A Guide to Global Warming Harvey (1993a_Energy_Policy_GWPs)

8/12/2019 A Guide to Global Warming Harvey (1993a_Energy_Policy_GWPs)

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A guide to global w arm ingpotentials GWPs)L D Danny Harvey

In order to quant i tat ive ly compare the green-house e f fec t of di f ferent greenhouse gases ag lobal warming po ten t ia l G W P) index has beenused which is based on the ratio of the radiativeforc ing o f an equal emis s ion o f two d i f f e ren tgases, integrated either over all t ime or up to anarbi trari ly determ ined t ime hor izon. The GW Pindex is analogous to the oz on e deple t ing poten -t ia l OD P) index . How ever , the G W P index issubject to major conceptual dif f iculties arisingf ro m the fac t tha t the a tm ospher ic l i fe span forpar t o f the emit ted C0 2 is, fo r al l pract icalpurposes, infinite. In addition, there are majoruncertainties in the atmospheric l i fespans andindirect heat ing e f fec ts of the important green-ho use gases, wh ich are review ed here . A nal ternative G W P ind ex is pro po sed which expli -cit ly takes into account the duration of capitalinves tments in the energy sector and is lesssensitive to uncertainties in atm osph eric l i fespansand radiat ive heat ing than the usual GWP indexfor t ime hor izons longer than the l i fespan of thecapita l inves tment . The e f f ec t o f the G W P indexpro pose d here , com pare d wi th previo us indices,is to shi f t a t tent ion awa y fr om shor t l ived gasessuch as me thane and toward C02 .Keywords Global warming potent ial index Capital investment;CO_,

H u m a n a c t iv i ti e s h a v e l e d t o i n c r e a s e s i n t h e c o n c e n -t r a t i o n o f a n u m b e r o f g a s e s w h i c h a r e e f f e c t i v e i na b s o r b i n g a n d r e e m i t t i n g r a d i a t i o n i n t h e i n f r a r e dp a r t o f t h e e l e c t r o m a g n e t i c s p e c t r u m , l e a d i n g to as t r e n g th e n i n g o f t h e s o - c a l l e d g r e e n h o u s e e f f ec t .T h e p r i m a r y g a s e s r e s p o n s i b l e f o r t h i s s tr e n g t h e n i n ga r e c a r b o n d i o x i d e C O 2 ) , m e t h a n e C H 4 ) , t r o p o -s p h e r i c o z o n e O 3 ) , t h e c h l o r o f l u o r o c a r b o n s C F C s )

L.D. Danny Harvey is w it h the D epar t m en t o f Ge o~ ap hy ,Univers i ty of To ronto , 100 S t Geor ge S t reet , Toro nto ,Canada M5S 1A1.

a n d n i t r o u s o x i d e N 2 0 ) . M a n y o f th e p r o p o s e dr e p l a c e m e n t s f o r C F C s , s u c h a s t h e H C F C s a n dH F C s , a r e a l s o g r e e n h o u s e g a s e s . O t h e r g a s e s , s u c ha s c a r b o n m o n o x i d e C O ) an d th e n it r o g e n o x i d e sN O x ) , a s w e l l a s h y d r o c a r b o n s H C s ) , a r e n o t

t h e m s e l v e s g r e e n h o u s e g a s e s b u t i n f l u e n c e t h e c o n -c e n t r a t i o n o f o n e o r m o r e g r e e n h o u s e g a s e s a n d t h u si n d i re c t ly a f f e c t t h e s t r e n g t h o f t h e g r e e n h o u s ee f f ec t .Cer ta in cos t s w i l l be en ta i l ed in r educ ing thee m i s s i o n s o f a n y o n e o f t h e s e g a s es w h e t h e r e c o n o -m i c o r o t h e r k i n d s o f c os t s) . I n d e v e l o p i n g a n o v e r a l ls t r a te g y t o r e d u c e e m i s si o n s o f g r e e n h o u s e g a s e s , ti s i m p o r t a n t t o b e a b l e to q u a n t i t a t i v e l y c o m p a r e t h ec o n t r i b u t i o n t o t h e g r e e n h o u s e e f f e c t o f a g i v e nq u a n t i t y o f e a c h o f t h e g a s e s. T h e r e la t iv e u n i tc o n t r i b u t i o n o f e a c h g a s t o t h e g r e e n h o u s e e f f e c t c a nt h e n b e c o m p a r e d w i t h th e u n i t c o s t o f e m i s s io nreduc t ion in in s tances w here the re i s a t r ade o f fb e t w e e n r e d u c i n g e m i s s i o n s o f o n e g a s m o r e a n da n o t h e r g a s l e s s , o r w h e r e m e a s u r e s t o r e d u c eemis s ions o f one gas migh t l ead to an inc reas e ine m i s s i o n s o f a n o t h e r g a s .U n f o r t u n a t e l y , t h e t a s k o f q u a n t i t a t i v e ly c o m p a r -i n g t h e g r e e n h o u s e e f f e c t iv e n e s s o f d i f fe r e n t g a s e s i sf r a u g h t w i t h n u m e r o u s d i f f i c u l t i e s , b o t h c o n c e p t u a land ana ly t i c . Thes e d i f f i cu l t i e s a r i s e f rom the f ac tt h a t t h e h e a t t r a p p i n g a b i l i ty o f a g i v e n g as d e p e n d so n a n u m b e r o f p a r a m e t e r s w h i c h c h a n g e t h ro u g ht ime , tha t ind i r ec t chem ica l e f f ec t s a r e invo lved and ,m o s t i m p o r t a n t l y , t h a t t h e a v e r a g e l i f e s p a n i n t h ea t m o s p h e r e o f e a c h g a s is d i f f e re n t . T h u s , i f e q u a lq u a n t i t ie s o f e a c h g a s a r e e m i t t e d i n t o t h e a t m o s -p h e r e a t a g i v e n ti m e o r c o n v e r s e ly , e q u a l e m i s si o n sa r e a v o i d e d ) , t h e r e la t i v e a m o u n t s o f t h e g a s e sr e m a i n i n g i n t h e a t m o s p h e r e c o n t i n u o u s l y c h a n g et h r o u g h t i m e .S e v e r a l a u t h o r s h a v e a t t e m p t e d t o o v e r c o m e t h ep r o b l e m o f d i f f e r i n g l i f e s p a n s b y i n t r o d u c i n g a ni n d e x b a s e d o n t h e r a t i o o f r a d ia t i v e f o r c in g b yd i f f e r en t g as es in teg ra ted ov e r a spec i f ic t imein te rva l . 1 Th e expe c ta t ion i s tha t s uch ind ices w i ll beu s e d b y p o l i c y m a k e r s d u r i n g t h e n e g o t i a t io n o f

4 0301 4215/93/010024 11 1993 Butterworth Heinemann Ltd


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overall limitations of greenhouse gases, either topermit trading between gases or to allow a givenstate to choose the specific combination of green-house gas emission reductions that it wishes inachieving a given reduction in total greenhouseheating. 2 The purpose of this paper is to argue thatthe conceptual difficulties and scientific uncertain-ties in computing GWPs are so large, and willremain so large, that such indices have no practicalvalue in the applications suggested above. Analternative conceptual framework to calculatingGWPs is proposed which is robust to scientificuncertainties and which can be applied to the morelimited problem of evaluating the net effect ofdifferent fossil fuel using technologies when alterna-tive technologies emit more than one greenhouse gasbut in different proportions.

