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Global warming potentials
r~IUTTERWO RTH [~E I N E M A N N
EnergyPoli,~'.Vol. 23. No. 2. pp. 149-158. 1995 Copyright © 1995 Elsevier Science Ltd Printed in Great Britain. All rights reserved 03014215/94 $ I0.00 + 0.(X)
Global warming potentials The case of emissions from dams
Luiz Pingueili Rosa and Roberto Schaeffer Energy Planning Program, Federal University of Rio de Janeiro, Centro de Tecnologia, Bloco C, Sala 211, C.P. 68565, Cidade Universit6ria, llha do Fundgm 21945-970 Rio de Janeiro, RJ, Brazil
The global warming potential index (GWP) has been proposed to quantitatively compare the integrated greenhouse effect of different gases. However, the use of the GWP index is still subject to major conceptual difficulties. Here we revise and generalize this index and then apply our alternative index to the case of emissions from some hydroelectric reservoirs in Brazil. Our results suggest that, though the cumulative heating effects of emissions from hydroelectric reservoirs may be far from negligible, for the cases studied hydroelectricity in general contributes less to the greenhouse effect over a long time horizon than fossil fuelled electricity generation. Keywords: G l o b a l w a n n i n g potential index; G r e e n h o u s e g a s e m i s s i o n s ; H y d r o e l e c t r i c r e s e r v o i r s
A global warming potential (GWP) index has been proposed by Lashof and Ahuja (1990) and adopted and developed by the Intergovernmental Panel on Climate Change (IPCC) (Houghton et al, 1990; 1992), to quantitatively compare the greenhouse effect of different gases. It is based on the ratio of the instantaneous radiative forcing of a pulse emission of a particular greenhouse gas and that of an equal and simultaneous emission of a reference gas (chosen to be carbon dioxide (CO2), the dominant gas in the greenhouse effect problem) integrated up to an arbitrarily determined time horizon. The GWP index has been developed for policy purposes as a measure of the possible warming effect on the surface - troposphere system arising from the emission of different gases and used to estimate national contributions to global warming. It is thought of as being a useful tool for settling international agreements aiming at reducing, or at least stabilizing, greenhouse emissions from the various countries. In theory, the GWP index should reflect both the direct and indirect radiative forcings exerted by greenhouse gases (Shine et al, 1990). But even all the effort made by the most recent IPCC assessment (Isaksen et
al, 1992) has not been able to solve this problem. In the particular case of methane (CH4) emissions, the guidelines set out by the IPCC 1992 report (Isaksen et al, 1992) have, among others, the limitations of not (1) considering the indirect effects arising from the oxidation of CH 4 - even though all carbon emitted into the atmosphere a s C H 4 is eventually oxidized to CO 2 and H20 assuming that only the oxidation of fossil fuel related CH 4 leads to a net production of CO 2 and (2) of considering in the calculation of the GWP index only the effect of an integrated time dependent response to a single pulse emission of CH 4 as compared with that of a simultaneous single pulse emission of CO 2. The problem of comparing the integrated radiative heating effect from constant continuous emissions of a particular gas with equally continuous emissions of CO 2 for a particular time interval has been the focus of a recent research (Harvey, 1993). But to determine the net effect of fuel switching which reduces emissions of one greenhouse gas but increases emissions of another, taking into account distinct patterns of gas emissions, a more general GWP index has to be developed. This paper generalizes the GWP index to allow, for example, for comparisons between switching from hydroelectricity
Energy Policy 1995 Volume 23 Number 2
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Global warming and emissions fivm dams: L P Rosa and R Schaeffer
to fossil fuelled electricity generation, which would probably lead to a decrease of CH 4 emissions due to the anaerobic decomposition of organic matter submerged inside water reservoirs (depending on the volume of biomass submerged in the reservoir and on the relative production of CH 4 and CO2, according to the fraction of the biomass that suffers either anaerobic or aerobic decomposition respectively while significantly increasing CO 2 emissions (Rosa and Schaeffer, 1994). Rather than comparing the ratio of the integrated forcing due to a single pulse emission of both CH 4 and CO 2 at time t = 0, this work investigates the ratio of instantaneous radiative heatings at the end of a given time horizon assuming various patterns of emission for both gases. The indirect effect of CH 4 oxidation to CO 2 is also included in the analysis, l
The G W P for CH 4 The GWP index proposed by Lashof and Ahuja (1990), and adopted by both IPCC 1990 and IPCC 1992 studies (Houghton et al, 1990; 1992), is given by ~ a i (t) c i (t) dt Gi =
(1) J'Ta2 (t) C2 (t) dt
w h e r e ag is the instantaneous radiative forcing from a unit increase in the concentration of gas i, c i (t) is the fraction of gas i remaining at time t, T is the integration time, and the subscript 2 designates the corresponding values for the reference gas, CO 2. The emission of both gases is assumed to be at t = 0, and their effects have been evaluated over 20, 100 and 500 years by both IPCC 1990 and IPCC 1992 reports, and up to infinity by Lashof and Ahuja (1990). Using a simple exponential decay model to represent the behaviour of both CO 2 and CH 4 in the atmosphere (c 2 and c4), with constant values for both a 2 and a 4 in Equation (1), we have X
J'Te-~'4t dt
G 4 :'-
-
f0r e - x y d t
~'2 1 -- e-K4 T x-)~4 1 - e-K2 r
(2)
where i = 4 for CH 4 and i = 2 for CO2; ~'i is the time constant for gas i decay (x i = 1/2.i is the atmospheric lifetime, or decay time, of gas i, with "c2 = 120 years
(Shine et al, 1990; Isaksen et al, 1992) and "~4 = 10.5 years (Isaksen et al, 1992)); T is the time horizon for taking into account the heating effect of the emissions; and finally x = a4/a 2, the instantaneous radiative heating per molecule of CH 4 relative to CO 2, is assumed constant. From the first IPCC study (Shine et al, 1990) it is possible to calculate x: 0.036 (4Q4
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Energy Policy 1995 Volume 23 Number 2
(3) 6.3 (In Q2 + aQ2 .) Q2
where Qg and AQg are the atmospheric volumetric concentrations and their volumetric concentrations changes of CO 2 (i = 2) in parts per million in volume (ppmv) (valid for concentrations less than 1000 ppmv) and CH 4 (i = 4) in parts per billion in volume (ppbv) (valid for concentrations greater than 5000 ppbv) respectively, neglecting here the methane-nitrous oxide overlap term (Hansen et al, 1988). The assumption that allows us to t a k e a 4 and a 2 out of the integrals in (1) is an approximation, since both a 2 and a 4 a r e also functions of time. If we use (3) with the current values (Isaksen et al, 1992) Q4 = 1717 ppbv and Q2 = 354 ppmv, taking equal volumetric concentration changes AQ4 = 1000 ppbv and AQ2 = 1 ppmv, we can calculate x = 21.65, which is very similar to x = 21 (ie C H 4 is about 21 times more effective, molecule for molecule, than CO2) reported by IPCC (Isaksen et al, 1992). It is therefore possible to use the same approximation of formula (3) to compute the integrated radiative forcing of carbon emissions for different time horizons, considering different scenarios for both CO 2 and C H 4 volumetric concentrations in the atmosphere.
A generalization of the G W P for CH 4 In order to compare the integrated radiative heatings of two gases with different simultaneous emissions N 4 and N 2, it is possible to define a relative carbon emission effect by
R
(4)
= G 4 --
N2 which
The indirect effect of CH 4 oxidation to CO 2 has been excluded from IPCC calculations of GWP indexes (Houghton et al, 1992) because most CH 4 emissions come from short-term recycled biogenic CO 2 which is taken out from the atmosphere (CH 4 emissions from land use, for example). In the case of hydroelectric reservoirs, however, CH 4 emissions arise from the decomposition of the biomass stock under water, and as such the indirect effect of CH 4 oxidation to CO 2 has also to be accounted for.
+ AQ4 - ~4)
x=
is a generalization
of expression
(2) when
N4 N2. A comparison between microbial decomposition of flooded forest biomass and C H 4 production by hydroelectric reservoirs and CO 2 production from fossil fuelled electricity generation asks for a generalization of formula (4). In this case, most of the C H 4 emissions
Global warming and emissions¢J'om dams: L P Rosa and R Schaeffer
from hydroelectric reservoirs is concentrated in time and decays during a period of a few years, while CO 2 emissions from fossil fuelled electricity generation remain continuous and constant over the lifespan of the capital investment. There will also be small CH 4 and CO 2 longterm emissions from the decomposition of residual stored biomass remaining in the reservoirs after the initial intense degradation, as well as from new biomass produced over time inside the reservoirs. However, these latter emissions are neglected here, because they are relatively small compared to the CO 2 emissions from fossil fuelled electricity generation, according to data obtained in field studies in some Brazilian as well as Canadian reservoirs (Rosa and Schaeffer, 1994). A considerable portion of those permanent contributions would in any case arise from the normal decomposition of biomass resources on those lands seasonally flooded by the river even if no hydroelectric reservoir had been built (Franken et al, 1992). In this case, it is possible to evaluate the ratio between the warming effects of CH 4 relative to CO 2 using
E2=
n~22( T ,
e-k2(r-T')-e-)v2r)
(9)
)22 where AN2/
n2-
dN 2
Ati
-
dt
(10)
is constant. It is easy to see that, with T = ~o,
n2T ' E2 = --
N2 -
)22
(11) )22
as in the previous case. By applying the full expression (9), R = E 4 / E 2 can still be calculated as in (4) X )22N4
R-
(12) f )24 N2
& R=--
(5)
E2
with e-L2(T - T ' ) _ e-~.2T
with
(13)
f= 1 T')2 2
E, =-a i i A~., I~Se-~/<' (i'd, a2
(6)
The meaning of the factor in (12)
i
x )22 where A/~,7 is the carbon mass of gas i emitted in time and T ' is the time horizon over which we consider the emissions. In principle we must choose T ' < T. In the hypothesis of choosing T = oo, as Lashof and Ahuja did (1990), we can combine (5) and (6) and have the same expression as in (4), but with N 2 = ~,jN2j and N 4 = ~,jN4j , giving the total carbon mass emitted as CO 2 and CH 4, respectively, and )22 G 4 = X --
f)24 is the generalized warming power of CH 4 relative to CO 2, like the G W P but, instead of being limited to simultaneous pulse emissions o f both gases, it allows emissions at different periods of time. Assuming T ' = T = 100 years, we have computed G 4' from (13) and (14), and compared it with the result when T = ~ in Table 1.
