An Updated Valuation of the Impacts of Global Warming

8/6/2019 An Updated Valuation of the Impacts of Global Warming

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ELSEVIER P11:S0301-4215(96)00064-X

Enetigl ' Poli0 . ~'ol. 24. No. 9. pp, 783 793 1996Copyrighl ,. 1996 Elsevier Science lidPrinted in Great Brilain. All rights rc,cr~cd0301-421 596 S15.00 + 0.00

P A G E 9 5An upda ted va luat ion of the im pacts o f g lobal warm ing

E r i c a L P l a m b e c k a n d C h r is H o p eJudge Institute of M anagement Studies, University of Cambridge, Cam bridge, UKA v i t a l m e a s u r e f o r g lo b a l w a r m i n g p o l i c y is t h e m a r g i n a l i m p a c t o f a t o n n e o f c a r b o n e m i t te d t o t h ea t m o s p h e r e . I n e c o n o m i c t e r m s , t h i s v a lu e c o r r e s p o n d s t o t h e c a r b o n t a x l ev e l n e e d e d t o i n t e r n a l i z ethe ex te rna l i t i e s a ssoc ia ted w i th c l ima te change . Th i s s tudy r e -eva lua tes the m arg ina l imp ac t o f CO 2e m i s s i o n s i n t h e l i g h t o f n e w s c i en t i fi c a n d e c o n o m i c u n d e r s t a n d i n g o f t h e c o o l i n g e f f e c ts o f s u l p h a t eaeroso l s and ozo ne dep le t ion , the r eg iona l d i s t r ibut ion o f g loba l warm ing dam ages , non-Unear ity indam age a s a func t ion o f t empera tur e r i se and the appropr ia te d i scount ra te . Copyright 1996 Else-vier Science Ltd.K e y w o r d ~ : Global warmin g; Model; Valuation

Global wanning policy must balance the cost of reducinggreenhouse gas emissions now against potential damagefrom future climatic change. A vital and ongoing debate in-volves valuing the impacts of carbon dioxide (CO2) emis-sions to the atmosphere (Nordhaus, 1991; Cline, 1992,Fankhauser, 1993, 1994b; Azar, 1994). One common meas-ure is the marginal impact of a tonne of carbon emitted tothe atmosphere. In economic terms, this value correspondsto the carbon tax level needed to internalize the externalitiesassociated with climate change. However, the complexity ofthe global warming phenomenon and difficulties in repre-senting the impacts of climate change by a single monetaryvalue make the marginal impact per tonne carbon (tC) ahighly uncertain value. The range of estimates is large;most values lie between US$5 and US$25/tC (Fankhauserand Pearce, 1993).Hope and Maul (1996) demonstrated that much of thisdisparity is explained by different assumptions about theeffectiveness of adaptation to climate change, the back-ground level of CO2 emissions, economic growth rates andthe discount rate. Another key factor is the treatment ofuncertainty.O ne of the two models used by Hope and Maul, PAGE,was developed for use by EU decision makers in 1991.Since then scientific knowledge of the global warmingproblem and methods for impact valuation have developedgreatly. For example, the sulphate aerosols produced byfossil fuel burning have been found to have a significant

cooling effect. Chlorofluorocarbons, once thought to be themost potent greenhouse gases, are now believed to haveonly a slight effect because they destroy ozone, itself astrong greenhouse gas. Case studies in various regions inthe world have improved our ability to measure damagesfrom global warming, and suggest that damage is likely tobe a non-linear function of temperature. The PAGE modelwas recently updated to reflect this information (Plambecket al , 1995). In this paper we use the updated PAGE model,PAGE95, to examine the effect of new scientific and eco-nomic knowledge on the predicted marginal impact pertonne of carbon, and take another look at the role of as-sumptions about the discount rate, economic growth rate,and the effectiveness of adaptation to climate change.T h e P A G E 9 5 m o d e lThe PAGE (Policy Analysis for the Greenhouse Effect) in-tegrated assessment model was developed in 1991 for useby European Union decision makers (Hope et al , 1993). Anupdated model version, PAGE95, accounts for recent devel-opments in the science and economics of global warming(Plambeck e t a l , 1995). Global warming pol icy decisionvariables are the level of greenhouse gas emiss ions overtime and the degree of adaptation to climate change. For aspecified global warming policy, PAGE95 estimates thecost of enacting that policy as well as the resulting climatechange impacts, the focus o f this paper.

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7 8 4 PAGE95: E L Plam beck and C HopePAGE95 conta ins equat ions tha t model :

( 1) E m i s s i o n s o f th e p r i m a r y g r e e n h o u s e g a s e s , C O 2and me thane . Ch lo ro f luoroca rbons (CFCs) and hydro-c h l o r o f l u o r o c a r b o n s ( H C F C s ) , d e c i s i o n v a ri a b le s i nt h e o r i g i n a l P A G E m o d e l , h a v e a r e d u c e d r o l e i nPAGE95. A l thou gh future emissions are l imi ted by in-ternat ional agreem ents to protec t the ozone layer , exist -i n g a t m o s p h e r i c c o n c e n t r a t i o n s a r e n o t e x p e c t e d t odec l ine s ign i f i can t ly i n t he nex t c en tu ry . HencePAGE95 m ode l s (H)CFCs a s a sma l l add i ti on to back-ground rad i a ti ve fo rc ing : sma l l due to t he co o l ing e f -fec t o f ozone dep le tion .( 2) T h e g r e e n h o u s e e f f e c t. A n t h r o p o g e n i c e m i s s i o n s o fg reenhouse gase s exceed the r a t e o f r emova l by chem-ica l and biologica l processes and accumulate in the a t -mosphe re . The g reenhouse gase s t r ap hea t i n t heatmosp here so tha t less of the inc oming solar radia tionis re-radia ted to space . This increases radia t ive forc ing,the net f lux of energ y to ear th . The ear th ' s tempera turer ises very slow ly as excess heat i s t ransferred f rom thea tmosphe re t o l and and ocean .(3) Co ol ing from sulphate aerosols . Sulphate aerosols re-s u l t f r o m f o s s i l f u e l c o m b u s t i o n a n d a r e c o m m o n l yknown as the cause of ac id ra in . They a lso backscat terincoming so l a r r ad i a ti on and in t er fe re wi th c loud fo r -mat ion, producing a reduct ion in radia t ive forc ing. Thiscounterac ts the greenhous e effec t.

(4) Regional tempera ture effec ts . Unl ike greenhouse gaseswhich rema in in t he a tmosphe re fo r decades and a reglobal ly mixed, sulphate aerosols have a very short a t-mosphe r i c l i f e time (abou t s ix days) and so t end to r e -main in the source region. Therefore sulphate aerosolcool ing is a regional phenomenon. For the e ight worldregions in PAGE95,1 tempera ture r i se i s compu ted fromt h e d i f f e re n c e b e t w e e n g l o b a l w a r m i n g a n d r e g io n a lsulphate aerosol coo l ing. Sulphate coo l ing is grea test inthe more indust r ia l ized regions, and tends to decreaseover t ime due to sulphate controls to prevent ac id ra inand n egat ive heal th effec ts .(5) Non -l inear i ty in the damage caused by global warming.Cl ima t i c change impac t s a re a po lynomia l func t ion o freg iona l t empera tu re i nc rea se above some to l e rab l elevel of tempera ture chang e, (T - Tto/)", where n i s anuncer ta in inpu t parameter .

(6) Reg iona l e cono mic g rowth . Impac t s a re eva lua t ed int e rms o f an annua l pe rcen tage loss o f GDP in each re -g ion , fo r a ma x imu m of two s ec to r s - i n th i s app li ca tiondefined as econom ic impacts and non-econ omic (envir-onm enta l and socia l ) impacts .Adap ta t ion to c l ima te change . Inves tm ent i n adap t ivemeasure s ( eg the bu i ld ing o f sea wa l l s ; deve lopment o fdrought resistant c rops) can increase the tolerable levelo f t e m p e r a t u r e c h a n g e (Trot) b e f o r e e c o n o m i c l o s s es

( 7 )

ICh i n a an d cen t ra l l y p l an n ed A s i a , In d i a an d S o u t h -eas t A s i a , La t i nAmerica , Africa and the Middle East , Eastern Europe and the FormerUSSR, USA , EU and other OECD nations.

