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TSINGHUA SCIENCE AND TECHNOLOGY<br />

ISSN 1007-0214 18/18 pp617-624<br />

Volume 11, Number 5, October 2006<br />

<strong>Parameter</strong> <strong>Uncerta<strong>in</strong>ty</strong> <strong>in</strong> <strong>CGE</strong> Model<strong>in</strong>g <strong>of</strong> <strong>the</strong> <strong>Macroeconomic</strong><br />

<strong>Impact</strong> <strong>of</strong> Carbon Reduction <strong>in</strong> Ch<strong>in</strong>a *<br />

WANG Can (王 灿) ** , CHEN J<strong>in</strong><strong>in</strong>g (陈吉宁)<br />

Department <strong>of</strong> Environmental Science and Eng<strong>in</strong>eer<strong>in</strong>g, Ts<strong>in</strong>ghua University, Beij<strong>in</strong>g 100084, Ch<strong>in</strong>a<br />

Abstract: Formal methods are used to characterize <strong>the</strong> uncerta<strong>in</strong>ty <strong>in</strong> <strong>the</strong> computable general equilibrium<br />

(<strong>CGE</strong>) model outputs to assess <strong>the</strong> use <strong>of</strong> <strong>the</strong> <strong>CGE</strong> model <strong>of</strong> Ch<strong>in</strong>a (<strong>in</strong>tegrated energy-economy-<br />

environment dynamic <strong>CGE</strong>, TED<strong>CGE</strong>) for carbon tax policy issues. Monte Carlo experiment was used for<br />

<strong>the</strong> parameter uncerta<strong>in</strong>ty propagation and unconditional sensitivity analysis, us<strong>in</strong>g <strong>the</strong> variance <strong>of</strong> <strong>the</strong> con-<br />

ditional expectation (VCE) as <strong>the</strong> importance <strong>in</strong>dex to identify critical uncerta<strong>in</strong>ties. The results illustrate <strong>the</strong><br />

statistical characteristics <strong>of</strong> TED<strong>CGE</strong> outputs and sensitivities <strong>of</strong> <strong>the</strong> TED<strong>CGE</strong> outputs to 50 uncerta<strong>in</strong> elas-<br />

ticities. The results show that <strong>the</strong> carbon tax level for a predef<strong>in</strong>ed emission reduction goal is quite sensitive<br />

to both capital-energy substitution elasticity and <strong>in</strong>ter-fuel substitution elasticity <strong>in</strong> <strong>the</strong> production function,<br />

while <strong>the</strong> key parameter for <strong>the</strong> GDP reduction rate was only <strong>the</strong> <strong>in</strong>ter-fuel substitution elasticity. Among <strong>the</strong><br />

various sectors, heavy <strong>in</strong>dustry and electricity are most vitally affected by a carbon tax.<br />

Key words: parameter uncerta<strong>in</strong>ty; unconditional sensitivity analysis; computable general equilibrium;<br />

Introduction<br />

carbon tax; mitigation<br />

Computable general equilibrium (<strong>CGE</strong>) models are<br />

among <strong>the</strong> most <strong>in</strong>fluential tools for a wide range <strong>of</strong><br />

applied economic analyses and policy evaluations.<br />

From <strong>the</strong> 1960s, with grow<strong>in</strong>g computational capacity,<br />

<strong>CGE</strong> model<strong>in</strong>g has evolved from small stylized models<br />

for highlight<strong>in</strong>g economic <strong>the</strong>ories to large methods<br />

simulat<strong>in</strong>g detailed economic relationships. By 1995,<br />

more than 600 <strong>CGE</strong> models had been published<br />

worldwide [1] . A number <strong>of</strong> policy issues have been addressed<br />

us<strong>in</strong>g <strong>CGE</strong> models <strong>in</strong>clud<strong>in</strong>g public f<strong>in</strong>ance<br />

and taxes, <strong>in</strong>ternational trade policies and tariffs, regional<br />

development, and energy and environmental<br />

policies [2-7] .<br />

*<br />

**<br />

Received: 2005-04-15; revised: 2005-07-11<br />

Supported by <strong>the</strong> Major Research Project <strong>of</strong> <strong>the</strong> Tenth Five-Plan<br />

(2001-2005) <strong>of</strong> Ch<strong>in</strong>a (No. 2004-BA611B)<br />

To whom correspondence should be addressed.<br />

E-mail: canwang@ts<strong>in</strong>ghua.edu.cn; Tel: 86-10-62785610<br />

With <strong>the</strong> <strong>in</strong>creas<strong>in</strong>g number <strong>of</strong> <strong>CGE</strong> applications,<br />

economists have critiqued <strong>CGE</strong> model<strong>in</strong>g as hav<strong>in</strong>g serious<br />

shortcom<strong>in</strong>gs due to weak empirical foundations<br />

[8] . The core <strong>of</strong> <strong>the</strong> critiques is that <strong>the</strong> parameter<br />

selection criteria are unsound. S<strong>in</strong>ce available data do<br />

not typically allow one to econometrically evaluate <strong>the</strong><br />

<strong>CGE</strong> models, calibration methods are still used by almost<br />

<strong>CGE</strong> modelers. In <strong>the</strong> calibration procedures,<br />

some parameters are referenced from literature surveys<br />

or chosen arbitrarily on <strong>the</strong> basis <strong>of</strong> subjective judgments<br />

when no published figures are available. The<br />

o<strong>the</strong>r parameters are <strong>the</strong>n set at values which force <strong>the</strong><br />

model to reproduce <strong>the</strong> reference balanced data (e.g.,<br />

<strong>the</strong> social account<strong>in</strong>g matrix) for <strong>the</strong> benchmark year [2] .<br />

Obviously, <strong>the</strong>re exists a large uncerta<strong>in</strong>ty attributable<br />

to <strong>the</strong> arbitrary selection <strong>of</strong> free parameters that fundamentally<br />

contribute to <strong>the</strong> calibration process, which<br />

leads to criticisms [8] .<br />

To circumvent this calibration weakness, a number<br />

<strong>of</strong> authors have suggested that <strong>CGE</strong> models should be


