TERJE BERNTSEN , JAN FUGLESTVEDT , GUNNAR MYHRE...

ABATEMENT OF GREENHOUSE GASES: DOES LOCATION MATTER?

TERJE BERNTSEN1, JAN FUGLESTVEDT1, GUNNAR MYHRE2,3,FRODE STORDAL2,3 and TORE F. BERGLEN3

1CICERO, Center for International Climate and Environmental Research, P.O. Box 1129 Blindern,N-0318 Oslo, Norway

E-mail: [email protected] Institute for Air Research (NILU), Norway

3Department of Geosciences, University of Oslo, Norway

Abstract. Today’s climate policy is based on the assumption that the location of emissions reductions

has no impact on the overall climate effect. However, this may not be the case since reductions of

greenhouse gases generally will lead to changes in emissions of short-lived gases and aerosols.

Abatement measures may be primarily targeted at reducing CO2, but may also simultaneously reduce

emissions of NOx, CO, CH4 and SO2 and aerosols. Emissions of these species may cause significant

additional radiative forcing. We have used a global 3-D chemical transport model and a radiative

transfer model to study the impact on climate in terms of radiative forcing for a realistic change

in location of the emissions from large-scale sources. Based on an assumed 10% reduction in CO2

emissions, reductions in the emissions of other species have been estimated. Climate impact for the

SRES A1B scenario is compared to two reduction cases, with the main focus on a case with emission

reductions between 2010 and 2030, but also a case with sustained emission reductions. The emission

reductions are applied to four different regions (Europe, China, South Asia, and South America). In

terms of integrated radiative forcing (over 100 yr), the total effect (including only the direct effect

of aerosols) is always smaller than for CO2 alone. Large variations between the regions are found

(53–86% of the CO2 effect). Inclusion of the indirect effects of sulphate aerosols reduces the net effect

of measures towards zero. The global temperature responses, calculated with a simple energy balance

model, show an initial additional warming of different magnitude between the regions followed by a

more uniform reduction in the warming later. A major part of the regional differences can be attributed

to differences related to aerosols, while ozone and changes in methane lifetime make relatively small

contributions. Emission reductions in a different sector (e.g. transportation instead of large-scale

sources) might change this conclusion since the NOx to SO2 ratio in the emissions is significantly

higher for transportation than for large-scale sources. The total climate effect of abatement measures

thus depends on (i) which gases and aerosols are affected by the measure, (ii) the lifetime of the

measure implemented, (iii) time horizon over which the effects are considered, and (iv) the chemical,

physical and meteorological conditions in the region. There are important policy implications of the

results. Equal effects of a measure cannot be assumed if the measure is implemented in a different

region and if several gases are affected. Thus, the design of emission reduction measures should be

considered thoroughly before implementation.

1. Introduction

Most of today’s climate policy is based on the assumption that the locationof emissions reductions has no impact on the overall climate effect (e.g. JointImplementation, international emission trading). However, this may not be the case

Climatic Change (2006) 74: 377–411

DOI: 10.1007/s10584-006-0433-4 c© Springer 2006

378 TERJE BERNTSEN ET AL.

if emissions of several gases and aerosols are reduced simultaneously. Abatementmeasures such as reduced fossil fuel consumption, biomass burning, or switchingfrom coal to natural gas not only reduce emissions of greenhouse gases (CO2,CH4, etc.) but may also reduce such components as NOx, CO, and volatile organiccarbon (VOCs), which may indirectly cause significant radiative forcing of climatethrough chemical processes in the atmosphere. These reductions combined witha reduction in the emissions of carbonaceous aerosols (soot and organic carbon)and SO2 giving sulphate particles, can affect the total change in radiative forcingof a given abatement measure significantly. Changed emissions of such short-livedspecies that occur alongside reductions of gases included in the Kyoto Protocolthrough technological couplings may thus affect the total effect of emissionreductions. Previous studies have shown that the magnitude of these indirecteffects varies widely from region to region because of differences in chemical andmeteorological/physical key parameters and in emission ratios (Lin et al., 1988;Johnson and Derwent, 1996; Fuglestvedt et al., 1999; Derwent et al., 2001; Wildet al., 2001; Berntsen et al., 2002). Thus, a regional variation in the effectivenessof abatement measures is expected depending on to which extent the measuresaffect emissions of ozone precursors, SO2 or aerosols in addition to the well mixedgreenhouse gases. Studying the more realistic situation where several componentsare reduced simultaneously because of coupled source strengths may provide newinformation about possible regional variations in abatement effectiveness, whichwill be important in the further development of international climate policy.

