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

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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 WGI, pp. 113-115). Only the mid-aerosol forcing values were used in SAR WGI (Section 6.3). Forcing values are given in Wm 2, 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

concentrations at relatively high levels. For the reference cases

we assume that halocarbon emissions remain constant at their Given the very large uncertainties in the roles of the non-co 2100 levels. Hence, eventually, concentrations will remain gases relative to CO2 under an “existing policies” assumption, constant in accordance with Article 2. We note, however, chat and given that no comprebensive studies have been carried out the constant-2100 emissions assumption leads to a potential to examine their roles under the assumption of concentration global mean forcing overestimate after 2100 of, eventually. up stabilization, we can only consider them in a sensitivity study to 0.4 W m2 context. We, therefore, begin with a set of reference cases in which the emissions of CO2 follow a range of stabilization For tropospberic ozone, in the absence of any projections, and pathways, the emissions of CH4, N20 and SO2 are assumed to again following SAR WGI (Section 6.3), we assume that the remain constant at their 1990 levels, and balocarbons follow the only forcing changes are those that arise from the ozone that is Montreal Protocol scenario used in the SAR WGI (Section 6.3) produced by methane induced changes in tropospberic chemglobal mean temperature and sea level calculations.

istry. This term amounts to around 0.15 W m-2 by 2100 under

IS92a, but is much less for the reference case of constant CH For halocarbons in the reference scenarios we assume that the emissions. Our assumption bere may be unrealistic if nitrogen. Moutreal Protocol applies strictly (see SAR WGI: Chapters 2 hydrocarbon, or other ozone precursors associated with ozone and 6) so that there is only a single future scenario for these concentrations increase due to a rise in anthropogenic pollution. gases. Because the total forcing for these gases over 1990–2100 (accounting for the effects of stratospheric ozone changes) is It should be noted that we are not suggesting that the reference relatively small (0.3 W m-2), uncertainties due to incomplete cases in any way reflect predictions of the future, especially compliance with the Protocol and/or future emissions of substi- with regard to future SO2 emissions, nor that they should be a tute (hydrofluorocarbon, HPC) or non-controlled gases may be target for policy. The point of the reference cases is to help even smaller. In the context of global climate change, therefore, assess the relative importance of CH, N20 and SO2 emissions and given that they are not addressed by SAR WGI, we have in determining future global mean temperature and sea level chosen not to include these uncertainties. However, should a change. comprehensive (multi-gas) framework for stabilization be adopted, a more detailed gas-by-gas assessment of balocarbon To quantify the sensitivity of equivalent CO2 to other gases we forcing may be required at a specific country level.

consider perturbations from the reference cases in which annual

CH, emissions increase or decrease linearly over 1990–2100 by Because the calculations performed bere run beyond 2100, a total of t100 Tg(CH2) (i.e., 175 TgC) relative to 1990 and some assumption must be made regarding halocarbon emis- remain constant thereafter, annual N20 emissions increase or sions after this date. If these emissions remain constant at their decrease linearly over 1990–2100 by a total of 12 Tg(N) rela2100 level, the forcing level would remain close to 0.3 W m2 tive to 1990 and remain constant thereafter, and annual SO, This would stabilize halocarbon (primarily HFCs) emissions increase or decrease linearly over 1990–2100 by Stabilization of Armospheric Greenhouse Gases: Physical. Biological and Socio-economic Impluwutions


350 per cent (i.e., 37.5 TgS) relative to their 1990 level and was obtained by summing the forcings due to all anthroremain constant thereafter. For all three gases, these scenarios pogenic trace gases (see Table 1). In global mean terms. this lead to concentration stabilization, effectively instantly for SO2 total forcing can be treated as if it came solely from over a few decades for CHs. and over a period of centuries for changes in CO2; i.e., from an "equivalent CO2 concentraN,0. To put these perturbations into a wider contexi, they are tion". The equivalent CO, concentration, Ceg. can be compared with IS92a, c and e in Table 2. Note again that these defined, therefore, from the relationship between actual perturbations should not be construed as representing particular CO2 concentration and radiative forcing. In SAR WGI, the future outcomes or policy targets.