o c c u r a n d a l s o r e d u c e t h e i n t e n s i t y o f b o t h n o n -economic and econom ic impac ts .Al l a spec t s o f t he g loba l wa rming p rob lem a re sub jec t t oprofo und uncer ta inty. To express the mo del resul ts in te rmsof a s ing le ' be s t guess ' cou ld be dange rous ly mi s l ead ing .Ins t ead , po l i cy shou ld be i n fo rmed by a r ange o f poss ib l eou tcomes . The re fo re PAGE95 repre sen t s more t han 70 keyi n p u t p a r a m e t e r s b y p r o b a b i l i t y d i s t r i b u t i o n s . R a n d o msampl ing i s used to bu i ld up an approx ima te p robab i l i t yd i s t r i b u t i o n f o r t h e m o d e l r e s u l t s . T h e c o m p r e h e n s i v escope and p robab i l i s ti c fo rm ula t ion o f t he mode l necess i -t a t e t he s imple s t c red ib l e equa t ions . These equa t ions andthe p robab i l i t y d i s t r i bu t ions fo r key inpu t pa rame te r s a red e s c r i b e d i n P l a m b e c k e t a l ( 1 9 9 5 ) a n d P l a m b e c k a n dHop e (1995).Calculating m arginal impactsThe marg ina l impac ts o f a t onne o f ca rbon emi t t ed a s CO 2i s c o m p u t e d b y c o m p a r i n g t h e i m p a c t s o f t w o p o l i c i e swhich d i f f e r on ly by a ' pu l se ' o f c a rbon emiss ions . The im-p a c t o f o n e t o n n e o f c a r b o n i s t o o s m a l l t o b e d e t e c t e d .H u m a n e m i s s i o n s o f C O 2 ar e s m a l l c o m p a r e d t o n a t u r a lcyc l e s , so t he pu l se needs t o be l a rge i n hum an t e rms to p ro -duce a meas ured effec t . Even I bi l l ion tonnes ( I GtC, abo ut15% of annua l wor ld emiss ions) , c anno t be de t ec t ed , and10 GtC is a t the l imi t of resolut ions of the PAGE95 mod el .For t h i s s tudy the po l ic i e s a re made to d i f f e r by a pu l se o f100 GtC. The inc rementa l impac t o f t h i s pu l se i s t hen d i -vided by 10 II to give the va luat ion of im pacts per tC. T hisprocedu re i s taken from H ope and M aul (1996).

The s t ruc tu re o f t he PAGE95 mode l , de s igned to l ook a tl o n g - t e r m p o l i c i e s , d o e s n o t a l l o w f o r a n i n s t a n t a n e o u spulse . Therefore the 100 GtC pulse i s emit ted over the 30-yea r pe r iod f rom 199 0-2020 , p eak ing in 2000 . Hop e andM aul (1996) observe tha t , for a posi tive discou nt ra te , thisw i l l p r o d u c e a s m a l l e r im p a c t v a l u a t i o n t h a n a n i n s t a n -t aneous pu l se i n the yea r 2000. For example , g iven an e f -f e c t iv e d i s c o u n t r a t e o f 2 % , t h e a p p r o x i m a t e l e v e l iz i n gfac to r i s 0 .77 . Howeve r , t he r e su l t s i n t h i s pape r a re no tmodi f i ed by any l eve l i z ing fac to r . Spread ing the pu l se i sr ea sonab le g iven the l ong t ime sca l e fo r po l i cy to a f fec tCO 2 emission s (eg a carbon tax leading to the replacem entof ineff ic ient fue l burning equ ipment) .

Hope and Maul (1996) used the o r ig ina l PAGE mode l t oe s t ima te t he marg ina l impac t pe r t onne o f ca rbon emiss ionsa t US$5 , wi th a 90% range f rom US$2 -7 pe r tC , ba sed onthe fo l lowing a ssumpt ions :(1) A h or i zon o f 2200 fo r ca l cu l a t ing impac t s t o a l low tothe long t ime lags in the na tura l systems. The impacts

were aggregated and discounted back to the base year ,1990, a t 5% per y ear ; this ra te ref lec ts the oppo rtuni tycost o f capi ta l.(2) Bus iness a s usua l (BAU) em iss ions o f CO 2 , me thane ,CFCs and H CFC s based upon IPCC scena rio IS92a up

to 2100. The second pol icy (BAU + PULSE) added the


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p u l s e o f 1 00 G t C o f C O 2 e m i s s i o n s o n t o t h e s e B A Uemis s ions , a s s hown in F igure 1 .

( 3 ) E c o n o m i c i m p a c t s w e r e t a k e n t o b e i n t h e ra n g e o f0 . 2 5 % t o 1 . 6% o f G D P p e r C p e r y e a r , w i t h a m o s tl i k e l y v a l u e o f 0 . 6 % , f o r a l l w o r l d r e g i o n s. N o n - e c o -nom ic impac t s w e re t aken to be s l igh t ly lower than eco-n o m i c i m p a c t s t o c o n f o r m t o t h e r e s u l t s f o u n d b yNo rdhau s in a po l l o f expe r t s (Nordhaus , 1994 ) . Bo the c o n o m i c a n d n o n - e c o n o m i c i m p a c ts g r e w a s a l i n ea rfunc t ion o f t empera tu re .

( 4 ) L a r g e a m o u n t s o f a d a p ta t i o n i n t h e d e v e lo p e d w o r l d ,s uch a s the bu i ld ing o f s ea wa l l s and the p reven t ion o fdeve lopment in vu lne rab le a rea s , tha t e l imina ted eco-nomic impac t a l toge the r fo r the f i r s t 2C tempera tu rer i s e , and reduced the rema in ing impac t s by 90% a f te r50 yea rs ; in the deve lop ing wor ld , adap ta t ion reducedimpac t s by 50% a f te r 50 yea rs (CRU/E RL , 1992). In a llreg ions , adap ta t ion w as l e s s e f fec t ive a t r educ ing non-economic impac t s , b r ing ing on ly 25% reduc t ion .

( 5 ) A w o r l d w i d e e c o n o m i c g r o w t h r at e o f 2 % p e r y e a r, im -p l y i n g t h a t b o t h t h e e c o n o m i c a n d n o n - e c o n o m i c i m -p a c ts o f a 1C temp era tu re r i s e al s o g rew a t 2% pe r yea rbe fo re adap ta t ion .

In a s e r i e s o f expe r imen ts in th is pape r , we exam ine the e f -fec t o f s ucces s ive upda te s to the PAGE mode l on the e s t i -m a t e d m a r g i na l i m p a c t p e r t o n n e o f c a r b o n e m i s si o n s. T h i sp roces s cu lmina te s in the PAGE 95 cur ren t be s t a s s e s s men to f m a r g i n a l i m p a c t . S u b s e q u e n t e x p e r i m e n t s w i t h P A G E 9 5e x p l o r e t h e s e n s i ti v i t y o f t h a t r e s u l t to a s s u m p t i o n s a b o u tt h e d i s c o u n t r a t e a n d t h e d e g r e e o f a d a p t a ti o n t o c l i m a t i cchange .T h e P A G E 9 5 u p d a t e d c l i m a t e m o d e lO z o n e d e p l e t i o n T h e f i r s t e x p e r i m e n t , C L I M A T E , i n -v o l v e s t he P A G E 9 5 u p d a t e d c l i m a t e m o d e l w i t h c o o l i n gf r o m o z o n e d e p l e t i o n a n d s u l p h a t e a e r o s o l s. A l l o t h e r a s -p e c t s o f t h e P A G E m o d e l r e m a i n a s i n H o p e a n d M a u l( 1 9 9 6 ) . F o r m o r e t h a n f i f t e e n y e a r s i t w a s t h o u g h t t h a tC F C s w e r e t h e m o s t p o t e n t g r e e n h o u s e g a s e s ( R a m a n a t h an ,1975). How eve r , r ecen t s tud ie s s how tha t the rad ia tive fo rc -ing e f fec t o f CFCs i s coun te rba lanced by the i r de s t ruc t ionof s t ra tos phe r ic ozon e , i t s e l f a g ree nhous e gas . F or th i s rea -s o n C F C s h a v e o n l y a s m a l l n e t w a r m i n g e f f e c t ( W i g l e yand Rape r , 1992) . In PAG E 95 ne t rad ia t ive fo rc ing f rom a l lha loca rbon s , inc lud ing CFC s , i s t aken to be 0 . 2 W /m 2 fo rthe pe r iod 1990- -208 0 and ze ro the re a f t e r ba s ed on the l a t-e s t s c ien t i f ic da ta (D an ie l e t a l , 1995) . T h i s change reducest h e p r e d ic t e d l e v e l o f g l o b a l w a r m i n g b y a s m u c h a s 1 0 %b y t h e y e a r 2 2 0 0 .S u l p h a t e a e r o s o l s T h e m o r e d r a m a t i c u p d a t e i s r e g i o n a lcoo l ing f rom s u lpha te ae ros o l s . Cur ren t re s ea rch ind ica te stha t an th ropog en ic ae ros o l s in the t ropos phe re , no tab ly s u l-p h a t e , h a v e a s i g n i f i c a n t c o o l i n g e f f e c t ( W i g l e y , 1 9 9 4 ;C h a r l s o n e t a l , 1992 ; T ay lo r and Penn e r , 1994 ) . Ae ros o l sa r e p r o d u c e d p r i m a r i l y b y m e t a l s m e l t in g a n d t h e c o m b u s -

40 -

30-

2 0 -.e

10- BA U

PAGE95: E L Plambeck an d C Hope 78 5

I r I2000 2050 2100 2150

Figur e ! Carb on emissions by policy and year2200

t ion o f b iomas s and fos s i l fue l s . T hes e ac t iv i t i e s p rod uceg a s e s c o n t a i n i n g s u l p h u r , c a r b o n a n d n i t r o g e n w h i c h a r econv e r ted to ae ros o l s ( s ma l l , s o l id pa r t i c l e s f ro m 10 3 to102 p .m in rad ius ) by chem ica l reac t ions in the a tm os phe re .T h e p r i m a r y a c t o r i s S O 2 gas which i s ox id ized to p roduces u lpha te (SO4++) ae ros o l , commonly known a s a con t r ibu -to r to ac id ra in . Ae ros o l s have been found to have a d i rec te f f e c t o n c l i m a t e b y r e f l e c t i n g i n c o m i n g s u n l ig h t , a n d a nind i rec t e f fec t invo lv ing c loud fo rma t ion .