618<br />

tested for robustness by implement<strong>in</strong>g sensitivity<br />

analyses around <strong>the</strong> elasticity mean values [9-14] . However,<br />

<strong>in</strong> practice most analyses use limited sensitivity<br />

analyses, with changes <strong>in</strong> a few key elasticities to exam<strong>in</strong>e<br />

<strong>the</strong> sensitivity <strong>of</strong> <strong>the</strong> results. This procedure has<br />

no way <strong>of</strong> ensur<strong>in</strong>g that <strong>the</strong> chosen elasticities are <strong>the</strong><br />

important ones, and its role is also limited by <strong>the</strong> discretionary<br />

character <strong>of</strong> <strong>the</strong> values selected. Few <strong>of</strong> <strong>the</strong><br />

exist<strong>in</strong>g studies ei<strong>the</strong>r systematically explore <strong>the</strong> parameter<br />

uncerta<strong>in</strong>ty propagation or exam<strong>in</strong>e <strong>the</strong> contribution<br />

<strong>of</strong> <strong>the</strong> various parameters to <strong>the</strong> uncerta<strong>in</strong>ty due<br />

to <strong>the</strong> expensive comput<strong>in</strong>g costs. Harrison et al. [11]<br />

and Alber et al. [13] conducted unconditional sensitivity<br />

analyses, but <strong>the</strong>y focused only on some <strong>of</strong> <strong>the</strong> elasticities<br />

and failed to describe <strong>the</strong> <strong>CGE</strong> output distributional<br />

<strong>in</strong>formation.<br />

The purpose <strong>of</strong> this paper is to study <strong>the</strong> problem <strong>of</strong><br />

measur<strong>in</strong>g <strong>the</strong> uncerta<strong>in</strong>ty <strong>of</strong> <strong>CGE</strong> model simulations<br />

associated with parameter uncerta<strong>in</strong>ty us<strong>in</strong>g <strong>the</strong> macroeconomic<br />

impacts <strong>of</strong> a carbon tax <strong>in</strong> Ch<strong>in</strong>a as <strong>the</strong><br />

policy question for <strong>the</strong> simulations. The study used<br />

Monte Carlo experiments for <strong>the</strong> parameter uncerta<strong>in</strong>ty<br />

propagation and employed an unconditional systematic<br />

sensitivity analysis procedure to identify critically sensitive<br />

elasticities. The analytical process adopted <strong>in</strong> this<br />

paper is extensively used <strong>in</strong> model evaluations [15] .<br />

1 <strong>CGE</strong> Model<br />

The model developed here, <strong>in</strong>tegrated energyeconomy-environment<br />

dynamic <strong>CGE</strong> (TED<strong>CGE</strong>), is a<br />

time recursive dynamic general equilibrium model,<br />

which has been presented <strong>in</strong> detail [16] . S<strong>in</strong>ce <strong>the</strong> dynamic<br />

mechanism has little impact on <strong>the</strong> elasticities’<br />

<strong>in</strong>fluence on <strong>the</strong> <strong>CGE</strong> output and it is very time consum<strong>in</strong>g<br />

compared with <strong>the</strong> static <strong>CGE</strong> model, <strong>the</strong> static<br />

version <strong>of</strong> <strong>the</strong> model was used to exam<strong>in</strong>e <strong>the</strong> elasticities’<br />

uncerta<strong>in</strong>ties.<br />

The TED<strong>CGE</strong> model provides a description <strong>of</strong> <strong>the</strong><br />

Ch<strong>in</strong>a economy <strong>of</strong> 1997, <strong>the</strong> latest year for which <strong>the</strong><br />

Ch<strong>in</strong>a <strong>in</strong>put/output is available. The model identifies<br />

10 production sectors (agriculture, heavy <strong>in</strong>dustry,<br />

light <strong>in</strong>dustry, transportation, construction, service<br />

electricity, coal, oil, and natural gas, which are numbered<br />

as 1 to 10, respectively), 10 consumption goods,<br />

and 2 household groups dist<strong>in</strong>guished by rural and urban<br />

residents. The model operates by simulat<strong>in</strong>g <strong>the</strong><br />

operation <strong>of</strong> markets for factors, products, and foreign<br />

Ts<strong>in</strong>ghua Science and Technology, October 2006, 11(5): 617-624<br />

exchange. The model is highly nonl<strong>in</strong>ear, with equations<br />

specify<strong>in</strong>g supply and demand behavior across all<br />

markets. The model pays particular attention to model<strong>in</strong>g<br />

<strong>the</strong> energy sector and its l<strong>in</strong>kages to <strong>the</strong> rest <strong>of</strong> <strong>the</strong><br />

economy. The energy use is disaggregated <strong>in</strong>to coal, oil,<br />

natural gas, and electricity <strong>in</strong> <strong>the</strong> model. Along with<br />

capital, labour, and <strong>in</strong>termediate <strong>in</strong>puts, <strong>the</strong> four energy<br />

<strong>in</strong>puts are regarded as <strong>the</strong> basic <strong>in</strong>puts <strong>in</strong>to <strong>the</strong> production<br />

function.<br />

The carbon tax revenue from <strong>the</strong> sectoral energy<br />

consumption is described by<br />

E<br />

10 10<br />

∑∑<br />

RC ( t) = tc ⋅ε⋅(1−fc ) ⋅θ ⋅VE ( t)<br />

⋅ο<br />

i= 1 j=<br />

7<br />

h= 1 j=<br />

7<br />

j j j ji j<br />

(1)<br />

The carbon tax revenue from residential energy use is<br />

described by<br />

2 10<br />

RC C ( t) = ∑∑ tc ⋅εj ⋅θj ⋅CI hj( t)<br />

⋅οj<br />

(2)<br />

VEji is <strong>the</strong> monetary consumption (RMB) <strong>of</strong> fuel j by<br />

sector i, CIhj is <strong>the</strong> residential consumption (RMB) <strong>of</strong><br />

fuel j by household h, tc denotes <strong>the</strong> carbon tax rate <strong>in</strong><br />

terms <strong>of</strong> RMB/t, εj denotes <strong>the</strong> CO2 emission coefficient<br />