T he pro lemGiven that the relative greenhouse effect of equalemissions of different gases changes over time, oneway to quantit atively compare two or more gases isto compare the i n t e g r a t e d heating effect over alltime. If C i t ) is the concentration of gas i at time tfollowing the emission of a unit amount at time t =0, f,-(t) is the heat trapping ability at the same timeper unit of concentration, and C c t ) and f c t ) thecorresponding quantities for CO2, then the GWP ofgas i relative to CO2 could be given by

f ~ f ) C i ) d tG W P = (1)f ~ f c t ) C ~ t ) d tA similar approach is used in calculating the ozonedepleting potential (ODP) of different ozone deplet-ing gases: one compares the ozone depletion per unitof concentration o f two gases, times the concentra-tion remaining after a pulse input, integrated over alltime as the concentrations of the gases continouslydecrease. For a linear response to CFC emissions,this is mathematically equivalent to considering acontinuous and equal rate of emission of two differ-ent gases, and comparing the steady state ozonedepletion resulting from the two gases.There is no conceptual problem in making thiscomparison (whether for ODPs or GWPs) as long asboth gases have a finite atmospheric lifetime or,conversely, as long as both gases asymptote to asteady state concentration given a continuous andconstant rate of emission. This condition is satisfiedfor all gases of interest e x c e p t CO2. Unlike all othergases emitted in the atmosphere by humans, CO2does not have a chemical or photochemical sink

A guide to global warmingpotentials (GW Ps)within the atmosphere itself. Removal of CO2 isthere fore depe ndent on exchanges with other carbonreservoirs. Following the emission of a hypotheticalpulse of CO2 into the atmosphere, about 10 will beremove d within two years through gaseous diffusioninto the mixed layer of the ocean. Subsequentremoval is dependent on downward mixing into thedeeper ocean. Ultimately, about 85 of the initialpulse can be removed by mixing into the ocean, butonly over a time period of several hundred to athous and years. 3 Anoth er 10 of the initial pulsewill be removed by dissolution of marine carbonatesediments, but over a period of several thousandyears. 4 The remaining 5 or so is removed over aperi od of 100 000 years t hrou gh silicate rockweathering. 5

Three-dimensional models of ocean chemistry,circulation and deep mixing have been used todetermine the rate of decrease in concentration C ofa pulse of CO2, neglecting the geological timescaleprocesses of carbonate sediment dissolution andsilicate rock weathering. The concentration of re-maining CO2 following a unit pulse emission can beconveniently approximated by: 6

tC c t ) = Z ~ = o A i exp (--~) (2)where the values of A i and zi are given in Table 1.Since t0 =*c this can be written as

C c( ' ) = A + y~4=lAi exp ( - ~-T (2)

As t--* ~ , a fraction A0 of the initial impulse remainsin the atmosphere, all other terms in Equation (3)decaying to zero. As previously noted , thi s CO2 willeventually be removed by carbonate dissolution andsilicate rock weathering, but t he rate of removal is soslow that it is zero for all practical purposes. Thus,for any constant non-negligible anthropogenic emis-sion rate, the CO2 concentration does not asymptoteT a M e 1 . V a l u e s o f t h e c o n s t a n t s A i a nd ~ i n E q u a t i o n ( 2 ) , a s g i v e ni n M a i e r R e i m e r a n d H a s s e l m a n n .

0 0 . 1 3 11 0 . 2 0 1 3 6 2 . 92 0 . 3 2 1 7 3 . 63 0 . 2 4 9 1 7 . 34 0 . 0 9 8 1 . 9

Source: E . M a i e r - R e i m e r a n d K . H a s s e l m a n n , T r a n s p o r t an ds t o r a g e o f C O 2 i n t h e o c e a n - a n i n o r g a n i c o c e a n c i r c u la t i o nc a r b o n c y c l e , ClimateDynamics, V o l 2 , 1 9 8 7 , p p 6 3 - - 9 0 .E N E R G Y P O L I C Y J a nu a ry 9 9 3 2 5


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A g u id e t o g lo b a l wa rmin g p o t en tia ls GW Ps)tO a s t ead y s t a t e va lue , b u t inc r eas es inde f in i t e ly .H e n c e , t h e l o w e r i n t e g r a l i n E q u a t i o n ( 1) is i n fi n it e .

T h is p r o b l e m h a s b e e n r e s o lv e d i n t w o w a y s . O n ei s to s e t Xo < ~ , w h ich i s equ iva le n t to a s s um ing tha tt h e a s y m p t o t i c a i r b o r n e f r a c ti o n A o d e c r e a s e st h r o u g h t i m e . L a s h o f a n d A j u h a , f o r e x a m p l e , s e tA o = 1 0 0 0 y e a r s . 7 H o w e v e r , a t t h e c e n t u r y t i m e s c a l eo f h u m a n i n t e r e s t, A o w i ll a c t u a l ly i n c r e a s e w i t h t i m eb e c a u s e o f th e i n c r e a s e i n t h e o c e a n i c b u f f e r f a c t o r ,s o th a t t h e a b i li t y o f t h e o c e a n t o a b s o r b C O 2 i n p u t st o t h e a t m o s p h e r e d e c r e a s e s . O n l y a t t h e t i m e s c a l eo f s e v e r a l o c e a n i c o v e r t u r n i n g s , o f th e o r d e r o fs e v e r a l t h o u s a n d y e a r s , w i ll A o d e c r e a s e , a s a r e su l to f d is s o l u t io n o f c a r b o n a t e s e d i m e n t s . R e s o l v i n g t h ep r o b l e m o f i n f in i te i n t e g r al s b y l e tt i n g A o d e c a y t oz e r o w i t h t h e r e l a t i v e l y s h o r t t i m e c o n s t a n t o f at h o u s a n d y e a r s s e r v e s t o u n d e r e s t i m a t e t h e l o n g -t e r m i m p o r t a n c e o f C O 2 e m i s s i o n s r e l a ti v e t o e m i s -s i o n s o f o t h e r g r e e n h o u s e g a s e s .