Aeros o l s abs orb and backs ca t t e r s o la r rad ia t ion . T h i s i scom mo nly re fe r red to a s the d i rec t coo l ing e f fec t . In the ab -s e n c e o f c l o u d s , r a d i a t i v e f o r c i n g d e c r e a s e s a s a l i n e a rf u n c t i o n o f t r o p o s p h e r i c a e r o s o l c o n c e n t r a t i o n . I n t h i spape r , we t ake the mag n i tude o f the d i rec t coo l ing e f fec t tob e i n t h e r a n g e f r o m - 0 . 3 t o - 0 . 9 W / m 2 i n a n n u a l g l o b a lmean fo rc ing fo r p re s en t concen t ra t ions , ba s ed on J ones e ta l ( 1 9 9 4 ) , C h a r l s o n e t a l ( 1 9 9 2 ) , K i e h l a n d B r i e g l i e b(1993) and T ay lo r and Pen ne r (1994 ) . T h i s i s no t in s ign i fi -c a n t c o m p a r e d t o t h e r a d i a ti v e f o r c i n g e f f e c t o f a n t h r o -p o g e n i c g r e e n h o u s e g a s e s , e s t i m a t e d a t 2 t o 2 . 5 W / m 2( H o u g h t o n et a l , 1990).

A e r o s o l s a l s o i m p a c t c l i m a t e i n d i r e c t l y t h r o u g h c l o u dfo rma t ion . Ae ros o l s ac t a s c loud condens ing nuc le i (CCN) ,i n c r e a si n g t h e o v e r a l l v o l u m e o f c l o u d s . C l o u d s a u g m e n tthe a lbedo ( re f l ec t iv i ty ) o f the a tmos p he re s o tha t mo re in -coming s un l igh t i s r e f l ec ted to s pace (Langne r and Rohde ,1991) . Second ly , by inc rea s ing the concen t ra t ion o f CCN,a e r o s o l s r e d u c e m e a n c l o u d d r o p l e t s i z e . T h i s i n t e r f e r e sw i t h r a i n f a l l a n d c h a n g e s t h e d i s t r i b u t i o n o f c l o u d s a n dw a t e r v a p o u r . A s w a t e r v a p o u r i s t h e p r i m a r y g r e e n h o u s eg a s t h i s p h e n o m e n o n w i l l p l a y a m a j o r r o l e i n c l i m a t echange . T h e ind i rec t e f fec t o f a e ros o l s on rad ia t ive fo rc ingi s more d i f f i cu l t to quan t i fy than the d i rec t e f fec t becaus ethe comple x in te rac t ions be tw een ae ros o l s , CCN , and c loudp r o p e r t ie s a r e p o o r l y u n d e r s to o d . T h e i n d i r ec t e f f e c t m i g h teven cons t i tu te a ne t inc rea s e in rad ia tive fo rc ing (Ch a r l s on


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7 8 6 PAGE95: E L Plambeck and C Hope

u )

8 050 / / " , , ,

/3 0 t / / " " " ,2 0 _ / " ' " . 71 0 -

0 1 l " T T1 9 9 0 2 0 2 0 2 0 5 0 2 0 8 0 2 1 1 0

Developed coun triesT I I

2 1 4 0 2 1 7 0 2 2 0 0Figu re 2 An nual emissions of sulphur by developed and developing countriesaaDeveloped countries includes the lbllowing PAGE95 regions: the European Union, USA , otherOECD countries, Eastern Europ e and the form er USSR. Developing countries includes thefollowing PAGE95 regions: China and centrallyplanned Asia, India and South-east Asia, Africaand the M iddle East and L atin America.Source: Spiroet al (1992); WEC (19 92); Ball and D owlatabadi (1994).

e t a l , 1 9 9 2 ). U s i n g a v e r s i o n o f t h e H a d l e y C e n t r e G C M ,J ones e t a l ( 1 9 9 4 ) es t ima te tha t the g loba l annua l mean o find i rec t r ad ia tive fo rc ing i s -1 . 3 W /m 2 fo r p re s en t concen-t ra tions . Bas ed on the s e re s u l t s , we t ake the ind i rec t e f fec tt o b e i n t h e r a n g e f r o m + 0 . 2 t o - 2 . 4 W / m 2 o f an n u a l g l o b a lmean fo rc ing fo r p re s en t concen t ra t ions .

C l i m a t e m o d e l l e r s a r e m o v i n g q u i c k l y t o i n c o r p o ra t eae ros o l s , ' t he mis s ing fo rc ing fac to r ' , s o tha t mode l re s u l t sw i l l r e f l e c t o b s e r v e d t e m p e r a t u r e s o v e r t h e p a s t c e n t u r y(Ma t thews , 1994 ) . Ae ros o l s can exp la in pas t ove re s t ima te so f h e a t in g b y g l o b a l c i r c u la t i o n m o d e l s ( G C M s ) ( H a d l e y ,1995) . Ae ros o l s can a l s o acc oun t fo r the p rev iou s ly inexp li -cab le dec rea s e o f t empera tu re in the nor the rn hemis phe re ( inwhich mo re than 90% of indus t r i a l SO 2 is emi t t ed ) tha t ha sno t occu r red in the s ou the rn hem is phe re (Wig ley , 198 9).

Unl ike g reenho us e gas e s s uch a s CO 2 , wh ich rema ins inthe a tmos phe re fo r cen tu r i e s , a e ros o l s a re rap id ly removedf r o m t h e a t m o s p h e r e t h r o u g h p r e c i p i t a t io n o r d r y d e p o s i -t ion . T he ave rage l i f e t ime i s on ly s ix days (Cha r l s on e t a l ,1992). As a resul t , the coo l ing effe c t of sulphate ae rosols isconcen t ra ted in the s ource reg ion . Greenho us e gas e s, wh icha r e u n i f o r m l y m i x e d t h r o u g h o u t t h e a t m o s p h e r e , c a n b em o d e l l e d a s a s i m p l e a d d i t i v e c o m p o n e n t i n m e a n g l o b a lf o r c in g , w h e r e a s m o d e l l i n g t h e e f f e c t o f a e r o s o l s r e q u i re sreg iona l s pec i f ic i ty . T he re fo re , PAGE 95 com pute s reg iona lt e m p e r a t u r e r i s e b a s e d o n t h e c h a n g e i n r ad i a t iv e f o r c i n gf rom reg iona l s u lphur emis s ions . 2 F igure 2 s hows the p ro -jec ted s u lphur emis s ions ove r t ime tha t we us e in th i s pape r .

T hes e a re de r ived f rom Sp i ro e t a l ( 1 9 9 2 ), W E C ( 1 9 9 2 ) a n dBa l l and Dowla tabad i (1994 ) .