<strong>of</strong> fuel j with <strong>the</strong> unit <strong>of</strong> t/TJ, fcj is <strong>the</strong> carbon<br />

fixed rate <strong>of</strong> fuel j, θj is <strong>the</strong> conversion factor <strong>of</strong> fuel j<br />

from a nom<strong>in</strong>al output value to a physical term with <strong>the</strong><br />

unit <strong>of</strong> TJ/RMB, and οj is <strong>the</strong> oxidation rate <strong>of</strong> fuel j.<br />

The CO2 emissions are calculated for each sector by<br />

means <strong>of</strong> <strong>the</strong> sectoral fuel consumption and <strong>the</strong> correspond<strong>in</strong>g<br />

emission coefficient. S<strong>in</strong>ce <strong>the</strong> fuel consumption<br />

is <strong>in</strong> terms <strong>of</strong> <strong>the</strong> correspond<strong>in</strong>g nom<strong>in</strong>al output<br />

ra<strong>the</strong>r than directly as <strong>the</strong> physical units <strong>in</strong> <strong>the</strong> model,<br />

<strong>the</strong> sectoral real fuel consumption is first translated<br />

<strong>in</strong>to physical terms, us<strong>in</strong>g a fuel-specific technical<br />

conversion factor, and <strong>the</strong>n fur<strong>the</strong>r converted <strong>in</strong>to CO2<br />

based on <strong>the</strong> fuel-specific emission coefficient. The<br />

fuel used by different sectors is assumed to have <strong>the</strong><br />

same quality, imply<strong>in</strong>g equivalent emission factors, <strong>in</strong><br />

terms <strong>of</strong> tons <strong>of</strong> CO2 per unit <strong>of</strong> fuel consumption, for<br />

a fuel used <strong>in</strong> production, households, or government.<br />

A carbon tax is <strong>in</strong>corporated <strong>in</strong>to <strong>the</strong> model as a means<br />

<strong>of</strong> achiev<strong>in</strong>g an assumed carbon reduction target. The<br />

tax applies to <strong>the</strong> consumption <strong>of</strong> primary fuels only;<br />

that is, <strong>the</strong> energy sectors only pay <strong>the</strong> carbon tax on<br />

<strong>the</strong>ir own use <strong>of</strong> fuels. The carbon tax revenue is recycled<br />

to <strong>the</strong> economy <strong>in</strong> several alternative ways, ensur<strong>in</strong>g<br />

that <strong>the</strong> total government revenue is neutral to <strong>the</strong><br />

carbon tax.


WANG Can (王 灿) et al:<strong>Parameter</strong> <strong>Uncerta<strong>in</strong>ty</strong> <strong>in</strong> <strong>CGE</strong> Model<strong>in</strong>g <strong>of</strong>… 619<br />

As with most <strong>CGE</strong> models, <strong>the</strong> substitution elasticities<br />

and <strong>the</strong> transformation elasticities are <strong>in</strong>corporated<br />

through constant elasticity <strong>of</strong> substitution (CES)<br />

and constant elasticity <strong>of</strong> transformation (CET) functions<br />

<strong>in</strong>to <strong>the</strong> production and import/export decisionmak<strong>in</strong>g<br />

procedure <strong>in</strong> TED<strong>CGE</strong>. Both <strong>the</strong>se functions<br />

have <strong>the</strong> same general form:<br />

( ( ) ) 1<br />

ρ<br />

−<br />

1 1<br />

2<br />

ρ<br />

Y = φ δ ⋅ X + −δ ⋅ X ρ (3)<br />

where Y is <strong>the</strong> aggregate composite <strong>of</strong> X1 and X2, φ is<br />

<strong>the</strong> efficiency parameter, δ is <strong>the</strong> share parameter, and<br />

ρ is <strong>the</strong> substitution parameter. The substitution parameter<br />

is a function <strong>of</strong> <strong>the</strong> substitution or transformation<br />

elasticity, ε, def<strong>in</strong>ed as ρ = ( ε −1) ε if Eq. (3) is a<br />

CES and as ρ = ( ε + 1) ε if Eq. (3) is a CET. The full<br />

model <strong>in</strong>cludes, as exogenous parameters, four substitution<br />

elasticities (three <strong>in</strong> <strong>the</strong> production function and<br />

one <strong>in</strong> <strong>the</strong> import demand function) and one transformation<br />

elasticity (<strong>in</strong> <strong>the</strong> export supply function) for<br />

each <strong>of</strong> <strong>the</strong> ten sectors, for a total <strong>of</strong> 50 elasticities to<br />

be def<strong>in</strong>ed <strong>in</strong> <strong>the</strong> calibration process.<br />

2 <strong>Uncerta<strong>in</strong>ty</strong> Analysis Method<br />

Monte Carlo simulations and unconditional systematic<br />

sensitivity analysis were used to determ<strong>in</strong>e <strong>the</strong> uncerta<strong>in</strong>ty<br />

<strong>in</strong> <strong>the</strong> <strong>CGE</strong> model results given <strong>the</strong> parameter<br />

uncerta<strong>in</strong>ty and to determ<strong>in</strong>e <strong>the</strong> importance <strong>of</strong> each<br />

<strong>in</strong>dividual parameter with respect to <strong>the</strong> uncerta<strong>in</strong>ty <strong>in</strong><br />

<strong>the</strong> outputs. These questions are answered through<br />

quantification <strong>of</strong> <strong>the</strong> uncerta<strong>in</strong>ty <strong>in</strong> <strong>the</strong> screened parameters<br />

<strong>in</strong> <strong>the</strong> form <strong>of</strong> prior probability distributions,<br />

based on random parameter values and repeated model<br />

simulations. Given a large number <strong>of</strong> simulations, <strong>the</strong><br />

probability distributions <strong>of</strong> <strong>the</strong> outcomes can be constructed<br />

as a histogram <strong>of</strong> <strong>the</strong> outcomes that approximate<br />

with arbitrarily small error <strong>the</strong> “true” probability<br />

density function <strong>of</strong> <strong>the</strong> model outcomes.<br />

S<strong>in</strong>ce all <strong>of</strong> <strong>the</strong> uncerta<strong>in</strong> elasticities <strong>in</strong> question are<br />