(7)
X4
The dependence of radiative forcings on time
equal to that of expression (2), with T = oo. In the case of CH 4 emissions concentrated in time, T ' << T, we will always have from (6) that N4 E 4 = .V - )24
(14)
G' 4 _
(8)
while, in the case of long-term constant-continuous CO 2 emissions up to T ' , by integrating (6) we have for T ' _
Climate forcing arising from increasing atmospheric greenhouse gases is still highly uncertain (Hansen et al, 1988). Our procedure to estimate the error of taking the variables a 2 and a 4 as independent of time in (6) for computing (12) and (14) could therefore be to assume a linear variation along the emission time
a i (t') = a i(O) (1 + qi t')
(15)
In our model, this variation does not matter for C H 4
Energy Policy 1995 Volume 23 Number 2
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Global warming and emissionsfrom dams: L P Rosa and R Schaeffer Table 1 Generalized global warming potential for methane and for carbon emissions from a hydroelectric power plant relative to a fossil fuelled equivalent power plant (mole basisp Warming index
Time horizon (years)
Source
G W P = 3.70 ° G 0' = 1.84 c G 4' = 5.72 ~ G" = 3.50 d
T T T T
Lashof and Ahuja (1990) Equation (14) Equation (14) Equation (41)
= = = =
oo and pulse at T' = 0 ~ and T' any (finite) T' = 100 ~o and T ' = 100
a Values include the warming potential of CO 2 produced from the oxidation of CH 4, b Residence times of CH 4 and CO 2 are 14.4 and 230 years respectively. c Residence times of CH 4 and CO 2 are 10.5 and 120 years respectively. d Residence times of CH 4 and CO n are 10.5 and 120 years respectively. We have also assumed z = 0.05 (CH~ emission from fossil fuel combustion equal to 5% of CO 2 emission), et = 13 (50% of the decomposition of biomass is aerobic and 50% is anaerobic), y = 1 (all CO 2 is oxidized in CH 4 in the atmosphere).
because most of the biomass in the reservoir is decomposed in a short period of time (within approximately seven years), and s o r l 4 t ' = 0 and a 4 ( t ' ) = a n ( 0 ) . However, it could be important for CO z, since emissions from fossil fuelled electricity generation remain constant over the lifetime of the plant (30 years approximately) and, assuming that the plant is replaced after that by a similar plant, even over the entire period of analysis considered (100 years, in the present exercise). The global warming index G 4 ' , given by (14), can be corrected in such a way that its new value, G4", is related to the previous one by 1
1 -- +
G" 4
G' 4
]'[2
~'4
X
~'2
f'
(16)
with
f,
T"-~ 2,fo 'dt' t' ffie-~
~22T'
The advantage of expressions (16) and (17) over the previous ones is that they provide a simple way to perform a sensitivity analysis. The problem is to forecast the future behaviour of ai (t') to fit the parameter 1]i in (15). Scenario IS92a in the IPCC 1992 report (Legett et al, 1992) assumes an annual growth rate in the concentration of CO 2 in the atmosphere of about 0.7% for the next 100 years. However, more optimistic scenarios in terms of lower CO 2 emissions can possibly be expected. The currently observed increase in the concentration of CO 2 in the atmosphere, Q2 (t'), of about 0.5% per year (Rodhe, 1990) is used here. The relationships between a i (t') and Oi (t') for C H 4 and CO 2 are given by the numerator and by the de-
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Energy Policy 1995 Volume 23 Number 2
nominator of the fraction in expression (3) respectively. For CO 2, we can approximate for AQ2 << Q2 In Q2 + AQ2 = AQ2
Q2
(18)
Q2
which allows us to write the radiative forcing as being inversely proportional to Q2, and so on a 2 (t') = a 2 (0) 1.005 - t '
(19)
which can be fitted by the linear function (15), with rli = -0.004 yr -l in a very good approximation for 0 < t < 100 years. For C H 4 , scenario IS92a from IPCC (Leggett et al, 1992) forecasts an annual growth rate in the concentration in the atmosphere, Q4 (t'), of about 0.6% for the next 100 years. The radiative forcing variation of CH 4 28 years from now (four times the decay time of the biomass decomposition in the reservoir, according to our model) is only 5%, which can be ignored, as we have done. For T = T ' = 100 years, we obtain G 4 " = 6.66 from expression (16). In the case of considering T = co and T' = 100 years, the value G 4' changes to G4" = 2.30. The problem is that the future behaviour of a i (t') is unknown, because there are many factors involved in its scenario determination, with large uncertainties (Ramanathan, 1976; Donner and Ramanathan, 1980; Steudler et al, 1989; Lelieveld and Crutzen, 1992). All we can do is to perform a sensitivity analysis. The conclusions are that the order of magnitude of the results does not change and the error obtained is lower when the index value is higher. Emissions from hydroelectric reservoirs
A more general and straightforward way to compute the effects for different time horizons of non-simultaneous greenhouse gas fluxes to the atmosphere from hydro-
Global warming and emissionsfrom dams: L P Rosa and R Schaeffer electric reservoirs compared to greenhouse gas emissions by fossil fuelled electricity generation is to write a set of differential equations to describe them as follows.
For hydroelectric reservoirs ( 1) CH 4 emissions N4, remaining at time t, arising from the microbial decomposition of organic matter, which decays with a time constant y, being ot the fraction of the total mass of degradable carbon in the vegetation and soil C that would decay in a reservoir within time T and be released as CH4:
(20)
-= o~C,ye-"[ t - ~ 4 N 4 dt
E2
irN2dt =
I y_~C e -~'r -. ~'2 - 7 / y
~C - ~22
~.4yyotC [1
-
} e-X: r ~2
(24)
e-Vr
E2 = I~N '2 dt = - -
~4--'Y
1 - e-X4 r
~ ' 4 - 'Y
k4(~'4 -- ~'2 )
(~'2 -- Y) (X4 -- k2)
1 - - e-X2r.] /
+ k2
(25)
,I
In spite of their apparent complex forms, expressions (23)-(25) can be greatly simplified if we assume T = ~, as Lashof and Ahuja (1990) did:
xaC (2) C O 2 emissions N 2, remaining at time t, arising from the decomposition of organic matter, assuming the same time constant ~, for the production rate decay and a different fraction [3 of C that would be released
13c E2 =
as C02: dN 2
(21)
= ~C]te -¢t _ ~ 2 N 2
(26)
E4-
E;-
(27)
X2 y o~C
(28)
dt (3) CO 2 produced in the atmosphere N 2, arising from the oxidation of a fraction y of the emitted CH 4 that is transformed in CO2:
dU; (22)
--....._2."= y ~ , 4 N 4 - ~ 2 N ' 2
dt Solving expressions (20), (21) and (22) with the appropriate initial conditions, we can compute the integrated effects of each component of the total forcing: 2
xoc,(1 eYT, enT)23, E4 = x IT N4dt
-~447 -~
~t
The reason why expressions (26)-(28) are so simple is because, when we extend the time horizon up to infinite, the integrated heating effects of both CH 4 and CO 2 always include the total area under the N i (t) curve.
For fossil fuelled electricity generation (1) CO 2 emissions N2, remaining at time t, arising from the combustion of fossil fuels, assuming a constant emission rate n 2 up to time T ' : dN = n 2 -- ~2N2
(29)
dt
~'4
2 Because the results of various net energy analyses for hydroelectric
power performed so far have shown net energy requirements (amount of primary energy, measured in terms of the energy provided by fossil fuels, required to produce one unit of electricity) in the range of 0.03 to 0.10, which are very low and compare quite well with requirements for fossil-fuelled power, we decided not to take into account emissions associated with the use of direct and indirect energy inputs for the construction of the plants. For more details on net energy analyses of electric power, see Gilliland et al, (1981); Gusdorf (1992); Mortimer (1991). With respect to emissions from the reservoir itself, because the 'without project' situation referred to earlier in the paper is also responsible for some greenhouse gas emissions (Franken et al, 1992), we decided in the derivation of 'with project' emissions not to take into account long-term residual emissions from the reservoir but only the initial, intense decomposition of the biomass flooded. Data collected in some field studies in some hydroelectric reservoirs in the Amazon region in Brazil (G. Tundisi, personal communication) proved us right.