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). Scenario Сн, N20 SO2 (Tg(CHỊ)) (Tg(N)) (% of 1990 level Although the equivalent CO, concept is pedagogically

useful and provides a means to compare the effects of CO2 IS920


with other gases. it does have disadvantages. An important

disadvantage arises from the non-linear relationship IS920


between radiative forcing and CO2 concentration. This non

linear relationship means that, at higher CO2 levels, it IS92e


requires a larger CO, change to increase radiative forcing

by the same amount. Because of this, radiative forcing Perturbation

changes can be added. but CO, equivalents can not be. We Case +100


have therefore retained the use of radiative forcing as our primary variable.

Table 2. Emissions changes over 1990–2100 for CH.. N,0 and SO, A further disadvantage of the equivalent CO2 concept is that, in under IS920. c and e compared with the perturbation values used in the context of impact assessments, it addresses only the climate this study (Units: CH4. Tg(CHA): N20. T8(N): SO2. percentage of change aspect. Other impacts of increasing CO, (e.g.. fertilizathe 1990 level of 75 T&S).

tion). sulphate aerosol (acidification), and ozone may also be important. Also with the equivalent CO2 concept, as with radia

tive forcing. a global aggregate measure subsumes information 2.2.4 Stabilizing Equivalent CO, Concentration

about regional aspects of climate change that are critical in

assessing impacts. It would be possible, for example, to impose Stabilizing the atmospheric concentrations of greenhouse gases. a forcing pattern on the climate system that had zero global an explicit goal of Anicle 2, would not necessarily result in mean forcing, but which would lead to large changes in regional stabilizing the human caused perturbation in radiative forcing. climate. This is because aerosols, which are not explicitly addressed by Article 2. also have radiative effects. If concentrations of both We now give equivalent CO2 results for different concentragreenhouse gases and aerosols are stabilized, this would stabi- tion stabilization levels. We consider S350. S450, S550. S650 lize the human perturbation in global mean radiative forcingo. 5750. and WRE1000, together with the constant 1990-level Note also that because aerosols are not uniformly mixed gases, emissions reference cases for CH4. N20 and SO2, and halothe geographical distribution of emissions of aerosols and their carbon emissions following the Montreal Protocol (see precursors can have important effects on regional climate. Section 2.2.2). To illustrate the dependence of equivalent Stabilizing the human perturbation in global mean radiative CO2 level on the pathway to CO2 stabilization, we also forcing is clearly different from stabilizing CO, concentration consider WRES50. These reference case results are given in alone. Thus, while mitigation efforts may target members of a Figure 7. where the forcing values are given relative to 1990 suite of greenhouse gases, impact studies must consider (some 1.3 W m 2 above the pre-industrial level). In the year climates influenced by multiple gases and aerosols. “Equivalent 2500. close to the point of equivalent CO2 stabilization, the CO2" is a technique for considering multiple radiative forcing equivalent CO, concentrations vary from 26 ppmv (S350) to components in the aggregate.

74 ppmv (WRE 1000) above the actual CO2 level. In all cases,

the forcing difference due to gases other than CO2 is the In the calculations of future global mean temperature and same: 0.66 W m 2 over 1990 to 2500. As noted above, this is sea level change given in SAR WGI (Chapters 6 and 7), the equivalent to differing amounts of CO2 at different concen. models were driven by the total radiative forcing, which tration levels because of the non-linearity of the equivalent

CO2/radiative forcing function. 6 This will not eliminale climate variability because the climate

system exhibits considerable natural variability. beyond anthro- Note that here the mid-1990 equivalent CO2 level is 342 ppmv, pogenic influences.

slightly below the actual CO2 level (354 ppmv). This is


Stabilization of Armospheric Greenhouse Gases: Physical. Biologicul and Socio-economic Implications