<strong>the</strong>oretically constra<strong>in</strong>ed to be at least non-negative<br />

and non-<strong>in</strong>f<strong>in</strong>ite, <strong>the</strong> beta family <strong>of</strong> distributions was<br />

chosen for its f<strong>in</strong>ite end-po<strong>in</strong>ts and its flexibility <strong>in</strong> represent<strong>in</strong>g<br />

different distribution shapes. A standard beta<br />

distribution, which is def<strong>in</strong>ed over <strong>the</strong> <strong>in</strong>terval (0, 1),<br />

can be transformed to any desired scale through a simple<br />

l<strong>in</strong>ear transformation. The beta distribution function<br />

has two parameters a and b and is def<strong>in</strong>ed as<br />

⎧ Γ ( a+ b) a−1 b−1<br />

⎪ x (1 − x) , 0 < x<<br />

1;<br />

f( x/ a, b)<br />

= ⎨Γ( a) Γ(<br />

b)<br />

⎪⎩ 0, o t h erwise<br />

(4)<br />

Different values for <strong>the</strong> two parameters a and b will<br />

def<strong>in</strong>e various shapes and variances <strong>of</strong> <strong>the</strong> distribution.<br />

For example, a and b are greater than or equal to 1 for<br />

symmetric and unimodal beta distributions. The population<br />

parameters <strong>of</strong> each elasticity’s distribution (e.g.,<br />

<strong>the</strong> m<strong>in</strong>imum and maximum end-po<strong>in</strong>ts) are chosen on<br />

<strong>the</strong> basis <strong>of</strong> empirical studies. The next section presents<br />

detailed <strong>in</strong>formation describ<strong>in</strong>g uncerta<strong>in</strong> elasticities<br />

exam<strong>in</strong>ed <strong>in</strong> this paper.<br />

Once <strong>the</strong> ranges and distributions <strong>of</strong> elasticities have<br />

been established, <strong>the</strong> next step is to determ<strong>in</strong>e an adequate<br />

sampl<strong>in</strong>g <strong>in</strong>tensity for <strong>the</strong> Monte Carlo experiment.<br />

The sampl<strong>in</strong>g <strong>in</strong>tensity is chosen to limit <strong>the</strong><br />

marg<strong>in</strong>s <strong>of</strong> error <strong>in</strong> <strong>the</strong> estimated means and variances<br />

<strong>of</strong> <strong>the</strong> model’s output variables <strong>of</strong> <strong>in</strong>terest to a prespecified<br />

level, chosen to be 1% <strong>in</strong> this paper.<br />

This approach quantifies <strong>the</strong> variations <strong>in</strong> <strong>the</strong> model<br />

response, but <strong>the</strong> procedure cannot identify <strong>the</strong> driv<strong>in</strong>g<br />

uncerta<strong>in</strong>ties, which are <strong>the</strong> uncerta<strong>in</strong> parameters that<br />

cause <strong>the</strong> most variance <strong>in</strong> <strong>the</strong> outputs <strong>of</strong> <strong>in</strong>terest. An<br />

unconditional sensitivity analysis, which focuses on<br />

<strong>the</strong> output uncerta<strong>in</strong>ty over <strong>the</strong> entire range <strong>of</strong> values<br />

<strong>of</strong> <strong>the</strong> <strong>in</strong>put parameters, is <strong>the</strong> preferable procedure for<br />

identify<strong>in</strong>g <strong>the</strong> driv<strong>in</strong>g uncerta<strong>in</strong>ties. The unconditional<br />

sensitivity analysis method was described by Saltelli et<br />

al. [16] In this study, <strong>the</strong> 50 elasticities <strong>of</strong> substitution are<br />

treated as uncerta<strong>in</strong> and allowed to take random values<br />

from <strong>the</strong>ir probability density functions. For each perturbation<br />

<strong>of</strong> <strong>the</strong> elasticities, <strong>the</strong> TED<strong>CGE</strong> model is recalibrated<br />

and <strong>the</strong>n solved for <strong>the</strong> benchmark and<br />

counterfactual equilibria. The actual data generat<strong>in</strong>g<br />

procedure for <strong>the</strong> calibration data set could be described<br />

as follows:<br />

For i = 1, 50 (50 elasticity parameters)<br />

Def<strong>in</strong>e s i po<strong>in</strong>ts that are <strong>the</strong> mean values <strong>of</strong> s i non-<br />

overlapp<strong>in</strong>g <strong>in</strong>tervals <strong>of</strong> equal probability, which are exhaus-<br />

tively divided <strong>in</strong> <strong>the</strong> sampl<strong>in</strong>g space <strong>of</strong> parameter i<br />

For j = 1, s i (fixed s i po<strong>in</strong>ts <strong>of</strong> parameter i)<br />

For n = 1, K (sampl<strong>in</strong>g <strong>in</strong>tensity)<br />

Select a random value from <strong>the</strong> beta distribution for<br />

each <strong>of</strong> <strong>the</strong> 49 elasticity parameters besides parame-<br />

ter i<br />

Calibrate <strong>the</strong> model based on <strong>the</strong> 50 parameters


620<br />

End<br />

value and <strong>the</strong> benchmark year data<br />

Run <strong>the</strong> model<br />

Calculate <strong>the</strong> mean and variance <strong>of</strong> <strong>the</strong> model output<br />

for <strong>the</strong> K runs<br />

End<br />

Calculate <strong>the</strong> importance measure <strong>of</strong> uncerta<strong>in</strong> parameter i<br />

(def<strong>in</strong>ed as <strong>the</strong> difference between <strong>the</strong> maximum and<br />

m<strong>in</strong>imum mean output value over its si po<strong>in</strong>ts)<br />

End<br />

Rank <strong>the</strong> parameters by <strong>the</strong> value <strong>of</strong> <strong>the</strong>ir importance measure<br />

All beta variates were generated based on a rejection-acceptance<br />

algorithm (algorithm BS) [17] . The random<br />

data matrix can ei<strong>the</strong>r be generated us<strong>in</strong>g a crude<br />

