Climate change and energy policy

Energy Policy,Vol. 25, No. I 1, pp. 923-939 1997

ELSEVIER PII:S0301-4215(97)00086-4

© 1997 Publishedby Elsevier Science Ltd Printed in Great Britain. All rights reserved 0301-4215/97 $17.00 + 0.00

Climate change and energy policy The impacts and implications of aerosols James Jason West*, Chris Hope and Stuart N Lane Departments of Civil and Environmental Engineering and Engineering and Public Policy, Carnegie Mellon University, Pittsburgh, PA, 15213, USA Judge Institute of Management Studies, University of Cambridge, Cambridge, CB2 lAG UK Department of Geography, University of Cambridge, Cambridge, CB2 3EN UK

Anthropogenic increases in aerosol concentrations are believed to significantly affect climate, notably by exerting a negative radiative forcing which counteracts, to some extent, the positive radiative forcing of greenhouse gases (GHGs). The potential effects of aerosols and their short atmospheric lifetimes raise issues which are critical to climate policy. This paper isolates the implications of aerosols by treating aerosol emissions as a policy variable separate from GHG emissions, but linked through energy policy. Using a simple climate model, results show that with no GHG abatement, changes in aerosol emissions can significantly affect net radiative forcing, but that the positive forcing of GHGs continues to dominate. Aerosols are also shown to reduce the difference in net radiative forcing between abatement and 'businessas-usual' policies, while the ability to reduce this effect through aerosol emissions from energy policy is limited. However, the conclusion that aerosols are beneficial to climate because they counteract greenhouse warming is then questioned; scenarios with high aerosol and GHG emissions are expected to yield both greater uncertainty in mean temperature and a greater likelihood of changes in other climate parameters. © 1997 Published by Elsevier Science Ltd. All rights reserved. Keywords: Aerosol radiative forcing; Climate change; Energy emissions policy

Introduction A growing consensus among climate scientists holds that anthropogenic emissions of aerosols have significantly influenced climate over the past century. Increases in aerosols, notably sulfates, are believed to have exerted a net cooling (negative radiative forcing) effect, which may explain overpredictions of historical temperature increases by climate models which use only greenhouse gases (GHGs). Consequently, not only must predictions of future climate change consider the effects of aerosols, but policy must also be sensitive to the climatic impacts of proposed aerosol emissions. Aerosols are believed to reduce the short-wave solar radiation received at the earth's surface through both direct and indirect effects. The direct effect of aerosols involves the backscatter and absorption of incident sunlight (McCormick and Ludwig, 1967); since aerosols generally increase

*Corresponding author: Tel: (412) 268-7889, Fax: (412) 268-3757, e-mail: [email protected]

the fraction of sunlight reflected, they decrease net radiative forcing. However, over highly reflective surfaces or where aerosols are highly absorptive (eg soot), aerosols may increase net radiative forcing (Isaksen et al, 1992). Through the indirect effect, aerosols increase the concentration of cloud condensation nuclei (CCN). This both increases the albedo (reflectivity) of clouds (Twomey et al, 1984; Charlson et al, 1987) and prolongs the lifetime of cloud droplets to affect both total reflection by clouds and the atmospheric budget of water vapor (Charlson et al, 1992). Recent estimates of aerosol radiative forcing show that it is significant in comparison to GHG forcing. For sulfate aerosols alone, estimates of the present globally averaged anthropogenic radiative forcing by the direct effect range from -0.3 to -0.9 W m -2 (Kiehl and Briegleb, 1993; Taylor and Penner, 1994), while Jones et al (1994) estimate an indirect effect of -1.3 W m -2. These compare to an estimated forcing of +2.5 W m-2 from anthropogenic GHGs (Shine eta/, 1990). Although most research has focused on sulfate aerosols, other aerosols, including aerosols from biomass burning (Penner et al, 1991; Penner et al, 1992), may also be significant.

924 Climate change and energy policy: J J West et al Recent research by Mitchell et al (1995) supports aerosol science by demonstrating that the inclusion of aerosols in a general circulation model (GCM) significantly improves the prediction of temperature increases over the past century. These findings, however, are based on fairly simple representations of aerosol effects, and accordance with historical temperature may reflect multiple errors which cancel. Aerosol effects are more substantially confirmed through other climate observations, including decreases in the daily and seasonal temperature ranges (Karl et al, 1991; Engardt and Rodhe, 1993) and correlations between sulfur emissions and temperature (Karl eta/, 1995). Despite this progress, however, modeling aerosol effects has proven extremely difficult, and the uncertainties in aerosol radiative forcing are among the most significant in climate research. Aerosols in future climate

Aerosols add a new level of complexity, not only to climate science, but also to policy. Critically important is the fact that aerosols have a much shorter atmospheric lifetime than GHGs (days vs decades). This will lead to three very important results: (1) Aerosol emissions~ are concentrated in heavily industrialized areas and, because of their short lifetimes, the resulting concentrations and radiative forcings are regional. Although GCM results show that the cooling effects of aerosols are distributed over a much larger area than the radiative forcing, this cooling tends to be centered near the area of emissions (Taylor and Penner, 1994; Mitchell et al, 1995). Patterns of climate change will therefore differ from previous predictions using only GHGs, with some regions possibly experiencing cooling. Critically, the greatest aerosol cooling is likely to occur in the most heavily industrialized regions, while GHGs produced by industrial activity will impact climate globally. (2) Without substantial abatement, differences in lifetimes will cause GHG concentrations to continue to build up while aerosol concentrations remain roughly proportional to emissions. The warming from GHGs is therefore expected to become more dominant with time unless increases in aerosol emissions significantly outpace those of GHGs. (3) Anthropogenic emissions of both CO2 and aerosols result largely from the combustion of fossil fuels, which accounts for about 84% of CO2 (Subak et al, 1993) and 92% of sulfur (Spiro et al, 1992). Consequently, an abatement policy to reduce CO2 emissions from energy sources, through reductions in demand or greater use of non-fossil energy, will also decrease sulfate aerosol emissions (Kaufman et al, 1991). Alternatively, CO2 emissions may be reduced by favoring fossil fuels which produce less CO2 per unit energy - switching from coal to oil, or oil to gas (Grubb, 1990) - and doing so will The term 'aerosolemissions'is usedto signifyboth directemissionsof aerosols and emissionsof aerosolprecursors, and includessulfate,carbonaceous, and other aerosols.