A n a l t e r n a t iv e s o l u t i o n t o t h i s p r o b l e m is t o r e t a into = ~ bu t to in t eg ra te the r ad ia t ive fo r c ing a r i s ingf r o m a n i m p u l s e i n p u t o n l y u p t o a t i m e h o r i z o n T .T h u s ,

S ~ f i t ) C i t ) d tG W P = (4)J orf (t) Cc (t) dtT h is a p p r o a c h h a s b e e n a d o p t e d b y t h e I n te r -g o v e r n m e n t a l P a n e l o n C l i m a t ic C h a n g e ( I P C C ) a n di n s u b s e q u e n t w o r k b y L a s h o f a n d c o w o r k e r s p u b -l is he d b y t h e A d v i s o r y G r o u p o n G r e e n h o u s e G a s e s( A G G G ) . s T he G W P s o d e fi n ed d e p e n d s o n t h echo ice o f t ime ho r i zon : fo r gas es hav ing a s ho r t e ra t m o s p h e r i c r e s i d e n c e t i m e t h a n t h e a v e r a g e f o rC O 2 , G W P d e c r e a s e s a s l o n g e r t i m e h o r i z o n s a rec o n s i d e r e d .T h e f o r e g o i n g o u t l i n e s t h e c o n c e p t u a l p r o b l e m i nd e t e r m i n i n g G W P s f o r d i f f e r e n t g a s e s. I n t h ea p p r o a c h e s u s e d s o f a r , t h e c o n c e p t u a l p r o b l e mr e d u c e s t o d e c i d i n g o n a c h o i c e o f t i m e h o r i z o n .S h o r t t im e h o r i z o n s a r e a p p r o p r i a t e i f o n e i s m o r ec o n c e r n e d w i t h p o t e n t i a l c l im a t i c c h a n g e s d u ri n g t h en e x t f e w d e c a d e s o r i f o n e i s c o n c e r n e d w i t h th e r a t eo f c l im a t i c c h a n g e , w h e r e t h e r a p i d b u i l d - u p o f s h o r tl iv e d g r e e n h o u s e g a s e s w o u l d t e n d t o p r o v o k e i n it ia lr a p i d r a t e s o f w a r m i n g . L o n g e r t i m e h o r i z o n s a r em o r e a p p r o p r i a t e i f o n e i s m o r e c o n c e r n e d a b o u tl o n g - t e rm , c h r o n i c e f f e c t s o f c l im a t i c w a r m i n g ( s u c ha s s e a le v e l ri s e ). T h e r e a r e , h o w e v e r , a w h o l e s e r i e so f unce r t a in t i e s d ea l ing w i th the ha rd s c ience - inpa r t i cu la r , de te rmin ing the d i r ec t and ind i r ec t r ad ia -t ive hea t ing e f f ec t s o f d i f f e r en t gas es ( the f,- va luesa b o v e ) a n d t h e t im e c o n s t a n t s f o r r e m o v a l fr o m t h ea t m o s p h e r e o f C O 2 a n d o t h e r g a s e s ( th e x v a l u e s

T a b l e 2 R a d i a t i v e h e a t i n g e ff e c t p e r m o l e c u l e a n d p e r u n i t m a s s o fv a r i o u s g r e e n h o u s e g a s e s r e l a t i v e t o C O 2B e a t i n gr e l a t i v e P e rt o C O 2 p e r u n i tT r a c e g a s m o l e c u l e m a s sCO: 1 1

CH4 26 72N2 0 206 206CFC-11 12 400 3970CFC-12 15 800 5750HCF C-22 10 700 5440CFaBr 16 000 4730

a b o v e ) . T h e s e u n c e r t a i n ti e s a r e b r i e f ly o u t l in e db e l o w .

U n c e r t a i n t i e s i n c a l c u l a t i o nR a d i a t i v e e f f e c t sT h e g l o b a l ly a n d a n n u a l l y a v e r a g e d t r a p p i n g o fi n f r a re d r a d i a t i o n a t t h e t r o p o p a u s e ( t he b a s e o f t h es t r a t o s p h e r e ) p r o v i d e s a f ir s t o r d e r e s t i m a t e o f th eo v e r a l l w a r m i n g e f f e c t o f a g i v e n g r e e n h o u s e g a s.T h i s t ra p p i n g i n v o l v e s t w o c o m p o n e n t s : a d e c r e a s ei n u p w a r d r a d i a t i o n f r o m t h e t r o p o s p h e r e ( w h i c hunder l i e s the s t r a to s phere ) and an inc reas e in dow n-w a r d r a d i a t io n f r o m t h e s t r a t o s p h e r e . T h e n e t e f f e c to n i n f r a r e d r a d i a t i o n d e p e n d s o n : abs o rp t ion and emis s ion coe f f i c i en t s a s a func -t io n o f g a s a m o u n t , p r e s s u r e , t e m p e r a t u r e , a n d

w a v e l e n g t h , w h i c h c a n b e d e t e r m i n e d t o h i g ha c c u r a c y f r o m l a b o r a t o r y s p e c t r o s c o p i cm e a s u r e m e n t s ;

t h e d e g r e e o f o v e r l a p i n t h e a b s o r p t i o n o f t h eg a s in q u e s t i o n w i t h a b s o r p t i o n b y o t h e r g a s e s ,a n d h e n c e o n t h e c o n c e n t r a t i o n s o f a l l o t h e rgas es ;

t h e a t m o s p h e r i c t e m p e r a t u r e p r o f il e a n dc loud ines s .

T h e a b s o r p t i o n c o e f f i c i e n t s o f m a n y g a s e s d o n o tva ry l inea r ly w i th gas conc en t r a t io n : the r ad ia t ivehea t ing o f a un i t o f a g iven gas thus dep end s on thep r e e x i s t i n g c o n c e n t r a t i o n o f th e g a s a n d t h u sc h a n g e s t h r o u g h t i m e . I t a l s o d e p e n d s o n t h e c o n -c e n t r a t i o n s o f o t h e r g a s e s w h i c h a b s o r b i n t h e s a m es pec t r a l r eg ions . F o r thes e r eas ons , the hea t t r ap -p ing ab i l i ty j~ dep end s on the s cena r io s o f fu tu r ec o n c e n t r a t i o n s o f m o s t o r a l l o f t h e g r e e n h o u s eg a s e s. A l l c a l c u la t i o ns o f G W P t o d a t e h a v e a s s u m e dt ime inva r i an t f i v a l u e s , a p p r o p r i a t e t o p r e s e n t d a ycond i t ions .T a b l e 2 c o m p a r e s t h e a v e r a g e r a d i a t i v e h e a t i n gp e r m o l e c u l e o f va r i o u s g a s e s re l a t iv e t o C O 2 , a s