(2) CH 4 emissions N 4, remaining at time t, arising from the production, transformation and transport of fossil fuels, assuming a constant emission rate zn 2, with 0
- 2/12 -- ~4N4
(30)
dt (3) CO 2 produced in the atmosphere N2', arising from oxidation of a fraction of the C H 4 emitted, as in the case of hydroelectric reservoirs:
dt
= Y~2N4 - ~ 2 N ~
Energy Policy 1995 Volume 23 Number 2
(31)
153
Global warming and emissions from dams: L P Rosa and R Schaeffer Solving Equations (29)-(31) with the appropriate initial conditions, we can also compute the integrated effects of each component of the total forcing: 3
1 -e-XJ}
n2TI1 E 2 = J'TN 2 d t = K2 ~
E4=xfTN4dt-
T~, 2
xzn2T(1
1--e-X4T )
~'4 Ed = ~ N 2 dt = yzn 2
(32)
(33)
T~4
Yzn2T ' E2 -
(37)
The relative carbon emission effect of a hydroelectric reservoir (taking into account both CH 4 and CO s emissions), taking the warming effect of the carbon emitted by a hypothetical fossil-fuelled power plant (considering both CH 4 and CO s emissions too) as a reference, can now be calculated. We must add (26)-(28) and divide the result by the sum of (35)-(37). We obtain:
T
h o~C
-~ +
R-
(38)
g n2T' 1
-- e-~'4 T
~'4
1 -- e-~'z r ]
+
~4 (~'4 -- ~'2 )
~'2 (~'4 -- ~'2 )
~'2
]
(34)
with
xz
1
We must compare (32) with the previous result given by (9). To do that we have to consider the situation in which CO s emissions from a fossil fuelled power plant cease at time T ' , while their warming effects are computed up to time T > T ' . Thus we have to take the integration in (32) with the upper limit T ' < T and add a new one, of the function N/(T ') e-~'itt - T'i from T ' up to T. The result agrees exactly with the previous one in (9). The same procedure can be used for all gas emissions described by (32)-(34). We have particular interest in extending their effects up to T = oo, which means taking the upper limit T ' in each integration and adding the new one, as above, from T ' up to T = o~. The results, however, are always simple because, in this case, we include all the areas under the c u r v e s N i (t), which means multiplying the total amount of gas emitted to, or transformed in, the atmosphere by its atmospheric lifetime % = 1/~,i in all the formulae: n2T'
(35)
E2~'2
&-
xzn2 T '
(36)
3 Because the net energy requirements for the construction of fossilfuelled power plants is quite small too (Gilliland et al, 1981 ; Mortimer, 1991), here too we decided not to take into account emissions associated with the use of direct and indirect energy inputs for the construction of the plants, even though we do include emissions associated with the production, transformation, transport and combustion of the fossil fuels used in the plants.
154
Energy Policy 1995 Volume 23 Number 2
yz
g =--+--+ ~2
--
~4
(39)
~'2
and
x h=--+
13
y +--
(40)
a '2 We note that n2T' and czC represent the total emission of CO 2 from a hypothetical fossil fuelled power plant and the total emission of CH 4 from an also hypothetical hydroelectric power plant, respectively. For effect of comparison we can rewrite (38) as (4) with, instead of G4, h at!
~
__
(41)
g which agrees with formula (14) when we take only the first terms of both (39) and (40). Table 1 shows the results of (41) in comparison with the previous result from (13) and (14). Although all parameters used in (41) have been arbitrarily chosen for the calculations, they seem reasonable for the present exercise. We believe that the method developed here for extending the GWP concept can be used to provide more comprehensive analyses when comparing the greenhouse forcing impacts of switching from hydroelectricity to fossil fuelled electricity generation than previous analyses performed so far (Leggett et al, 1992; Oud, 1993; Rudd et al, 1993). If we take Equation (38), it is possible to conclude that there will be net radiative heating savings by switching from fossil fuelled electricity generation to hydroelectricity if R < 1, and an increase in the greenhouse effect from switching back.
Global warming and emissions fi'om dams: L P Rosa and R Schaeffer Table 2
A comparison of total integrated radiative forcing from greenhouse gas emissions from three hypothetical hydroelectric reser-
voirs and fossil fuelled power plants a Plants
Hydro Ac Hydro Bf Hydro Cg Fossil-fuelled
Energy produced
Flooded area
Energy density
(TWh pa)
(kin 2)
(TWh pa)/km 2
Integrated radiative forcing equivalent b (Mt C pa)/TWh Over 50 years Over 100 years
0.95 17.35 48.18 -
560 2,430 1,100 -
0.0017 0.0071 0.0438 -
1.78 d-I 2.25 c 0.42 d- 2.93 e 0.07 a- 0.48 e 2.52 h- 5.80 i
0.91 d_ 6.27 e 0.22 ~- 1.50e 0.04 d- 0.25 ~ 4.45h-10.23 i
The hypothetical reservoirs in the Amazon region of Brazil have low, medium and high ratios of energy produced to flooded area (energy density). b Although cumulative forcing is normally expressed in W/m 2 pa units, we have decided to use Mt C pa equivalent units as a basis for comparison. c Hypothetical hydroelectric power plant similar to Samuel hydroelectric power plant in the Amazon region in Brazil in terms of amount of energy produced and flooded area. Methane emissions only. d Biomass value of 155.1 t/ha dry weight (Brown and Lugo, 1984), with a carbon stock of 45% of the dry biomass, 10% of which assumed to decompose anaerobically. e Biomass value of 361.5 t/ha dry weight (Setzer and Pereira, 1991), with a carbon stock of 45% of the dry biomass, 30% of which assumed to decompose anaerobically. f Hypothetical hydroelectric power plant similar to Tucurui hydroelectric power plant in the Amazon region in Brazil in terms of amount of energy produced and flooded area. Methane emissions only. g Hypothetical hydroelectric power plant similar to the proposed Belo Monte hydroelectric power plant in the Amazon region in Brazil in terms of amount of energy produced and flooded area. Methane emissions only. h Combined cycle natural gas fired power station with an assumed 45% thermal efficiency and burning gas with CO 2 emissions of 13.7 (kg C)/109 J of fuel (Rosa and Schaeffer, 1994). Coal fired power station with an assumed 35% thermal efficiency and burning coal with CO 2 emissions of 25.2 (kg C)/10 9 J of fuel (Marland, 1983).
Some applications There is some consensus that greenhouse gas emissions to the atmosphere from some hydroelectric reservoirs may be far from negligible (Oud, 1993; R u d d e t al, 1993). There is less agreement, however, on the cumulative heating effects of those emissions over time per unit of energy produced compared to greenhouse gas emissions by fossil fuelled electricity generation (Rosa and Schaeffer, 1994). It may be worthwhile, therefore, to investigate how hydroelectric reservoirs in the Amazon region in Brazil stand against fossil fuelled power plants with respect to the cumulative heating effects of greenhouse gas emissions. Table 2 presents a comparison between the integrated radiative forcing from CH 4 emissions from three hypothetical hydroelectric reservoirs similar (in terms of characteristics of reservoirs and extent and types of landscape flooded) to three real hydroelectric reservoirs in the Amazon region in Brazil (two existing and one projected) with low, medium and high ratios of energy produced to flooded area, and CO 2 emissions from combined cycle natural gas and coal fired power stations over two different time horizons. The reason for assuming hypothetical, rather than real, hydroelectric power stations is to allow a wide range of estimates using different biomass densities (since forest biomass varies greatly in different parts of the region) (Fearnside, 1992) as well as different rates for bacterial decomposition of forest biomass to C H 4. By applying our generalized GWP index we have been able to express the C H 4 fluxes as ' C O 2 equivalents' and com-
pare their warming effects with those from the CO 2 fluxes. After flooding, the CH 4 fluxes from reservoirs have been estimated by assuming that 45% of the biomass values in the region (155.1-361.5 t/ha dry weight (Brown and Lugo, 1984; Setzer and Pereira, 1991)) is carbon, 10-30% of which assumed to decompose anaerobically decaying exponentially over time with a decay time of seven years. The CO 2 fluxes from fossil fuelled power stations have been estimated by assuming carbon contents of 13.7-25.2 kg C / 1 0 9 J of fuel for natural gas and coal respectively (Marland, 1983), and thermal efficiencies of 45-35% for combined cycle natural gas fired and coal fired power stations, respectively. Results are strongly dependent on assumptions and on time horizons. For reservoirs with medium and high ratios of energy produced to flooded area (hydros B and C), the estimated integrated radiative forcings per unit of energy produced were 0.42-2.93 (Mt C pa)/MWh and 0.07-0.48 (Mt C pa)/MWh equivalent units respectively over 50 years, and 0.22-1.50 (Mt C pa)/MWh and 0.04~.25 (Mt C pa)/MWh equivalent units respectively over 100 years. These estimates are clearly much lower than the cumulative heating effects of electrical generation by fossil fuelled power plants with the sole exception of hydro B which, for the high biomass density case with high rate of CH 4 production in the flooded area over 50 years, is similar to the greenhouse effect of electrical generation by combined cycle natural gas fired power generation.
Energy Policy 1995 Vohmw 23 Number 2
155
Global warming and emissions from dams: L P Rosa and R Schaeffer 5 4.5 -c>,
CO2
4
0
3.5
g
3
~ 2.5 E 2 E o ..Q
1.5
o
\cH,
0.5
~
0
20
:
I
p
40
60
80
I
100 120 Time (yr)
I
t
I
I
140
160
180
200
Figure 1 C H 4 emissions from the anaerobic decomposition of biomass in the Tucurui hydroelectric power plant compared to CO 2 emissions from the combustion of fuel oil in a fossil fuelled equivalent power plant over a time horizon of 100 years 3.5
350
~
3
3oo
v
2.5
250 ~
2
E 200 '=
.~ 1.5
150 .=_
E
1
100 E
0.5
50 c£
E "= t-
o
~
t-.