also decrease sulfur emissions. While these reductions may stabilize CO2 concentrations over the long term, aerosol concentrations and their negative radiative forcing will decrease almost immediately (Charlson et al, 1991). An abatement strategy might therefore exacerbate warming in the short term relative to 'business-as-usual' alternatives- a possibility demonstrated by Wigley ( 1991). Finally, just as changes in policies addressing CO2 emissions through control of fossil fuel combustion affect aerosol radiative forcing, so will changes in aerosol emissions for other policy purposes. Most notably, reducing aerosol emissions to ameliorate acid rain or improve urban air quality may also cause a relative increase in net radiative forcing (Wigley, 1989). In wealthier industrialized nations, there is already significant and mounting pressure to reduce aerosol emissions, and smokestack emission controls have been introduced. Objectives

These new complexities in the understanding of climate change have profound implications for policy, both in determining what level of abatement is preferred in different regions, and in determining how abatement can be best achieved for a minimal climate impact. But despite the recognition of aerosols, policy analyses o f climate change which now include the effects of aerosols, including the IPCC92 scenarios (Leggett et al, 1992), do not account for aerosol emissions as a separate policy variable. Likewise most policy discussions determine aerosol emissions directly from GHG emissions policies (Wigley, 1991). This research proposes that: (0 rather than being consequences of GHG emissions policies, aerosol emissions can, to some extent, be determined by policy independently from GHG emissions through the application of different policy levers; and (it) the climatic effects of changing aerosol emissions, apart from GHG emissions, may be significant. The objective of this paper, therefore, is to apply a simple model to evaluate the radiative forcing and climate outcomes of different combinations of GHG and aerosol emissions which represent a range of plausible and illustrative energy policy scenarios. Specifically, this paper will perform two experiments. The first will evaluate the climatic impacts of changes in aerosol emissions under 'business-as-usual' GHG emissions. The second will evaluate claims that fossil fuel abatement may lead to a short-term warming relative to do-nothing policies, and will test whether this result is inevitable or whether different policy levers addressing aerosol emissions can effectively reduce it.

Modeling methods The model used in this study is an adaptation of the climate module of the Policy Analysis for the Greenhouse Effect (PAGE) integrated assessment model (Hope eta/, 1993; Plambeck et al, 1995; Plambeck and Hope, 1995). The model is modified to express aerosol effects differently (using the same functional forms, and including the effects of biomass burning), and to calculate radiative forcing and temperature change hemispherically rather than by economic region.

Climate change and energy policy: J J West et al -3-

Table 1 Uncertain parameters. Triangular distributions are used for each a

-2.§-

i

925

Uncertain parameter (units)

Proportion of CO2 emitted to air (%) 55 Half-life of CO2 atmospheric residence 100 (years) Stimulation of natural CO2 (M tonnes °C-1) -6000 Present-day aerosol direct effectb (W m-2) -0.6 Present-day aerosol indirect effectb (Wm-2) -1.2 Present-day biomass burning effectb -0.5 (W m-2) Climate sensitivity - equilibrium warming from doubling the pre-industrial CO2 2 concentrationb (°C) Half-life of global warming (years) 25

-1.5 •

-0.S

0 0

Low

I 50

I 100

150

EmlNIons per I'leml~hem ('rgS/yr)

Figure I

Modeled indirect radiative forcing in each hemisphere for a range of emissions and assumed present-day forcing (dQ). Present-day anthropogenic emissions are: Northern Hemisphere, 65; Southern Hemisphere, 8 TgS yr-1. Forcings in the two hemispheres match at any emissions level because natural emissions are taken as equal

Given future emissions under different policies, atmospheric concentrations of CO 2 and C H 4 are modeled using their residence time (half-life) in the atmosphere. For CO2, additional calculations are made of the stimulation of emissions by temperature changes, assuming that net stimulated emissions are related linearly with the change in temperature. This stimulation parameter typically takes a negative value to represent increased plant uptake through CO2 fertilization. Radiative forcing is calculated from CO2 and CH 4 concentrations using established relations. Radiative forcing from other greenhouse gases (N20 and halocarbons) is not modeled explicitly, but is represented by an 'extra forcing' trajectory, which is assumed the same for all policies. While radiative forcing by G H G s is considered uniform globally, aerosol forcing from the direct and indirect effects, and the aerosol effect of biomass burning, are calculated hemispherically as functions of anthropogenic sulfur emissions. The direct effect is calculated as a linear function of emissions, as in Wigley (1991), with a linear function also used for the aerosol forcing of biomass burning. For the indirect effect, a logarithmic function is chosen to simulate the saturation of the indirect effect as aerosol number concentration increases, 2 based on evidence in Charlson et al (1992). This function (Figure 1) accounts for both anthropogenic and natural emissions and is used by Wigley (1991), Wigley and Raper (1992), and Raper et al (1996). Note that because present anthropogenic emissions are much lower in the Southern Hemisphere, the current marginal indirect forc-

2 As aerosol number concentration increases, the number of CCN approaches a maximum for a given water content,

Mode

High

80 120

90 150

-2000 -0.3

2000 0.0

-0.5 -0.2

-0.1 +0. I

3.5

5.5

45

75

aA triangular distribution has a probability density function with a triangular shape. The mode is the peak of the triangle (the most probable value) while the low and high values represent the extreme probabilities (see Morgan and Henrion, 1990). bpresent-day values for the aerosol forcingeffects and climate sensitivity are selected based on Raper et al (1996) and the discussion in the next section.

ing (the change in forcing per unit change in emissions) is estimated to be 2.8 times greater. Both the direct and indirect effects consider only sulfate aerosols and neglect other aerosols, as with much current modeling work. However, Penner (1995) shows that including carbonaceous aerosols may nearly double the direct effect, as they are emitted at nearly the same rate as sulfate aerosols. This study therefore assumes either that the contribution of other aerosols to radiative forcing is negligible, or that other aerosol emissions, and their climate impacts, will be proportional to those of sulfates and are included in the presumed present-day effects. Further, the effect of biomass burning is treated separately, even though it acts through the direct and indirect effects. Separate calculations of the biomass burning and industrial effects is justified because emissions occur in different locations and because biomass burning emits a different mixture of aerosol types. Temperature change is found in each hemisphere by computing the equilibrium temperature rise from the net radiative forcing) and by using the half-life of temperature response to allow consideration of the thermal lag. This treatment of hemispheric-mean temperature fails to account for regional and temporal distributions of average temperature, and does not account for changes in other climate parameters, as discussed later. Assumed distributions for all uncertain parameters are shown in Table 1, with other parameter values and initial conditions given in Table 2. A full description of model equations is provided in West (1995).