Monte Carlo algorithm or some form <strong>of</strong> stratified sampl<strong>in</strong>g,<br />

such as Lat<strong>in</strong> hypercube sampl<strong>in</strong>g (LHS). LHS<br />

was used <strong>in</strong> this study to reduce <strong>the</strong> computation cost<br />

us<strong>in</strong>g s<strong>of</strong>tware developed <strong>in</strong> Visual Basic to call for <strong>the</strong><br />

execution <strong>of</strong> <strong>the</strong> general algebraic model<strong>in</strong>g system<br />

(GAMS) solution <strong>of</strong> TED<strong>CGE</strong>.<br />

3 Data and Results<br />

The model was calibrated to a set <strong>of</strong> data for Ch<strong>in</strong>a <strong>in</strong><br />

1997. The data was aggregated <strong>in</strong>to three broad categories:<br />

detailed economic accounts, which are ideally<br />

ma<strong>in</strong>ta<strong>in</strong>ed <strong>in</strong> <strong>the</strong> form <strong>of</strong> a social account<strong>in</strong>g matrix<br />

(SAM); structural parameters; and some subsidiary<br />

data. The National Bureau <strong>of</strong> Statistics <strong>of</strong> Ch<strong>in</strong>a<br />

(NBSC) [18] discovered <strong>the</strong> <strong>in</strong>come and expenditures for<br />

each household category, <strong>the</strong> imports and exports for<br />

Ts<strong>in</strong>ghua Science and Technology, October 2006, 11(5): 617-624<br />

each sector, <strong>the</strong> labor and capital under each <strong>of</strong> <strong>the</strong><br />

production sectors as well as <strong>the</strong>ir level <strong>of</strong> output, <strong>the</strong><br />

transformation matrix between <strong>in</strong>dustrial output and<br />

consumer goods, <strong>the</strong> <strong>in</strong>vestments by sector, and government<br />

revenues and expenditures. Before calibration,<br />

a number <strong>of</strong> adjustments to <strong>the</strong> orig<strong>in</strong>al <strong>in</strong>put/output<br />

(I/O) table were necessary with <strong>the</strong> SAM used to impose<br />

a general equilibrium structure on <strong>the</strong> economy,<br />

s<strong>in</strong>ce many <strong>in</strong>consistencies or “residuals” may exist <strong>in</strong><br />

<strong>the</strong> I/O table. In addition, <strong>the</strong> 50 elasticities <strong>of</strong> substitution<br />

<strong>in</strong> <strong>the</strong> model were predef<strong>in</strong>ed based on literature<br />

values before calibration.<br />

3.1 Uncerta<strong>in</strong>ties <strong>in</strong> <strong>the</strong> <strong>in</strong>put parameters<br />

Statistical <strong>in</strong>formation for <strong>the</strong> probability density functions<br />

for each parameter such as <strong>the</strong> mode (most likely<br />

value), <strong>the</strong> endpo<strong>in</strong>ts (<strong>the</strong> 0.00 and 1.00 fractiles) and<br />

<strong>the</strong> level <strong>of</strong> variance <strong>of</strong> each uncerta<strong>in</strong> parameter were<br />

derived from exist<strong>in</strong>g literature data. All <strong>the</strong> elasticity<br />

parameters selected as uncerta<strong>in</strong> <strong>in</strong> <strong>the</strong> model are<br />

summarized <strong>in</strong> Table 1 with <strong>the</strong>ir means, endpo<strong>in</strong>ts,<br />

standard deviations, and relative errors (def<strong>in</strong>ed as<br />

standard deviation divided by <strong>the</strong> mean). Details on<br />

how and where <strong>the</strong> <strong>in</strong>formation was obta<strong>in</strong>ed can be<br />

found <strong>in</strong> Wang [16] . Note that <strong>the</strong> distributions shown<br />

here are not meant to precisely capture accurate values<br />

<strong>of</strong> <strong>the</strong> <strong>in</strong>put uncerta<strong>in</strong>ties, but ra<strong>the</strong>r to demonstrate<br />

how much variation <strong>in</strong> <strong>the</strong> outcomes can be caused by<br />

conservative estimates <strong>of</strong> <strong>the</strong> <strong>in</strong>put uncerta<strong>in</strong>ties.<br />

Table 1 Mean and standard deviations <strong>of</strong> <strong>the</strong> uncerta<strong>in</strong> parameters <strong>in</strong> <strong>the</strong> TED<strong>CGE</strong> model<br />

<strong>Parameter</strong>s Mean<br />

Lower<br />

bound<br />

Upper<br />

bound<br />

Standard<br />

deviation<br />

Relative<br />

error (%)<br />

Beta distribution parameter<br />

a b<br />

Eee 1.00 0.5 1.5 0.25 25 1.5 1.5<br />

Eke 0.50 0.2 1.4 0.26 40 1.5 2.5<br />

Ecl 0.55 0.2 0.9 0.18 32 1.5 1.5<br />

Eid 1.50 0.1 4.0 0.73 42 2.5 3.5<br />

Eed 2.50 0.1 4.0 0.73 31 3.5 2.5<br />

Note: Eee, elasticity substitution between energies; Eke, elasticity substitution between capital and energy aggregate; Ecl, elasticity substitution be-<br />

tween capital-energy aggregate and labor; Eid, elasticity substitution between imported goods and domestic production; Eed, elasticity transfer between<br />

export supply and domestic demand.<br />

Figures 1 and 2 give examples <strong>of</strong> <strong>the</strong> probability distributions<br />

for Eid and Eed. The TED<strong>CGE</strong> reference runs<br />

(neglect<strong>in</strong>g <strong>the</strong> uncerta<strong>in</strong>ties) used a nom<strong>in</strong>al value <strong>of</strong><br />

1.5 for <strong>the</strong> elasticity <strong>of</strong> import substitution (Eid) and 2.5<br />

for elasticity <strong>of</strong> export transfer (Eed) for all sectors. The<br />

modes <strong>of</strong> <strong>the</strong> distributions were chosen to reproduce<br />

<strong>the</strong> reference assumptions. Both <strong>of</strong> <strong>the</strong>se elasticities<br />

were assumed to have <strong>the</strong> same endpo<strong>in</strong>ts. The lowest