F i g u r e 3 c o n t r a s t s t h e t e m p e r a t u r e r i s e p r e d i c t e d b yP A G E 9 5 w i t h t h e r es u lt s f r o m H o p e a n d M a u l ( 1 9 9 6 ). F o rb o t h p o l i c y s c e n a r io s t h e m e a n t e m p e r a t u r e r i s e p r e d i c t e dby PAGE 95 i s s ign i f i can t ly lower than in PAGE (Hope andMaul , 1996). T he d i f fe re nce is mos t im por tan t in the ea r lyyea rs , when the s u lpha te coo l ing e f fec t i s g rea te s t in p ro -p o r t i o n t o g r e e n h o u s e w a r m i n g . D u e t o t h e i r s h o r t a t m o -s phe r ic l i f et ime , s u lpha te ae ros o l s do no t accu mu la te in thea tmos ph e re ove r t ime a s does CO2; s u lpha te ae ros o l coo l -ing i s rough ly p ropor t iona l to the ra te o f emis s ion . T he re -f o r e t h e g r e e n h o u s e e f f e c t w i ll d o m i n a t e i n t h e l o n g t e r munle s s s u lpha te ae ros o l emis s ions inc rea s e d ram a t ica l ly re l -a t ive to the g reenh ous e gas e s . T h i s i s ve ry un l ike ly . Fos s ilf u e l c o m b u s t i o n i s th e p r i m a r y s o u r c e o f b o t h s u l p h a t eae ros o l s and C O 2 emis s ions , s o the two a re c los e ly l inked .H o w e v e r , u n l ik e C O 2 , s u l p h at e s c a n b e r e m o v e d f r o m t h eexhaus t s t ream. P a r t i cu la r ly in wea l thy coun t r i e s , conce rnsabou t ac id ra in and hea l th e f fec t s have p rom pted inves tm en tin s u lphur con t ro l t e chno log ie s . Dec i s ion make rs a re ve ryun l ike ly to choos e to inc rea s e s u lpha te emis s ions in o rde rto comba t g loba l wa rming . As F igure 2 s hows , we a s s umet h a t a g g r e g a t e w o r l d s u l p h a t e a e r o s o l e m i s s i o n s d e c r e a s ea f t e r 204 0 , caus ing the l eve l o f ne t rad ia t ive fo rc ing in thet w o m o d e l s t o c o n v e r g e . H o w e v e r , a s a r e s u l t o f e a r t h ' sthe rma l l ag , t empera tu re s in PA GE 95 a re s t i ll s ign i f i can t lyl o w e r t h r o u g h t h e y e a r 2 2 0 0 .

2The direct cool ing ef fect is modelled as a l inear reduct ion in radiat ivelbrcing as a funct ion of anthropogenic su lphur f lux for each region. Theindirect ef fect on radiat ive lbrcing is mod elled as a logar i thmic funct ion of

th e r a t io o f an th r o p o g en ic s u lp h u r to th e n a tu r a l s u lp h u r f lu x. R eg io n a ltemperature r ise is calculated f rom thc d if ference between pos i t ive radia-t iv e f o r c in g f r o m th e g r een h o u s e e f f ec t an d n eg a t iv e r ad ia t iv e f o r c in gfrom aerosols , a l lowing tbr thermal lag.


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7 ~ . . . . . . . . . . . . P A G E B A U + P U L S E! . . . . . . . . P A G E B A U6 - i . . . . . . . . . . P A G E 9 5 B A U + P U L S E ~ " "

- - P A G E 9 5 B A U . ~ , ~ " " "S ~ , ,.,, ~.~ J ' ~

3 -i . . : . '. : . " " . . - " "

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

1 ~ . ' ; -"" -"

0 - ~ . . . .i

2 0 0 0 2 0 2 0 2 0 4 0 2 0 6 0 2 0 8 0 2 1 0 0 2 1 2 0 2 1 4 0 2 1 6 0 2 1 8 0 2 2 0 0

Figure 3 Mean emperature rise above the 1990 level in EU in CLIMATEexperimentby policy and yearap u l s e o f 1 0 0 G t C em i t t ed f ro m 1 9 9 0 -2 0 2 0 .Source: H o p e an d M au l (1 9 9 6 ) an d P A G E 9 5 ru n s .

The experiment CLIMATE computes the marginal im-pact per tonne of carbon emissions using the PAGE95 up-date climate model with cooling from ozone depletion andsulphate aerosols. All other aspects of the PAGE model re-main as in Hope and Maul (1996). Marginal impact resultsfor the experiment CLIMATE appear in Table 1. 'Net pre-sent value of impacts' refers to the sum of economic andnon-economic impacts through the year 2200, discountedand aggregated back to 1990. The columns 'minimum' and'maximum' refer to the 5% and 95% point on the probabil-ity distribution of results. Recall that Hope and Maul(1996) estimated the marginal impact per tonne of carbonemissions at US$5, with a 90% range from US$2-7 per tC.The CLIMATE experiment suggests that the cooling effectsof ozone depletion and sulphate aerosols decreases the esti-mated marginal impact per tonne of carbon by US$1-2.This result is not surprising. Natural and economic systemsare thought to be robust; that is, impacts will not occur forsufficiently small or gradual increases in temperature. Bydepressing temperature, ozone depletion and sulphateaerosols are expected to reduce and delay the onset of im-pacts from the CO 2 pulse.C l i m a t e s e n s i t i v i ~ t o C O 2 c o n c e n t r a ti o nNevertheless, the true implications of the scientific find-ings on aerosol cooling may be counterintuitive. Climatesensitivity to increased atmospheric CO 2 concentration isusually estimated from GCMs calibrated to reproduce ob-served temperature trends over the past century. However,by leaving out a substantial cooling factor (sulphateaerosols) GCMs probably underestimated climate sensitiv-ity to CO 2. Therefore greenhouse warming is likely to be

greater than was previously expected. As discussed previ-ously, sulphate aerosol cooling will not significantly coun-teract greenhouse warming in the long term. Hence recentscientific findings on cooling from sulphate aerosols mayactually increase the estimate of marginal impact per tonneof carbon emissions.The second experiment, CLIMATE2, investigates thispossibility by also varying a key input parameter in PAGE:the equilibrium warming caused by a doubling o f atmo-spheric CO2 concentration (AT2co2). Previous experimentsin Hope and Maul (1996) and CLIMATE used the IPCC1992 estimate of ATzco2 in the range 1.5 to 4.0C, withmost likely value 2.5C. This figure was derived fromGCMs without sulphate aerosol cooling; an increased valueis appropriate for use in the PAGE95 climate model withsulphate aerosols. In the experiment CLIMATE2 ATzco2ranges from 1.5 to 6.0C, with most likely value 3.0C.This increase in AT2co2 is conservative. From the estimatesof aerosol cooling in the forthcoming IPCC report, Raper e t

T a b l e I T o t a l a n d m a r g i n a l i m p a c t s in C L I M A T E e x p e r i m e n t( 1 9 9 0 - 2 2 0 0 )M i n i m u m a M e a n M a x i m u m b

N et p r e s en t v a l u e o f i m p ac t sB A U + 1 00 G t C e m i s s i o n s(US$ t r i l l ion) 0 .9 2 .5 5 .5B A U em i s s i o n s (U S $ t r il l io n ) 0 .8 2 .2 4 .6M arg i n a l i m p ac t s (U S $ p e r t C ) t 3 6aM i n i m u m = 5 p e rcen t ag e p o i n t s o n d i s t ri b u t i o n o f r e s u l ts .b M a x i m u m = 9 5 p e rcen t ag e p o i n t s o n d i s t r i b u t io n o f r e s u l t s .Source: P A G E 9 5 ru n s .


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7 8 8 PAGE95:E L Plambeck and C Hope

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12

10

- 2

9 5 % p o in t: B A U + PULSE

s s s j s S S S ~ S

. ~ . - f . f 6% point: B A U ~

I I [ I I I I I ]

2 00 0 2 0 2 0 2 0 4 0 2 0 6 0 2 0 8 0 2 1 0 0 2 1 2 0 2 1 4 0 2 1 6 0 2 1 8 0 2 2 0 0

Figure 4 90% range and mea n temperature rise above the 1990 level in the EU in theCLIM ATE2 experiment by policy and yearaapulse of 100 GtC emitted from 1990-2020.Source: PAGE95 runs.

o f a e r o s o l c o o l i n g i n t h e f o r t h c o m i n g I P C C r e p o r t, R a p e r e ta l (1995) conc lud e tha t the va lue o f AT 2co2 mus t be a t l e a s t4 . 5 C t o e x p l a i n t h e o b s e r v e d t e m p e r a t u r e r i s e o f 0 .5 Cove r the pas t c en tu ry . Wes t e t a l (1995) r e c o m m e n d a ra n g eof 2 . 0 to 5 . 5C , wi th a b e s t e s t ima te o f 3 . 5C fo r AT 2co2.F i g u r e 4 s h o w s t h e 9 0 % r a n g e a n d m e a n t e m p e r a t u r e ri s ep red ic ted by PAG E 95 w i th inc rea s ed c l ima te s ens i t iv i ty fo reach po l i cy . No te tha t the l eve l o f t empera tu re inc rea s e i sh i g h l y u n c e r t a i n , a n d t h a t t h e r a n g e o f p o s s ib l e v a l u e s i sl a rge compa red to the d i f fe rence be tween po l i c ie s.