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determined by the IPCC except for CH4, which isbased on Lelieveld and Crutzen. 9 The values givenin Table 2 assume the gases to be uniformly mixed inthe atmosphere. This is a valid assumption for thegases shown in Table 2, but not for O3, whoseconcentration varies substantially in all three dimen-sions. The effectiveness of a given tropospheric gasin reducing upward infrared radiation at the tropo-pause is greater the colder it is relative to thesurface; removal of an ozone molecule in the uppertroposphere (which is coldest relative to the surface)causes a much greater decrease in the greenhouseeffect than removal of an ozone molecule near thesurface. In the case of stratospheric ozone, an ozonedecrease in the lower stratosphere is more effectivein reducing downward radiation at the tropopausethan a decrease in the middle or upper stratosphere.Offsetting this decrease to varying degrees is anincrease in downward solar radiation at the tropo-pause. H ence, the net effect of ozone losses dependscritically on the vertical distribution of changes, andcan be one of hea ting or cooling. 1A t m o s p h e r i c l i f e t i m eC 0 2 . Observations from polar ice cores indicatethat the atmospheric CO2 concentration was close toconstant for several hundred years prior to theindustrial revolut ion.~ Since 1800, the atmosphericCO2 concentra tion has increased by 25 and thereis no doubt that this increase is due to anthropogenicactivities. However, the observed increase - theso-called a i r b o r n e f r a c t i o n - is only about 50 of thecumulative emissions since 1800. The remaining50 has already been absorbed by some combina-tion of the oceans and terrestrial biosphere. Unfor-tunately, we are not able to determine the relativeimportance of these two sinks. Some evidence indi-cates that the oceans could not have absorbed morethan 1/3 of the non-airborne CO2 and that thenorthern temperate latitude biosphere is a majorcarbon sink. 12 This, however, is contradic ted byisotopic a3C data, which indicate that the northernhigh latitude oceans are a larger sink than thenorthern biosphere. 13

Determi nation of the current CO2 sinks, and howthese sinks are likely to change with increasing CO2emissions, is crucial to the calculation of GWPs. Ifthe primary sink is the terrestrial biosphere thencontinuing forest des truction , nutrient limitations ortemperature induced increases of respiration coulddramatically increase the lifetime of CO2 in theatmosphere.~4 If the primary sink is the oceans , thenclimatic change-induced changes in ocean circulationor biological product ivity could significantly alter the

A guide to global warming potentials GW Ps)atmospheric CO2 lifetime. Sarmiento and Toggweil-er, and Baes and Killough concluded that decreasedoceanic overturning would reduce atmospheric CO2,while Baes, and Broecker and Takahashi suggest theopposite.15 Uncert ainty in the removal rates of COzfrom the atmosphere is a major source of uncertain-ty in the calculation of GWPs over a time horizon ofthe next few decades.C H 4 . The primary sink of methane is reaction withatmospheric OH, with soils serving as an additionalsmall sink. a6 The average lifetime of a methanemolecule is 8-12 years, based on the global averageOH concentration, which is computed from thedistribution and concentration trend for methylchloroform. 17 However, the exact value is sensitiveto the atmospheric OH distribution. It is assumedthat CHaCC13 is destroyed only by reaction withOH, but recent work suggests that some CH3CCI 3 istaken up by the ocean, is Such additional loss couldmean that the inferred OH concentration is 5-20too large, and hence that the lifetime of moleculeslost solely by reaction with OH is 5-20 longer thancurrently estimated. In addition, a recent revision ofthe CH4-OH rate constant suggests a further 25increase in the estimated lifetime of CH4.19The OH concentration itself depends in complexways on emissions of CH~, thus introducing a feed-back between CH4 concentration and its GWP.Uncertainties in the CH4-OH coupling introduce afurther uncertainty in the CH4 GWP of about20-30 over a 50 year time horizon. 2 Other chem-ical effects on the CH4 GWP are discussed below.N 2 0 . The major sink of N20 is photochemicaldestruction in the stratosphere, and its estimatedlifetime has recently been revised from 150 years to110 years. 21 The main sources appear to be theoceans, denitrification in aerobic soils, and combus-tion and biomass burning. However, we cannot atpresent balance the N20 budget, which introducesuncertainties in determining its present atmosphericlifetime. As with CO2, it is possible that changes inocean circulation or biological productivity as cli-mate changes could significantly alter its atmospher-ic lifetime,H a l o c a r b o n s . Although the most ozone damagingchemicals will be strongly controlled over the com-ing years, many of the proposed substitutes areimportant greenhouse gases. Most of these arehydrochlorofluorocarbons (HCFCs), and are des-troyed principally by reaction with troposphericOH. 22 Their lifetime will therefo re change in re-

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latitudes during this period masked half of theincremental heating due to increase of CO2 duringthe same period. 32 Limi ted data from a single south-ern hemisphere mid-latitude location also imply asurface cooling due to ozone changes, while indirectevidence suggests that ozone changes at low lati-tudes have also had a net cooling effect. Since thedirect heating effect of CFC increases between 1970and 1980 was only 40 that due to the CO,_increase, 33 the net effect of CFCs could be one ofcooling. As discussed above, the net effect of ozonechanges depends critically on the vertical distribu-tion of changes, as ozone losses above 30 km have anet warming effect due to the fact that the increasedpenetration of solar radiation more than compen-sates the reduction in infrared emission to the tropo-sphere. Further complicating the picture is the factthat stratospheric ozone loss allows an increase inpenetration of ultraviolet radiation to the tropo-sphere, leading to increased tropospheric ozone andproduction of OH, thereby tending to reduce themean atmospheric methane lifespan. 3'* There are,therefore, large uncertainties in the determination ofthe net radiative effect of ozone changes and hencein the net effect of CFCs.CO CO emissions, like those of methane, affecttropospheric OH and ozone concentrations and thusindirectly affect greenhouse heating. Three-dimensional model simulations suggest that anthro-pogenic CO emissions have decreased atmosphericOH at low latitudes and in the southern hemisphere,but have increased atmospheric OH north of20 N. 3s The mean atmospher ic lifetime of CO isabout two months, which is comparable to atmos-pheric mixing times, so that the average global effectof CO emissions depends on where the emissionsoccur. As with methane, CO is ultimately oxidizedto C02.NOx NOx emissions contribute to troposphericozone formation and hence indirectly add to thegreenhouse effect, but also tend to increase atmos-pheric OH, which will tend to shorten the lifetime ofCH4 and other greenhouse gases, thereby reducingthe greenhouse effect. The extent and even occur-rence of ozone formation associated with NOx emis-sions is highly dependent on the regional atmospher-ic chemistry, so that, like CO, there is no singleGWP for NOx which can be applied everywhere.Furthermore, NO~ emissions in the upper tropo-sphere (from aircraft) are about 20 times moreeffective in producing 03 than are surface emissions,and upper tropospheric ozone is about 1.3 times