WANG Can (王 灿) et al:<strong>Parameter</strong> <strong>Uncerta<strong>in</strong>ty</strong> <strong>in</strong> <strong>CGE</strong> Model<strong>in</strong>g <strong>of</strong>… 621<br />

endpo<strong>in</strong>t was set at 0 and <strong>the</strong> highest was set at 4.0.<br />

The result<strong>in</strong>g distribution for import substitution elasticity<br />

was modeled by a beta distribution with parameters<br />

a = 2.5 and b = 3.5, while <strong>the</strong> beta parameters for<br />

export transfer elasticity were a = 3.5 and b = 2.5.<br />

Fig. 1 Probability distribution for elasticity <strong>of</strong> import<br />

substitution<br />

Fig. 2 Probability distribution for elasticity <strong>of</strong> export<br />

transfer<br />

3.2 <strong>Uncerta<strong>in</strong>ty</strong> <strong>in</strong> <strong>the</strong> TED<strong>CGE</strong> outputs<br />

Figure 3 shows <strong>the</strong> probability distributions for carbon<br />

tax rates required to achieve <strong>the</strong> carbon reduction goals<br />

<strong>of</strong> 10%, 20%, 30%, and 40% compared with <strong>the</strong> BAU<br />

scenario for 2010 <strong>in</strong> Ch<strong>in</strong>a. 5000 repetitions <strong>of</strong> <strong>the</strong><br />

Monte Carlo experiment were first used to formulate<br />

<strong>the</strong> probability distributions. The Monte Carlo experiments<br />

were <strong>the</strong>n repeated to get ano<strong>the</strong>r probability<br />

distribution. The errors <strong>in</strong> <strong>the</strong> means and variances<br />

from <strong>the</strong>se two experiments were found to be less than<br />

1%, which means that <strong>the</strong> 5000 Monte Carlo simulations<br />

give sufficiently accurate results. The implications<br />

<strong>of</strong> <strong>the</strong> uncerta<strong>in</strong>ties are important when evaluat<strong>in</strong>g<br />

<strong>the</strong> performance <strong>of</strong> a carbon tax policy for a carbon<br />

mitigation strategy, but even <strong>in</strong> <strong>the</strong> absence <strong>of</strong> a policy<br />

<strong>the</strong> uncerta<strong>in</strong>ties <strong>in</strong> <strong>the</strong> carbon tax rates have implications<br />

for <strong>in</strong>terpret<strong>in</strong>g results. What is <strong>of</strong>ten called a<br />

marg<strong>in</strong>al abatement cost curve for <strong>the</strong> <strong>in</strong>ternational<br />

carbon emission trad<strong>in</strong>g model is <strong>in</strong> fact only one series<br />

<strong>of</strong> values from a series <strong>of</strong> distributions <strong>of</strong> possible<br />

values, given <strong>the</strong> uncerta<strong>in</strong> parameters exist<strong>in</strong>g <strong>in</strong> <strong>the</strong><br />

model. Policy analysis with models on carbon emission<br />

trad<strong>in</strong>g should be viewed <strong>in</strong> this context.<br />

Fig. 3 Probability distribution <strong>of</strong> carbon tax rate response<br />

to carbon reduction rates<br />

The mean (bold middle l<strong>in</strong>e), <strong>the</strong> 95% and 100%<br />

confidence <strong>in</strong>tervals, and <strong>the</strong> relative error for <strong>the</strong> marg<strong>in</strong>al<br />

abatement cost curve <strong>in</strong> 2010 for Ch<strong>in</strong>a calculated<br />

by TED<strong>CGE</strong> are illustrated <strong>in</strong> Fig. 4. The results<br />

show <strong>in</strong>creas<strong>in</strong>g means and variances for <strong>the</strong> abatement<br />

cost as <strong>the</strong> carbon reduction goal <strong>in</strong>creases. In<br />

addition, <strong>the</strong> relative error <strong>in</strong> <strong>the</strong> TED<strong>CGE</strong> output is<br />

much smaller than <strong>the</strong> error <strong>in</strong> <strong>the</strong> <strong>in</strong>put parameters<br />

(see Table 1).<br />

Fig. 4 Marg<strong>in</strong>al abatement cost curve with parameter<br />

uncerta<strong>in</strong>ties<br />

The results show that if <strong>the</strong> <strong>in</strong>put parameter statistical<br />

<strong>in</strong>formation is fully provided, <strong>the</strong> <strong>CGE</strong> model outputs<br />

also exhibit statistical characteristics reflect<strong>in</strong>g <strong>the</strong><br />

uncerta<strong>in</strong>ties transmitted from <strong>the</strong> <strong>in</strong>puts to <strong>the</strong> simulation<br />

results. Although parameter uncerta<strong>in</strong>ties throw<br />

doubt on <strong>the</strong> reliability <strong>of</strong> <strong>the</strong> <strong>CGE</strong> model simulation<br />

results, <strong>the</strong> <strong>in</strong>put uncerta<strong>in</strong>ties are reduced to some extent<br />

through <strong>the</strong> <strong>CGE</strong> model<strong>in</strong>g procedure.


622<br />

3.3 Unconditional sensitivity analysis results<br />

One <strong>of</strong> <strong>the</strong> most important contributions <strong>of</strong> <strong>the</strong> uncerta<strong>in</strong>ty<br />

analysis is <strong>the</strong> identification <strong>of</strong> <strong>the</strong> driv<strong>in</strong>g uncerta<strong>in</strong>ties,<br />

<strong>the</strong> uncerta<strong>in</strong> parameters that cause <strong>the</strong><br />

most variance <strong>in</strong> <strong>the</strong> outputs <strong>of</strong> <strong>in</strong>terest. Figure 5 shows<br />

<strong>the</strong> effects <strong>of</strong> perturbations <strong>of</strong> each elasticity parameter<br />

<strong>in</strong> its space with random values <strong>of</strong> o<strong>the</strong>r elasticity<br />

Ts<strong>in</strong>ghua Science and Technology, October 2006, 11(5): 617-624<br />

parameters selected from <strong>the</strong>ir probability density<br />

functions. The carbon tax rate and GDP reduction rate<br />

calculated by <strong>the</strong> model when assum<strong>in</strong>g a counterfactual<br />