o
0 0
20
40
60
80
100 120 Time (yr)
140
160
180
200
Figure 2 C H 4 emissions from the anaerobic decomposition of biomass in the Tucurui hydroelectric power plant compared to CO 2 emissions from the combustion of fuel oil in a fossil fuelled equivalent power plant remaining in the atmosphere at time t. Scales are different as indicated in vertical axes For a reservoir with a low ratio of energy produced to flooded area (hydro A), however, the estimated integrated radiative forcings per unit of energy produced were 1.78-12.25 (Mt C pa)/TWh equivalent units over 50 years and 0.91-6.27 (Mt C pa)/TWh equivalent units over 100 years. These estimates may be lower, similar or higher than the greenhouse effect of fossil fuelled electrical generation, depending on biomass density and CH 4 production in the flooded area, fossil fuelled electrical generation technology and the time horizon of concern. In any case, however, the longer the planning horizon the better hydroelectricity becomes and the worse thermoelectricity becomes (this is a direct consequence of both the distinct pattern of emissions between hydroelectricity and thermoelectricity, and also a consequence of the different atmospheric lifetime of CH 4 as compared to CO2). For a graphical representation of the method
156
Energy Policy 1995 Volume 23 Number 2
employed in our previous analysis, we decided to compare the Tucurui hydroelectric power plant (Table 2) with a hypothetical oil fired equivalent power station typical for Brazil, supposing that both plants generate the same amount of electrical energy during a 100 year period. Emissions are shown in Figure 1, where CH 4 fluxes from the reservoir have been estimated by assuming that 45% of the biomass value in the reservoir region (226 t/ha dry weight (Fearnside, 1990)) is carbon, 30% of which is decomposing anaerobically and decaying exponentially over time with a decay time of seven years. The CO z fluxes from the oil fired equivalent power station have been estimated by assuming a carbon content of 20.0 kg C/109 J of fuel (Marland, 1983), and a thermal efficiency of 27.5%, typical in Brazil today (MME, 1993). Figure 2 shows the quantities of each gas remaining in the atmosphere at time t. We have also calculated the
Global warming and emissions from dams. L P Rosa and R Schaeffer 1800
.40000
J
1600
5 ~ 14oo #.~ 12oo
35000 "5 30000 ~
OH4
5.~
CO 2
25000 ~ ~_
lOOO
~o
800
~E
600
~I
400
--
200
2 0 0 0 0 : 5 >" 15000 10000 ~ 0 ~0 5000 ---= I
0 20
o
40
60
80
100 120 Time (yr)
140
160
0 200
180
Figure 3 Integrated radiative forcing of C H 4 emissions (E4) from the anaerobic decomposition of biomass in the Tucurui hydroelectric power plant compared to integrated radiative forcing of CO 2 emissions (E2) from the combustion of fuel oil in a fossil fuelled equivalent power plant. Scales are different as indicated in vertical axes 4.5 4 3.5 3 2.5 2 1.5 1
0.5 0
I
0
0
0
I
I
I
I
r
I
0
Time (yr)
Figure 4 Ratios R = E4/E 2 between the integrated warming effects of C H 4 emissions from the anaerobic decomposition of biomass in the Tucurui hydroelectric power plant and CO 2 emissions from the combustion of fuel oil in a fossil fuelled equivalent power plant warming effect E 2 and E 4, expressed as the integrated product of the radiative forcings with the fraction of CO 2 and CH 4 remaining in the atmosphere, respectively (Figure 3). The CH 4 warming effect saturates after some 60 years, while the CO 2 warming effect continues to increase even after 200 years (as a matter of fact, it stabilizes only after about 500 years). Finally, we can see in Figure 4 the ratio R = E4/E 2 between the warming effect of CH 4 relative to CO 2, defined by expressions (5) and (6) respectively. With its power density of 0.39 W/m 2 (energy density of 0.0071 TWh pa/km 2, for a capacity factor of 50% for its installed capacity of 3960 MW), the R index becomes lower than 1 after about 23 years. This means that, with respect to greenhouse gas emissions, the Tucurui hydroelectric power plant is clearly more beneficial to the global environment than an oil fired power station.
Conclusions The GWP generalization introduced here seems to represent a useful conceptual extension from previous studies (Lashof and Ahuja, 1990; Shine et al, 1990; Isaksen et al, 1992; Harvey, 1993; Oud, 1993; Rudd et al, 1993). Rather than viewing the GWP index as the ratio of the integrated radiative forcing due to a single pulse emission of two different gases at time t = 0, the new index defined can be seen as the ratio of radiative heatings at the end of a given time horizon, assuming distinct and continuous emissions during an entire time interval. The ratio between area flooded and energy produced has already been a matter of concern in the context of the overall environmental and social impacts of hydroelectricity worldwide in general (Gleick, 1992; Lele and Subramanian, 1988) and in Brazil in particular (Rosa
Energy Policy 1995 Volume 23 Number 2
157
Global warming and emissions from dams: L P Rosa and R Schaeffer
and Schaeffer, 1990). We have shown here that this ratio, together with other local characteristics in the area of the reservoir, may also be important in determining the magnitude of greenhouse gas emissions. Though we cannot ignore some of the serious environmental impacts that may arise from a hydroelectric reservoir, the evidence gathered here seem to suggest that, in the case of Brazil, hydroelectricity can still be seen as a realistic mitigation option for global climate change. Our results suggest that, though hydroelectric reservoirs may be significant sources of greenhouse gases and the cumulative heating effects of their emissions may be far from negligible, for the cases studied hydroelectricity seems in general to contribute less to the greenhouse effect over a long time horizon than fossil fuelled electricity generation. This is not to mention the issue recently raised that man-made lakes may be the single largest (and negative) contribution to the sea level (Chao, 1994; Rodenburg, 1994; Sahagian et al, 1994). The method proposed here is now being used to test whether a redirection of the energy policy in Brazil from hydroelectricity toward fossil fuelled electricity generation, as supported by some international and bilateral funding agencies, represents a step in the right direction toward reducing the Brazilian contribution to global wanning. Acknowledgements This paper benefited enormously from discussions with Marco Aur61io dos Santos and Rafael Schechtman. This work was supported in part by Conselho Nacional de Desenvolvimento Cientffico e Tecnol6gico-CNPq and ELETROBRAS-Centrais E16tricas Brasileiras S.A. References Brown, S and Lugo, A E (1984) 'Biomass of tropical forests: a new estimate based on forest volumes' Science 223 1290-1293 Chao, B F (1994) 'Man-made lakes and sea-level rise' Nature 370 (6487) 258 Donner, L and Ramanathan, V (1980) 'Methane and nitrous oxide: their effects on the terrestrial climate' Journal of the Atmospheric Sciences 37 (1) 119-124 Feamside, F (1990) 'Deforestation in Brazilian Amazonia as source of greenhouse gases' Regional Conference on Global Wanning, USP, Sao Paulo, Brazil Fearnside, P M (1992) 'Forest biomass in Brazilian Amazonia: comments on the estimate by Brown and Lugo' lnterciencia 17 (1) 1927 Franken, R O G, Vierssen, W and Lubberding, H J (1992) 'Emissions of some greenhouse gases from aquatic and semi-aquatic ecosystems in the Netherlands and options to control them' The Science of
158
Energy Policy 1995 Volume 23 Number"2
the Total Environment 126 (3) 277-293 Gilliland, M W, Klopatek, J M and Hildebrand, S G (1981) 'Net energy: results for small-scale hydroelectric power and summary of existing analyses' Energy 6 (10) 1029-1040 Gleick, P H (1992) 'Environmental consequences of hydroelectric development: the role of facility size and type' Energy 17 (8) 735-747 Gusdorf, J (1992) 'Energy paybacks and renewable breeders' Energy 17(12) 1137-1151 Hansen, J, Fung, I, Lacis, A, Rind, D, Lebedeff, S, Ruedy, R, Russel, G and Stone, P (1988) 'Global climate changes as forecast by Goddard Institute for Space Studies three-dimensional model' Journal of Geophysical Research 93 9341-9364 Harvey, L D D (1993) 'A guide to global warming potentials (GWPs)' Energy Policy 21 ( 1) 24-34 Houghton, J T, Jenkins, G J and Ephraums, J J (eds) (1990) Climate Change: The IPCC Scientific Assessment Cambridge University Press, Cambridge Houghton, J T, Callander, B A and Varney, S K (eds) (1992) Climate Change 1992: The Supplementary Report to the IPCC Scientific Assessment Cambridge University Press, Cambridge Isaksen, I S A, Ramaswamy, V, Rodhe, H and Wigley, T M L (1992) 'Radiative forcing of climate', in Houghton et al, 1992, pp 47-67 Lashof, D A and Ahuja, D R (1990) 'Relative contributions of greenhouse gas emissions to global warming' Nature 344 (6266) 529-531 Leggett, J, Pepper, W J and Swart, R J 'Emissions scenarios for the IPCC: an update' in Houghton et al, 1992, pp 69-95 Lele, S M and Subramanian, D K (1988) 'A hydro-wood net-energy approach to hydro project design' Energy 13 (4) 367-381 Lelieveld, J and Crutzen, P J (1992) 'Indirect chemical effects of methane on climate warming' Nature 355 (6358) 339-342 Marland, G (1983) 'Carbon dioxide emission rates for conventional and synthetic fuels' Energy 8 (12) 981-992 Minist6rio das Minas e Energia (MME), Brazil (1993) Balan~.o Energ~tico 1993 Brasflia, p 111 Mortimer, N D (1991) 'Energy analysis of renewable energy sources' Energy Policy 19 (4) 374-385 Oud, E (1993) 'Global wanning: a changing climate for hydro' Water Power & Dam Construction 20--23 Ramanathan0 V (1976) 'Radiative transfer within the earth's troposphere and stratosphere: a simplified radiative-convective model' Journal of the Atmospheric Sciences 33 1330-1346 Rodenburg, E (1994) Nature 370 (6487) 258 Rodhe, H (1990) 'A comparison of the contribution of various gases to the greenhouse effect' Science 248 (4960) 1217-1219 Rosa, L P and Schaeffer, R (1990) 'Brazilian energy policy', in Santos, L A O and Andrade, L M M (eds) Hydroelectric Dams on Brazil's Xingu River and Indigenous Peoples Cultural Survival, Cambridge, MA pp 47-52 Rosa, L P and Schaeffer, R (1994) 'Greenhouse gas emissions from hydroelectric reservoirs" Ambio 23 (2) 164-165 Rudd, J W M, Harris, R, Kelly, C A, and Hecky, R E (1993) 'Are hydroelectric reservoirs significant sources of greenhouse gases?' Ambio 22 (4) 246-248 Sahagian, F W, Schwartz, F W and Jacobs, D K (1994) 'Direct anthropogenic contributions to sea-level rise in the twentieth century', Nature 367 (6458) 54-57 Setzer, A W and Pereira, M C (1991) Amazonian biomass burning in 1987 and an estimate of their tropospheric emissions' Ambio 20 (1) 19-22 Shine, K P, Derwent, R G, Wuebbles, D J a n d Morcrette, J-J '(1990) Radiative forcing of climate', in Houghton et al, 1992, pp 41-68 Steudler, P A, Bowden, R D, Melillo, J M and Aber, J D (1989) 'Influence of nitrogen fertilization on methane uptake in temperate forest soils' Nature 341 (6240) 31 4-316