In this paper we explore whether reduction in other species (in particular gasesand aerosols not included in the Kyoto Protocol) as a consequence of CO2 abate-ment measures will lead to significant regional differences in the total efficiencyof such reductions. It is a well established fact that reductions in SO2 followingreductions in greenhouse cases can partly offset the impacts of the reductionsthrough reduced cooling by sulphate aerosols (e.g. Wigley, 1991, West et al., 1997;Hayhoe et al., 2002). However, the regional aspects of the net effect of mitigationmeasures have not been studied in detail previously. We use a global chemicaltransport model (CTM) to study the chemical processes, and a radiative transfermodel to calculate the radiative forcing of ozone and aerosols. Concentrationchanges for CO2 have been calculated by the model described by Joos et al. (1996),while methane and N2O responses to emission changes have been calculatedwith a simple box model with variable lifetime of methane as given by the CTM.Standard concentration-forcing relations from IPCC (2001) have been used forCO2, CH4, and N2O. A simple energy balance upwelling/diffusion climate modelhas been used to estimate global mean temperature changes. We compare potentialfuture climate effects of the emission reduction including both short-lived andlong-lived components. The results of such comparisons can depend strongly onthe metric used (e.g. Smith, 2003). To emphasize this we perform the comparisonusing different metrics based either on the standard procedure of integrated

ABATEMENT OF GREENHOUSE GASES: DOES LOCATION MATTER? 379

radiative forcing (similar to the Global Warming Potentials), or with a metricthat integrates a non-linear damage function with discounting (e.g. Kandlikar,1995).

Previous studies using both simple and sophisticated models for atmosphericprocesses have shown that the effects of emissions may depend on the location(horizontal and vertical) and timing (season) of the emissions. For instance, emis-sions of NOx from aircraft in the upper troposphere and lower stratosphere have amuch stronger effect on local ozone (O3) concentrations than if the same amountof NOx were emitted close to ground level (Fuglestvedt et al., 1996; Brasseur et al.,1998; IPCC 1999). In addition to the differences in chemical efficiency, ozonechanges in these altitudes give a larger radiative forcing compared to an equalozone change at lower altitudes (Wang et al., 1980; Lacis et al., 1990; Hansen et al.,1997).

For emissions from ground sources, large variations in both chemical efficiencyand radiative forcing can also be expected horizontally due to strong non-linearrelations in atmospheric chemistry and large regional differences in both chemical(e.g. NOx, VOCs) and physical (e.g. UV, temperature, humidity, albedo, convection,clouds) key parameters. As shown by Isaksen et al. (1978) and Lin et al. (1988) thereis a strong non-linear relation between levels of NOx and tropospheric ozone pro-duction. Due to its catalytic role in the production of ozone, NOx emitted in or trans-ported to the remote troposphere is more efficient in producing ozone than if it wereintroduced to the troposphere in a polluted region and oxidized there. In a study of ef-fects of NOx reductions, Fuglestvedt et al. (1999) found a much higher sensitivity forupper tropospheric O3 to reductions in NOx from ground sources in Southeast Asiaand Australia than in regions at middle and high latitudes like U.S.A., Europe andScandinavia. Differences in seasonal variation of O3 production efficiency were alsoevident.

Emissions of ozone precursors also affect the oxidizing capacity of theatmosphere mainly through perturbations of the hydroxyl (OH) radical. In termsof radiative forcing of climate, the main effect is to change the lifetime of methane.While the larger part of the ozone perturbation occurs within a few months Prather(1996) have demonstrated that the changes in methane occur on a timescalecorresponding to the ‘primary mode’ of the tropospheric chemistry system (about14 yr, Wild et al., 2001; Derwent et al., 2001). The effect of NOx emissions aloneon the levels of methane have been found to reduce its lifetime and to vary withrespect to location of emissions in a similar way as for ozone, i.e. with highersensitivities in low NOx backgrounds regions (Fuglestvedt et al., 1999; Wild et al.,2001). Following pulse emissions of NOx from surface (different latitudes) as wellas free tropospheric sources (lightning and aircraft), Wild et al. (2001) find thatwhen integrated (for more than 50 yr) the net RF (ozone(+) and methane(−)) isslightly negative (except for aircraft), but can be the difference between two largenumbers.


2. Experimental Design

A large number of possible measures or combinations of measures to meet obli-gations under the Kyoto Protocol can be envisaged. The aim of this study is notto explore the whole ‘measure space’, but to look more closely at one reasonablyrealistic case to see whether other species affected by reduction measures will in-crease or decrease the total efficiency of emission reductions, and whether there aresignificant regional variations in the efficiency of such reductions. We have cho-sen to study an idealized case where the reductions in emissions of other speciesthan CO2 do not vary between regions. In this way we can study the chemical andphysical effects of geographical location in isolation from other factors such asdifferences in technology and emissions. Since reductions are likely to be carriedout through removal of the oldest and least efficient installations, a more completeanalysis would require a bottom-up analysis of potential measures in different re-gions. However, since we do not adopt very large emission changes and currentclimate policy do not seem to indicate large emission changes, the results from oursimulations can be scaled within reasonable limits to represent the effects of othermore realistic combinations of measures.