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Figure 7. Radiative forcing from 1990 to 2100 (relative to 1990) for Figure 8. The effect of different non-CO2 gas emission profiles on CO2 concentrations following the S3SO. SASO. SSSO, WRESSO. S650, radiative forcing (and equivalent CO2) for the S450 and $650 concenS750 and WRE1000 profiles (see Figure 4) and constant 1990 emis- tration profiles (see Figure 4). The short dashed lines give the sions of CH.. N20 and SO2. For halocarbons, a single emissions "COz-alone“ results; the solid lines the "reference case” (see Figure 7) scenario consistent with compliance with the Montreal Protocol is and the long dashed lines give results where CH. NGO and SO2 emisassumed. These assumptions are referred to in the text and in later sions increase according to 1992a 10 2100 and then stabilize (the captions as the “reference case”. Equivalent CO, levels are shown by "IS92a case“). Note that, initially, the radiative forcing for the referthe dots on the right-hand axis. For the S450 (S650) profile for ence case is less than for the "IS92a case”. This is due to the negative example, the CO2 concentration in 2100 is 450 (575) ppmv (from forcing effect of aerosols. Note also that, for the CO.,-alone cases, the Figure 4), but the additional effect of other greenhouse gases and SO2 equivalent CO, levels are less than the actual CO2 levels because of gives an equivalent CO2 concentration of 473 (604) ppmv. These differences in their 1990 values. 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.

because, in 1990, the negative forcing due to aerosols more than (constant CH4. N20, and SO2 emissions), and the extended offsets the positive forcing due to non-CO2 greenhouse gases. IS92a case. Results are shown at the date of CO2 stabilization This value is, however, quite uncertain due mainly to uncer- (which varies according to stabilization level). tainties in the magnitude of aerosol forcing. For aerosol forcing uncertainties of t0.5 W m2 in 1990, the 1990 equivalent CO2 The above calculations are presented to illustrate the impor. level varies between 316 and 370 ppmv.

tance of other gases in determining the equivalent CO

level, and the overall level of uncertainty involved in deterThe overall sensitivity to the assumptions regarding the emis- mining their contribution. None of the cases studied (CO2 sions of non-CO2 gases is shown in Figure 8. for S450 and alone, constant 1990 emissions, or IS92a based emissions S650. Here, the same reference cases (Figure 7) are shown for CH.. N20 and SO2) should be taken as a particular together with cases where IS92a emissions are used for CHg. fulure scenario, nor as a policy recommendation. The results N20 and SO2 out to 2100 with constant emissions thereafter. In show that the concentration stabilization levels chosen for this second case, the eventual forcing increment from 1990 due CHA. N20 and SO2 may have a significant influence on to non-CO2 gases is 1.13 W m-2 (compared with 0.66 W m-2 for future equivalent CO2 changes and on the equivalent CO2 the reference case). The equivalent CO2 levels in 2100 are stabilization level. Individual sensitivities are addressed in 491 ppmv (S450) and 627 ppmv (S650) compared with 473 the next section. ppmv (S450) and 604 ppmv (S650) for the reference cases. Figure 8 also shows the forcing due to CO2 alone.

As a final point in this section, we note that equivalent CO2

levels do not stabilize in our examples, even by 2500. Small The results presented in Figures 7 and 8 are characterized and but noticeable forcing changes (of order 0.1-0.3 W m-?) occur summarized in Table 3. This shows radiative forcing changes after the point of CO2 stabilization (viz. 2100 in S450, 2200 in from 1765 and equivalent CO2 levels for CO2 stabilization S650), due mainly to the long lifetime of N,O, which leads to levels of 350 ppmv to 1 000 ppmv under three different significant concentration changes for this gas after emissions assumptions regarding the forcing effects of other gases: no stabilize in 2100. Changes after 2500, however, are very other-gas effects (i.e., CO, changes alone), the reference case small.

Stabilization of Amospheric Greenhouse Gases: Physical. Biological and Socio-econontic Implications


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AF (W m2)
CO2 equiv.
AF (W m-2)
CO2 equiv.
AF (W m-2)
CO2 equiv.
AF (W m-2)
CO2 equiv.
AF (W m-2
CO2 equiv.
AF (W m2)
CO2 equiv.


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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 "CO, only" column assumes changes after 1990 are in CO, only (as in SAR WGI). Note that the equivalent CO2 level al CO2 stabilization in these cases differs from the CO2 stabilization level because of differences between the 1990 CO, and equivalent CO, levels.

2.2.5 Equivalent CO, Sensitivities

relative to 1990 in annual SO2 emissions (i.e., 137.5 TgS) lead

to forcing differentials of -0.37/+0.45 W m2, which translates Section 2.2.4 provides estimates of equivalent CO, that include to equivalent CO2 concentration differentials of -27/+36 ppmv the collective effects of CH4. N20. SO, and the halocarbons. for $450 and -40/+52 ppmv for $650 (note that the sign of the Here we consider the influences of CH.. N20, and SO2 sepa- forcing or concentration differential is opposite to the sign of rately. To do this, we use emissions perturbations about the the emissions perturbation). constant 1990 emissions reference cases.