Results Before applying the model to project future policy outcomes, the model is first run over the past century to find parameter

3 Net radiative forcing in each hemisphere is a simple sum of positive and negative forcings.

926

Climate change and energy policy: J J West et al

Table 2

,y.ll.I. / /

Model parameters and initial conditions

Parameter (units) Half-life of CH 4 in atmosphere (years) Fraction of CO2 that stays in the atmospherea 1990 temperature (above pre-industrial) (°C)

Natural sulfur emissions(TgS yr-l)b 1990 Anthropogenicsulfuremissions (TgS yr-1)c 1990 Biomassburning (dimensionless)~

Value

-NH -SH - NH

10 0.2 0.6 0.6 24

- SH

24

- NH

67

- SH

8

- NH - SH

0.57 0.43

/

5

/:1/

:

//i

/

!

/

j....."

;S;S;'-.-" """

aUsedto representthe new ocean-atmosphereequilibriumpartition for CO2 hFromM/511er(1995), and assumedconstant in the future CFromSpiro et al (1992) '/The hemisphericratio for biomassburning uses the ratio of sulfur emissions attributed to biomassburning by Spiro et al (1992), and this ratio is assumedconstant in the future.

0 0

I -0.25

I -0.5

I

-0.75

I -1

I -1.25

1 -1.5

I -1.75

-2

Aerosol Rsdlalive Forcing (WIm^2)

2 Change of global mean temperature from 1900 to 1990 (dT) modeled as a function of the total globally-averaged present day aerosol radiative forcing and climate sensitivity (for doubled CO2) Figure

values which simulate the observed global mean temperature rise of 0.5 + 0.15°C since 1900 (Raper et al, 1996). In particular, the relationship between climate sensitivity and aerosol forcing is analysed, following the approach of Wigley (1989), since climate predictions are sensitive to the large uncertainties in these parameters. Further, Raper et al (1996) show that the inclusion of aerosols in climate models may require climate sensitivity to be increased above the 2.5°C (for twice the pre-industrial concentration of CO2) suggested by Houghton et al (1990). Using the mid-range estimates of aerosol radiative forcing by the IPCC (direct effect,-0.3 W m-2; indirect effect, --0.8 W m-2; and biomass burning, -0.2 W m -2, all globally averaged), 4 Raper et al (1996) require a climate sensitivity greater than 4.5°C to produce a temperature rise of 0.5°C over the past century. Reducing the indirect aerosol effect to -0.4 W m -2, climate sensitivity values of 2.5 to 4.5°C give plausible temperature results, with a best estimate of ,,~ 3.5°C. For this exercise, an indirect aerosol effect of -0.5 W m -2 is used along with -0.3 W m -2 for the direct effect and -0.2 W m -2 for biomass burning. The ratio of these values is used to divide the total aerosol radiative forcing among the three separate effects; this ratio is important since the nonlinearity of the indirect effect makes its radiative forcing proportionally greater in 1900 than at present (Jones et al, 1994). Historical changes in G H G radiative forcing are taken from Shine et al (1990), and are well-established. Historical sulfur emissions use the assumptions of Wigley and Raper (1992), and biomass burning is assumed to increase linearly from one-fourth its current extent in 1900 to the presentP The thermal lag parameter is set at its modal value of 45 years. The results (Figure 2) show that for the assumed total aerosol forcing of -1.0 W m -2, the observed temperature rise of 0.5°C is generated by a climate sensitivity of 3.4°C -

a result remarkably similar to that of Raper e t al (1996). The results further show that aerosol forcings larger than -1.5 W m -2 produce the observed temperature change only when large climate sensitivities are chosen. However, this analysis does not consider the uncertainty in the half-life of global warming, or different ratios among the three aerosol effects, which may be significant. Further, it does not consider other changes which may have significantly affected climate - land use, surface albedo, solar irradiance, and anthropogenic changes in types of aerosols (notably the fraction of soot and the introduction of taller smokestacks which allows greater chemical conversion to aerosols) - and it is unknown what climate would have been without human intervention. While this analysis is based on global mean temperature, the model appears to overpredict the hemispheric difference in warming; at the 3.4°C climate sensitivity, the model predicts a temperature rise of 0.8°C in the Southern Hemisphere and 0.2°C in the Northern Hemisphere, while there is little observed difference (Raper et al, 1996). This is partially a result of calculating temperature independently in each hemisphere, with no allowance for differences in thermal inertia or heat flow between hemispheres. In projecting policies into the future, the model may therefore overestimate hemispheric differences in temperature. Based on this analysis, distributions for climate sensitivity and the present-day aerosol radiative forcings are selected (Table 1). For the indirect effect, a wide distribution with a long tail towards greater values is chosen to represent the wide range of estimates. Experiment 1 - 'Business-as-usual' G H G emissions

4 Cited in Raper et al (1996). 5 Past divisions of sulfur emissions and biomass burning between hemispheres are assumed the same as at present.

Experiment 1 isolates the effects of sulfur emissions on climate when no substantial reduction in G H G emissions is assumed. The 'business-as-usual' scenario employed is the IPCC92a

Climate change and energy policy." J J West et al Table 3 sources

Emission factors per unit energy for different energy a

Energy source

Solids - coal, lignite, peat, and oil shale Liquids - crude petroleum and natural gas liquids Gases - natural gas

CO z (tonnes C TJ -1)

Sulfur (kg S TJ -1)

23.1

451

18.9

178

13.2

0

aThese values represent the current aggregatedaverageemission factors for each energy source, found by dividing the emissions attributed to each energy source (Subak etal, 1993;von Hippel etal, 1993; Spiro etal, 1992) by the global energy consumptionfrom each source (UNSTAT, 1992). The emission factors are global aggregates, including commercial energy and other private and industrial uses.