The purpose of the experimental design was to simulate the transient totaleffects (in terms of RF) of a realistic climate mitigation measure in response tothe demands of the Kyoto Protocol. As the point of departure, we consider a 10%reduction in total man-made CO2 emissions in Europe, obtained through measuresaimed at large-scale sources only. These sources are based on the definition ofsectors by the European Environment Agency (EEA, 1999) and comprise publicpower, cogeneration and district heating (sector 1 of EEA (1999)) and industrialcombustion (sector 3 of EEA (1999)). EEA (1999) gives the total emissions fromsectors 1 and 3 of CO2, N2O, CH4, ozone precursors (NOx, CO, and VOCs),and SO2. Emissions of black- and organic carbon aerosols are based on emissionfactors from Cooke et al. (1999). The emissions used are summarized in the nextsection.

To evaluate the validity of the assumption that ‘location does not matter’ we haveselected four regions of the world (Figure 1), and performed separate calculationswith the Oslo-CTM2 model with equal emission reductions (in absolute terms, massunits) in these four regions. To evaluate the net effect of these mitigation measures,we have calculated differences in radiative forcing for the perturbations of CO2,N2O, CH4 (including lifetime effects), tropospheric O3, and sulphate, black carbon(BC) and organic carbon (OC) aerosols.

Since we look at measures regulating large-scale sources only, we makethe assumption that the reductions are obtained by investment in some kind ofnew technical equipment with a specific lifetime. In our reference calculationwe choose this to be 20 yr, starting in 2010. At the end of the 20-yr period(i.e. in 2030) all emissions are set back to the baseline emissions (see below).The rationale for this assumption is that we want to study the effect of specific


Figure 1. The four regions for which the effect of equal emission perturbations are calculated.

measures. It may be argued that new technology in large-scale facilities may lastfor more than 20 yr. A discussion of how the length of this period affects theresults is therefore included in the Section 4. If sectors with a large number ofsmaller sources (e.g. transportation or domestic sources) were targeted through forexample increased taxation, one would expect that the reductions in the emissionswould be reversed with a timescale of much less than 20 yr when the tax was liftedagain.

Another potentially important assumption is the choice of baseline emissions.Since the aim of the study is to test effects of an important assumption behind theso-called flexible mechanisms of the Kyoto Protocol, we have chosen a baselinescenario that assumes international cooperation. Of the IPCC SRES marker scenar-ios (IPCC, 2000), the A1B scenario is the one that assumes the most widespreadinternational cooperation. We have therefore used this emission scenario (it includesall the gases of interest, but not the carbonaceous aerosols) as our baseline. Figure 2shows a schematic illustration of how the emission perturbations are projected tochange over time.

For the long-lived gases (CO2, N2O and CH4) we perform transient calculationsof the concentrations following the 20-yr long reduction in emissions for theperiod 1990–2110. Emissions are kept constant between 2100 and 2110. Thedetails of these calculations are given in Section 3.1. The perturbations inducedby the short lived gases and aerosols (tropospheric ozone, OH, sulphate andcarbonaceous aerosols) are assumed to appear instantaneously after the emissionreductions start, and vanish again immediately after 20 yr. Figure 3 shows aschematic illustration of this assumption. Due to computational restrictions wecannot perform transient CTM calculations of the effect of the short-lived gasesand aerosols. Instead we have performed time-slice calculations for 1990 and


Figure 2. Schematic illustration of projected emissions. The shape of the baseline emissions (solid

line) and the magnitude of the reductions are different for the different species (given by the SRES

A1B and the perturbations (see Table I)).

Figure 3. Schematic illustration of the CTM calculations, and assumption about temporal behavior

of the perturbations in the concentrations of the short-lived species.

2020 baseline conditions (diamond symbol in Figure 3), and for the 2020 cases(four regions) with reduced emissions (square symbol). The square box given bythe dashed line shows the perturbation of the concentrations of the short-livedspecies.


2.1. EMISSIONS

The 1990 and 2020 base emissions are taken from the A1B marker scenario ofIPCC (2000). It includes the geographical distribution of the emissions of short-lived gases on a 1◦ × 1◦ grid. (sres.ciesin.org/final data.html). The 1990 emissionsof BC from fossil fuel (FF) sources are taken from Cooke et al. (1999), and BC frombiomass burning (BMB) are taken from the Global Emissions Inventory Activity(GEIA, weather.engin.umich.edu/geia). Total emissions of BC from FF sourcesare 5.12 Tg(C)/yr. Total (FF + BMB) monthly emissions for OC are taken fromLiousse et al. (1996). For each grid cell, the fraction of OC coming from BMB isassumed to be equal to the fraction of BC from BMB. For the 2020 baseline case,the emissions of BC and OC from fossil fuel sources are scaled according to thechange in SO2 emissions.

Table I shows the annual emissions of gases and aerosols used in this study. Thelast column shows the reductions following the 10% reduction in CO2 emissionsin Europe. EEA (1999) gives the total European (28 countries) emissions of CO2,N2O, CH4, CO, NOx, VOC, and SO2 for 11 different sectors. Using these EEAnumbers we find that a 10% total reduction of CO2 emissions in Europe is equalto a 19.3% reduction from sectors 1 and 3 (i.e. no reductions in other sectors). Theemission perturbations in Table I are then derived by reducing the total emissionsof CO2, N2O, CH4, CO, NOx, VOC, and SO2 from EEA sectors 1 and 3 by 19.3%.