CO2 emission reduction goal <strong>of</strong> 20% <strong>in</strong> 2010 were<br />

selected as typical TED<strong>CGE</strong> outputs <strong>in</strong> Figs. 5a and 5b.<br />

Only <strong>the</strong> <strong>in</strong>fluential parameters are named <strong>in</strong> Fig. 5.<br />

Fig. 5 Effects <strong>of</strong> perturbations <strong>of</strong> each elasticity parameter <strong>in</strong> its space with o<strong>the</strong>r elasticities randomized from <strong>the</strong>ir<br />

probability density functions. Only <strong>the</strong> most important factors are named. See <strong>the</strong> notes with Table 2 for explanation <strong>of</strong><br />

<strong>the</strong> sector <strong>in</strong>dicators.<br />

For a model with 50 uncerta<strong>in</strong> <strong>in</strong>put parameters such<br />

as TED<strong>CGE</strong>, only a few factors are likely to have a<br />

sizeable <strong>in</strong>fluence. Figure 5 demonstrates that <strong>the</strong> TED-<br />

<strong>CGE</strong> model results are sensitive to only a few <strong>of</strong> <strong>the</strong> parameters<br />

when <strong>the</strong> critical parameters <strong>of</strong> various endogenous<br />

variables are varied. For example, <strong>the</strong> uncerta<strong>in</strong>ty<br />

<strong>in</strong> <strong>the</strong> elasticity <strong>of</strong> substitution between capital<br />

and energy <strong>in</strong> three sectors (Eke(2), Eke(6), and Eke(10))<br />

and <strong>the</strong> elasticity <strong>of</strong> substitution between different energy<br />

sources <strong>in</strong> two sectors (Eee(2) and Eee(10)) are <strong>the</strong><br />

primarily parameters <strong>in</strong>fluenc<strong>in</strong>g <strong>the</strong> variances <strong>in</strong> <strong>the</strong><br />

carbon tax rate correspond<strong>in</strong>g to a specific carbon emission<br />

reduction rate, while <strong>the</strong> key parameters for <strong>the</strong><br />

GDP reduction rate are only <strong>the</strong> elasticity <strong>of</strong> substitution<br />

between <strong>the</strong> different energy sources <strong>in</strong> <strong>the</strong> heavy <strong>in</strong>dustry<br />

and natural gas sectors (Eee(2) and Eee(10)).<br />

A simple measure <strong>of</strong> importance <strong>of</strong> an <strong>in</strong>put factor,<br />

α, was def<strong>in</strong>ed based on <strong>the</strong> so-called variance <strong>of</strong> <strong>the</strong><br />

conditional expectation (VCE) <strong>of</strong> outcome. Assume<br />

that we are <strong>in</strong>terested <strong>in</strong> describ<strong>in</strong>g <strong>the</strong> importance <strong>of</strong><br />

an elasiticity parameter α <strong>in</strong> TED<strong>CGE</strong>. The importance<br />

<strong>of</strong> α with regard to <strong>the</strong> output uncerta<strong>in</strong>ty can be assessed<br />

by consider<strong>in</strong>g <strong>the</strong> conditional probability distributions<br />

<strong>of</strong> <strong>the</strong> TED<strong>CGE</strong> outputs conditioned on α.<br />

The importance <strong>of</strong> a parameter is assumed to be related<br />

to how much it affects <strong>the</strong> model output. Intuitively,<br />

parameter α is important if fix<strong>in</strong>g its value substantially<br />

reduces <strong>the</strong> (conditional) output variance relative<br />

to <strong>the</strong> marg<strong>in</strong>al output variance. Hence, <strong>the</strong> variance<br />

ratio can be used as an appropriate measure <strong>of</strong> importance.<br />

In this study, <strong>the</strong> magnitude <strong>of</strong> <strong>the</strong> VCE relative<br />

to <strong>the</strong> total variance <strong>in</strong> <strong>the</strong> output is def<strong>in</strong>ed as importance<br />

<strong>in</strong>dex:<br />

IM<br />

αi<br />

( Y )<br />

[ ]<br />

( )<br />

[ ]<br />

Var ⎡ α E α ⎤ VCE α<br />

=<br />

⎣ ⎦<br />

= (5)<br />

Var Y Var Y<br />

where Varα [E(Y|α)] is <strong>the</strong> variance <strong>of</strong> <strong>the</strong> conditional<br />

expectation <strong>of</strong> <strong>the</strong> model output <strong>of</strong> Y, conditioned on α,<br />

that is, <strong>the</strong> variability <strong>in</strong> E(Y|α) as α varies. In fact, this<br />

<strong>in</strong>dex is termed <strong>the</strong> correlation ratio and had been studied<br />

and applied by many <strong>in</strong>vestigators for sensitivity<br />

analysis [19] . The unconditional systematic sensitivity<br />

analysis results can be used to calculate <strong>the</strong> importance<br />

<strong>in</strong>dex for each uncerta<strong>in</strong> elasticity parameter. Table 2<br />

lists <strong>the</strong> top ten elasticities with <strong>the</strong> largest and smallest<br />

sensitiveness to <strong>the</strong> endogenous variables <strong>of</strong> <strong>the</strong> carbon<br />

tax rate and GDP reduction rate.


WANG Can (王 灿) et al:<strong>Parameter</strong> <strong>Uncerta<strong>in</strong>ty</strong> <strong>in</strong> <strong>CGE</strong> Model<strong>in</strong>g <strong>of</strong>… 623<br />

Table 2 Order <strong>of</strong> <strong>the</strong> sensitivities <strong>of</strong> elasticities to <strong>the</strong> <strong>CGE</strong> model outputs<br />

Top ten sensitive elasticities Top ten <strong>in</strong>sensitive elasticities<br />