A guide to global warming potentials GW Ps)T a b l e 3 A t m o s p h e r i c l if e s p a n s a n d g l o b a l w a r m i n g p o te n t i a ls p e ru n i t m a s s a s c o m p u t e d b y t h e I P C C u p d a t e f o r t i m e h o r i zo n s of 20 ,1 0 0 a n d 5 0 0 y e a r s

A v e r a g e T i m el i f e s p a n h o r i z o nG a s y e a r s ) 2 0 y ea rs 1 0 0 y ea rs 5 0 0 y ea rsCH4 10.5 35 11 4N2 0 132 260 270 170CFC -11 55 4500 3400 1400CFC -12 116 7100 7100 4300H C F C - 2 2 1 5 . 8 4 2 0 0 1 6 00 5 4 0

more effective, on a molecule-per-molecule basis, intrapping infrared radiation than surface ozone. Con-sequently, NO~ emissions from high flying aircraftare calculated to have about 30 times the warmingeffect of equal emissions from surface sources. 36Direct GWPsTable 3 gives GWPs fo r CH4, N20 , CFC-11, CFC-12and HCFC-22 as computed by the IPCC update ; 37these GWPs include only direct radiative effects. Arevision in the estimated lifespan of CO2, orclimate-carbon cycle feedbacks, could dramaticallychange all of the GWPs given in Table 3.

Potent ia l appl ica tions o f W PsGiven the enormous uncertainties in the calculationof GWPs and their dependenc e on future scenarios,it is difficult to see how they could be used in anyrigorous way for policy analysis. The US administra-tion advocates a basket approach to any greenhousegas emission reductions, whereby any nation canreduce the mix of greenhouse gases giving an agreednet effect. This requires the ability to quantitativelycompare different greenhouse gases. As indicatedabove, we are far from the point where reliableintercomparisons can be made for a given timehorizon. Indeed, it is extremely unlikely that we willbe able to predict all of the changes affecting theatmospheric lifetime of CO2 and other greenhousegases; such changes as do occur are likely to occur ina non-uniform manner, implying strong non-linearities in the response to successive emissionincrements. Oceanic circulation, changes in whichcould significantly affect the mean atmospheric CO2lifetime, can change dramatically following subtlechanges in precipitation and evaporation fields. 3s Inaddition, the problem of which time horizon to useremains and is not subject to scientific determina-tion. A further problem with this potential applica-tion of GWPs is the great difficulty in quantifyingand monitoring the sources and sinks of most gases,as discussed by Victor. 39

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A guide to g lobal warm ing poten t ia ls (GW Ps)A n o t h e r p o t e n t i a l a p p l i c a t i o n o f G W P s i s i n a s s es -

s i n g t h e n e t e f f e c t o f m e a s u r e s w h i c h r e d u c e e m i s -s i o n s o f o n e g r e e n h o u s e g a s b u t i n c r e a s e e m i s s io n so f a n o t h e r g a s . F o r e x a m p l e , s w i tc h i n g f r o m c o a l t ona tu ra l gas fo r e l ec t r ic i ty gen era t io n m i g h t l e a d t o a ni n c r e a s e o f m e t h a n e e m i s s io n s d e p e n d i n g o n t h er e l a ti v e m a g n i t u d e o f m e t h a n e l e a k a g e f r o m n a t u r a lg a s d i s t ri b u t i o n a n d m e t h a n e s e e p a g e f r o m c o a lmin ing ) w h i le s ign i f i can t ly r educ ing CO 2 emis s ions .Th i s po ten t i a l app l i ca t ion s ugges t s an a l t e rna t iveG W P i n d e x ti e d t o t h e l i f et i m e o f th e p a r t i c u l a ri n v e s t m e n t d e c i s i o n u n d e r c o n s i d e r a t i o n .

n a l t er n a t iv e G W P i n d e xT h e c h a n g e i n c o n c e n t r a t i o n y ( t ) of a g iven gasfo l low ing a con t inuous emis s ion a t a r a t e x ( t ) i s g ivenb y t h e c o n v o l u t i o n o f t h e e m i s s i o n r a t e w i t h t h ei m p u l s e r e s p o n s e , C ( t ) . Tha t i s ,

y ( t ) = f t o X ( t ) C ( t - t ) d t 5 )E q u a t i o n 5 ) a s s u m e s a l i n e a r r e l a t io n s h i p b e t w e e ne m i s s i o n a n d c o n c e n t r a t i o n , s o t h a t s u c c e s s iv ee m i s s i o n p u l s e s h a v e t h e s a m e i n c r e m e n t a l e f f e c to n c o n c e n t r a t i o n . F o r t h e c a s e o f C H 4 , C ( t ) c a n b er e p r e s e n t e d b y a s i m p l e e x p o n e n t i a l d e c a y ,e x p ( - t / x ) , s o t h a t E q u a t i o n 5 ) y i e ld s

ty cH 4 c l e x pfor a cons tant emiss ion ra te xcr~4. As t - -~ o~ , them e t h a n e c o n c e n t ra t io n a p p r o a c h e s t h e s t e a d y s t a teva lue X X cn4. F o r the cas e o f CO 2 , C ( t ) c a n b e g i v e nb y E q u a t i o n 3 ) , s o t h a t th e C O 2 c o n c e n t r a t i o nchange i s

ty c o t - -x c o 2 1 e x pw h ere X co2 is a cons tan t CO emis s ion r a te . I f ther a d i a t i v e f o r c i n g p e r m o l e c u l e i s c o n s t a n t i n t i m e ,t h e G W P a s d e f i n e d b y E q u a t i o n 4 ) is m a t h e m a t i -c a ll y e q u i v a l e n t t o c o m p a r i n g t h e r a d i a t i v e h e a t i n go f t w o g a s e s a t ti m e T a f t e r a c o n t i n u o u s a n d e q u a le m i s s i o n o f b o t h g a s e s u p t o t i m e T . T h u s , f o r C H 4t h e G W P a s g i v e n b y E q u a t i o n 4 ) is e q u i v a l e n t , f o rt h e s e c o n d i t i o n s , t o

y c . 4 T ) f cr a4Y c o 2 T ) f c o 2

w h e r e fc n 4 a n d fco are t h e t i m e i n v a r i a n t r a d i a t iv e

f o r ci n g s p e r u n i t c o n c e n t r a t i o n . I n t h e m o r e g e n e r a lc a s e w h e r e t h e r a d i a t i v e f o r c in g p e r m o l e c u l e v a r i e sw i t h t im e , t h e G W P i s g i v e n b y