For BC and OC, which are not included in the EEA emission inventory, submi-cron emission factors (BC/CO2 and OC/CO2) for industrial combustion in semi-developed countries from Cooke et al., (1999) have been used. The emission factorsfor semi-developed countries were derived by Cooke et al. by taking the maximumin the range of the emission factors found for developed countries. Since it is likelythat older facilities with the highest emissions are closed down first as part of cli-mate mitigation measures, using the values for semi-developed countries seems

TABLE I

Global emissions for 1990 and 2020 based on SRES A1B, and estimated changes

in global emissions following a 10% decrease in European CO2 emissions

Global 1990 Global 2020 Emission perturbations

CO2 (Pg(C)/yr) 7.10 12.6 −0.13 −1.03%

CH4 (Tg/yr) 506 632 −0.125 −0.02%

N2O (Tg(N)/yr) 12.9 13.7 −0.025 −0.18%

NOx (Tg(N)/yr 43.1 58.2 −0.365 −0.63%

CO (Tg/yr) 895 1053 −2.06 −0.20%

NMVOC (Tg/yr) 247 308 −0.19 −0.06%

SO2 (Tg(S)/yr) 73.1 102 −2.2 −2.2%

BC (Tg(C)/yr) 5.91 7.07 −0.054 −0.76%

OC (Tg(C)/yr) 81.1 99.7 −0.123 −0.12%


appropriate for this study. Generally the combustion in large-scale facilities is moreefficient (or includes technology to reduce emissions of e.g. NOx and N2O) thanthe combustion in smaller installations (i.e. motor vehicles, domestic heating, etc).This can be seen from the relative emission changes given in Table I. For all com-ponents except SO2, the relative perturbations are smaller than for CO2. In the caseof SO2, the burning of sulphur-containing coal in large-scale combustion explainsthis difference.

All 2020 simulations with the CTM have been carried out with a fixed con-centration of methane of 2026 ppbv, taken from concentrations given for the A1Bscenario (IPCC, 2001).

2.2. MODEL DESCRIPTION

Simple globally averaged models are used to calculate the atmospheric con-centrations of the long-lived species (CO2, N2O and CH4), while a global 3-DCTM is used to calculate the concentrations of the short-lived species (ozone,and the sulphate and carbonaceous aerosols). The CTM is also used to calculateatmospheric lifetimes for CH4 based on the distribution of OH radicals in theCTM.

2.2.1. Models for CO2, N2O, and CH4

Transient simulations (1990–2110) of the concentrations and radiative forcing ofCO2, N2O and CH4 are performed using the modules of CICERO’s simple climatemodel (Fuglestvedt and Berntsen, 1999; Fuglestvedt et al., 2000). The atmosphericconcentration of CO2 is calculated using an efficient and accurate representation ofthe carbon cycle developed by Joos et al. (1996). The model uses an ocean mixed-layer pulse response function that characterizes the surface to deep ocean mixingin combination with a separate equation describing the air-sea exchange based onthe HILDA model (Siegenthaler and Joos, 1992).

For N2O, a fixed lifetime (e-folding time) of 120 yr is used to calculate theconcentrations. Methane concentrations are calculated using the lifetimes, includingthe small changes driven by regional emissions of reactive gases, as calculated bythe full 3-D CTM. Between 1990 and 2020 a linear interpolation of the 1990 and2020 lifetimes is used, while after 2020 the 2020 lifetimes are used. The globalscale feedback of methane on its own lifetime through its effects on OH (Berntsenet al., 1992; Prather, 1996) is accounted for through the adjustment of the lifetimeproposed by Osborn and Wigley (1994)

τatm = τ 0atm

(C

C0

)N

where τatm is the methane lifetime, and N = 0.3, C0 = 1700 ppbv, and τ 0atm is the

methane lifetime when C = C0.


The radiative forcing of the CO2, CH4, and N2O perturbations are calculatedusing the parameterizations of the relations between global mean concentrationsand RF given in IPCC (2001).

2.2.2. The Oslo CTM-2 ModelThe OSLO-CTM2 is an off-line chemical transport model that uses pre-calculatedmeteorological fields to drive the chemical turnover and distribution of tracers inthe troposphere (Sundet, 1997; Kraabøl et al., 2002). The horizontal resolution ofthe model is determined by the input data and computer time available. The inputdata set is based on ECMWF forecast data with a T63 (1.87◦ × 1.87◦) horizontalresolution, truncated to T21 (5.6◦ × 5.6◦) for the simulations in this study. In thevertical, the model has 19 levels from the surface up to 10 hPa. The meteorologicalinput data have been generated for year 1996 by running the Integrated ForecastSystem (IFS) model at ECMWF in a series of forecasts starting from the analyzedfields every 24 h. Each forecast is run for 36 h, allowing for 12 h of spin-up.Linking together all the forecasts gives us a continuous record of input data. Dataare sampled every 3 h. The advection of chemical species is calculated by thesecond-order moment method, which is able to maintain large gradients in thedistribution of species (Prather, 1986). Vertical mixing by convection is based onthe Tiedtke mass flux scheme (Tiedtke, 1989). Turbulent mixing in the boundarylayer is treated according to the Holtslag K-profile scheme (Holtslag et al., 1990).