In addition to the influence of emissions uncertainties, the effect For CH. (Figure 9a), a perturbation in annual emissions of SO, on equivalent CO2 concentrations is sensitive to the from 1990 10 2100 of 175 TgC (1100 Tg(CH2)) changes highly uncertain relationships between SO2 emissions and radiative forcing by approximately 10.20 W m? at concen- radiative forcing. SO2-derived sulphate aerosol affects radiative tration stabilization. This translates to equivalent CO2 forcing both directly, under clear-sky conditions, and indirectly, differentials of approximately 115 ppmv for $450 and through changes in cloud albedo. The central estimate of direct +22 ppmv for $650. For annual N20 emissions, a perturba- sulphate aerosol forcing for 1990 was calculated in SAR WGI tion of 12 Tg(N) from 1990 to 2100 changes forcing by as - 0.4 W m?, an estimate of -0.8 W m-2 was used in Section 10.16 W m2 at concentration stabilization, and gives 6.3 of SAR WGI for the indirect forcing. When combined with concentration differentials of 112 ppmy for $450 and a carbonaceous (sool) aerosol forcing of +0.1 W m? this gives 118 ppmv for $650 (see Figure 9b).

a total sulphate aerosol forcing of - 1.1 W m? To assess the

sensitivity to uncertainties in this quantity, we use the range of Sulphur dioxide sensitivities occur in two ways. First, there is 20.1 W m- for direct forcing and 10.4 W m2 for indirect the basic sensitivity to emissions uncertainties (Figure 10a). Al forcing (giving a total sulphate (plus sool) aerosol forcing range concentration stabilization, perturbations of 150 per cent of -1.1 10.5 W m ).


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

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Figure 9. (a) The sensitivity of radiative forcing (and equivalent CO, concentration) to CH, emissions for the S450 and S650 concentration profiles (see Figure 4). The "CH, low"/"CH, high“ curves assume annual CH, emissions decreasesincrease linearly by 100 Tg(CH2) over 1990 to 2100 (see Table 4); (b) The sensitivity of radiative forcing (and equivalent CO2 concentration) to N,O emissions for the S450 and S650 concentration profiles (see Figure 4). The “N2O low"/"N,O high" curves assume annual N2O emissions decrease increase linearly by 2 Tg(N) over 1990 to 2100 (see Table 4).

Radiative forcing (


Equivalent CO, (ppm)

The way this emissions/forcing uncertainty manifests itself initially in our calculations is in the 1990 equivalent CO, level. As noted earlier, whereas the “best guess" value of Ceg(1990) is Figure 10. (a) Sensitivity of radiative forcing (and equivalent CO2 342 ppmv, the range corresponding to +0.5 W m2 in the 1990 concentration) to SO2 emissions for the $450 and S650 concentration aerosol forcing level is 316-370 ppmv. For future forcing. if we profiles. The solid lines give the "reference" cases; the shor long use the reference case of no change in SO2 emissions, then the dashed lines show the "high SOz/low SO2" cases where emissions emissions/forcing uncertainty has no effect - zero emissions

increase/decrease linearly by + 50 per cent over 1990-2100; (b) change means zero forcing no matter what the emissions/forcing Sensitivity of radiative forcing (and equivalent CO2 concentration) to relationship is. The 1990 forcing uncertainty is simply propa- sulphate aerosol forcing in 1990 (relative to pre-industrial times) of -0.6, gated "as is" into the future (Figure 106).

- 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 If, however, future SO2 emissions increase or decrease from radiative forcing (and equivalent CO2 concentration) of sensitivity to their 1990 level (as in the emissions perturbation cases SO2 emissions and 1990 aerosol forcing for the S650 concentration considered in Figure 10a), then the emissions/forcing uncer- profile only. E highE low indicates increasing/decreasing emissions of tainty does affect future aerosol forcing. This is illustrated in SO2 from 1990 to 2100 (these are the same as the corresponding curves Figure 10c, where (for the S650 case only) we show the uncer- in Figure 10a); Q high/Q low indicates high/low 1990 aerosol forcing tainties associated with both emissions and forcing together. (these are the same as the corresponding curves in Figure 106).

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