EnergyPoli,~'.Vol. 23. No. 2. pp. 149-158. 1995 Copyright © 1995 Elsevier Science Ltd Printed in Great Britain. All rights reserved 03014215/94 $ I0.00 + 0.(X)
Global warming potentials The case of emissions from dams
Luiz Pingueili Rosa and Roberto Schaeffer Energy Planning Program, Federal University of Rio de Janeiro, Centro de Tecnologia, Bloco C, Sala 211, C.P. 68565, Cidade Universit6ria, llha do Fundgm 21945-970 Rio de Janeiro, RJ, Brazil
The global warming potential index (GWP) has been proposed to quantitatively compare the integrated greenhouse effect of different gases. However, the use of the GWP index is still subject to major conceptual difficulties. Here we revise and generalize this index and then apply our alternative index to the case of emissions from some hydroelectric reservoirs in Brazil. Our results suggest that, though the cumulative heating effects of emissions from hydroelectric reservoirs may be far from negligible, for the cases studied hydroelectricity in general contributes less to the greenhouse effect over a long time horizon than fossil fuelled electricity generation. Keywords: G l o b a l w a n n i n g potential index; G r e e n h o u s e g a s e m i s s i o n s ; H y d r o e l e c t r i c r e s e r v o i r s
A global warming potential (GWP) index has been proposed by Lashof and Ahuja (1990) and adopted and developed by the Intergovernmental Panel on Climate Change (IPCC) (Houghton et al, 1990; 1992), to quantitatively compare the greenhouse effect of different gases. It is based on the ratio of the instantaneous radiative forcing of a pulse emission of a particular greenhouse gas and that of an equal and simultaneous emission of a reference gas (chosen to be carbon dioxide (CO2), the dominant gas in the greenhouse effect problem) integrated up to an arbitrarily determined time horizon. The GWP index has been developed for policy purposes as a measure of the possible warming effect on the surface - troposphere system arising from the emission of different gases and used to estimate national contributions to global warming. It is thought of as being a useful tool for settling international agreements aiming at reducing, or at least stabilizing, greenhouse emissions from the various countries. In theory, the GWP index should reflect both the direct and indirect radiative forcings exerted by greenhouse gases (Shine et al, 1990). But even all the effort made by the most recent IPCC assessment (Isaksen et
al, 1992) has not been able to solve this problem. In the particular case of methane (CH4) emissions, the guidelines set out by the IPCC 1992 report (Isaksen et al, 1992) have, among others, the limitations of not (1) considering the indirect effects arising from the oxidation of CH 4 - even though all carbon emitted into the atmosphere a s C H 4 is eventually oxidized to CO 2 and H20 assuming that only the oxidation of fossil fuel related CH 4 leads to a net production of CO 2 and (2) of considering in the calculation of the GWP index only the effect of an integrated time dependent response to a single pulse emission of CH 4 as compared with that of a simultaneous single pulse emission of CO 2. The problem of comparing the integrated radiative heating effect from constant continuous emissions of a particular gas with equally continuous emissions of CO 2 for a particular time interval has been the focus of a recent research (Harvey, 1993). But to determine the net effect of fuel switching which reduces emissions of one greenhouse gas but increases emissions of another, taking into account distinct patterns of gas emissions, a more general GWP index has to be developed. This paper generalizes the GWP index to allow, for example, for comparisons between switching from hydroelectricity
Energy Policy 1995 Volume 23 Number 2
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Global warming and emissions fivm dams: L P Rosa and R Schaeffer
to fossil fuelled electricity generation, which would probably lead to a decrease of CH 4 emissions due to the anaerobic decomposition of organic matter submerged inside water reservoirs (depending on the volume of biomass submerged in the reservoir and on the relative production of CH 4 and CO2, according to the fraction of the biomass that suffers either anaerobic or aerobic decomposition respectively while significantly increasing CO 2 emissions (Rosa and Schaeffer, 1994). Rather than comparing the ratio of the integrated forcing due to a single pulse emission of both CH 4 and CO 2 at time t = 0, this work investigates the ratio of instantaneous radiative heatings at the end of a given time horizon assuming various patterns of emission for both gases. The indirect effect of CH 4 oxidation to CO 2 is also included in the analysis, l
The G W P for CH 4 The GWP index proposed by Lashof and Ahuja (1990), and adopted by both IPCC 1990 and IPCC 1992 studies (Houghton et al, 1990; 1992), is given by ~ a i (t) c i (t) dt Gi =
(1) J'Ta2 (t) C2 (t) dt
w h e r e ag is the instantaneous radiative forcing from a unit increase in the concentration of gas i, c i (t) is the fraction of gas i remaining at time t, T is the integration time, and the subscript 2 designates the corresponding values for the reference gas, CO 2. The emission of both gases is assumed to be at t = 0, and their effects have been evaluated over 20, 100 and 500 years by both IPCC 1990 and IPCC 1992 reports, and up to infinity by Lashof and Ahuja (1990). Using a simple exponential decay model to represent the behaviour of both CO 2 and CH 4 in the atmosphere (c 2 and c4), with constant values for both a 2 and a 4 in Equation (1), we have X
J'Te-~'4t dt
G 4 :'-
-
f0r e - x y d t
~'2 1 -- e-K4 T x-)~4 1 - e-K2 r
(2)
where i = 4 for CH 4 and i = 2 for CO2; ~'i is the time constant for gas i decay (x i = 1/2.i is the atmospheric lifetime, or decay time, of gas i, with "c2 = 120 years
(Shine et al, 1990; Isaksen et al, 1992) and "~4 = 10.5 years (Isaksen et al, 1992)); T is the time horizon for taking into account the heating effect of the emissions; and finally x = a4/a 2, the instantaneous radiative heating per molecule of CH 4 relative to CO 2, is assumed constant. From the first IPCC study (Shine et al, 1990) it is possible to calculate x: 0.036 (4Q4
150
Energy Policy 1995 Volume 23 Number 2
(3) 6.3 (In Q2 + aQ2 .) Q2
where Qg and AQg are the atmospheric volumetric concentrations and their volumetric concentrations changes of CO 2 (i = 2) in parts per million in volume (ppmv) (valid for concentrations less than 1000 ppmv) and CH 4 (i = 4) in parts per billion in volume (ppbv) (valid for concentrations greater than 5000 ppbv) respectively, neglecting here the methane-nitrous oxide overlap term (Hansen et al, 1988). The assumption that allows us to t a k e a 4 and a 2 out of the integrals in (1) is an approximation, since both a 2 and a 4 a r e also functions of time. If we use (3) with the current values (Isaksen et al, 1992) Q4 = 1717 ppbv and Q2 = 354 ppmv, taking equal volumetric concentration changes AQ4 = 1000 ppbv and AQ2 = 1 ppmv, we can calculate x = 21.65, which is very similar to x = 21 (ie C H 4 is about 21 times more effective, molecule for molecule, than CO2) reported by IPCC (Isaksen et al, 1992). It is therefore possible to use the same approximation of formula (3) to compute the integrated radiative forcing of carbon emissions for different time horizons, considering different scenarios for both CO 2 and C H 4 volumetric concentrations in the atmosphere.
A generalization of the G W P for CH 4 In order to compare the integrated radiative heatings of two gases with different simultaneous emissions N 4 and N 2, it is possible to define a relative carbon emission effect by
R
(4)
= G 4 --
N2 which
The indirect effect of CH 4 oxidation to CO 2 has been excluded from IPCC calculations of GWP indexes (Houghton et al, 1992) because most CH 4 emissions come from short-term recycled biogenic CO 2 which is taken out from the atmosphere (CH 4 emissions from land use, for example). In the case of hydroelectric reservoirs, however, CH 4 emissions arise from the decomposition of the biomass stock under water, and as such the indirect effect of CH 4 oxidation to CO 2 has also to be accounted for.
+ AQ4 - ~4)
x=
is a generalization
of expression
(2) when
N4 N2. A comparison between microbial decomposition of flooded forest biomass and C H 4 production by hydroelectric reservoirs and CO 2 production from fossil fuelled electricity generation asks for a generalization of formula (4). In this case, most of the C H 4 emissions
Global warming and emissions¢J'om dams: L P Rosa and R Schaeffer
from hydroelectric reservoirs is concentrated in time and decays during a period of a few years, while CO 2 emissions from fossil fuelled electricity generation remain continuous and constant over the lifespan of the capital investment. There will also be small CH 4 and CO 2 longterm emissions from the decomposition of residual stored biomass remaining in the reservoirs after the initial intense degradation, as well as from new biomass produced over time inside the reservoirs. However, these latter emissions are neglected here, because they are relatively small compared to the CO 2 emissions from fossil fuelled electricity generation, according to data obtained in field studies in some Brazilian as well as Canadian reservoirs (Rosa and Schaeffer, 1994). A considerable portion of those permanent contributions would in any case arise from the normal decomposition of biomass resources on those lands seasonally flooded by the river even if no hydroelectric reservoir had been built (Franken et al, 1992). In this case, it is possible to evaluate the ratio between the warming effects of CH 4 relative to CO 2 using
E2=
n~22( T ,
e-k2(r-T')-e-)v2r)
(9)
)22 where AN2/
n2-
dN 2
Ati
-
dt
(10)
is constant. It is easy to see that, with T = ~o,
n2T ' E2 = --
N2 -
)22
(11) )22
as in the previous case. By applying the full expression (9), R = E 4 / E 2 can still be calculated as in (4) X )22N4
R-
(12) f )24 N2
& R=--
(5)
E2
with e-L2(T - T ' ) _ e-~.2T
with
(13)
f= 1 T')2 2
E, =-a i i A~., I~Se-~/<' (i'd, a2
(6)
The meaning of the factor in (12)
i
x )22 where A/~,7 is the carbon mass of gas i emitted in time and T ' is the time horizon over which we consider the emissions. In principle we must choose T ' < T. In the hypothesis of choosing T = oo, as Lashof and Ahuja did (1990), we can combine (5) and (6) and have the same expression as in (4), but with N 2 = ~,jN2j and N 4 = ~,jN4j , giving the total carbon mass emitted as CO 2 and CH 4, respectively, and )22 G 4 = X --
f)24 is the generalized warming power of CH 4 relative to CO 2, like the G W P but, instead of being limited to simultaneous pulse emissions o f both gases, it allows emissions at different periods of time. Assuming T ' = T = 100 years, we have computed G 4' from (13) and (14), and compared it with the result when T = ~ in Table 1.
