concentrations of other gases, particularly gas) by exchange with the stratosphere where it is slowly with regard to the concept of "lifetime”. The processes that destroyed by photochemical decomposition. Like methane, its control the lifetimes of the key gases are reviewed in detail in lifetime is controlled by its destruction rate, and, like methane, SAR WGI (Chapter 2) and IPCC94, which provides vital back- it is destroyed rather than exchanged with other reservoirs of ground material for this brief review.

N20. Stabilization of the N20 concentration requires reduction

of sources, and such reductions would need to extend over Most carbon reservoirs exchange CO2 with the atmosphere: lengthy periods to influence concentrations because of the they both absorb (oceans) or assimilate (ecosystems), and -120-year lifetime of this gas. On the other hand, atmospheric release (oceans) or respire (ecosystems) CO2. The critical aerosol concentration adjusts within days to weeks to a change point here is that anthropogenic carbon emitted into the in emissions of aerosols and aerosol precursor gases. atmosphere is not destroyed but adds to and is redistributed among the carbon reservoirs. These reservoirs exchange carbon between themselves on a wide range of time-scales 2.2 Description of Concentration Profiles, Other Trace determined by their respective turnover times. Turnover Gas Scenarlos and Computation of Equivalent CO, times range from years to decades (carbon turnover in living plants) to millennia (carbon turnover in the deep sea and in 2.2.1 Emission Consequences of Stabilization long-lived soil pools). These time-scales are generally much longer than the average time a particular CO2 molecule 2.2.1.1 Concentration Profiles Leading to Stabilization spends in the atmosphere, which is only about four years. The large range of turnover times bas another remarkable conse- In this Technical Paper, we evaluate the 11 illustrative CO, quence: the relaxation of a perturbed atmospheric CO2 concentration profiles (stabilizing at 350 to 1 000 ppmv, concentration towards a new equilibrium cannot be described referred to as the "S" and "WRE" profiles) as discussed in SAR by a single time constant. Thus, attempts to characterize the WGI. These profiles prescribe paths of concentration with time, removal of anthropogenic CO2 from the atmosphere by a leading gradually to stabilization at the prescribed level single time constant (e.g., 100 years) must be interpreted in a (Figure 4). The WRE profiles prescribe larger increases in CQ, qualitative sense only. Quantitative evaluations based on a concentration earlier in time wben compared with the S single lifetime are erroneous.

profiles, but lead to the same stabilized levels (Wigley, et al.,

1996). The concentration profiles can also be used as input to In contrast to CO2, aerosols and non-CO2 grecnhouse gases compute a range of allowed emissions over time. Deduced such as the balocarbons, methane and N20 are destroyed (e.g., emissions, in turn, can be used as inputs to economic models to by oxidation, photochemical decomposition, or, for aerosols, by compute the mitigation costs associated with reducing emisdeposition on the ground). The time sucb'a molecule (or parti- sions to follow a specified concentration profile. It should be cle) spends on average in the atmosphere (i.e., its turnover time) noted that this approach does not allow calculation of, or imply is equal or roughly similar to the adjustment time.

anything about, optimal paths of emissions.

Methane is emitted to the atmosphere from a range of sources (see SAR WGI) and is destroyed mainly through oxidation by 2.2.1.2 Emissions Implications of Stabilization of CO2 the hydroxyl radical (OH) in the atmosphere and by soil micro

Concentrations organisms. The adjustment time of a perturbation in atmospberic methane is controlled by its oxidation (to CO, and In this analysis, we again consider the S350–750 profiles and water vapour) rather than by exchange with other reservoirs, the WRE350-1000 profiles described in [PCC94 (Chapter 1) which could subsequently re-release methane back to the and SAR WGI (Section 2.1), but more completely than was atmosphere. Methane's lifetime is complicated by feedbacks possible in either of those documents. First, we present between methane and OH, such that increasing the methane graphs showing CO2 concentrations versus time (Figure 4) concentration changes the methane removal rate by -0.17 to and the corresponding emissions versus time for all 11 +0.35 per cent per l per cent increase in methane (SAR WGI: profiles together with, for comparison, the IS924, c, and Section 2.2.3.1). Many other feedback processes in the CH - scenarios (Figure 5). Note that CO2 emissions for the S92a co_03--0H-NO, UV system also influence the lifetime and e scenarios are higher in year 2000 than are emissions for

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