T h e m a r g i n a l i m p a c t r e s ul t s f o r e x p e ri m e n t C L I M A T E 2a p p e a r i n T a b l e 2 . I n c r e a s e d c l i m a t e s e n s i t i v i t y to a t m o -s phe r ic CO 2 y ie lds an e s t ima ted m arg ina l im pac t o f US$5p e r t C , in a g r e e m e n t w i t h H o p e a n d M a u l ( 1 9 9 6 ) a n d 2 5 %h i g h e r t h a n i n t h e p r e v io u s e x p e r i m e n t C L I M A T E . W e o b -s e r v e t h a t c o o l i n g f r o m o z o n e d e p l e t i o n a n d s u l p h a t eae ros o l s reduces the e s t ima ted m arg ina l impac t pe r tonne o fc a r b o n , b u t o n l y i f t h e t e m p e r a t u r e s e n s i t i v i t y t o a t m o -s phe r ic C O 2 i s no t ad jus ted . C lea r ly , the marg ina l impac tva lua t ion pe r tonne o f ca rbon ma y r i s e i f, a s s ugges ted byR a p e r et al (1995) c l ima te s ens i t iv i ty i s even g rea te r thana s su m e d i n C L I M A T E 2 .

E x p a n s i o n o f t h e 9 0 % r a n g e f o r t h e m a r g in a l i m p a c t p e rt o n n e o f ca r b o n , f ro m U S $ 2 - 7 p e r t C i n H o p e a n d M a u l(1996) to US $2- 12 pe r tC i l lu s t ra te s a ve ry impor tan t po in tabou t recen t s c ien t i f i c f ind ings on ae ros o l coo l ing . T he c l i -m a t e s y s t e m i s e v e n m o r e c o m p l e x t h a n w a s p r e v i o u s l ythough t . Mo de l l e r s mus t a s s e s s the s t reng th o f two compe t -i n g p h e n o m e n a , g r e e n h o u s e w a n n i n g a n d a e r o s o l co o l in g ,f rom the h i s to r i ca l t emp era tu re record . T he re fo re , a l though

the mean re s u l t s have no t changed , we can be fa r l e s s ce r -t a in o f ou r ca lcu la t ions . T he marg ina l im pac t s caus ed by atonne o f ca rbon emis s ions may be s ign i f i can t ly l a rge r thane rs twhi le p red ic t ions .Regional differences in the impacts o f climate cha ngeI n t h e t h i r d e x p e r i m e n t , I M P A C T S , P A G E 9 5 i s r u n a s i nC L I M A T E 2 w i t h t h e a d d it i o n o f a n u p d a t e d v a l u a t i o n o f t h ereg iona l impac t s o f g loba l wann ing . M uch o f the re s ea rchon va lu ing the impac t s o f c lima te change has focus ed on theU S A a n d o t h e r O E C D c o u n t r i e s , e g E P A ( 1 9 8 9 ) a n dC R U / E R L ( 1 9 9 2 ). H o w e v e r , t h e l e ve l o f d a m a g e s f r o mg l o b a l w a r m i n g i s e x p e c t e d t o v a r y w i d e l y a m o n g g e o -g raph ica l reg ions. D i f fe ren t a rea s may b e m ore o r l e s s vu l -n e r a b l e t o c l i m a t i c c h a n g e . F o r e x a m p l e , h e a t s t r e s s a n ddrough t a re expec ted to be mos t ex t reme in the in te r io r o fcon t inen t s , wh i le i s land na t ions and low ly ing coas ta l a rea s

T a b l e 2 T o t a l an d m a r g i n a l i m p a c t s in C L I M A T E 2 e x p e r i m e n t(1 9 9 0 -2 2 0 0 )M i n i m u m a M e a n M a x i m u m b

Net present value o f impactsBAU + 100 GtC emissions(US$ trillion) 1. [ 3.7 8.4BAU emissio ns (US$ trill ion) 0.9 3.2 7.2Marginal impacts (US$ per tC) 2 5 12aMinimum = 5 percentag e points on distribution of results.bMaximum = 95 percen tage points on distribution of results.Source: PAGE95 runs.


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PAGE95: E L Plambeck and C Hope 7 8 9T a b l e 3 R eg i o n a l i mp a ct fa c to rs (a s co mp a red to th e EU) fo r ex p er i me n t I M P A C T S

Regional im pact factorsR e gi on s M i n i m u m M o s t l ik e l y M a x i m u mUSA 0.75 1 .20 1 .40O E C D e x c e p t U S A a n d E U 0 . 75 2 . 2 0 2 . 6 0F o rm er U S S R an d E as t e rn E u ro p e ~ ) .3 0 0 .0 0 0 .3 0C h i n a an d cen t r a l l y p l an n ed A s i a 1 .0 0 4 .0 0 4 .8 0In d i a an d S o u t h -eas t A s i a 1 .0 0 6 .6 0 7 .9 0A f r i ca an d t h e M i d d l e E as t 1 .0 0 4 .5 0 5 .4 0L a t i n A m er i ca 1 .0 0 3 .3 0 4 .0 0Source: Tol ( 1995); F ank hau ser ( 1994b): C RU /ER L (1992) .

s uch a s Bang lad es h w i l l s u f fe r mo s t f rom s ea l eve l r i s e. Im-pac t s a re expec ted to be re la t ive ly l a rge in the l e s s deve l -o p e d c o u n t r i e s d u e t o t h e r e l a t iv e i m p o r t a n c e o f c l i m a t edependen t s ec to rs s uch a s ag r icu l tu re . Fur the rmore , lo s s o fl i f e i s l i k e l y t o b e p r o p o r t i o n a l l y g r e a t e r i n d e v e l o p i n gc o u n t r i e s b e c a u s e o f p o o r n u t r i t i o n a n d h e a l t h i n f r a s tr u c -tu re . Wi th l imi ted f inanc ia l r e s e rves , deve lop ing coun t r i e shave l e s s capac i ty fo r adap ta t ion . Accord ing to Fankhaus e r( 1 9 9 4 c ) d a m a g e t o d e v e l o p i n g c o u n tr i e s w i ll b e 5 0 % h i g h e rthan the OE CD ave rage . Howeve r , no t a l l ag ree . Manne e ta l ( 1 9 9 5 ) o b s e r v e t h a t w i l l i n g n e s s t o p a y t o a v o i d n o n -e c o n o m i c ( e c o l o g i c a l a n d s o c ia l ) d a m a g e s f r o m g l o b a lw a r m i n g i n c r ea s e s w i t h i n c o m e . T h e r e f o r e th e v a l u a t io n o fn o n - e c o n o m i c d a m a g e s s h o u l d b e h i g h e r i n th e d e v e l o p e dcoun t r i e s .M o s t a t t e m p t s t o q u a n t i f y d a m a g e h a v e f o c u s e d o n t h eb e n c h m a r k o f a d o u b l i n g o f a t m o s p h e r i c C O 2 c o n c e n tr a t io nand the a s s oc ia ted t empera tu re r i s e o f 2 .5C . PAG E 95 hastwo unce r ta in inpu t pa rame te rs rep re s en t ing the pe rcen tageG D P l o s s p e r 2 . 5 C i n t h e e c o n o m i c a n d n o n - e c o n o m i cs ec to rs . T hes e va lues a re e s t ima ted fo r the focus reg ion , theE U . I n t e r m s o f th e p e r c e n t a g e G D P l o s t p e r 2 . 5 C , n o n -e c o n o m i c i m p a c t s w i l l r a n g e f r o m 0 . 3 t o 3 . 5 w i t h m o s tl ike ly va lue 0 . 7 ; e con om ic impac t s wi l l r ange f rom 0 . 3 to1 . 5 w i t h m o s t l i k e l y v a l u e 0 . 6 . E c o n o m i c a n d n o n - e c o -nom ic impac t s in the o the r reg ions a re compute d a s a mul t i -p l e o f t h e E U v a l u e s . F o r e x a m p l e , p r e s e n t G D P l o s t p e r2 . 5 C i n I n d i a a n d S o u t h - e a s t A s i a i s b e t w e e n 1 a n d 7 . 9t imes the va lue fo r the E U, w i th mo s t l ike ly va lue 6 . 6 . Ins o m e c a s e s , b e n e f i t s a r e e x p e c t e d t o o c c u r a s a r e s u lt o fwarm ing (eg ag r icu l tu re in the fo rme r USS R) . T h i s i s rep -r e s e n t e d b y a n e g a t i v e v a l u e f o r G D P l o s s ( se e T a b l e 3 ).T h i s v a l u a t i o n o f i m p a c t s d e r i v e s f r o m T o l (1 9 9 5 ) ,F a n k h a u s e r ( 1 9 9 4 b ) a n d C R U / E R L ( 1 9 9 2 ) .T h e r e s u l ts o f th e e x p e r i m e n t I M P A C T S a p p e a r i n T a b le4 . Upda t ing the reg iona l damage e s t ima te s y ie lds a s l igh treduc t ion in the e s tima ted range o f marg ina l imp ac t s . How -eve r , the me an m arg ina l im pac t inc rea s e s to US$8 pe r tC inI M P A C T S a s c o m p a r e d t o U S $ 5 p e r t C i n t h e e x p e r i m e n tCLIMAT E 2, p r imar i ly due to l a rge damages in the l e s s de -ve loped coun t r i e s .Non-line ari O, in c l imatic impactsWe obs e rved p rev ious ly tha t mos t a t t empts to quan t i fy thei m p a c t s o f c l im a t e c h a n g e f o c u s o n a b e n c h m a r k w a r m i n go f 2 . 5 C , y e t t h i s b e n c h m a r k i s l i k e l y to b e s u r p a s s e d