f ~ f i ( t ) C i ( T - t ) d tG W P ( t ) = f r f c (t ) C ( T _ t , ) d t, 6)

w here )~ t ) a n d f c ( t ) a r e t h e t i m e d e p e n d e n t r a d i a t i v efo rc ings fo r gas i and CO 2 , and x i ( t ) = x c ( t ) = X o a n dt h u s ca n c el f ro m t h e n u m e r a t o r a n d d e n o m i n a t o r o ft h e a b o v e e x p r e s s i o n .C a s t i n g t h e G W P i n t h e f o r m g i v e n b y E q u a t i o n6 ) r e p r e s e n t s a n i m p o r t a n t c o n c e p t u a l s h i f t f r o mt h a t o f E q u a t i o n 4 ). R a t h e r t h a n v ie w i ng t h e G W P

as the r a t io o f the in teg ra te d r ad ia t ive hea t ing due toa s ing le pu l s e em is s ion o f bo th gas es a t t ime t = 0 , i tc a n b e v i e w e d a s b e i n g a p p r o x i m a t e l y e q u a l t o t h era t io o f in s tan taneo us r ad ia t ive fo rc ings a t the en d o fa g i v e n t i m e p e r i o d , a s s u m i n g e q u a l a n d c o n t i n u o u semis s ions du r ing the e n t i r e t ime in te rva l o f in te r es t .

I n the cas e o f inves tm en t dec i s ions invo lv ing emis -s i o n s o f t w o o r m o r e g a s e s o v e r a f i n it e p e r i o d o ft i m e , t h e t i m e h o r i z o n T s h o u l d b e c h o se n e q u a l t othe l i f e s pan o f the end -us e t ec hno logy . I f one i si n t e r e s t e d i n t h e r e l a t i v e g r e e n h o u s e i m p a c t o fs w i tch ing f rom o i l to na tu ra l gas fo r au tomob i le s ,h o m e h e a t i n g , o r e le c t r ic i t y g e n e r a t i o n , t a k i n g i n t oa c c o u n t t h e g r e e n h o u s e f o r c in g o f C H 4 , t h e n T = 1 0 ,2 0 a n d 4 0 y e a r s r e s p e c t i v e ly a r e r e a s o n a b l e c h o i c e s .

I f o n e is c o n c e r n e d w i t h t h e g r e e n h o u s e i m p l ic a -t i o n s o f a g i v e n i n v e s t m e n t d e c i s io n b e y o n d t h el i f e time T o f the dec i s ion , then i t i s app rop r ia te toa s s u m e t h a t t h e e m i s s i o n ra t e x ( t ) = 0 for t > T. Th atis , i n a s s e s s i n g t h e r e l a ti v e g r e e n h o u s e i m p a c t o fd i f fe r e n t i n v e s t m e n t o p t i o n s , o n e s h o u l d a s s u m e t h a te m i s s i o n s o f a l l t h e a s s o c i a te d g r e e n h o u s e g a s e s o c c u ro n l y f o r a s l o n g a s t h e l i f e ti m e o f t he i n v e s t m e n td e c i s i o n , e v e n i f o n e i s i n t e re s t e d i n l o n g e r t i m eh o r i z o n s . I f i t i s dec id ed to r ep lac e an o ld fo s s il f ue lu s i ng e n d - u s e t e c h n o l o g y a t t h e e n d o f it s e c o n o m i -ca l ly u s e fu l l if e w i th a ne w fos s i l f ue l u s ing t echn o lo -g y , t h a t r e p r e s e n t s a s e p a r a t e i n v e s t m e n t d e c i s i o n ,t h e i m p a c t o f w h i c h s h o u l d n o t b e i n c o r p o r a t e d i nana lys i s o f the g ree nho us e imp l ica t ions o f the f i r s ti n v e s t m e n t d e c i s io n . F u r t h e r m o r e , i t is p o s s i b l e th a ta fo ss i l f ue l t ech no log y in s ta l l ed now m igh t ber e p l a c e d w i t h a n o n - f o ss i l f ue l t e c h n o l o g y a t t h e e n dof i ts l i fe .B a s e d o n t h e a b o v e d i s c u ss io n , th e G W P f o r t i m e st _> T is given by

S ~ f i ( t ) C i ( t - t ) d tGWP t) = S T f c ( t ) C c ( t _ t ) d t t > - T 7)o f w h ich Equ a t io n 6 ) is a spec ia l cas e .

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A g u i d e o g l o b a l w a r m i n g o t e n ti a ls ( G W P s )T a b l e 4. Global w a r m i n g p o t e n t ia l s p e r u n i t m a s s f o r C H 4 a n d H C F C 2 2 f o r d i f fe r e n t t im e h o r i z o n s a n d i n v e s t m e n t l i f e ti m e s a s c o m p u t e dusing Equation 7).

T i m ehor iz onye ar s2OInvestmentlifetimeyears) 10 20CH4 26.7 42.7HCFC-22 3093 4036

40 100 500

10 20 40 10 20 40 100 10 20 40 5006.0 9.3 28.3 1.1 1.1 1.2 15.3 1.0 1.0 1.0 6.11104 1509 2949 36 51 110 1628 0.0 0.0 0.0 580

For purposes of illustrating the difference in GWPfor time horizons t beyond the lifetime T of aninvestment decision, assume that j~ t) and f c t ) areconstant. We shall consider the direct GWP ofmethane and HCFC-22 using the relative radiativeheatings and lifetimes given in Tables 2 and 3respectively, and taking into account the CO2 pro-duced from oxidation of methane. The GWP formethane, assuming that emissions occur only duringthe lifetime T of an investment decision, is given by

f C H 4 f ~ C c H 4 ( t- t )x o d t + fc f t o C c ( t - t ) x c ( t ) d tG W P ( t) = f ~ f~ C c ( t- t ) x o d t 8 )

where the CO2 source X c t ) due to methane oxida-tion is given by

f t o a exp ( - - ) )

x c t ) = 9)( ) ( .oexp - - exp -- - 1t > T

Table 4 gives the GWP, as computed by Equation8), for time horizons t of 10, 20, 40, 100 and 500

years. The IPCC/AGGG method implicitly assumesan investment lifetime equal to the time horizon,while for the new method, results are given assuminginvestment lifetimes T of 10, 20 and 40 years.For time horizons equal to the lifetime of theparticular investment decision, the G WP index pro-posed here is equivalent to that of the IPCC. TheGWP values given in Table 4 for this case dif fer fromthe IPCC values given in Table 3 because of the useof a different model for absorption of CO2 by theoceans and because of the inclusion here of the CO2oxidation product in the case of methane. For timehorizons beyond the i nvestment lifetime, the GWPsrapidly fall to one in the case of CH4, or zero in the

case of HCFC-22 indicating in both cases, that onlyCO2 is important), whereas they remain large usingthe IPCC/AGGG approach. The GWPs rapidlyapproach one or zero using the new approach be-cause of the short atmospheric lifespan of CH4 andHCFC-22 relative to CO2 and relative to the timesince the cessation of emissions. For example, theaverage lifespan of CH4 is 10 years, compared toremoval time constants of up to 363 years for CO2.A 100 year time horizon is six mean lifespans beyondthe last emission of CH4 for a 40 year investmentlife, so that there is negligible CH4 remaining. Sincethe cumulative carbon emission is the same for CH4and CO2 emissions, and all the CH 4 is ultimatelyconverted to CO2, the GWP rapidly asymptotes to avalue of 1.0.