scenario (Leggett et al, 1992), which uses best-guess assumptions of population, economic growth, technological improvements, and resource availability, with trajectories of fuel consumption for each fuel source in each economic region determined from a global energy economics model (Pepper et al, 1992). This scenario includes sulfur reductions currently agreed upon in industrialized nations, and assumes future levels of controls elsewhere. To consider the climatic effects of different levels of sulfur emissions, two alternative policies are constructed, each with the same G H G emissions as the IPCC92a scenario but different sulfur emissions. Sulfur emissions are varied by changing the mixture of fossil fuels consumed - varying quantities of solids, liquids, and gases 6 while producing the same trajectories of CO2 emissions as the IPCC92a scenario - and by assuming different fractions of sulfur removal by smokestack control technologies. Emissions of sulfur and CO2 from energy and the fraction of sulfur removal are calculated using the globally-averaged emissions per unit energy for different energy sources (Table 3). These emission factors involve a large degree of aggregation, and do not account for regional differences in the sulfur content of fuels and efficiency of energy production. The calculated sulfur emissions factors also include existing emission controls, ignoring spatial differences in the extent of emissions controls. Using the sulfur emissions factors, the percent reduction in sulfur emissions in 2100 implied in the IPCC92a scenario is calculated as 6 2 ° above the fraction of sulfur presently removed. 7 The two policy alternatives are constructed to represent the range of plausible sulfur emissions from energy sources for the same CO2 emissions trajectory. Solid fuel consumption and total sulfur emissions for these policies are shown in Figure 3: (1) Low Sulfur. The IPCC92a scenario is based on the modal resource constraints for liquids and gases reported by Masters et al (1991), with quantities of liquids and

6 Solids include coal, lignite, peat and oil shale. Liquids include crude petroleum and natural gas liquids. Gases include only natural gas. 7All fractions of sulfur removal are of the fraction not presently removed, since present removal is included in the emission factors.

927

gases declining towards 2100. IPCC92a is therefore taken as the maximum liquid and gas consumption while respecting these constraints. The Low Sulfur policy therefore uses the same consumption of fossil fuels as IPCC92a, and increases the fraction of sulfur removal by smokestack controls, reaching a maximum of 90% removal in 2100. s (2) High Sulfur. Sulfur emissions are increased by consuming more solid fuels and less liquids and gases than the IPCC92a scenario. The trajectories for liquids and gases are based on the IPCC92c scenario (a scenario with low emissions which is chosen arbitrarily), with the solids combustion calculated at each time step to maintain CO2 emissions. Smokestack emissions controls are reduced substantially from IPCC92a, but increase towards the end of the simulation (to 40% removal) to cap global sulfur emissions at three times the present level (225 TgS yr-I). While sulfur emissions of three times current levels may not be tolerated in industrialized nations, the increase of emissions elsewhere may not make this scenario implausible. Because the emission factors for CO 2 are based on other studies (Subak et al, 1993; von Hippel et al, 1993), the IPCC92 emissions of CO2 from energy sources are scaled (reduced by 10%) for consistency. The present hemispheric split in fuel consumption, for each fuel source, is found from the UNSTAT (1992) national energy statistics inventory. 9 Future biomass burning is assumed the same under all policies, growing proportionally with the total carbon released through biomass burning in the IPCC92a scenario. Emissions of methane, biomass burning, and CO2 and sulfur from sources other than energy are taken directly from the IPCC92a scenario, while the 'extra forcing' trajectory is assumed to increase from +0.15 W m -2 in 1990 to +0.35 W m -2 in 2100. Figures 4 and 5 show the modeled net radiative forcing and temperature for Experiment 1 when modal parameter values are used. The modal results for IPCC92a compare well with those of Mitchell et al (1995); the results estimate a lower global average radiative forcing (3.7 vs 4.0 W m -2) and a higher temperature change (1.9°C vs 1.8°C) in 2050, as expected given the larger aerosol forcing and climate sensitivity in this study. The model also compares well with the temperature changes of Raper et al (1996) when they use a high climate sensitivity of 4.5°C (a 3.6°C rise for both models in 210010), but overestimates temperature for lower climate sensitivities. The results suggest that changes in aerosol emissions

8This analysis does not consider the reduced efficiencyof energyproduction, and thus greater CO2 emissions, from sulfur removal (Vernon, 1989). 9 The future division between hemispheres is determined using the energy trajectories by economic region in Pepper et al (1992), for each energy source. Consumption in Australia and New Zealand is assumed to change proportionally with OECD consumption, while the rest of the Southern Hemisphere changes in proportion to the 'Other' category. ~oThe temperature change calculated by Raper et al (1996) is -3.0"C from 1990. We added 0.6°C as the rise since pre-industrial times for comparison.

928

Climate change and energy policy: J J West et al

t;it

...e_ iPCC92a i 4 - Low Sulfur I High Sulfur ]

E

r

8,.

A.,.ooA ..... .o,&'''°° ...

i

1980

;;..a

°"

@

. . . . . . . . . . . .

..¢p°o-O .............

d)-.............

I

I

I

I

I

2000

2020

2040

2060

2080

2100

Year

Figure 3 Normalized global solid fuel consumption (solid lines) and anthropogenic sulfur emissions (dotted lines) under the IPCC92a scenario and the policy alternatives constructed for Experiment 1. All values are relative to 1990 levels, where 1990 solid fuel consumption is 97.5 EJ yr-~ and 1990 sulfur emissions are 75 TgS yr-'. Solids consumption for the IPCC92a scenario and the Low Sulfur policy are indistinguishable

~6

SS

S~

rd 4 ¸

It I1=CC92a i - Q -- Low Sulfur ] -- t - . H i g h Sulft=" I

]!a

~,

.jt.. -~lf"

° ° o A o°

0 1980

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I

I

I

I

2000

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2040

2060

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2100

Year

~6

J_5 f,

: IPCC,92a I - ,D - Low Sulfur I - - ~" -.High Su~ur I

2

0 1980

I 2000

I 2020

I 2040

I 2060

I 2080

2100

Year

Figure 4 Net radiative forcing (relative to pre-industrial) in each hemisphere in Experiment I

significantly affect climate, with a much greater effect in the Northern Hemisphere. Differences in aerosol emissions alone among the different policies, account for differences in radiative forcing of 1.8 W m -2, and in temperature of 1.0*C in

2100 in the Northern Hemisphere. The figures show, furthermore, that the magnitude of temperature change is greater in the Southern Hemisphere than in the Northern Hemisphere, although as discussed earlier, the model may

Climate change and energy policy: J J West et al 929

4.