T a b l e 4 T o t a l a n d m a r g i n a l im p a c t s i n I M P A C T S e x p e r i m e n t( 1 9 9 0 - 2 2 0 0 )M i n i m u m a M e a n M a x i m u m b

N et p r e s en t v a l u e o f i m p ac t sB A U + 1 0 0 G t C em i s s i o n s(US$ t r i l lion) 2 .8 6 .8 14 .2B A U em i s s i o n s (U S $ t r il l io n ) 2 .5 6 .0 1 3 .0M arg i n a l i m p ac t s (U S $ p e r t C ) 3 8 1 2aM i n i m u m = 5 p e rcen t ag e p o i n t s o n d i s t r i b u ti o n o f r e s u l t s .b M a x i m u m = 9 5 p e rcen t ag e p o i n t s o n d i s t r i b u ti o n o f r e s u l t s .Source: P A G E 9 5 r u n s .

T a b l e 5 T o t a l a n d m a r g i n a l im p a c t s i n N O N - L I N E A R e x p e r i m e n t( 1 9 9 0 - 2 2 0 0 )M i n im u m a M e a n M a x i m u m b

N et p r e s en t v a l u e o f i m p ac t sB A U + 1 0 0 G t C em i s s i o n s(US$ t r i l lion) 1 .8 6 .7 16 .7B A U em i s s i o n s (U S $ t r il l io n ) 1 .2 5 .9 1 4 .9M arg i n a l i m p ac t s (U S $ p e r t C ) 6 8 1 8aM i n i m u m = 5 p e rcen t ag e p o i n t s o n d i s t r i b u ti o n o f r e s u l t s .b M a x i m u m = 9 5 p e rcen t ag e p o i n t s o n d i s t r ib u t i o n o f r e s u l ts .Source: P A G E 9 5 ru n s .

with in the nex t cen tu ry . A cur ren t i s s ue in po l i cy ana lys i s i show to ex t rapo la te in o rde r to p red ic t damages be fo re anda f t e r t h e b e n c h m a r k o f 2 . 5 C . I m p a c t a r e u s u a l l y a s s u m e dt o b e a p o l y n o m i a l f u n c t io n o f t e m p e r a t u r e r i s e w i t h p o w e rbe twe en 1 and 3 (Nordhaus , 1993a , 1993b ; Peck and T e i s -be rg , 1993a, 1993b) . A po l l o f expe r t s s ugges t a power o f1 .3 a s the mos t l ike ly va lue (Fankh aus e r , 1994 b) . In thef i f t h e x p e r i m e n t , N O N - L I N E A R , b o t h e c o n o m i c a n d n o n -e c o n o m i c i m p a c ts g r o w n o n - l i n e a r l y w i t h t e m p e r a t u r e r i se( reca l l tha t a l inea r mode l was us ed in the p rev ious expe r i -m e n t s ) . T h e i m p a c t f u n c t i o n i s a p o l y n o m i a l f u n c t i o n o ft empera tu re r i s e above the to le rab le l eve l ( T - T t d )" , w h e r en i s an unce r ta in inpu t pa rame te r wi th min imum 1 , max i -m u m 3 a n d m o s t l i k e l y v a lu e 1 .3 . T h e i m p a c t f u n c t i o n i sca l ib ra ted to g ive the s am e re s u l t s a s a l inea r func t ion fo r a2 . 5C tem pera tu re r i se .T h e r e s ul t s f o r th e e x p e r i m e n t N O N - L I N E A R a p p e a r i nT ab le 5 . For the range o f emis s ions we exam ine , non- l inea r -i t y i n t h e i m p a c t f u n c t i o n d o e s n o t c h a n g e t h e e s t i m a t e dm e a n m a r g in a l i m p a c t o f a t o n n e o f c a r b o n e m i s s io n s . A ss hown in F igure 5 , fo r t empera tu re r i s e above the to le rab le


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790 PAGE95: E L Plambeck and C Hope

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2 3 4 5 6

Cubic function

Linear function

CF i g u r e 5 I m p a c t s b y t e m p e r a t u r e r is e a n d f o r m o f d a m a g e f u n c t i o n aaTolerable temperature rise before impacts occur is 2C.

t ion fa l l s be low the l inear funct ion. How ever , for large tem-pera ture r i se the non-l inear damage funct ion dominates. Inshort , non-l inear i ty in the damage funct ion decreases dam-ages in the ear ly years but increases dam ages la ter on. In theexperiment NO N-L INE AR the net e ffec t i s sl ight. However,the int roduct ion o f an addi t ional uncer ta in parameter to rep-re sen t t he cu rva tu re o f t he damage func t ion y i e lds an in -crease in the range o f est imated marginal impacts .Reg io n a l eco n o mic g ro wth a n d t ime-va r ia b le d i sco u n t ingPrev ious expe r imen t s a ssumed a un i fo rm , wor ldwide eco-n o m i c g r o w t h o f 2 % p e r y e a r a n d a d i s c o u n t r a te o f 5 % .The f i f t h expe r imen t , REGIONAL, i s d i s t i ngu i shed f romNO N-L INEA R b y the use o f t ime- and reg ion-spec if i c va l-ues fo r t he economic g rowth ra t e taken f rom EM F (1994) .These values appear in Table 6 . Furthermore , the discountra t e i s t ime va r i ab l e and l i nked to economic g rowth ( seeTable 7).

In econom ic g rowth theory the d i scoun t r a te r( t) is givenby Ram sey ' s ru le :

r ( t ) = y .g ( t ) + pwhere y i s the negat ive of the e last ic i ty of marginal ut i l i tyo f consum pt ion , p t he pure r a t e o f t ime p re fe rence , and g( t)i s t he pe r cap i t a r e l a ti ve g rowth ra t e o f consumpt ion . Theterm y . g ( t ) i s posi t ive und er the standard co ndi t ions tha t theeconomy grows, and tha t marginal ut i l i ty i s posi t ive but i t sder iva t ive neg at ive (Azar , 1994, p 1256). Th e value y i susua l ly se t t o one , co r re spond ing to a l oga r i t hmic u t i l i t yfunc t ion . The pe r cap i t a r e l a t ive g row th ra te o f consump-t ion ma y be com puted a s :

d (C/P)g ( t ) = d t / ( C / P )w h e r e C( t ) i s t he g loba l consumpt ion , and P ( t ) t he wor ldp o p u l a t i o n a t t i m e t . F o r e x p e r i m e n t a l p u r p o s e s i n t h i sstudy, g( t ) i s assumed to be equ ivalent to the world wid e percap i ta econom ic g rowth ra t e ( i e consum pt ion accoun t s fo r af ixed percentage o f tota l product ion) .

A t ime var iable discount ra te should be used in c l imatechange ana lys i s because economic and popu la t ion g rowthra t e s, and h ence the va lue g ( t ) , a re h igh ly va r i ab l e i n t helong term. In par t icular , economic growth ra tes wi l l be af-f ec t ed by aba t ement po l i c ie s and warm ing impac t s . Neve r -the le ss , to da t e m os t g loba l wa rming ana lyse s have used af ixed d i scoun t r at e , t he l eve l o f wh ich s t rong ly cond i t i onsthe resul ts (see Haraden, 1993; Fankha user , 1994a) .

The use o f va ri ab le d i scoun t ing based on econom ic andpopu la t ion growth ra tes st i ll requires a di f f icul t dec isio n onthe pure r a t e o f t ime p re fe rence , p . The ap propr i a t e puret i m e p r e f e re n c e f o r t h e s t u d y o f g l o b a l w a r m i n g i s h o t l yd i spu ted . Accord ing to Aza r (1994) and Cl ine (1992) t heuse o f a pos i t ive pure t ime p re fe rence , p , i s une th i ca l be -cause i t impl ies tha t the ut i l i ty of the current genera t ion iswor th m ore than tha t o f fu tu re gene ra tions . Howeve r , on ly aposi t ive ra te i s consistent wi th savings and interest ra te da ta(Fankhause r , 1994b) . Many g loba l wa rming op t imiza t ionmo dels (Nordhaus, 1992, 1993a, 1993b; Pec k and Teisberg,1992, 1993a, 1993b; Manne e t a l , 1993) use a 3% ra te ofpure t ime preference , and tha t va lue i s a lso assumed in thee x p e ri m e n t R E G I O N A L .