The chemical scheme includes 62 chemical compounds and 130 gas phase re-actions in order to describe the photochemistry of the troposphere (Berntsen andIsaksen, 1997; Berntsen and Isaksen, 1999). Recently, the scheme was extended toinclude sulphur chemistry, which has been coupled to the photochemistry (Berglenet al., 2004). The coupling of sulphur and the oxidant chemistry means that oxida-tion limitations (i.e. removal of H2O2 before all of the SO2 oxidized) can be treatedproperly.

The scheme is solved using the Quasi Steady State Approximation (Hesstvedtet al., 1978). Photodissociation rates are calculated on-line, following the approachdescribed in Wild et al. (2000). NOx emissions from lightning are coupled on-lineto the convection in the model using the parameterisation proposed by Price andRind (1993) and the procedure given by Berntsen and Isaksen (1999).

Carbonaceous aerosols are implemented following Cooke et al. (1999). BothBC and OC are separated into a hydrophobic fraction and a hydrophilic fraction.Emissions of BC are assumed to be 80% hydrophobic, while for OC this figureis 50%. Hydrophobic aerosols are aged (oxidized or coated by a hydrophiliccompound) in the atmosphere and then become hydrophilic with an exponentiallifetime of 1.15 days. Dry deposition of hydrophilic carbonaceous aerosols iscalculated with a deposition velocity of 0.025 cm/s over dry surfaces (land)and 0.2 cm/s over oceans. For hydrophobic aerosols, a deposition velocity of0.025 cm/s is applied for all surfaces. The hydrophilic aerosols are also removedby wet deposition. These aerosols are assumed to be 100% absorbed in the cloud


droplets, and are removed according to the fraction of the liquid water content(LWC) of a cloud that is removed by precipitation.

For the simulations in this study, the CTM was run for 18 months with a 6-monthspin-up period, for the six simulations (1990, SRES A1B 2020 (reference) and 2020(with emission reductions in Europe, China, South Asia and South America)).

For ozone, output of monthly mean fields was used for the radiative forcingcalculations. For the aerosols (sulphate, BC, and OC aerosols), three-dimensionalfields of concentrations were sampled every 3 h because of the effects of humidityand clouds on radiative forcing. Data for humidity and clouds (for the RF calcula-tions) were taken from the input data for the CTM described above.

2.2.3. Radiative Transfer ModelsThe radiative transfer calculations of radiative forcing from ozone changeswere made using schemes for thermal infrared radiation and solar radiation asdescribed in Berntsen et al. (1997) and Myhre et al. (2000). The thermal infraredscheme is an absorptivity/emissivity broadband model and the solar scheme isa multi-stream model using the discrete ordinate method (Stamnes et al., 1988).Radiative transfer calculations of aerosols are performed with a multi-streammodel using the discrete ordinate method (Stamnes et al., 1988; Myhre et al.,2002). The solar scheme treats the gas absorption with the exponential sum fittingmethod.

The optical properties of the aerosols are calculated with Mie Theory. Forsulphate aerosols the optical properties are taken from Myhre et al. (2002) andfor BC from Myhre et al., (1998). BC aerosols are assumed to be externally mixed.For OC we have used a size distribution from Penner et al. (1998) and assumption ofpure scattering aerosols in the calculation of the optical properties. No hygroscopicgrowth is taken into account for the organic carbon aerosols.

Aerosols are known to cause indirect radiative effects through modificationsof clouds. It is not unlikely that the magnitude of these effects is different for thedifferent regions we consider in our analysis. To estimate the indirect RF of sulphateaerosols we have used the standard IPCC formulations used in SCMs (IPCC, 1997).Most of our analysis (cf. Section 4) is done with only the direct effect of aerosolsand the simple order-of-magnitude estimates of the indirect forcing is only added toillustrate the large uncertainties. Organic carbon aerosols can give a negative indirectradiative forcing through the same mechanism as sulphate aerosols, but most GCMstudies indicate that it is smaller than for sulphate (Kristjansson, 2002). Black carbonare less hydrophilic so their indirect effects are probably smaller and linked to theamount of sulphate available, however, it has been suggested that they may causea semi direct effect through absorption of solar radiation leading to evaporation ofcloud droplets and inhibiting formation of clouds (Hansen et al., 1997; Ackermanet al., 2000; Lohmann and Feichter, 2001). Due to the large uncertainties and theneed to perform very costly GCM calculation to do these calculations properly (ourfocus is on regional differences which we believe are even more uncertain than


the global numbers) we have not made separate calculations of indirect effects ofaerosols on radiative forcing.

3. Results

The following section describes how applying the same package of absolute emis-sion reductions (column 4 of Table I) to the four regions defined above affectsperturbations of the chemical composition and radiative forcing. All differences,including regional differences, shown in Figures 4–14 refer to the impact of theseemission reductions.