An interesting property of the GWP index prop-osed here is that, for gases whose atmosphericlifetime is short compared to that of CO2, it is lesssensitive to changes in the relative radiative forcingper molecule or greenhouse gas lifetimes for timehorizons longer than the investment lifetime. Giventhe large uncertainties in these parameters, thisrobustness is important from a policy point of view.

P r a c t ic a l i m p l i c a ti o n s o f W P sIt is argued above that the scientific uncertainties inthe calculation of GWPs, coupled with the greatdifficulties in measuring anthropogenic emissions ofmost gases, preclude the use of GWPs as part of aninternational agreement involving overall green-house forcing limits or trading between gases. Theonly foreseeable application of GWP is in determin-ing the net effect of fuel switching in order to screenpotential greenhouse gas emission reduction op-tions. In this case, the great uncertainty associatedwith GWPs for time horizons comparable to theinvestment lifetime can be accounted for by allowingonly those fuel switching options which are calcu-lated to reduce total greenhouse forcing by a largefraction. However, when one screens fuel switchingoptions in terms of cost-effectiveness as part of an

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A guide to global warmingpotentials GWPs)i n t e g r a t e d g r e e n h o u s e g a s e m i s s i o n r e d u c t i o ns t r a t e g y , o n e f i n d s th a t t h e n e t g r e e n h o u s e r e d u c t i o no f m a n y o p t i o n s w h i c h p a s s t h e e c o n o m i c t e s t i s s ol a rg e t h a t e v e n t h e m o s t e x t r e m e G W P v a l u e s d o n o tc a u s e t h e f u e l s w i t c h i n g o p t i o n t o b e r e j e c t e d .

T h e e f f e c t i v e C O 2 e m i s s i o n f a c t o r f o r a m i x o fg r e e n h o u s e g a s e s i s g i v e n b y

F eff = E( F c o2 + Z F i G W P i ) 1 0 )w h e r e E i s t h e f o s si l f u e l c o n s u m p t i o n ( G J ) a t t h ep o i n t o f u s e , FCO i s t he CO s emis s ion f ac to r (kg /G J ) , a n d F i a n d G W P i a r e t h e e m i s s i o n f a c t o r s( k g / G J ) a n d G W P s f o r t h e o t h e r e m i t t e d g a s es . T h eCO 2 emis s ion f ac to r s fo r coa l , o i l , and na tu ra l gasa r e 8 8 - 9 5 k g / G J , 6 8 - 7 3 k g / G J a n d 4 9 . 5 k g / G Jr e s p e c t i v e l y . 4 I n c o m p a r i n g d i f f e r e n t f u e l s , a c c o u n ta l s o m u s t b e t a k e n o f t h e e m i s s i o n s o f all g a se sa s s o c i a t e d w i t h t h e e x t r a c t io n , p r o c e s s i n g , a n d t r a n s -p o r t a t i o n o f t h e f u e l t o t h e p o i n t o f u se . T h i sr e q u i r e s m u l t i p ly i n g t h e a b o v e C O 2 e m i s s i o n f a c t o r sb y a v e r a g e m a r k u p f a c t o r s o f 1 . 04 , 1 . 1 3 a n d 1 . 1 8 f o rcoa l , o i l and na tu ra l gas r e s p ec t ive ly . 41 M etha nee m i s s i o n f a c t o r s r a n g e f r o m 0 . 0 1 9 k g / G J f o r t y p i c a ll ign i t e s to 0 .554 kg /G J fo r typ ica l b i tuminous coa l s( b a s e d o n B a r n e s a n d E d m o n d s , 42 a s s u m i n g h e a t i n gva lue s o f 7000 B tu / lb fo r l ign i te and 12000 B tu / lb fo rb i t u m i n o u s c o a l ) , w h i l e e a c h p e r c e n t a g e l e a k a g e o fn a t u r a l g a s c o r r e s p o n d s t o a n e m i s s i o n f a c t o r o f0 . 1 8 2 k g / G J ( b a s e d o n t h e h i g h e r h e a t i n g v a l u e f o rm e t h a n e o f 5 5 M J / k g a n d a s s u m i n g n a t u r a l g a s t o b e1 0 0 m e t h a n e ) . E x a m p l e s o f f u e l s w i tc h i ng w i t hl a r g e e f f e c t i v e C O , e m i s s i o n r e d u c t i o n s a r e g i v e nb e l o w .C o a l t o n a t u r a l g a s f o r e l e c tr i c it y g e n e r a t i o nI f n a t u r a l g a s c o m b i n e d cy c l e ( 4 6 - 4 8 e f f ic i e n c y ) o rc o g e n e r a t i o n ( 6 5 - 9 5 m a r g i n a l e f f ic i e n c y ) r e p l a c e sc o n v e n t i o n a l c o a l - f i r e d e l e c t r i c i t y g e n e r a t i o n ( 3 3e f f i c i e n c y ) , C O 2 e m i s s i o n p e r k i l o w a t t h o u r o f e le c -t r ic i t y is r e d u c e d b y a f a c t o r o f tw o t o f o u r . B e c a u s ea l m o s t t w o t o t h r e e t i m e s m o r e c o a l p r i m a r y e n e r g yi s u s ed than na tu ra l gas p r imary ene rgy in th i sc o m p a r i s o n , m e t h a n e e m i s s i o n s w i l l a l s o b e r e d u c e di f t h e m e t h a n e e m i s s i on f a c t o r f o r n a t u r a l g a s is n om o r e t h a n t w o t o th r e e t i m e s l a r g e r th a n f o r c o a l.N a t u r a l g a s e m i s s i o n f a c t o r s i n W e s t e r n c o u n t r i e sa r e u n d o u b t e d l y s m a l l e r t h a n f o r b i t u m i n o u s c o a l ,w h i c h i s t h e c o a l m o s t o f t e n u s e d f o r e l e c t r i c i t yg e n e r a t i o n .E l e c t r i c r e s i s t a n c e t o h i g h e f f ic i e n c y n a t u r a l g a s f o rh e a t i n gS w i tch ing f rom e lec t r i c r e s i s t ance hea t ing to h igh