T h e r e s u lt s o f t h e e x p er i m e n t R E G I O N A L a p p e ar i nTable 8 . These f igures represent the PAGE95 current bestest imate for marginal impacts . The ext rem ely large increase


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T a b l e 6 R eg i o n a l a n n u a l eco n o m i c g ro wth ra te sPAGE95: E L Plambeck and C Hope 79 1

Y e a r s E U , U S A O t h e r O E C D F o rm er USSR , C en tra l l y p l a n n ed In d i a , So u th - ea s tEa s tern Eu ro p e A s i a , C h i n a A s i a , A fr i ca1990--2000 2.5 2 .7 -1 .5 4 .0 3 .82 0 0 0 -2 0 2 0 2 .3 2 .3 4 .3 3 .5 4 22 0 2 0 -2 0 4 0 1 .5 1 .5 3 .6 3 .3 3 .6204 0-2 060 1 .7 1 .7 2 .7 3 . t 3 . I2 0 6 0 -2 0 8 0 1 .2 1 .2 2 .0 3 .0 2 .820 80- 21 00 1 .1 I . I 2 .0 3 .0 2 .8210 0-2 125 1 . I I . I 1 .0 2 .0 2 .02 1 2 5 -2 1 5 0 0 .8 0 .8 1 .0 2 .0 2 .02 1 5 0 -2 2 0 0 0 .8 0 .8 0 .8 1 .0 1 .0Source: E M F (1 9 9 4 ) .

T a b l e 7 T i m e v a r i a b l e d i s co u n t ra teYear r( t)1 9 9 0 -2 0 0 0 3 .9 32 0 0 0 - 2 0 2 0 4 . 5 92 0 2 0 - 2 0 4 0 4 . 5 62 0 4 0 - 2 0 6 0 4 . 5 82 0 6 0 - 2 0 8 0 4 . 7 02 0 8 0 - 2 1 5 0 4 . 9 42 1 5 0 - 2 2 0 0 3 . 9 0Source: W o r l d p e r cap i t a g ro w t h r a t e + 3 % t i m e p re f e r en ce (E M F , 1 9 94 ).

T a b l e 8 T o t a l a n d m a r g i n a l i m p a c t s in R E G I O N A L e x p e r i m e n t( 1 990 - 2 2 0 0 )

M i n im u m a M e a n M a x i m u m bN et p r e s e n t v a l u e o f i m p a c t sB A U + 1 00 G t C e m i s s i o n s(US $ t r i l l ion) 5 .0 19 .8 45 .5B A U em i s s i o n s (U S $ t r i ll i o n ) 4 .0 1 7 .7 4 0 .7M arg i n a l i m p ac t s (U S $ p e r t C ) 1 0 2 1 4 8aM i n i m u m = 5 p e rce n t ag e p o i n t s o n d i s t r i b u t i o n o f r e s u lt s .b M a x i m u m = 9 5 p e rcen t ag e p o i n t s o n d i st r i b u t io n o f r e s u l t s.Source: P A G E 9 5 r u n s .

T a b l e 9 T o t a l a n d m a r g i n a l i m p a c t s i n P T P 2 e x p e r i m e n t( 1 990 - 2 2 0 0 )

M i n i m u m a M e a n M a x i m u m bN et p r e s en t v a l u e o f i m p ac t sB A U + 1 00 G t C e m i s s i o n s(U S$ tri l l ion) 19.0 58.1 103.5BA U em iss io ns (US $ t r i l lion) 17 .0 53 .5 94 .1M arg i n a l i m p ac t s (U S $ p e r t C ) 2 0 4 6 9 4aM i n i m u m = 5 p e rcen t ag e p o i n t s o n d i s t r i b u t i o n o f r e s u lt s .b M a x i m u m = 9 5 p e rcen t ag e p o i n t s o n d i s t r ib u t i o n o f r e s u l ts .Source: P A G E 9 5 r u n s .

in the va lua t ion o f marg ina l im pac t , f rom a mean va lue o fU S $ 8 p e r tC i n N O N L I N E A R t o a m e a n v a l u e o f U S $ 21p e r t C i n R E G I O N A L o c c u r s b e c a u s e b o t h e c o n o m i cgrowth and damages t end to be concen t ra ted in the deve l -op ing coun t r i e s .

T he nex t two expe r imen ts , PT P2 and PT P0 , a re equ iva -l e n t t o R E G I O N A L e x c e p t f o r th e u s e o f a p u r e r a t e o f t im epre fe rence p = 2% and p = 0% , re s pec t ive ly . T he re s u l t s o fthe expe r imen ts PT P2 and P T P0 ap pea r in T ab le s 9 and 10.T h e m e a n v a l u e o f i m p a c t s r is e s t o U S $ 4 6 a n d U S $ 4 4 0 , r e -s pec t ive ly . C lea r ly , the marg ina l im pac t pe r tonn e o f ca rbonemis s ions i s h igh ly s ens i t ive to the cho ice o f a pu re ra te o ft ime p re fe rence . T h i s i s due to the long - te rm na tu re o f theg l o b a l w a r m i n g p r o b l e m . A p u l s e o f C O 2 e m i s si o n s a f f e c t sthe c l ima te fo r many decades i f no t c en tu r i e s .Adaptation to climate changeT h e f i n a l e x p e r i m e n t N O - A D A P T i s t h e s a m e a s R E -G I O N A L e x c e p t t h a t n o a d a p t a t i o n i s u s e d t o r e d u c e t h ei m p a c t s o f c l i m a t e ch a n g e . R e s u l ts f o r e x p e r i m e n t N O -A D A P T a p p e a r i n T a b l e 1 1 . W i t h o u t a d a p t a t i o n t h e e s t i -m a t e d m a r g i n a l i m p a c t p e r t o n n e o f c a r b o n i s U S $ 3 2 p e rt C , a n i n c r e a s e o f U S $ 1 1 p e r t C c o m p a r e d t o t h e e x p e r i -m e n t R E G I O N A L w h i c h a s s u m e d a g g r e ss i v e a d a pt a ti o n .F r e q u e n t l y i n t h e l i t e r a t u r e i m p a c t v a l u a t i o n s a r e m a d ewi thou t s t a t ing a s s umpt ions on the degree o f adap ta t ion toc l im a t e c h a n ge . T h e e x p e r i m e n t N O - A D A P T d e m o n s t r a te sthe need to c la r i fy a s s umpt ions abou t adap ta t ion in fu tu rework . No te tha t marg ina l impac t e s t ima te s in th i s s tudy dono t co ns ide r the cos t o f adap t ive meas ure s . T h i s i s ju s t i f i edbecaus e aggre s s ive adap ta t ion has been s hown to be op t i -ma l fo r bo th po l i cy s cena r ios cons ide red in the s tudy (Hop e

T a b l e 1 0 T o ta l a n d ma rg i n a l i mp a ct s in PT P0 ex p er i m en t( 19911-2200)

M i n i m u m a M e a n M a x i m u m bN et p r e s en t v a l u e o f " i m p ac t sB A U + 1 0 0 G t C em i s s i o n s(US $ t r i l l ion) 237 .0 965.0 2156.0B A U em i s s i o n s (U S $ t r il l io n ) 1 9 8 .0 9 2 1 .0 2 0 5 8 .0M arg i n a l i m p ac t s (U S $ p e r t C ) 3 9 0 4 4 0 9 8 0aM i n i m u m = 5 p e rcen t ag e p o i n t s o n d i s t r i b u t i o n o f r e su l t s .b M a x i m u m = 9 5 p e rcen t ag e p o i n t s o n d i s t r i b u t io n o f r e s u l ts .

T a b l e 11 T o t a l a n d m a r g i n a l im p a c t s i n N O - A D A P T e x p e r i m e n t( 1 990 - 2 2 0 0 )M i n im u m a M e a n M a x i m u m b

N et p r e s en t v a l u e o f i m p ac t sB A U + 1 0 0 G t C em i s s i o n s(U S$ tri l l ion) 10.0 31 I 69.6B A U em i s s i o n s (U S $ t r il l io n ) 7 8 2 7 .9 6 4 .0M arg i n a l i m p ac t s (U S $ p e r t C ) 2 2 3 2 5 6aM i n i m u m = 5 p e rcen t ag e p o i n t s o n d i s t r i b u t io n o f r e s u l t s .b M a x i m u m = 9 5 p e rcen t ag e p o i n t s o n d i s t r i b u ti o n o f r e s u l t s ,

S o u rce : P A G E 9 5 ru n s . Source: P A G E 9 5 ru n s .


