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

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The bold curve in the centre is the reference SO2 emissions case of stabilization. To do so comprehensively requires, at least. (no change from 1990), for which there is no emissions/forcing that regional-scale changes in temperature and sea level, and uncenainty band. The upper three curves correspond to the case changes in other climate variables (such as rainfall or soil moisof decreasing SO2 emissions (by 50 per cent over 1990-2100) ture) be considered. However, climate models are not yet and give results for low, mid and high values of the 1990 sufficiently accurate to allow confident prediction of such sulphate aerosol forcing level (-1.1 10.5 W m ). High 1990 regional, multivariate influences. forcing leads to a larger departure from the reference case. The lower three curves are for the case where SO2 emissions The present analysis includes CO2, together with a number of increase by 50 per cent over 1990-2100. Here, the high 1990 possible combinations of other gas influences, as shown in forcing case must again lead to a larger (this time, negative) Table 4. This approach was chosen to give some insights into departure from the reference case.

the sensitivities of temperature and sea level to the assumptions regarding fulure greenhouse gas and SO2 emissions. The

approach is nol meant to span the full range of possibilities. For 2.3 Temperature and Sea Level Consequences of each combination we compute four variables: Stabilizing Co, Concentrations

(a) Radiative forcing (W m2); 2.3.1 Temperature and Sea Level Analyses: Methodology

(b) The equivalent CO2 concentration associated with the The CO2 concentration stabilization profiles described above particular combination of other gases; together with the scenarios introduced for other gases have been used as inputs to simplified climate models that assess the (c) Global mean temperature changes; global mean temperature and sea level consequences. This is only a first step towards addressing the full climate implications (d) Global mean sea level changes.

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Tropospheric 03

As SAR WGI: no direct changes after 1990. CHg-induced changes included with CHA

• With emissions adjusted to balance the 1990 budget, as in SAR WGI: Chapter 6. 175 TgS yr as in the IS92 scenarios.

A synthesis of emissions as given in Chapter 2 of SAR WGI, with other minor species as given in Chapter 6. Stratospheric ozone effects accounted for as in Chapter 6.

Table 4. Emissions cases considered in the sensitivity studies.

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

Results for (a) and (b) have been given in Section 2.2; this The primary calculations use the reference case of constant section considers the global mean temperature and sea level 1990 level emissions for CH, N20. and SO2 (see Table 4). This implications. Rates of change may be estimated graphically facilitates the comparison between different CO, stabilization from the results provided.

levels and pathways, and is consistent with the equivalent Co,

results given earlier. Emissions for these gases under the ISM In addition, we need to consider uncertainties in the response scenarios differ markedly from the reference case (see Tables 1 of the climate system to external forcing, due largely to and 2). In addition to the reference cases, we assess the sensiuncertainties in the climate sensitivity (we consider three tivity of the various temperature and sea level results to the cases, following SAR WGI (Section 6.3); viz. AT 20 = 1.5.2.5 emissions levels of CH4, N20, and SO2, by considering differ. and 4.5°C), and sea level rise uncertainties due to uncertain- ent emissions cases for these gases. ties in modelling ice-melt (SAR WGI: Chapter 7). For the latter, we span the range by considering low (AT2x = 1.5°C, We have noted above that the future emissions trajectories of combined with low ice-melt), mid (2.5°C, mid ice-melt), and the non-Co, trace gases (CH. N20, SO2) can have a marked higb sea level rise cases (4.5°C, high ice-melt). This gives effect on the total forcing associated with any CO2 stabilization three sets of climate/sea level output for each forcing case. profile. For example, if the actual CO2 concentration were to The results given use the Wigley and Raper (1992) models stabilize at 450 ppmv, and methane emissions continue to (see also Raper, et al., 1996) as employed in SAR WGI increase, the radiative forcing would be substantially higher (Section 6.3). In SAR WGI a model developed by de Wolde than that associated with CO, alone. Higher temperature and and colleagues (e.g., de Wolde, et al., 1995) was used, but sea level changes would also be expected, as sbown below. their climate model has a fixed sensitivity for temperature change at doubled CO2 of 2.2°C (AT2x = 2.2), which Global mean temperature and sea level change results for 1990 precludes its use in the present context. For information on to 2100 are sbown in Figures 11 to 15 (for results to 2300 see model structure and intermodel differences, see [PCC TP Appendix 1). These are changes from the present only (nomiSCM (1997).

nally from 1990). To obtain the anthropogenic change in global

mean temperature from 1880, based on the central estimate of Because of the large number of model simulations and the historical forcing used in SAR WGI, 0.2-0.5°C should be Qumber of response variables, we present only a subset of the added. To obtain the change from pre-industrial times, a further results here to illustrate the possible consequences. (Because of 0.1-0.2°C sbould be added. the potential interest in the detailed results, full results from all carbon cycle and climate model calculations will be made avail- It sbould be noted that global mean quantities are only indica able electronically via the World Wide Web (or alternatively, on of the overall magnitude of potential future climate diskette).)