3.1. CHEMICAL PERTURBATIONS

3.1.1. Long-Lived SpeciesThe transient behavior of the atmospheric CO2 abundances given in Figure 4, closelyresembles the A1B results for the ISAM and BERN-CC carbon cycle models (IPCC,2001). For 2100 our simulations give 698 ppmv, while ISAM and BERN-CC give717 and 703 ppmv respectively. In response to the reduced emissions between2010 and 2030 the concentration is reduced by 0.88 ppmv (−0.19%) by the endof this period. However, even if the emissions after 2030 are equal to the referencescenario, this initial perturbation does not recover, but stabilizes at a reduction ofabout 0.6 ppmv. This is because of the non-linear CO2 chemistry of the ocean (cf.Joos et al., 1996), which becomes more important when CO2 levels are high. After80 yr (2110), 72% of the maximum perturbation (0.63 ppmv out of 0.88 ppmv)

Figure 4. Calculated concentration of CO2 and N2O for the SRES A1B scenario (thick solid line, left

axis) and deviations from the reference scenario for the perturbation (right axis).


still remains in the atmosphere. For comparison, using the two different linearatmospheric response functions given by Hasselmann et al. (1997), 58% and 45%of the initial perturbation would remain in the atmosphere after 80 yr.

The concentration of N2O increases from 310 to 383 ppbv from 1990 to 2110as a result of the A1B emissions. The maximum reduction (2030) is 0.096 ppbv,and the perturbation is reduced after 2030 with an e-folding time of 120 yr.

Methane is a long-lived gas, but due to its chemical reaction with the OH radical,the levels of this gas will be sensitive to emissions of gases that affect the OH levelsand thus the location of the emissions of these gases (e.g. Fuglestvedt et al., 1999).Using the A1B emission scenario for anthropogenic methane emissions and a fixednatural source of 300 Tg/yr the concentration peaks around 2050 at a level of2460 ppbv and then returns to current levels by the end of the century (Figure 5).The calculated perturbations are driven by two factors, the perturbed emissions(−0.125 Tg/yr) between 2010 and 2030, and the change in lifetime due to thechanges in the emissions of NOx, CO, and VOCs. The atmospheric lifetimes givenin Table II are calculated by the 3-D CTM, and used together with a fixed lifetimeof 150 yr for uptake in soils, in the simple methane model described above.

TABLE II

Calculated global atmospheric lifetimes for CH4 in the 6 CTM simulations

1990 2020 Europe China South Asia South America

τatm (years) 7.9056 7.9272 7.92619 7.92616 7.9319 7.93981

�τatm (years) −0.00110 −0.00114 0.0046 0.0125

�τatm is the difference between the reference 2020 simulation (A1B) and the simulations with

emission reductions in the four different regions.

Figure 5. Calculated methane concentration for the SRES A1B scenario (thick solid line, left axis)

and deviations from the reference scenario for the four different perturbations (right axis). Note that

the perturbations for Europe and China are almost identical and cannot de distinguished in the figure.


The abundance of the OH radical that determines the lifetime of methane isgoverned by a complex photochemical system (Crutzen, 1987; Berntsen et al.,1992; Poppe et al., 1993; Karlsdottir and Isaksen, 2000). Equal reductions in theemissions of NOx, CO and VOCs in the four regions give very different responses(enhancement of OH for Europe and China, and reductions for South Asia and SouthAmerica) due to differences in the background levels of primarily NOx, differencesin humidity, and differences in amounts of sunlight to drive the photochemistry. Anincrease in OH, in turn, leads to a reduction in the lifetime of CH4, and vice versa.The net effect of these two driving forces varies between the regions. For emissionreductions in Europe and China, the reductions in the lifetime and emissions ofCH4 both contribute to reducing the concentrations. For South Asia and SouthAmerica on the other hand, the increase in the lifetime dominates, giving a positiveperturbation of methane.

3.1.2. OzoneFigure 6 shows the zonally and annually averaged change in ozone concentrationsfor the four regions. It can be clearly seen that the sensitivity increases as one goessouth from Europe, to China, South Asia and into the southern hemisphere to South

Figure 6. Annual averaged change in zonal mean ozone concentrations (pptv) in 2020 due to reduc-

tions in emissions of ozone precursors in the four different regions.


America. For Europe and China the background levels of pollutants are sufficientlyhigh that reduced emissions of ozone precursors cause an increase in ozone closeto the ground. The sensitivities, defined as the change in total tropospheric ozoneburden per change in NOx emission (Tg(O3)/Tg(N)/yr), are 0.30 (Europe), 0.40(China), 0.87 (South Asia), and 1.81 for South America. A previous study usingmore simplified emission perturbations, added to 1990 emission estimates (Berntsenet al., 2002) showed sensitivities for a South East Asia region (10–30◦N, 100–120◦E) of 1.15 and 1.23 Tg(O3)/Tg(N)/yr using two different global CTMs. Thelower sensitivity found in our current study is due to higher NOx background dueto enhanced emissions of NOx in South East Asia in the A1B scenario for 2020 andthe fact that our China region includes northern China, while the South East Asiaregion of Berntsen et al. (2002) included parts of Indo China.