e f f i c i e n c y ( 9 2 ) n a t u r a l g as h e a t i n g r e d u c e s C O 2e m i s s i o n s b y a l m o s t a f a c t o r o f f o u r i f t h e e l e c t r ic i t yi s coa l f i r ed . Th i s r educ t ion i s s o l a rge , and ther e d u c t i o n i n p r i m a r y e n e r g y u s e s o l a r g e , t h a t as i g n if i ca n t n e t g r e e n h o u s e e m i s si o n r e d u c t i o n o c c u r se v e n f o r n a tu r a l g a s l e a k a g e r a t e s as l a r ge a s 1 - 2 .M i d - e f f i c i e n c y o i l t o h i g h e f fi c i e n c y n a t u r a l g a s f o rh e a t i n gS w i tch ing f rom a mid -e f f i c i ency (78 ) oi l f u rna ce toa h igh e f f i c i ency na tu ra l gas fu rnace w i ll r ed uce CO 2e m i s s i o n s b y a b o u t 4 0 . T h i s is a l a rg e e n o u g hreduc t ion to g ive a s ign i f i can t ne t bene f i t a f t e ra l l o w i n g f o r l i m i te d ( n o m o r e th a n 1 ) n a t u r a l g a sl e a k a g e a n d a n e x t r e m e ( t w i c e t h e d i r ec t ) G W P f o rm e t h a n e .E l e c t r i c c h i l le r s t o a d v a n c e d n a t u r a l g a s a b s o r p t i o nch i l l er sA n a l t e rna t ive to e l ec t r i c ch i l l e r s ( coe f f i c i en t o fp e r f o r m a n c e ( C O P ) = 3 - 4 ) is a n a d v a n c e d n a t u r a lg a s a b s o r p t i o n c h i l le r ( C O P = 2 - 2 . 5 ) . A l t h o u g h t h ee l e c t r i c c h i l l e r h a s a h i g h e r C O P , g r e a t e r p r i m a r yene rgy i s r equ i r ed pe r un i t o f coo l ing i f t he e l ec t r i c -i t y i s d e r i v e d f r o m a c o n v e n t i o n a l c o a l f i r e d p o w e rp lan t . Th e CO 2 emis s ion r educ t ion fo r th i s s w i tchr a n g e s f r o m 6 4 t o 7 8 . I f n a t u r a l g a s a b s o r p t i o nch i l l e rs d i s p lace e l ec t r i c ch i l le r s pow ered by e l ec t r i c -i t y f r o m c o g e n e r a t i o n , o n t h e o t h e r h a n d , t h e r e m a yb e n o C O z e m i s s io n r e d u ct io n . H o w e v e r , a b s o r p t i o nc h i l l e r s d o n o t r e q u i r e C F C s o r c h l o r i n e c o n t a i n i n gC F C s u b s t i t u t e s ; a l t h o u g h t h e G W P f o r t h e s e g a s e sm i g h t v e r y w e l l b e n e g a t i v e ( i f t h e e f f e c t o f o z o n el o s s i s i n c l u d e d ) , o p t i o n s w h i c h d o n o t r e q u i r ech lo r ine con ta in ing s u bs t i tu t e s a r e l ike ly to be in -c r e a s i n g l y f a v o u r e d t o p r o v i d e g r e a t e r p r o t e c t i o n t ot h e o z o n e l a y e r ( m a n y p r o p o s e d C F C s u b s t i t u t e shave a s ign if i can t O D P on a 10 - -20 yea r t ime ho r i -z o n , w i t h s m a l l O D P s o n l y o n m u c h l o n g e r t i m eh o r i z o n s ) .

I t s h o u l d b e n o t e d t h a t e v e n i n c a s e s w h e r e f u e ls w i t c h i n g f o r h e a t i n g a n d c o o l i n g w o u l d i n c r e a s emethane emis s ions , i t i s s t i l l pos s ib le to ach ieves i m u l t a n e o u s r e d u c t i o n s i n C O 2 a n d m e t h a n e i f f u e ls w i t c h in g i s c o m b i n e d w i th o t h e r m e a s u r e s s u c h a st h e r m a l e n v e l o p e i m p r o v e m e n t s a n d r e d u c t io n o fi n t e r n a l c o o l i n g l o a d s t h r o u g h a d o p t i o n o f m o r ee f f i c i en t l i gh t ing and mach ines . S uch ' bund l ing ' o fm e a s u r e s i s a t t r a c t i v e p u r e l y a s a C O 2 e m i s s i o nr e d u c t i o n m e a s u r e b e c a u s e s a v i n g s i n d o w n s i z i n g o fh e a t i n g a n d c o o l i n g e q u i p m e n t w h e n t h e y a r e d u ef o r r e p l a c e m e n t c a n o f f s e t p a r t o f t h e c o s t o f e n -v e l o p e o r l i g h t i n g i m p r o v e m e n t s . T h u s , i n m o s tcas es invo lv ing fue l s w i tch ing i t is pos s ib le to ach ieve

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s i m u l tan e ou s r e d u c t i on s i n b o th C O a n d m e t h a n eth u s r e n d e r i n g th e GWP i n d e x i r r e l e van t to th ed e c i s i on o f wh e th e r or n o t to swi tc h .

C o n c l u s i o n sT h e c o m p u t a t i o n o f g l o b a l w a rm i n g p o t e n t i a l s(GWPs) i s subject to large uncertaint ies as wel l asconceptual di ff icul t ies , in cont rast to the relat ivelysimple case of ozone deplet ing potent ials (ODPs)which were used in formula t ing in t e rna t io na l agree-men ts to protect the oz one layer. It is difficult to seehow GWPs could be used in any pract ical interna-t ional t reaty or in t rading be twe en gr een hou se gases.On the other hand, i t i s important to be able toquant i tat ive ly compar e the t rade offs associated wi thinves tment dec i s ions which s imul t an eous ly increaseemissions of one gr eenh ouse gas but decrease emis-sions of another. In this case, however, an al terna-t ive GWP t ied to the l i fespan of the investmentdecision i s more appropriate and i s less sensi t ive touncertain t ies in atmos pheri c chemist ry , radiat iveforcing and atmospheric gas l i fespans. The effect ofthe al ternat ive GWP index i s to shi f t a t tent ion morestrong ly toward CO2. In retro spe ct , this is notsurprising, given the uni que nes s of CO2 amo nggreenhouse gases ari s ing from the absence of achemical or photochemical s ink in the atmosphere,and the much longer t imescales associated wi th i t sf inal removal from the atmosphere.

This paper benefited from discussions with Martin Hoffert andTyler Volk in a crowded New York deli.

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