(7)
X4
The dependence of radiative forcings on time
equal to that of expression (2), with T = oo. In the case of CH 4 emissions concentrated in time, T ' << T, we will always have from (6) that N4 E 4 = .V - )24
(14)
G' 4 _
(8)
while, in the case of long-term constant-continuous CO 2 emissions up to T ' , by integrating (6) we have for T ' _
Climate forcing arising from increasing atmospheric greenhouse gases is still highly uncertain (Hansen et al, 1988). Our procedure to estimate the error of taking the variables a 2 and a 4 as independent of time in (6) for computing (12) and (14) could therefore be to assume a linear variation along the emission time
a i (t') = a i(O) (1 + qi t')
(15)
In our model, this variation does not matter for C H 4
Energy Policy 1995 Volume 23 Number 2
151
Global warming and emissionsfrom dams: L P Rosa and R Schaeffer Table 1 Generalized global warming potential for methane and for carbon emissions from a hydroelectric power plant relative to a fossil fuelled equivalent power plant (mole basisp Warming index
Time horizon (years)
Source
G W P = 3.70 ° G 0' = 1.84 c G 4' = 5.72 ~ G" = 3.50 d
T T T T
Lashof and Ahuja (1990) Equation (14) Equation (14) Equation (41)
= = = =
oo and pulse at T' = 0 ~ and T' any (finite) T' = 100 ~o and T ' = 100
a Values include the warming potential of CO 2 produced from the oxidation of CH 4, b Residence times of CH 4 and CO 2 are 14.4 and 230 years respectively. c Residence times of CH 4 and CO 2 are 10.5 and 120 years respectively. d Residence times of CH 4 and CO n are 10.5 and 120 years respectively. We have also assumed z = 0.05 (CH~ emission from fossil fuel combustion equal to 5% of CO 2 emission), et = 13 (50% of the decomposition of biomass is aerobic and 50% is anaerobic), y = 1 (all CO 2 is oxidized in CH 4 in the atmosphere).
because most of the biomass in the reservoir is decomposed in a short period of time (within approximately seven years), and s o r l 4 t ' = 0 and a 4 ( t ' ) = a n ( 0 ) . However, it could be important for CO z, since emissions from fossil fuelled electricity generation remain constant over the lifetime of the plant (30 years approximately) and, assuming that the plant is replaced after that by a similar plant, even over the entire period of analysis considered (100 years, in the present exercise). The global warming index G 4 ' , given by (14), can be corrected in such a way that its new value, G4", is related to the previous one by 1
1 -- +
G" 4
G' 4
]'[2
~'4
X
~'2
f'
(16)
with
f,
T"-~ 2,fo 'dt' t' ffie-~
~22T'
The advantage of expressions (16) and (17) over the previous ones is that they provide a simple way to perform a sensitivity analysis. The problem is to forecast the future behaviour of ai (t') to fit the parameter 1]i in (15). Scenario IS92a in the IPCC 1992 report (Legett et al, 1992) assumes an annual growth rate in the concentration of CO 2 in the atmosphere of about 0.7% for the next 100 years. However, more optimistic scenarios in terms of lower CO 2 emissions can possibly be expected. The currently observed increase in the concentration of CO 2 in the atmosphere, Q2 (t'), of about 0.5% per year (Rodhe, 1990) is used here. The relationships between a i (t') and Oi (t') for C H 4 and CO 2 are given by the numerator and by the de-
152
Energy Policy 1995 Volume 23 Number 2
nominator of the fraction in expression (3) respectively. For CO 2, we can approximate for AQ2 << Q2 In Q2 + AQ2 = AQ2
Q2
(18)
Q2
which allows us to write the radiative forcing as being inversely proportional to Q2, and so on a 2 (t') = a 2 (0) 1.005 - t '
(19)
which can be fitted by the linear function (15), with rli = -0.004 yr -l in a very good approximation for 0 < t < 100 years. For C H 4 , scenario IS92a from IPCC (Leggett et al, 1992) forecasts an annual growth rate in the concentration in the atmosphere, Q4 (t'), of about 0.6% for the next 100 years. The radiative forcing variation of CH 4 28 years from now (four times the decay time of the biomass decomposition in the reservoir, according to our model) is only 5%, which can be ignored, as we have done. For T = T ' = 100 years, we obtain G 4 " = 6.66 from expression (16). In the case of considering T = co and T' = 100 years, the value G 4' changes to G4" = 2.30. The problem is that the future behaviour of a i (t') is unknown, because there are many factors involved in its scenario determination, with large uncertainties (Ramanathan, 1976; Donner and Ramanathan, 1980; Steudler et al, 1989; Lelieveld and Crutzen, 1992). All we can do is to perform a sensitivity analysis. The conclusions are that the order of magnitude of the results does not change and the error obtained is lower when the index value is higher. Emissions from hydroelectric reservoirs
A more general and straightforward way to compute the effects for different time horizons of non-simultaneous greenhouse gas fluxes to the atmosphere from hydro-
Global warming and emissionsfrom dams: L P Rosa and R Schaeffer electric reservoirs compared to greenhouse gas emissions by fossil fuelled electricity generation is to write a set of differential equations to describe them as follows.
For hydroelectric reservoirs ( 1) CH 4 emissions N4, remaining at time t, arising from the microbial decomposition of organic matter, which decays with a time constant y, being ot the fraction of the total mass of degradable carbon in the vegetation and soil C that would decay in a reservoir within time T and be released as CH4:
(20)
-= o~C,ye-"[ t - ~ 4 N 4 dt
E2
irN2dt =
I y_~C e -~'r -. ~'2 - 7 / y
~C - ~22
~.4yyotC [1
-
} e-X: r ~2
(24)
e-Vr
E2 = I~N '2 dt = - -
~4--'Y
1 - e-X4 r
~ ' 4 - 'Y
k4(~'4 -- ~'2 )
(~'2 -- Y) (X4 -- k2)
1 - - e-X2r.] /
+ k2
(25)
,I
In spite of their apparent complex forms, expressions (23)-(25) can be greatly simplified if we assume T = ~, as Lashof and Ahuja (1990) did:
xaC (2) C O 2 emissions N 2, remaining at time t, arising from the decomposition of organic matter, assuming the same time constant ~, for the production rate decay and a different fraction [3 of C that would be released
13c E2 =
as C02: dN 2
(21)
= ~C]te -¢t _ ~ 2 N 2
(26)
E4-
E;-
(27)
X2 y o~C
(28)
dt (3) CO 2 produced in the atmosphere N 2, arising from the oxidation of a fraction y of the emitted CH 4 that is transformed in CO2:
dU; (22)
--....._2."= y ~ , 4 N 4 - ~ 2 N ' 2
dt Solving expressions (20), (21) and (22) with the appropriate initial conditions, we can compute the integrated effects of each component of the total forcing: 2
xoc,(1 eYT, enT)23, E4 = x IT N4dt
-~447 -~
~t
The reason why expressions (26)-(28) are so simple is because, when we extend the time horizon up to infinite, the integrated heating effects of both CH 4 and CO 2 always include the total area under the N i (t) curve.
For fossil fuelled electricity generation (1) CO 2 emissions N2, remaining at time t, arising from the combustion of fossil fuels, assuming a constant emission rate n 2 up to time T ' : dN = n 2 -- ~2N2
(29)
dt
~'4
2 Because the results of various net energy analyses for hydroelectric
power performed so far have shown net energy requirements (amount of primary energy, measured in terms of the energy provided by fossil fuels, required to produce one unit of electricity) in the range of 0.03 to 0.10, which are very low and compare quite well with requirements for fossil-fuelled power, we decided not to take into account emissions associated with the use of direct and indirect energy inputs for the construction of the plants. For more details on net energy analyses of electric power, see Gilliland et al, (1981); Gusdorf (1992); Mortimer (1991). With respect to emissions from the reservoir itself, because the 'without project' situation referred to earlier in the paper is also responsible for some greenhouse gas emissions (Franken et al, 1992), we decided in the derivation of 'with project' emissions not to take into account long-term residual emissions from the reservoir but only the initial, intense decomposition of the biomass flooded. Data collected in some field studies in some hydroelectric reservoirs in the Amazon region in Brazil (G. Tundisi, personal communication) proved us right.