All northern hemisphere regions show a seasonal cycle that peaks during summer(in terms of tropospheric column changes) and extends into the autumn (Septemberfor Europe, November for South Asia). The smallest perturbation occurs duringFebruary and March. For South America the maximum change is found in May andJune, with a minimum in September; however, the seasonal cycle is less pronouncedthan for the other regions.

Several studies have shown that the radiative forcing of ozone is particularlystrong for changes in the upper troposphere (Wang and Sze, 1980; Lacis et al.,1990; Forster and Shine, 1997; Hansen et al., 1997). The strong vertical mixingby deep convection in the tropics gives rise to significant enhancements in the12–16 km regions for emission perturbations in South Asia and South America.

3.1.3. Sulphate AerosolsThe changes in annual mean column burden of sulphate for emission perturbationsin the four regions are shown in Figure 7. In the CTM the emission reductions (−2.2Tg(S)/yr in all regions) caused the largest local reduction in the annual sulphateburden when the reductions were carried out for South Asia, reaching more than1 mg(S)/m2 over north eastern parts of the region. For all regions the mid-latitudewesterlies cause a plume extending eastwards from the source regions. The plumeis more extensive for the subtropical regions (South Asia and South America) due todecreasing precipitation as the air mass approaches the subtropical high-pressuresystems. For all regions even for Europe, there are also well identified plumesmoving west in the tropical easterlies.

Figure 8 shows the annual cycle in the reductions of the global sulphate bur-den caused by the emission reductions in each region. The annual mean changeis largest for South Asia with −13.2 Gg(S) compared to the other regions wherethe reductions were 8.9, 6.9 and 12.2 Gg(S) for Europe, China, and South Americarespectively. The seasonal cycle is strongest for Europe for three reasons: Less effi-cient oxidation of SO2 to sulphate during winter (less chemical oxidant productionin the atmosphere), less ventilation out of the planetary boundary layer which in-creases the dry deposition of SO2, and more efficient wet scavenging of SO4 during


Figure 7. Annual averaged reduction in the column of sulphate aerosols. Isolines at −1500, −750,

−500, −300, −100, −50, −25, −10, −5, −2, −1, and 0.0 μg(S)/m2.

Figure 8. Seasonal cycle in the reductions of global mean sulphate burden (Gg(S)) for an equal change

in SO2 emissions (−2.2 Tg(S)/yr) in the four regions.


winter by large-scale frontal precipitation. For South Asia the perturbations areparticularly large during the winter monsoon period from November to May. Dur-ing these months the reductions in the plume extending south westwards towardsAfrica, which can also be seen in the annual average in Figure 7, are particularlypronounced. This is consistent with the measurements from the INDOEX experi-ment (Lelieveld et al., 2001). Changes in burden during summer are about 50% ofthe winter changes due to shorter lifetimes of the water soluble sulphate aerosolsduring the rainy season.

3.1.4. Carbonaceous AerosolsThe changes in the annual mean burdens of BC aerosols and OC aerosols givenin Figures 9 and 10, respectively, show many features similar to those of sulphateaerosols (Figure 7), with plumes at mid-latitude to the east, and less pronouncedwesterly plumes in the tropics. For both types of carbonaceous aerosols, reducingthe emissions in South Asia causes the largest local reduction in burden (152 and344 μg(C)/m2 for BC and OC, respectively). In terms of contribution to the globalannual burdens, South Asia is also the region with the largest effect. For BC aerosols,

Figure 9. Annual averaged reduction in the column of black carbon aerosols. Isolines at −150, −75,

−50, −25, −10, −5, −2, −1, −0.5, −0.2, and 0.0 μg(C)/m2.


Figure 10. Annual averaged reduction in the column of organic carbon aerosols. Isolines at −150,

−75, −50, −25, −10, −5, −2, −1, −0.5, −0.2, and 0.0 μg(C)/m2.

the equal reductions of the emissions by 0.054 Tg(C)/yr, lead to a reductions in theglobal annual mean burden by 3.2, 2.6, 1.9, and 1.6 μg(C)/m2 for South Asia, SouthAmerica, Europe, and China respectively. For OC the corresponding numbers are6.6, 5.0, 3.3 and 3.3 μg(C)/m2.

3.2. RADIATIVE FORCING

The total global and annual mean radiative forcing and the relative contribution fromthe different gases and aerosols caused by a particular climate mitigation measurewill vary considerably over time. Table III shows the radiative forcing in 2030 atthe end of the 20-yr mitigation period (see Figure 2), when the contribution fromthe short-lived species is at a maximum. The effects of the temporal evolution of theradiative forcing are discussed in the next section. There is a clear difference in theeffect of reduction in well-mixed and short-lived components, with large regionaldifferences in the radiative forcing for the short-lived species. Methane is a well-mixed gas in the atmosphere. However, the loss of methane through reaction withthe OH radical is controlled by short-lived components and thus causes different