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79 2 PAGE95:E L Plambeck and C Hopeet al, 1 9 9 3) . T h e c o s t o f a d a p t a t i o n i s sl i g h t c o m p a r e d t op o t e n t i a l i m p a c t s f r o m c l i m a t e c h a n g e . F u r t h e rm o r e , d i f -fe ren t po l i cy l eve rs a re need ed to in f luence adap ta t ion andg r e e n h o u s e g a s a b a t e m e n t , s o t h e i n c r e m e n t a l c o s t s o fadap ta t ion and g reenho us e gas emis s ions s hou ld be cons id -e red s epa ra te ly .D i s c u s s i o n s a n d c o n c l u s i o n sO u r c u r r e n t b e s t e s t im a t e o f m a r g i n a l i m p a c t s f r o mPAGE 95 i s US$21 pe r tC , w i th a 90% u nce r ta in ty range o fUS $10 -4 8 pe r tC . T o pu t th i s meas ure in to con tex t , US$21p e r t C c o r r e s p o n d s r o u g h l y t o a p e t r o l e u m t ax o f U S $ 2 p e rba r re l , o r a pe t ro l t ax o f US 1 .2 pe r l i t r e. Ou r e s t ima te o ft h e m a r g i n a l i m p a c t o f a t o n n e o f c a r b o n i s b a s e d o n t h efo l lowing key a s s umpt ions and inpu t s .(1 ) An upda ted c l ima te mode l wi th coo l ing f rom s u lpha te

a e r o s o l s a n d o z o n e d e p l e t i o n , a n d i n c r e a s e d c l i m a t es ens i t iv ity to a tmos ph e r ic C O 2 concen t ra t ion . Impac t sg row a s a non- l inea r func t ion o f t empera tu re .

( 2 ) N o n - e c o n o m i c i m p a c t s a r e s li g h t l y g r e a t e r t h a n e c o -nom ic impac t s and a l s o more unce r ta in . E xpre s s ed a s ape rcen tage GD P los s, bo th econom ic and non-econom icdama ges a re l a rges t in the deve lop ing coun t r i e s . E co-nomic g rowth i s reg ion and t ime-s pec i f i c . S ince bo thc l ima t ic change impac t s and economic g rowth t end tob e c o n c e n t r a t e d i n t h e d e v e l o p i n g c o u n t r i e s , t h i s i n -c rea s e s the e s t ima ted marg ina l im pac t pe r tC .

(3 ) La rge amou nts o f adap ta tion to c l ima te change , s uch asthe bu i ld ing o f s ea wa l l s and the p reven t ion o f deve lop-men t in vu lne rab le a rea s , pa r t i cu la r ly in the deve lopedwor ld .

(4 ) A t ime va r iab le d i s coun t ra te compu ted a s the s um ofw o r l d p e r c a p i t a e c o n o m i c g r o w t h a n d a p u r e r a t e o ft i m e p r e f e r e n c e o f 3 % .T h e ' b e s t ' m a r g i n a l i m p a c t e s t i m a t e i s m o s t s e n s i t i v e t oa s s u m p t i o n s a b o u t a d a p t a t io n , n o n - l i n e a ri t y , a n d t h e d i s -coun t ra t e . In the ca s e o f ze ro adap ta t ion to c l ima te change ,m a r g i n a l i m p a c t s r i s e b y 5 0 % t o U S $ 3 2 p e r t C . A n i m -p o r t a n t i n t e r a c t i o n e f f e c t w a s o b s e r v e d b e t w e e n t h e d e -g r e e o f a d a p t a t i o n a n d n o n - l i n e a r i t y i n d a m a g e s a s af u n c t i o n o f t e m p e r a t u r e r i s e a b o v e s o m e t o l e r a b l e l ev e l .R e c a l l t h a t t h e n e t e f f e c t o f n o n - l i n e a r i t y i n th e d a m a g efunc t ion i s s l igh t unde r aggre s s ive adap ta t ion becaus e thed e c r e a s e i n p r e d i c t e d d a m a g e s e a r l y i n th e t i m e h o r i z o n i sb a l a n c e d b y t h e i n c r e a s e i n p r e d i c t e d d a m a g e s l a t e r o n .T h i s t r a d e - o f f i s h i g h l y s e n s i t i v e t o t h e t o l e r a b l e l e v e l o ft e m p e r a t u r e i n c r e a s e s b e f o r e d a m a g e s o c c u r , w h i c h i nt u r n d e p e n d s o n t h e d e g r e e o f a d a p t a t io n . F o r e x a m p l e , i nt h e a b s e n c e o f a d a p t a t io n t h e t o l e r a b l e l e v e l o f t e m p e r a -t u r e i n c r e a s e i s a s s u m e d t o b e z e r o , a s c o m p a r e d t o 2 Cf o r t h e d e v e l o p i n g c o u n t r i e s u n d e r a g g r e s s iv e a d a p t a t io n .N o n - l i n e a r i t y i n t h e d a m a g e f u n c t i o n m a k e s a s u b s t a n t ia lc o n t r i b u t i o n t o t h e d i f f e r e n c e i n m a r g i n a l i m p a c t s u n d e raggre s s ive and ze ro adap ta t ion . In a s imi la r manne r , non-l i n e a r it y i n t h e d a m a g e f u n c t i o n i n c r e a s e s t h e i m p o r t a n c eo f t h e d i s c o u n t r a t e .

T he mos t in f luen t i a l a s s umpt ion examined in th i s s tudyi s the d i s coun t ra t e. Redu c ing the pure ra te o f t ime p re fe r -e n c e c o m p o n e n t o f t h e d i sc o u n t r at e f r o m 3 % t o 2 % d o u b -le s the mean marg ina l imp ac t to US$4 6 pe r tC . I f the ze rora te o f pu re t ime p re fe ren ce i s chos en to s a t i s fy in te rgene ra -t iona l equ i ty then the mean m arg ina l impac t inc rea s e s by ano r d e r o f m a g n i tu d e t o U S $ 4 4 0 p e r t C .

In add i t ion to the i s s ue o f in te rgene ra t iona l equ i ty , theex i s t ence o f s econd a ry bene f i t s to CO 2 aba temen t s ugges t st h a t o u r b e st m a r g i n a l i m p a c t e s t i m a t e o f U S $ 2 1 p e r t Cmay be cons e rva t ive , even though i t i s a t the uppe r end o ft h e r a n g e o f e st i m a t e s i n th e l i t e r a t u re ( U S $ 5 - 2 5 p e r t c )(Fankhaus e r and Pea rce , 1993) . Fos s i l fue l s a re the p r ima rys ource , no t on ly o f CO 2 , bu t o f o th e r a i r po l lu tan t s - CO,S O x , N O x , p a r t i c u l a t e s a n d v o l a t i l e o r g a n i c c o m p o u n d s .T he re fo re a ca rbon t ax to reduce fos s i l fue l us e wi l l incurs econda ry bene f i t s in improved a i r qua l i ty and red uced ac idra in . Fur the rmore , r ecyc l ing the ca rbon t ax revenues to re -duce o the r d i s to r t iona ry t axes s uch a s VAT or payro l l t axeswi l l s t imula te emp loym ent and gene ra l ly inc rea s e the s oc ia lwe l fa re (Ba rke r et al, 1993).F ina l ly , the 90% range fo r the marg ina l impac t pe r tonneo f c a r b o n e m i s s i o ns f o u n d i n th i s s tu d y , U S $ 1 0 - 4 8 p e r t C ,i s v e r y l a r g e i n c o m p a r i s o n t o t h e u n c e r t a i n r a n g e o fU S $ 2 - 7 e s t i m a t e d i n H o p e a n d M a u l ( 1 9 9 6 ) o r t h e a c -c e p t e d r a n g e o f v a l u e s in t h e l i te r a t u re , U S $ 5 - 2 5 p e r t C .N e w s c i e n t i f i c a n d e c o n o m i c k n o w l e d g e , i n p a r t i c u l a rabou t s u lpha te ae ros o l s and non- l inea r i ty in damages a s afunc t ion o f t empera tu re r i s e , s ugges t s tha t c l ima te - -hum anin te rac tions a re even more co mp lex and d i f f i cu l t to p red ic tthan was p rev iou s ly though t . T h i s inc rea s e in unce r ta in ty i sre f l ec ted in the re s u l t s o f ou r s tudy .

In conc lus ion , in read ing any s tudy on the va lua t ion o fg loba l wa rming impac t s , po l i cy make rs a re adv i s ed to ca re -f u l l y c o n s i d e r t h e t re a t m e n t o f u n c e r t a i n t y a s w e l l a s a s -s umpt ions abou t adap ta t ion to c l ima te change , non- l inea r i tyin damage a s a func t ion o f t empera tu re r i s e , s econda ry ben-e f i t s t o C O 2 a b a t e m e n t , a n d t h e d i s c o u n t r a t e . W e h a v es h o w n t h a t t h e s e a s su m p t i o n s , o f t e n h i d d e n i n t h e s m a l lp r in t o r no t repor ted a t a l l, have a p ro fo und e f fec t upon themarg ina l im pac t ca lcu la tions .

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