. s '~ ~3,

d j '~ jD o..' S / SS °'" . ' " S o...~"

!, 0 1980

°.oR'"

I 2000

I 2020

* IPCC92a - o - Low Sulfur -- ~- - .High Sulfur

o.A "'°

I 2040

I 2060

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2100

Yelu"

jJ

4

o°,°"

13

* IPCC92a I - o - Low Sulfur - - 6 . . , High Sulfur

i2 0 1980

l

I

I

I

I

2000

2020

2040

2060

2080

2100

Year

Figure 5 Temperature change (relative to pre-industrial) in each hemisphere for Experiment 1

overpredict hemispheric differences in temperature. Finally, note that even with the extremely large increase of sulfur emissions under the High Sulfur policy, the positive forcing of GHGs dominates the climate response. This is due primarily to the longer atmospheric lifetimes of GHGS, but also results from the saturation of the indirect aerosol effect. Experiment 2 - Fossilfuel abatement Experiment 2 explores the contention that the abatement of fossil fuel combustion will lead to warming in the short term relative to business-as-usual policies (Wigley, 1991). The model is used to estimate the climate response from abatement scenarios relative to the IPCC92a scenario, and further, to explore the climatic impacts of different means of achieving CO2 abatement through energy policy. The abatement scenario employed is the IPCC92c scenario. Note that the IPCC92 scenarios are not policies - they reflect a range of assumptions about population growth, economic activity, and energy sources, and not the use of policy levers to affect emissions. For this experiment, the low trajectories of fossil fuel use in the IPCC92c scenario may be considered the result of abatement policies enacted through market-based controls, supply restrictions, or other policy levers, rather than assumptions of low population and economic growth. The IPCC92c scenario has a sufficiently low trajectory of emissions such that C O 2 concentrations

are expected to stabilize at roughly 500 to 550 ppmv between years 2100 and 2150. As in Experiment 1, two alternative abatement policies are constructed to represent the range of plausible sulphur emissions, but with the same trajectories of CO2 emissions as IPCC92c. H The IPCC92c scenario assumes the low estimate of resource availability for liquids and gases reported by Masters et al (1991), and so has a high trajectory of solid fuel combustion over the next century. This scenario is therefore used to represent abatement through reduction of liquids and gases. The policy alternatives to the IPCC92c scenario are (see Figure 6): (1) Low Sulfur. Consumption of liquids and gases is maximized while respecting resource constraints by basing consumption trajectories on the IPCC92a scenario. A more substantial reduction in the use of solids is limited by the resource availability of liquids and gases. The fraction of sulfur removed by smokestack controls is doubled over that of the IPCC92c scenario, reaching a maximum of 70% in 2100. (2) High Sulfur. Fossil fuel trajectories are constructed to represent an extreme reduction in liquids and gases,

1z Alternative policies with the same energy production as the IPCC92c scenario (rather than the same G H G emissions) were also constructed, but with very similar results which are not presented.

930 Climatechange and energypolicy: J J West et al 2.00

1.75

1.50

1.25

• .o .....

o .....

o,.

""''-

-.-,pec~

...,

-=,-Law

I

Sulfur

. . - ~ - H g h S u fur

1,00 "o . . . . .

D- . . . .

D.....

O.....

O.....

D.,

0.75

0.50

0.25 1980

I 2000

I 2020

I 2940

I 2060

I 2080

I 2100

Year

Figure 6 Normalized global solid fuel consumption (solid lines) and anthropogenic sulfur emissions (dotted lines) under the IPCC92c scenario and the policy alternatives constructed for Experiment 2. All values are relative to 1990 levels, where 1990 solid fuel consumption is 97.5 EJ yr-t and 1990 sulfur emissions are 75 TgS yr-1

but consider the time lag of converting infrastructure, the demands for liquid fuels in transportation, and the growth in use of gases recovered from biological processes. In addition, the smokestack emissions controls in the IPCC92c scenario are removed throughout the future, so that the fraction removal is the same as at present. Emissions of methane, biomass burning, and CO2 and sulfur from sources other than energy are the same as the IPCC92c scenario, while the 'extra forcing' term is the same as for the IPCC92a scenario. The fraction of each fuel source in the Southern Hemisphere at each time step is calculated as in Experiment 1, and assumed the same for all policies. Because of differences in the CO2 efficiency of different energy sources, the total fossil fuel energy production for the Low Sulfur policy exceeds that of the IPCC92c scenario by a maximum of 11%, while production under the High Sulfur policy is at most 4% less. The most likely scenario to generate the given CO2 emissions trajectory would lie between the IPCC92c scenario and the Low Sulfur policy. Finally, note that these policies, as in Experiment 1, do not consider potential changes in emissions of GHGs other than CO2. There is, therefore, an even greater range of aerosol emissions possible for the same trajectories of total G H G forcing. Figures 7 and 8 show net radiative forcing and temperature projected over the next century for modal parameter values. The results support claims that abatement may lead to a warming in the short term, as the CoalReduction policy (and to a lesser extent the IPCC92c scenario) produces a slightly greater net radiative forcing and temperature than the IPCC92a scenario early in the simulation (until 2040), but only in the Northern Hemisphere. This effect is not witnessed for the High Sulfur policy due to its maintenance of sulfur emissions, and thus negative radiative forcing. These results are

qualitatively similar to those of Wigley (1991) but are less distinct, despite the larger total aerosol forcing assumed in this study. This is primarily due to the fact that Wigley (1991) considered more extreme policies.12 More generally, little difference is witnessed in the Northern Hemisphere radiative forcing from the various policies until year 2050, when the abatement policies begin to diverge from the IPCC92a scenario. The divergence is greater in the Southern Hemisphere, but again is not significant until 2050. This lack of divergence is due to the reduction in aerosol radiative forcing in the short term under the abatement policies, and is consistent with other findings (Wigley and Raper, 1992). Further, the difference in radiative forcing and temperatures among the alternative abatement policies is small but significant, with a maximum difference in forcing of 0.8 W m -2 and in temperature of 0.5°C in the Northern Hemisphere. Finally, while all previous results show only modal values, Figure 9 presents the uncertainty in Northern Hemisphere temperature. Uncertainty is considered by randomly selecting values from the distributions of each parameter, using Latin hypercube sampling (see Morgan and Henrion, 1990). Calculated uncertainties are based on fifty simulations, estimating the 5% and 95% outcomes using the third and fortyeighth outcomes when simulations are ranked in terms of Northern Hemisphere temperature in 2100. Although some combinations of parameter values may be rather implausible (for example, high aerosol forcing with a low climate sensitivity and high half-life of global warming), the parameter distributions are chosen to avoid implausible scenarios. The results show that extremely large temperature changes are more likely under the IPCC92a scenario, as expected, while

12This analysis does not test the robustnessof the conclusionsrelative to the extent of abatement, which may be significant.