(2) CH 4 emissions N 4, remaining at time t, arising from the production, transformation and transport of fossil fuels, assuming a constant emission rate zn 2, with 0
- 2/12 -- ~4N4
(30)
dt (3) CO 2 produced in the atmosphere N2', arising from oxidation of a fraction of the C H 4 emitted, as in the case of hydroelectric reservoirs:
dt
= Y~2N4 - ~ 2 N ~
Energy Policy 1995 Volume 23 Number 2
(31)
153
Global warming and emissions from dams: L P Rosa and R Schaeffer Solving Equations (29)-(31) with the appropriate initial conditions, we can also compute the integrated effects of each component of the total forcing: 3
1 -e-XJ}
n2TI1 E 2 = J'TN 2 d t = K2 ~
E4=xfTN4dt-
T~, 2
xzn2T(1
1--e-X4T )
~'4 Ed = ~ N 2 dt = yzn 2
(32)
(33)
T~4
Yzn2T ' E2 -
(37)
The relative carbon emission effect of a hydroelectric reservoir (taking into account both CH 4 and CO s emissions), taking the warming effect of the carbon emitted by a hypothetical fossil-fuelled power plant (considering both CH 4 and CO s emissions too) as a reference, can now be calculated. We must add (26)-(28) and divide the result by the sum of (35)-(37). We obtain:
T
h o~C
-~ +
R-
(38)
g n2T' 1
-- e-~'4 T
~'4
1 -- e-~'z r ]
+
~4 (~'4 -- ~'2 )
~'2 (~'4 -- ~'2 )
~'2
]
(34)
with
xz
1
We must compare (32) with the previous result given by (9). To do that we have to consider the situation in which CO s emissions from a fossil fuelled power plant cease at time T ' , while their warming effects are computed up to time T > T ' . Thus we have to take the integration in (32) with the upper limit T ' < T and add a new one, of the function N/(T ') e-~'itt - T'i from T ' up to T. The result agrees exactly with the previous one in (9). The same procedure can be used for all gas emissions described by (32)-(34). We have particular interest in extending their effects up to T = oo, which means taking the upper limit T ' in each integration and adding the new one, as above, from T ' up to T = o~. The results, however, are always simple because, in this case, we include all the areas under the c u r v e s N i (t), which means multiplying the total amount of gas emitted to, or transformed in, the atmosphere by its atmospheric lifetime % = 1/~,i in all the formulae: n2T'
(35)
E2~'2
&-
xzn2 T '
(36)
3 Because the net energy requirements for the construction of fossilfuelled power plants is quite small too (Gilliland et al, 1981 ; Mortimer, 1991), here too we decided not to take into account emissions associated with the use of direct and indirect energy inputs for the construction of the plants, even though we do include emissions associated with the production, transformation, transport and combustion of the fossil fuels used in the plants.
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Energy Policy 1995 Volume 23 Number 2
yz
g =--+--+ ~2
--
~4
(39)
~'2
and
x h=--+
13
y +--
(40)
a '2 We note that n2T' and czC represent the total emission of CO 2 from a hypothetical fossil fuelled power plant and the total emission of CH 4 from an also hypothetical hydroelectric power plant, respectively. For effect of comparison we can rewrite (38) as (4) with, instead of G4, h at!
~
__
(41)
g which agrees with formula (14) when we take only the first terms of both (39) and (40). Table 1 shows the results of (41) in comparison with the previous result from (13) and (14). Although all parameters used in (41) have been arbitrarily chosen for the calculations, they seem reasonable for the present exercise. We believe that the method developed here for extending the GWP concept can be used to provide more comprehensive analyses when comparing the greenhouse forcing impacts of switching from hydroelectricity to fossil fuelled electricity generation than previous analyses performed so far (Leggett et al, 1992; Oud, 1993; Rudd et al, 1993). If we take Equation (38), it is possible to conclude that there will be net radiative heating savings by switching from fossil fuelled electricity generation to hydroelectricity if R < 1, and an increase in the greenhouse effect from switching back.
Global warming and emissions fi'om dams: L P Rosa and R Schaeffer Table 2
A comparison of total integrated radiative forcing from greenhouse gas emissions from three hypothetical hydroelectric reser-
voirs and fossil fuelled power plants a Plants
Hydro Ac Hydro Bf Hydro Cg Fossil-fuelled
Energy produced
Flooded area
Energy density
(TWh pa)
(kin 2)
(TWh pa)/km 2
Integrated radiative forcing equivalent b (Mt C pa)/TWh Over 50 years Over 100 years
0.95 17.35 48.18 -
560 2,430 1,100 -
0.0017 0.0071 0.0438 -
1.78 d-I 2.25 c 0.42 d- 2.93 e 0.07 a- 0.48 e 2.52 h- 5.80 i
0.91 d_ 6.27 e 0.22 ~- 1.50e 0.04 d- 0.25 ~ 4.45h-10.23 i
The hypothetical reservoirs in the Amazon region of Brazil have low, medium and high ratios of energy produced to flooded area (energy density). b Although cumulative forcing is normally expressed in W/m 2 pa units, we have decided to use Mt C pa equivalent units as a basis for comparison. c Hypothetical hydroelectric power plant similar to Samuel hydroelectric power plant in the Amazon region in Brazil in terms of amount of energy produced and flooded area. Methane emissions only. d Biomass value of 155.1 t/ha dry weight (Brown and Lugo, 1984), with a carbon stock of 45% of the dry biomass, 10% of which assumed to decompose anaerobically. e Biomass value of 361.5 t/ha dry weight (Setzer and Pereira, 1991), with a carbon stock of 45% of the dry biomass, 30% of which assumed to decompose anaerobically. f Hypothetical hydroelectric power plant similar to Tucurui hydroelectric power plant in the Amazon region in Brazil in terms of amount of energy produced and flooded area. Methane emissions only. g Hypothetical hydroelectric power plant similar to the proposed Belo Monte hydroelectric power plant in the Amazon region in Brazil in terms of amount of energy produced and flooded area. Methane emissions only. h Combined cycle natural gas fired power station with an assumed 45% thermal efficiency and burning gas with CO 2 emissions of 13.7 (kg C)/109 J of fuel (Rosa and Schaeffer, 1994). Coal fired power station with an assumed 35% thermal efficiency and burning coal with CO 2 emissions of 25.2 (kg C)/10 9 J of fuel (Marland, 1983).
Some applications There is some consensus that greenhouse gas emissions to the atmosphere from some hydroelectric reservoirs may be far from negligible (Oud, 1993; R u d d e t al, 1993). There is less agreement, however, on the cumulative heating effects of those emissions over time per unit of energy produced compared to greenhouse gas emissions by fossil fuelled electricity generation (Rosa and Schaeffer, 1994). It may be worthwhile, therefore, to investigate how hydroelectric reservoirs in the Amazon region in Brazil stand against fossil fuelled power plants with respect to the cumulative heating effects of greenhouse gas emissions. Table 2 presents a comparison between the integrated radiative forcing from CH 4 emissions from three hypothetical hydroelectric reservoirs similar (in terms of characteristics of reservoirs and extent and types of landscape flooded) to three real hydroelectric reservoirs in the Amazon region in Brazil (two existing and one projected) with low, medium and high ratios of energy produced to flooded area, and CO 2 emissions from combined cycle natural gas and coal fired power stations over two different time horizons. The reason for assuming hypothetical, rather than real, hydroelectric power stations is to allow a wide range of estimates using different biomass densities (since forest biomass varies greatly in different parts of the region) (Fearnside, 1992) as well as different rates for bacterial decomposition of forest biomass to C H 4. By applying our generalized GWP index we have been able to express the C H 4 fluxes as ' C O 2 equivalents' and com-
pare their warming effects with those from the CO 2 fluxes. After flooding, the CH 4 fluxes from reservoirs have been estimated by assuming that 45% of the biomass values in the region (155.1-361.5 t/ha dry weight (Brown and Lugo, 1984; Setzer and Pereira, 1991)) is carbon, 10-30% of which assumed to decompose anaerobically decaying exponentially over time with a decay time of seven years. The CO 2 fluxes from fossil fuelled power stations have been estimated by assuming carbon contents of 13.7-25.2 kg C / 1 0 9 J of fuel for natural gas and coal respectively (Marland, 1983), and thermal efficiencies of 45-35% for combined cycle natural gas fired and coal fired power stations, respectively. Results are strongly dependent on assumptions and on time horizons. For reservoirs with medium and high ratios of energy produced to flooded area (hydros B and C), the estimated integrated radiative forcings per unit of energy produced were 0.42-2.93 (Mt C pa)/MWh and 0.07-0.48 (Mt C pa)/MWh equivalent units respectively over 50 years, and 0.22-1.50 (Mt C pa)/MWh and 0.04~.25 (Mt C pa)/MWh equivalent units respectively over 100 years. These estimates are clearly much lower than the cumulative heating effects of electrical generation by fossil fuelled power plants with the sole exception of hydro B which, for the high biomass density case with high rate of CH 4 production in the flooded area over 50 years, is similar to the greenhouse effect of electrical generation by combined cycle natural gas fired power generation.
Energy Policy 1995 Vohmw 23 Number 2
155
Global warming and emissions from dams: L P Rosa and R Schaeffer 5 4.5 -c>,
CO2
4
0
3.5
g
3
~ 2.5 E 2 E o ..Q
1.5
o
\cH,
0.5
~
0
20
:
I
p
40
60
80
I
100 120 Time (yr)
I
t
I
I
140
160
180
200
Figure 1 C H 4 emissions from the anaerobic decomposition of biomass in the Tucurui hydroelectric power plant compared to CO 2 emissions from the combustion of fuel oil in a fossil fuelled equivalent power plant over a time horizon of 100 years 3.5
350
~
3
3oo
v
2.5
250 ~
2
E 200 '=
.~ 1.5
150 .=_
E
1
100 E
0.5
50 c£
E "= t-
o
~
t-.