To carbon tax rate To GDP loss rate To carbon tax rate To GDP loss rate<br />

Elasticity IMαi Elasticity IMαi Elasticity IMαi Elasticity IMαi<br />

E ke(2) 0.36 E ee(2) 0.36 E id(5) 0.02 E ed(9) 0.02<br />

E ee(2) 0.31 E ee(10) 0.19 E ee(5) 0.02 E ee(1) 0.02<br />

E ke(10) 0.26 E ee(7) 0.10 E ke(4) 0.02 E ke(9) 0.02<br />

E ke(6) 0.16 E ke(2) 0.09 E id(9) 0.02 E ed(1) 0.02<br />

E ee(10) 0.12 E ke(10) 0.09 E ed(10) 0.02 E ed(7) 0.03<br />

E ke(3) 0.11 E ee(6) 0.09 E ed(7) 0.03 E ed(4) 0.03<br />

E cl(2) 0.10 E cl(7) 0.09 E ed(9) 0.03 E id(10) 0.03<br />

E ee(6) 0.09 E ee(9) 0.09 E ee(1) 0.03 E cl(1) 0.04<br />

E cl(10) 0.08 E id(4) 0.09 E ke(9) 0.03 E ee(4) 0.04<br />

E ed(6) 0.08 E ed(6) 0.08 E ed(4) 0.03 E id(7) 0.04<br />

Notes: Figures at <strong>the</strong> end <strong>of</strong> <strong>the</strong> code <strong>in</strong>dicate <strong>the</strong> sector: 1, Agriculture; 2, Heavy <strong>in</strong>dustry; 3, Light <strong>in</strong>dustry; 4, Transportation; 5, Construction;<br />

6, Service; 7, Electricity; 8, Coal; 9, Oil, and 10, Natural gas.<br />

Table 2 shows that <strong>the</strong> most sensitive elasticity parameters<br />

are generally Eke (elasticity substitution between<br />

capital and energy aggregate) and Eee (elasticity<br />

substitution between different energy sources). The<br />

elasticity parameters <strong>in</strong> <strong>the</strong> <strong>in</strong>ternational trade function<br />

(i.e, Eed and Eid) have relatively weak effects on <strong>the</strong><br />

outputs as shown <strong>in</strong> <strong>the</strong> top ten <strong>in</strong>sensitive elasticities<br />

<strong>in</strong> Table 2. When consider<strong>in</strong>g sectoral elasticities, <strong>the</strong><br />

elasticity parameters <strong>in</strong> heavy <strong>in</strong>dustry (Sector 2), service<br />

(Sector 6), and natural gas sector (Sector 10) are<br />

<strong>the</strong> most significant contributors listed <strong>in</strong> <strong>the</strong> left side <strong>of</strong><br />

Table 2. That means <strong>the</strong> elasticities <strong>of</strong> <strong>the</strong>se three sectors<br />

cause <strong>the</strong> most variations <strong>in</strong> <strong>the</strong> outputs. The elasticities<br />

<strong>in</strong> <strong>the</strong> agriculture, transportation, and oil production sectors<br />

(Sectors 1, 4, and 9) have little <strong>in</strong>fluence on <strong>the</strong> output<br />

as shown on <strong>the</strong> right side <strong>of</strong> Table 2.<br />

4 Conclusions<br />

Computable general equilibrium model<strong>in</strong>g has become<br />

an effective technique for evaluat<strong>in</strong>g a wide range <strong>of</strong><br />

policy questions. While uncerta<strong>in</strong>ties about <strong>the</strong> <strong>in</strong>put<br />

values <strong>in</strong> a <strong>CGE</strong> model may limit <strong>the</strong> credibility <strong>of</strong> its<br />

conclusions, relatively few applications have explicitly<br />

treated <strong>the</strong> uncerta<strong>in</strong>ties. This paper describes formal<br />

methods for assess<strong>in</strong>g this type <strong>of</strong> uncerta<strong>in</strong>ties and illustrates<br />

its use <strong>in</strong> a TED<strong>CGE</strong> model applied to a carbon<br />

tax policy issue. The method relies on build<strong>in</strong>g<br />

probability density functions <strong>of</strong> <strong>the</strong> <strong>CGE</strong> model output<br />

us<strong>in</strong>g crude Monte Carlo experiments. In contrast to<br />

<strong>the</strong> traditional op<strong>in</strong>ions, <strong>the</strong> results <strong>in</strong>dicate that not<br />

only can uncerta<strong>in</strong>ty <strong>in</strong> <strong>the</strong> <strong>CGE</strong> model be described<br />

given full statistical <strong>in</strong>formation on <strong>the</strong> <strong>in</strong>put parameters,<br />

but that <strong>the</strong> uncerta<strong>in</strong>ties have important quantitative<br />

and qualitative consequences. The <strong>in</strong>put uncerta<strong>in</strong>ty<br />

can be reduced to some extent through <strong>the</strong> <strong>CGE</strong><br />

model<strong>in</strong>g procedure. The results also <strong>in</strong>dicate that <strong>the</strong><br />

<strong>CGE</strong> model results were sensitive to only some <strong>of</strong> <strong>the</strong><br />

parameters when <strong>the</strong> critical parameters for different<br />

endogenous variables are varied. The carbon tax level<br />

correspond<strong>in</strong>g to a predef<strong>in</strong>ed carbon reduction rate <strong>in</strong><br />

TED<strong>CGE</strong>, for example, was quite sensitive to both <strong>the</strong><br />

capital-energy substitution elasticity and <strong>the</strong> <strong>in</strong>ter-fuel<br />

substitution elasticity <strong>in</strong> <strong>the</strong> production sector, while<br />

<strong>the</strong> key parameter affect<strong>in</strong>g <strong>the</strong> GDP reduction rate<br />

was only <strong>the</strong> <strong>in</strong>ter-fuel substitution elasticity. The results<br />

also show that <strong>the</strong> heavy <strong>in</strong>dustry and electricity<br />

sectors are <strong>the</strong> most important sectors affect<strong>in</strong>g <strong>the</strong><br />

carbon tax level.<br />

Acknowledgements<br />

The manuscript was mostly prepared dur<strong>in</strong>g <strong>the</strong> first author’s<br />

stay as a guest researcher at <strong>the</strong> Energy Project <strong>of</strong> International<br />

Institute for Applied Systems Analysis (IIASA). The authors<br />

gratefully acknowledge Dr. Leo Schrattenholzer, Dr. Leonardo<br />

Barreto, and Dr. Ji Zou for <strong>the</strong>ir valuable comments.


624<br />

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