Climate change and energy policy." J J West et al 931 7. 6

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estimated confidence intervals are roughly proportional to the magnitude of temperature change.

Discussion Based on this analysis, a number of general observations can be made. First, although aerosols appear to have counteracted the effects of GHGs in determining temperature over the past century, the magnitudes of the radiative forcings and of the climate sensitivity remain uncertain. Figure 2 demonstrates that introducing aerosols into climate models may require increases in climate sensitivity in order to reproduce historical increases in temperature, while many recent estimates of aerosol radiative forcing may be unrealistically large. 13 Alternatively, other processes may be responsible for the discrepancies between observations and predictions. Clearly, much work in aerosol science and modeling remains before historical climate is better explained and the confidence interval of future predictions is narrowed.

53 Critics of climate science may note that because research funding is allocated on the basis of what specialities are perceived to be most important, scientists may well have an institutionalized incentive to overstate the importance of their own research. It may not be surprising, one could argue, that aerosol science has fairly consistently overestimated impacts relative to what is now the general opinion. See Boehmer-Christiansen (1994) for the view that climate scientists act to further their own funding interests.

Second, Experiment 1 shows that because aerosols have a significant climatic effect, large-scale changes in aerosol emissions can affect climate outcomes. Under business-as-usual G H G emissions, this experiment demonstrates that a wide range of sulfur emissions trajectories can realistically be constructed, and that the climatic impacts resulting from these policies differ significantly, particularly in the Northern Hemisphere. However, as G H G emissions build up and the indirect aerosol effect tends toward saturation, GHGs dominate the climate response, even when very high emissions of sulfur are assumed. Third, Experiment 2 supports claims that the abatement of CO2 emissions through a reduction in fossil fuel combustion will lead to a simultaneous reduction in aerosol emissions, and therefore cause a warming in the short term relative to 'business-as-usual' policies. However, this experiment estimates that the relative warming is minimal and within wide uncertainty bounds. Further, this result is only witnessed in the Northern Hemisphere, while the High Sulfur policy is shown to produce enough negative radiative forcing to avoid this result. A more robust conclusion is that because of aerosols, abatement and business-as-usual policies show little divergence in net radiative forcing and temperature projections before 2050, and that the range of climate outcomes among the different policies is narrowed. This experiment further shows that because of the reduction in aerosol emissions required by the abatement strategies, even large changes

932

Climate change and energy policy: J J West et al

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in the means of abatement, and thus sulfur emissions, have rather small impacts on net radiative forcing and temperature changes. These results are derived from a climate model which calculates aerosol forcing as a hemispheric average using simple functions of emissions. However, aerosol emissions are spatially heterogeneous, and the resulting radiative forcing is dependent upon the location of emissions for several reasons. First, aerosol forcing is enhanced in the tropics and subtropics because of greater solar insolation and longer aerosol lifetimes due to greater convective mix-

ing (Charlson et al, 1991). Second, aerosol forcing is greatest in humid regions because the direct effect is highly dependent upon relative humidity (Pilinis et al, 1995), and the indirect effect requires the presence of water vapor to form clouds. Consequently, emissions over humid tropical regions likely cause greater forcing, while emissions in less industrialized regions, particularly the Southern Hemisphere, will result in a greater indirect effect because they are further from saturation. Aside from projecting hemispheric trends in emissions, the model does not consider changes in aerosol forcing due to changes in the spatial distribution

Climate change and energy policy." J J West et al 933 of emissions. This may be most important for increases in emissions from the projected industrialization in Southern Asia, which could increase the global average radiative forcing per unit emissions.

Implications for decision-making The policy significance of Experiment 2 is apparent as the lack of divergence of policy outcomes makes abatement policies difficult to justify in an economic decision-making context where time horizons are short and future costs and benefits are discounted (Nordhaus, 1991; Manne and Richels, 1992). Likewise, in a political context, policy-makers will be reluctant to choose an abatement policy if it will lead to no apparent difference, or even exacerbate warming, in the short term - especially if that 'short term' is on the order of 50 years. The experiments also raise interesting but potentially difficult issues for equity. First, while aerosols will likely provide the greatest cooling near industrial sources, the warming due to G H G s from those same industrial sources will be experienced globally. In the experiments, this is seen in the hemispheric differences in radiative forcing and temperature, but heterogeneities will also exist within each hemisphere.14 The result is potentially inequitable, with those most responsible for altering climate, bearing perhaps the least change.15 While this creates a dilemma for geographical or spatial equity, the effect of aerosols to reduce the divergence of abatement and business-as-usual policies puts intergenerational equity at the forefront of decision-making; that is, if the benefits of abatement are not witnessed until far in the future, then decisions will be highly sensitive to how future well-being is weighed against present well-being. The outcomes of the two experiments show that decisions regarding aerosol emissions can have a significant impact on climate, irrespective of G H G emissions, but with less of an impact under abatement policies. This research therefore asserts that rather than being consequences of G H G emissions policies, aerosol emissions need to be addressed directly by policy and in conjunction with G H G emissions policies. Still, the policies developed in the two experiments represent extremes in physical capabilities, irrespective of how they may be achieved through policy. If policy is indeed to manage emissions of both aerosols and G H G s in some co-ordinated manner, a large degree of government control and international agreement is necessary. For this control over the fuel mix, economic measures (taxes and subsidies) may be effective, but greater control through regulatory measures (permits) may be required. Consequently, recent proposals to encourage CO2 abatement through carbon taxes may affect the fuel mix to generate undesirable aerosol emis-

t4Although, as previouslynoted, the modelmayoverestimatehemispheric differences in temperature. ~sThis inequity may be exacerbatedby the fact that developingnations will be less capable of adapting to climaticchange, in part becausetheir economies depend more on agricultural production.