change: regional temperature changes may differ markedly from the global mean change, and changes in other variables,

such as precipitation, are not related in any simple or direct way 2.3.2 Implications of Stabilization of Greenhouse Gases to global mean temperature change (see SAR WGI: Chapter 6). for Temperature and Sea Level

Regional sea level changes may also differ from the global

mean due to land movement and/or oceanic circulation effects The results presented bere provide a more unified view of the (see SAR WGI: Chapter 7). issues related to stabilization than is available from any single chapter in SAR WGI. The bulk of these results are for a climate Figures lla and b show temperature and sea level changes sensitivity (AT22 of 2.5°C, a mid-range value. If the true value from the present for CO, stabilization levels of 350, 450, were lower or higher, the results would scale accordingly, as 550, 650, 750 and 1000 ppmv using the reference case for discussed below. In addition, we emphasize that the results other gases (constant 1990 level emissions for CH . N20 and shown are globally averaged: both impacts and mitigative SO2). A climate sensitivity of 2.5°C and mid ice-melt paraactions are sensitive to regional patterns of climate and sea level meter values (see SAR WGI: Chapters 6 and 7) are used in change, because regional opportunities and vulnerabilities are these calculations, which are directed towards showing how highly variable

temperature and sea level changes vary according to the

chosen stabilization level. For the 550 ppmy case, both the The temperature and sea level results given here "S" and "WRE” results are given to illustrate the sensitivity computed using relatively simple models. As discussed in of the changes to the pathway taken towards stabilization. IPCC TP SCM (1997). these models are designed to repro- Out to around 2050, the WRESSO results show greater duce, with reasonable fidelity, the globally averaged warming and sea level rise than even the S750 case (but not behaviour of complex models. They have also been compared the 1000 ppmv case, because this was constrained to lie to historical and/or present day observations. They. in always equal to or above the WRESSO CO, concentration). common with more complex models, do not include all possi- Rates of change may be derived from Figures lla and b, over ble interactions and climate feedbacks, but they do reflect our the next fifty years rates of temperature change range from

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

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Figure 11. (a) Projected global mean temperature when the concen- Figure 12. (a) The effect of different non-CO2 gas emission profiles tration of CO2 is stabilized following the S profiles and the WRESSO on global temperature change for the S450 and S650 concentration and 1 000 profiles shown in Figure 4. CHA. N20 and SO2 emissions profiles (see Figure 4). The solid lines give the "reference “ results, are assumed to remain constant at their 1990 levels and halocarbons the shon dashed lines the “CO2 alone" results and the long dashed follow an emissions scenario consistent with compliance with the lines give results where CH4, N20 and SO2 emissions increase Montreal Protocol (i.e., the reference case). The radiative forcing (and according to IS92a to 2100 (the “IS92a case"). The climate sensitivity equivalent CO2) from which the global temperatures were derived is assumed to be the mid-range value of 2.5°C: (b) As for (a), but for were shown earlier in Figure 7. The climate sensitivity is assumed to global sea level change. Central values of the ice-melt parameters are be the mid-range value of 2.5*C. For comparison, results for the IS920. c and e emissions scenarios are shown for the year 2100. To obtain the anthropogenic change in global mean temperature from importance of other gases is clearly seen from this Figure. 1880, based on the central estimate of historical forcing used in SAR Differences between the reference case and the case with IS920 WGI. 0.2-0.5'C should be added. To obtain the change from pre- emissions for other gases exceed the differences between S450 industrial times, a further 0.1 -0.2°C should be added: (b) As for (a). and S650 out to around 2050. The IS92a results are (to around but for global sea level change using central ice-melt parameters. All 2050) lower than the others due to the global mean offsetting results were produced using the Wigley and Raper simple climate/sea effect of increasing SO2 emissions in this scenario: but this level model (see IPCC TP SCM. 1997).

hides important regional details and it does not necessarily mean that the severity of climate changes associated with this

case (in the sense of their impacts) would be less. Figures 120 and b illustrate how the emissions of non-CO2 gases might influence future global mean temperature and sea The results in Figures 11 and 12 are for “best guess" climate and level change (for CO2 stabilization levels of 450 ppmv and ice-melt model parameters only. Figure 13 shows 450 ppmv and 650 ppmv). The cases shown are the reference case used in 650 ppmv results for different climate sensitivities (1.5, 2.5 and Figure 12; the case where all emissions (other than CO2) follow 4.5°C) coupled (for sea level rise) with low, mid and high iceIS920 to 2100, and the case where only CO2 changes are melt model parameters respectively. Uncertainties related to considered from 1990 — i.e., where the radiative forcings for model parameter uncertainties for any given stabilization level all other gases remain at their 1990 levels. Only the last case are much larger than the differences between the 450 ppmv and was considered in SAR WGI (see Figures 6.26 and 7.12). The 650 ppmv stabilization level results, particularly for sea level.