o
0 0
20
40
60
80
100 120 Time (yr)
140
160
180
200
Figure 2 C H 4 emissions from the anaerobic decomposition of biomass in the Tucurui hydroelectric power plant compared to CO 2 emissions from the combustion of fuel oil in a fossil fuelled equivalent power plant remaining in the atmosphere at time t. Scales are different as indicated in vertical axes For a reservoir with a low ratio of energy produced to flooded area (hydro A), however, the estimated integrated radiative forcings per unit of energy produced were 1.78-12.25 (Mt C pa)/TWh equivalent units over 50 years and 0.91-6.27 (Mt C pa)/TWh equivalent units over 100 years. These estimates may be lower, similar or higher than the greenhouse effect of fossil fuelled electrical generation, depending on biomass density and CH 4 production in the flooded area, fossil fuelled electrical generation technology and the time horizon of concern. In any case, however, the longer the planning horizon the better hydroelectricity becomes and the worse thermoelectricity becomes (this is a direct consequence of both the distinct pattern of emissions between hydroelectricity and thermoelectricity, and also a consequence of the different atmospheric lifetime of CH 4 as compared to CO2). For a graphical representation of the method
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Energy Policy 1995 Volume 23 Number 2
employed in our previous analysis, we decided to compare the Tucurui hydroelectric power plant (Table 2) with a hypothetical oil fired equivalent power station typical for Brazil, supposing that both plants generate the same amount of electrical energy during a 100 year period. Emissions are shown in Figure 1, where CH 4 fluxes from the reservoir have been estimated by assuming that 45% of the biomass value in the reservoir region (226 t/ha dry weight (Fearnside, 1990)) is carbon, 30% of which is decomposing anaerobically and decaying exponentially over time with a decay time of seven years. The CO z fluxes from the oil fired equivalent power station have been estimated by assuming a carbon content of 20.0 kg C/109 J of fuel (Marland, 1983), and a thermal efficiency of 27.5%, typical in Brazil today (MME, 1993). Figure 2 shows the quantities of each gas remaining in the atmosphere at time t. We have also calculated the
Global warming and emissions from dams. L P Rosa and R Schaeffer 1800
.40000
J
1600
5 ~ 14oo #.~ 12oo
35000 "5 30000 ~
OH4
5.~
CO 2
25000 ~ ~_
lOOO
~o
800
~E
600
~I
400
--
200
2 0 0 0 0 : 5 >" 15000 10000 ~ 0 ~0 5000 ---= I
0 20
o
40
60
80
100 120 Time (yr)
140
160
0 200
180
Figure 3 Integrated radiative forcing of C H 4 emissions (E4) from the anaerobic decomposition of biomass in the Tucurui hydroelectric power plant compared to integrated radiative forcing of CO 2 emissions (E2) from the combustion of fuel oil in a fossil fuelled equivalent power plant. Scales are different as indicated in vertical axes 4.5 4 3.5 3 2.5 2 1.5 1
0.5 0
I
0
0
0
I
I
I
I
r
I
0
Time (yr)
Figure 4 Ratios R = E4/E 2 between the integrated warming effects of C H 4 emissions from the anaerobic decomposition of biomass in the Tucurui hydroelectric power plant and CO 2 emissions from the combustion of fuel oil in a fossil fuelled equivalent power plant warming effect E 2 and E 4, expressed as the integrated product of the radiative forcings with the fraction of CO 2 and CH 4 remaining in the atmosphere, respectively (Figure 3). The CH 4 warming effect saturates after some 60 years, while the CO 2 warming effect continues to increase even after 200 years (as a matter of fact, it stabilizes only after about 500 years). Finally, we can see in Figure 4 the ratio R = E4/E 2 between the warming effect of CH 4 relative to CO 2, defined by expressions (5) and (6) respectively. With its power density of 0.39 W/m 2 (energy density of 0.0071 TWh pa/km 2, for a capacity factor of 50% for its installed capacity of 3960 MW), the R index becomes lower than 1 after about 23 years. This means that, with respect to greenhouse gas emissions, the Tucurui hydroelectric power plant is clearly more beneficial to the global environment than an oil fired power station.
Conclusions The GWP generalization introduced here seems to represent a useful conceptual extension from previous studies (Lashof and Ahuja, 1990; Shine et al, 1990; Isaksen et al, 1992; Harvey, 1993; Oud, 1993; Rudd et al, 1993). Rather than viewing the GWP index as the ratio of the integrated radiative forcing due to a single pulse emission of two different gases at time t = 0, the new index defined can be seen as the ratio of radiative heatings at the end of a given time horizon, assuming distinct and continuous emissions during an entire time interval. The ratio between area flooded and energy produced has already been a matter of concern in the context of the overall environmental and social impacts of hydroelectricity worldwide in general (Gleick, 1992; Lele and Subramanian, 1988) and in Brazil in particular (Rosa
Energy Policy 1995 Volume 23 Number 2
157
Global warming and emissions from dams: L P Rosa and R Schaeffer
and Schaeffer, 1990). We have shown here that this ratio, together with other local characteristics in the area of the reservoir, may also be important in determining the magnitude of greenhouse gas emissions. Though we cannot ignore some of the serious environmental impacts that may arise from a hydroelectric reservoir, the evidence gathered here seem to suggest that, in the case of Brazil, hydroelectricity can still be seen as a realistic mitigation option for global climate change. Our results suggest that, though hydroelectric reservoirs may be significant sources of greenhouse gases and the cumulative heating effects of their emissions may be far from negligible, for the cases studied hydroelectricity seems in general to contribute less to the greenhouse effect over a long time horizon than fossil fuelled electricity generation. This is not to mention the issue recently raised that man-made lakes may be the single largest (and negative) contribution to the sea level (Chao, 1994; Rodenburg, 1994; Sahagian et al, 1994). The method proposed here is now being used to test whether a redirection of the energy policy in Brazil from hydroelectricity toward fossil fuelled electricity generation, as supported by some international and bilateral funding agencies, represents a step in the right direction toward reducing the Brazilian contribution to global wanning. Acknowledgements This paper benefited enormously from discussions with Marco Aur61io dos Santos and Rafael Schechtman. This work was supported in part by Conselho Nacional de Desenvolvimento Cientffico e Tecnol6gico-CNPq and ELETROBRAS-Centrais E16tricas Brasileiras S.A. References Brown, S and Lugo, A E (1984) 'Biomass of tropical forests: a new estimate based on forest volumes' Science 223 1290-1293 Chao, B F (1994) 'Man-made lakes and sea-level rise' Nature 370 (6487) 258 Donner, L and Ramanathan, V (1980) 'Methane and nitrous oxide: their effects on the terrestrial climate' Journal of the Atmospheric Sciences 37 (1) 119-124 Feamside, F (1990) 'Deforestation in Brazilian Amazonia as source of greenhouse gases' Regional Conference on Global Wanning, USP, Sao Paulo, Brazil Fearnside, P M (1992) 'Forest biomass in Brazilian Amazonia: comments on the estimate by Brown and Lugo' lnterciencia 17 (1) 1927 Franken, R O G, Vierssen, W and Lubberding, H J (1992) 'Emissions of some greenhouse gases from aquatic and semi-aquatic ecosystems in the Netherlands and options to control them' The Science of
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Energy Policy 1995 Volume 23 Number"2
the Total Environment 126 (3) 277-293 Gilliland, M W, Klopatek, J M and Hildebrand, S G (1981) 'Net energy: results for small-scale hydroelectric power and summary of existing analyses' Energy 6 (10) 1029-1040 Gleick, P H (1992) 'Environmental consequences of hydroelectric development: the role of facility size and type' Energy 17 (8) 735-747 Gusdorf, J (1992) 'Energy paybacks and renewable breeders' Energy 17(12) 1137-1151 Hansen, J, Fung, I, Lacis, A, Rind, D, Lebedeff, S, Ruedy, R, Russel, G and Stone, P (1988) 'Global climate changes as forecast by Goddard Institute for Space Studies three-dimensional model' Journal of Geophysical Research 93 9341-9364 Harvey, L D D (1993) 'A guide to global warming potentials (GWPs)' Energy Policy 21 ( 1) 24-34 Houghton, J T, Jenkins, G J and Ephraums, J J (eds) (1990) Climate Change: The IPCC Scientific Assessment Cambridge University Press, Cambridge Houghton, J T, Callander, B A and Varney, S K (eds) (1992) Climate Change 1992: The Supplementary Report to the IPCC Scientific Assessment Cambridge University Press, Cambridge Isaksen, I S A, Ramaswamy, V, Rodhe, H and Wigley, T M L (1992) 'Radiative forcing of climate', in Houghton et al, 1992, pp 47-67 Lashof, D A and Ahuja, D R (1990) 'Relative contributions of greenhouse gas emissions to global warming' Nature 344 (6266) 529-531 Leggett, J, Pepper, W J and Swart, R J 'Emissions scenarios for the IPCC: an update' in Houghton et al, 1992, pp 69-95 Lele, S M and Subramanian, D K (1988) 'A hydro-wood net-energy approach to hydro project design' Energy 13 (4) 367-381 Lelieveld, J and Crutzen, P J (1992) 'Indirect chemical effects of methane on climate warming' Nature 355 (6358) 339-342 Marland, G (1983) 'Carbon dioxide emission rates for conventional and synthetic fuels' Energy 8 (12) 981-992 Minist6rio das Minas e Energia (MME), Brazil (1993) Balan~.o Energ~tico 1993 Brasflia, p 111 Mortimer, N D (1991) 'Energy analysis of renewable energy sources' Energy Policy 19 (4) 374-385 Oud, E (1993) 'Global wanning: a changing climate for hydro' Water Power & Dam Construction 20--23 Ramanathan0 V (1976) 'Radiative transfer within the earth's troposphere and stratosphere: a simplified radiative-convective model' Journal of the Atmospheric Sciences 33 1330-1346 Rodenburg, E (1994) Nature 370 (6487) 258 Rodhe, H (1990) 'A comparison of the contribution of various gases to the greenhouse effect' Science 248 (4960) 1217-1219 Rosa, L P and Schaeffer, R (1990) 'Brazilian energy policy', in Santos, L A O and Andrade, L M M (eds) Hydroelectric Dams on Brazil's Xingu River and Indigenous Peoples Cultural Survival, Cambridge, MA pp 47-52 Rosa, L P and Schaeffer, R (1994) 'Greenhouse gas emissions from hydroelectric reservoirs" Ambio 23 (2) 164-165 Rudd, J W M, Harris, R, Kelly, C A, and Hecky, R E (1993) 'Are hydroelectric reservoirs significant sources of greenhouse gases?' Ambio 22 (4) 246-248 Sahagian, F W, Schwartz, F W and Jacobs, D K (1994) 'Direct anthropogenic contributions to sea-level rise in the twentieth century', Nature 367 (6458) 54-57 Setzer, A W and Pereira, M C (1991) Amazonian biomass burning in 1987 and an estimate of their tropospheric emissions' Ambio 20 (1) 19-22 Shine, K P, Derwent, R G, Wuebbles, D J a n d Morcrette, J-J '(1990) Radiative forcing of climate', in Houghton et al, 1992, pp 41-68 Steudler, P A, Bowden, R D, Melillo, J M and Aber, J D (1989) 'Influence of nitrogen fertilization on methane uptake in temperate forest soils' Nature 341 (6240) 31 4-316