sions; such carbon taxes may be best instituted in co-ordination with controls on aerosol emissions,16 In addition, aerosols cause other environmental problems including acidification, which impacts terrestrial and freshwater ecosystems, and poor urban air quality, which causes reduced visibility and health impacts (Weliburn, 1988). These impacts are considered sufficiently damaging to warrant significant legislative action in Europe and North America (BoehmerChristiansen and Skea, 1991). Because aerosols are now recognized to also affect climate, policy can no longer address climate change and acidification (and urban air quality) as separate problems, In evaluating abatement policies for climate change, the damages of acidification should be incorporated; it may be possible to construct some trade-off between the damages of acidification and any 'benefits' of aerosols in reducing temperature change. Conversely, policies which address acidification cannot be considered complete unless their effects on climate are also analysed. Further, since the effects of aerosols on climate extend well beyond the range of their deposition, aerosols are no longer simply a regional problem. Policy therefore needs to extend its scale to consider the climatic effects of aerosols hemispherically or globally. Finally, because these issues are linked, models aiding policy decisions should be expanded to reflect these linkages. Climate models which incorporate the effects of aerosols could be coupled with models of regional acidification which estimate damage from spatially non-uniform loadings (Alcamo et al, 1987; Metcalfe and Whyatt, 1994). However, to reasonably predict acidification impacts, a much finer spatial resolution is required than most climate models are capable of. 17

Net radiative forcing and climate policy While policy needs to be sensitive to the climatic impacts of the combined G H G and aerosol forcings, it is not readily apparent, in either experiment, what the desired aerosol forcing should be. In both experiments, increasing aerosol emissions decreases net radiative forcing and thus temperature change; as long as the positive forcing of G H G s dominates the climate response, aerosol emissions are seen as beneficial. Clearly, anthropogenic aerosols cause other environmental problems. But, from a purely climatic perspective (ignoring these other effects), are aerosol emissions truly beneficial? Are the climatic impacts resulting from high concentrations of both G H G s and aerosols indistinguishable from a case of low G H G and aerosol concentrations, even if they have the same net radiative forcing? There are two general reasons to believe that they are not.

~6In practice, a carbon tax would particularly reduce coal combustion which would also reduce aerosolemissions.If the projected reduction in aerosol emissionsis the desired outcome, then no other aerosol emissions control is required. ~7Acid rain and climatechange are also physicallylinked in the following ways: sulfur emissionscontrol in power plants addressing acid rain will decrease the efficiencyof energy production and thus increase the COz emission per unit of useful energy (Vernon, 1989); acid rain and climate change will cause combined ecosystem stresses, with possible synergisticeffects(CO2fertilizationcould, however,increaseplant growth, but doing so may also be an ecosystemstress); and ecosystemdamage from acid rain will contribute to biogenicCO2 emissions.

934

Climate change and energy policy." J J West et al

First, it is not apparent that the short-wave forcing of aerosols compensates simply for the long-wave forcing of G H G s (ie they combine as a simple sum) in determining global or hemispheric average temperature. Charlson and Wigley (1994) question such a simple compensation because the two processes have different spatial distributions, and because of the diurnal and seasonal variability in aerosol forcing; aerosol forcing is much more variable, both spatially and temporally, than G H G forcing. In addition, the two effects arise through different mechanisms (long-wave vs short-wave forcing) and occur at different elevations (Cox et al, 1995). Once heat from these variable forcings is redistributed through the climate system, the net effect on mean temperature may not be determined by net radiative forcing, but by some function of the positive and negative forcings in which either the G H G warming or aerosol cooling is proportionally greater than the ratio of radiative forcings.~S Until greater understanding is gained of how estimates of temperature using net radiative forcing might be in error, the estimates of expected mean temperature in the last section (Figures 5 and 8) do not change. However, the uncertainty distributions in Figure 9 include only the uncertainty in parameter values in the model presented, and do not consider uncertainty in the model itself. Because net radiative forcing may not be the appropriate determinant of mean temperature, the inclusion of model uncertainty suggests that uncertainty distributions should be wider than shown. In particular, the uncertainty in mean temperature is expected to be greater for a policy with high G H G and aerosol concentrations (IPCC92a) than for an abatement policy, with a greater likelihood of both greater and lesser temperature changes. Second, the modeling approach in this study uses global mean temperature implicitly as a determinant of climate 'damages' (both environmental and economic), with no allowance for changes in other climate parameters. While it has long been recognized that other climate parameters may change along with mean temperature, aerosols suggest that other parameters may change even if mean temperature is constant, as there are a number of effects that aerosols alone may have. Aerosols are believed to be partially responsible for observed decreases in diurnal and seasonal temperature ranges (Karl et al, 1995), and because aerosols prolong cloud lifetimes, they may considerably alter patterns of precipitation (Charlson et al, 1992). Further, the spatial heterogeneity of aerosol forcing may alter the distribution of temperature changes, particularly by causing hemispheric temperature differences (Wigley, 1991). This in turn may change atmospheric and oceanic circulation as heat is redistributed,

18 This possibility is shown by Taylor and Penner (1994), who find significantly different climate sensitivities associated with GHGs and aerosols. Their results, however, have recently been proven erroneous, and Cox et al (1995) find that for small foreingsover 35 model years,the sensitivities are essentially the same. The argument that positive and negativeforcingsmaynot combinesimplyto determinemean temperature response may help to explain (or complicate further!) the relationship between aerosol radiative forcing and climate sensitivityin simulating temperature over the past century (Figure 2).

thus altering predominant weather patternsA 9 Likewise, atmospheric circulation may be affected as aerosols reduce the solar radiation received at the surface, changing the vertical temperature profile and reducing convection (Penner, 1995). For example, Taylor and Penner (1994) find a significant cooling in the Gulf of Alaska not associated directly with aerosol emissions, but with a change in atmospheric circulation. Likewise, Lal et al (1995) find that the inclusion of aerosols in a G C M weakens the strength of the Indian monsoon, while the monsoon is strengthened when only CO2 is considered. Research is only beginning to consider whether and how aerosols have contributed to such regional changes in climate. 2° As a result of these climate influences, a policy with high G H G and aerosol concentrations (IPCC92a) is expected to be more likely to induce regional climate changes (and changes in climate parameters other than mean temperature) and their associated damages,2j than an abatement policy. With these climate changes due to aerosols in mind, it is clear that although aerosols may counteract G H G s to some extent in determining mean temperature, aerosol emissions can also be viewed as a further perturbation of the climate system. It must therefore be recognized that the conclusion that aerosols are beneficial to the climate system22 is a direct result of the use of net radiative forcing as a determinant of climate change and climate damages, and that this perspective is to some extent misleading. Consequently, policy must consider not only the net radiative forcing, but also the disaggregated positive and negative radiative forcings presented in Figure 10. This presentation of radiative forcing makes the divergence of policy outcomes much more apparent, and conveys a very different message to policy. Most current models of climate change for policy analysis, like the model in this study, determine temperature from net radiative forcing and generally do not account for changes in other climate parameters in determining damages. This is primarily because aerosol radiative forcing and its impacts on climate are still poorly understood, and because climate