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

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Figure 13. (a) The effect of climate sensitivity uncertainties on global mean temperature for the S450 and S650 CO2 concentration profiles and the reference case for non-CO2 gases. The range of climate sensitivity (AT2r) is 1.5 to 4.5°C with a mid-range value of 2.5°C. For the same range in climate sensitivity, the global mean temperature change from 1990 10 2100 for the IS92a emissions scenario is between 1.4 and 2.9°C with a mid-range value of 2.0°C: (b) As for (a), but for global mean sea level change. The low, mid and high values of climate sensitivity are combined with low, mid and high ice-melt parameters. respectively, to give extreme ranges. For the same range in climate sensitivity and ice-melt parameteres, the global mean sea level rise from 1990 to 2100 for the IS92a emissions scenario is between 19 and 86 cm with a mid-range value of 49 cm.

Figure 14. (a) Sensitivity of global mean temperature change to CHA emissions for the S450 and S650 concentration profiles (see Figure 4). The solid lines give the "reference” results; the "CH, low"/"CHA high" curves assume annual CH4 emissions decrease/increase linearly by 100 Tg(CH2) over 1990 to 2100 (see Table 4). The radiative forcing (and equivalent CO,) from which the global temperatures were derived were shown earlier in Figure 9a; (b) As for (a), but for global sca level change. Central values of the ice-melt parameters are assumed.

For planning purposes, reducing model parameter uncenainties SO2 emissions case in Figure 15. (The same sensitivity cases would clearly be advantageous. These are uncontrollable aspects were considered in the assessment of forcing and equivalent of the climate/sea level system, however, while the stabilization CO2 uncertainties in Section 2.3.1.) N20 sensitivity is not level is potentially controllable. The comparison in Figure 13. shown because, for the +2 Tg(N) perturbations considered therefore, provides a graphic illustration of the extent of poten- previously, this is appreciably less in the near-term than for CH, tial control relative to overall uncertainties in the climate and sea due to the long lifetime of N20 relative to CH. (compare level projections.

Figures 9a and 9h).

Figures 14 and 15 show the sensitivity of the 450 ppmv and in the context of this sensitivity analysis, the long-term effects 650 ppmv results to gas-specific uncertainties in future emis- of CH, and So, for the considered perturbations are relatively sions: a change over 1990–2100 of 4100 Tg(CH2) about the small compared with the differences between the results for reference CH, emissions case in Figure 14, and a change over different stabilization levels (see Figures A4 and AS in 1990–2100 of 350 per cent (i.e., 37.5 TgS) about the reference Appendix 1). However, the short-term effects are, relatively, Stabilisation of Atmospheric Greenhouse Gases: Physical, Biological and Socio-economic Implications

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1990 2000 2010 2020 2030 2040 2050 2060 2070 2080 2090 2100

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much larger (compare Figures 11 and A2). This is because both

CH, and Soz-derived aerosol have much shorter response 2.

times than CO2. The full differential effects on climate related
to different CO, stabilization targets therefore take much longer
to manifest themselves compared with the more rapid responses
10 CH4 and SO2 emissions changes.
Although we cannot yet characterize the differences among
stabilization levels and pathways in terms of their degree of
risk, it is clear, as noted in SAR WGI (Section 6.3) and in
Wigley, et al. (1996) that the choice of both stabilization level
and pathway affects the magnitudes and rates of future climate
and sea level change. Future emissions of other greenhouse
gases also influence future climate and sea level appreciably,
generally leading to larger changes than from CO2 emissions

alone. Thus, mitigation of these other-gas emissions is a valusoso able component of a programme designed to prevent dangerous

interference with the climate system. In the long-term (beyond 2100). uncertainties in the future emissions of CH4, N20 and SO, have effects that are generally less than those associated with the differences between different CO2 stabilization levels. In the short-term (to around 2050), however, the importance of other-gas emissions is, relatively, much larger. Uncertainties in future CH4 and SO2 emissions lead to climate change uncertainties that exceed those due to different CO, concentration

profiles 1990 2000 2010 2020 2030 2040 2050 2060 2070 2080 2090 2100

The situation with regard to SO2 emissions is more complex

than that for greenhouse gas emissions because of their Figure 15. (a) Sensitivity of global mean temperature change to SO, extreme spatial heterogeneity

. The cooling effect of SO2 emisemissions for the S450 and 5650 concentration profiles (see Figure 4). sions cannot be considered as merely offsetting the warming As in Figure 10a, the solid lines give the "reference" cases, the short effect of greenhouse gas emissions. dashed lines show the "high SO2“ cases where emissions increase linearly from 75 TgS/yr in 1990 to 112.5 TgS/yr in 2100 and the long dashed lines show the "low S02" cases where emissions decrease linearly to 37.5 TgS/yr in 2100. The radiative forcing (and equivalent CO2) from which the global temperatures were derived were shown earlier in Figure 101. (b) As for (a), but for global sea level change. Central values of the ice-melt parameters are assumed.

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