t9 Basedon these effects,one could argue that policymust also consider the spatial distribution of aerosols. Although the effects of spatially heterogeneous forcings on regional climate are not well understood, a risk-averse policy might well be one which reduces peaks in aerosol forcing over industrialized regions. Transferring emissions to lessindustrialized regions, however,may increase the radiative forcing per unit emissions. 2oThe most notable regionalclimatechange this century is the prolonged drought in the Sahelregionof Africa, the causesof which are still highly uncertain. 2, The 'damages' from such changes in climate depend on both the magnitude of change and ability of natural ecosystems and human economicsystemsto adapt to the changes Somechangesmaybe beneficial, depending on one's point of view. 22Such a conclusion would seemto suggestincreasingaerosolemissions to reduce temperature change through 'climate manipulation' or 'geoengineering' strategies. Both experiments show, however, that due to differencesin atmosphericlifetimes,evenextremeassumptionsin energy policyare incapableof continuing to significantlycounteract greenhouse warming. Further, because one could also take the view that aerosols perturb rather than repair climate, any such strategy should be viewed skeptically. Finally, in addition to questions of scientificunderstanding, geoengineeringalso raisesseriousquestions about ethics concerning the relationship between humankind and nature.

Climate change and energy policy." J J West et al 935

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Conclusions The recognition that aerosols have a significant effect on climate is an acknowledgment that human perturbations to the climate system are likely greater than previously believed, although some anthropogenic changes may have counteracted each other to some degree. The magnitude of these changes and the extent to which they have counteracted one another

through interactions in the climate system remain poorly understood, hindering the ability to predict future climate under different combinations of positive G H G and negative aerosol forcings. This research demonstrates that for a 'business-as-usual' G H G emissions trajectory, a wide range of possible aerosol emissions can be realistically constructed, and that the differences in climate response among these policies are significant. This research therefore asserts that rather than being consequences of G H G emissions policies, aerosol emissions and their climatic impacts should be considered both separately in policy formulation, and in conjunction with G H G emissions policy. Results from Experiment 2 support claims that the reduction of aerosol emissions from CO2 abatement policies will lead to a warming in the short term, relative to business-asusual alternatives, although this effect is found to be minimal. More generally, aerosols are shown to reduce the difference in climatic outcomes between policy options, particularly in the short term, while the ability to reduce this effect through changes in aerosol emissions is limited. Still, this experiment demonstrates that simple claims that abatement will cause a relative warming fail to account for the many ways policy can achieve G H G abatement and influence aerosol emissions. These conclusions are reached using a climate model in which temperature is calculated simply from net radiative forcing, with temperature used implicitly as a measure of climate damages. This approach is simplistic both because it assumes that positive and negative radiative forcings cancel simply in determining mean temperature, and because it fails to account for changes in other climate parameters possibly induced by aerosols. Therefore, while scenarios with high aerosol and GHG emissions may result in similar expected

936

Climate change and energy policy." J J West et al

temperatures as a b a t e m e n t scenarios, both the u n c e r t a i n t y in m e a n temperature a n d the likelihood of changes to other climate parameters are expected to be greater. Policy is therefore advised that rather t h a n being a solution to the greenhouse problem, aerosols can also be viewed as a further perturbat i o n to the climate system, a n d that an assured m a i n t e n a n c e o f climate stability is best achieved by reducing emissions of b o t h G H G s a n d aerosols.

Acknowledgements This research was completed with the s u p p o r t of the Winston Churchill F o u n d a t i o n o f the U n i t e d States, a n d u n d e r a N a t i o n a l Science F o u n d a t i o n G r a d u a t e Research Fellowship. The work o f Erica P l a m b e c k to update the P A G E model is gratefully acknowledged, as is the advice o f Hadi D o w l a t a b a d i a n d Mike Hulme.

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Appendix Figures A.I through A.5 show the assumptions for consumption of solid, liquid, and

gaseous fossil fuels in the IPCC92a scenario, the IPCC92c scenario and the alternative

sulfur emission policies constructed from these scenarios.

700 600

soo 400

o_

L0u011

300 200 Ul

..~....A..--~---~....~..-..-_--.,,a___

100 ,--"

0 1980

".....

I 2000

t 2020

I ....... 2040

I 2060

" .....

i 2080

.

7.7

2100

Year Figure A.I Profiles of fossil fuel energy consumption for solids, liquids and gases under the IPCC92a scenario and the Low Sulfur policy in Experiment 1. From Pepper et al (1992)

Climate change and energy policy." J J West et al

938

700 "c"

g ==

600

soo I

400

: Solids I - a - Liquids I - - ~r -. Gases

¢~ 300

I

== 200

I,U

100 0 1980

I

I

I

I

I

2000

2020

2040

2060

2080

21 O0

Year

Figure A.2 Profiles of fossil fuel energy consumption under the High Sulfur policy in Experiment 1. This policy produces the same CO 2 emissions at each time step as the IPCC92a scenario (Figure A.1)

200 "¢" 175 150 P

I- *- Li ids I

-~. 100

==

8

:~

A...~k...A....A.,oeA,,

75

I:"~"-Gases

"12~''~''"~12,~ '

1

&-..

50

b'~emt

25

L

0

1980

I

I

1

I

I

2000

2020

2040

2060

2080

2100

Year

Figure A.3

(1992)

Profiles of fossil fuel energy consumption for solids, liquids and gases under the IPCC92c policy. From Pepper et al

Climate change and energy policy." J J West et al 939

200 ~" 175 --~150

s El . - " l ~

125

I

g loo

m

~, Solids I - ¢n - Liquids I - - ~- -. Gases I

25

0 1980

I 2000

I 2020

I 2040

I

2060

I,,,

2080

2100

Year Figure A.4 Profiles of fossil fuel energy consumption under the Low Sulfur policy in Experiment 2. This policy produces the same CO2 emissions at each time step as the IPCC92c scenario (Figure A.3)

200

~_o 175 m~. 150

m=x~,125 Solids I - a - Uquids I - - ~- - -Gases I

=.100

==

"El,.

75 "''&'"

so Q t.-

ILl

" " ~ r - . . =~...

"~-,

25 0 1980

I 2000

I 2020

I

2040

I 2060

I 2080

2100

Year

Figure A.5 Profiles of fossil fuel energy consumption under the High Sulfur policy in Experiment 2. This policy produces the same CO2 emissions at each time step as the IPCC92c scenario (Figure A.3)