TABLE III

Change in global annual radiative forcing (mW/m2) at the end of the period of

reduced emissions (2030) for the different forcing agents

Europe China South Asia South America

CO2 −9.8 −9.8 −9.8 −9.8

N2O −0.28 −0.28 −0.28 −0.28

Methane −0.22 −0.22 0.28 0.97

Ozone −0.35 −0.52 −1.1 −2.2

Sulphate aerosols 11 (24) 7.5 (20) 17 (30) 19 (32)

Black carbon aerosols −2.0 −1.6 −3.0 −2.9

Organic carbon aerosols 0.51 0.40 0.90 0.66

Short-lived components (ozone, sulphate, BC, and OC) have this forcing for the

whole period (2010–2030, see Figure 3), while transient calculations are performed

for the long-lived species according to their concentrations (Figures 4 and 5). The

numbers in parenthesis for SO4 include a contribution 12.8 mW/m2 from indirect

effects.

radiative forcing for emission changes in the four regions. Methane is the onlycomponent that has different sign of the forcing for the different emission regions,for reasons explained in Section 3.1.1.

Of the radiative forcing mechanisms considered here, CO2 and sulphate arethe two dominating components. They are of the same order of magnitude, butof opposite sign. For the short-lived components, the magnitude of the radiativeforcing is generally greatest in South Asia and South America.

There are several reasons why the short-lived components (ozone, sulphate, BC,and OC) vary in terms of their global mean radiative forcing. Of primary importanceare the regional differences in the change in the burden of the components asdiscussed in Section 3.1. In addition, there are different mechanisms that influencethe magnitude of the radiative forcing of the short-lived components. The radiativeforcing of tropospheric ozone depends strongly on the vertical distribution (Laciset al., 1990) and the horizontal distribution (Berntsen et al., 1997) in the changein the abundance, as well as the background ozone concentration. The radiativeforcing of sulphate depends significantly on the distribution of relative humidityand clouds. Clouds reduce the magnitude of the radiative forcing, while uptakeof water with increasing relative humidity strengthens the radiative forcing due tosulphate (Haywood et al., 1997; Myhre et al., 2002). At mid and high latitudes theseasonal cycle in perturbations (see Figure 8 for sulphate) is also important. Athigher latitudes (Europe and to a lesser extent in China), the larger changes duringsummer increase the radiative forcing for these regions. For OC, clouds have astrong effect on reducing the magnitude of the radiative forcing and may causesignificant spatial inhomogenities. The radiative forcing due to BC is also verydependent on the cloud distribution: BC above the cloud layer has a much greater


forcing than BC in a clear sky, and BC below the cloud layer has a reduced forcingcompared to BC in a clear sky. For all the aerosol components, the surface albedois also important because the magnitude of the forcing increases for scatteringaerosols (sulphate and OC) in regions with low surface albedo and increases forabsorbing aerosols (BC) in regions with high surface albedo.

To isolate the other effects from the differences in burden we show in Table IV theradiative forcing normalized to the changes in global annual atmospheric burden.Differences are now much smaller between the regions than shown in Table III,but still we find high values for sulphate for South America. There is a region offthe west coast of South America with high relative humidity and a small amountof low stratocumulus clouds that largely explains the high normalized values forthe aerosols. Indirect effects on clouds could offset this since it would probably bebelow average in this region.

The normalized forcings given in Table IV agree well with the numbers givenin IPCC (2001). The mean normalized forcing for tropospheric ozone for ninestudies is 0.042 Wm−2/DU, with a range from 0.033 to 0.056 Wm−2/DU. For thenormalized direct forcing of sulphate aerosols the 19 studies referenced in IPCCgive a mean of −215 W/g, a median of −171 W/g and a range from −110 to−460 W/g. For fossil fuel BC (externally mixed), the IPCC gives numbers fromfour studies ranging from 1123 to 1500 W/g. The lower value is from Myhre et al.(1998) using the same optical properties as in this study. For fossil fuel OC, onlythree studies are given in IPCC, with values from −60 to −340 W/g.

For all four short-lived components, the differences between the regions in thechange in global burden appear to be more important than the differences in thenormalized radiative forcings given in Table IV.

4. Potential for Climate Change

Comparing the potential climate effects of the mitigation measures applied to thevarious regions is not straightforward since the nature of the radiative forcing

TABLE IV

Radiative forcing for the short-lived species normalized to the change in global annual

atmospheric burden of each of the short-lived species (e.g. dSO4 is the change in global

annual atmospheric burden (g/m−2) for sulphate) for each region

Europe China South Asia South America

RF/dO3 (Wm−2/DU) 0.035 0.040 0.038 0.037

RF/dSO4 (W/g) −213 −186 −224 −268

RF/dBC (W/g) 1090 1010 930 1120

RF/dOC (W/g) −152 −122 −136 −131

Note that W/g is equal to Wm−2/gm−2, as a unit for RF normalized to the mean change in

atmospheric burden.


differs between the regions, and for the different forcing agents it differs overtime. Fuglestvedt et al. (2003) thoroughly reviews the complex questions relatedto the assumptions behind the choice of indices to compare emissions of differentclimate forcing agents.

To compare the potential climate effects of the emission reductions studied in thiswork, we have calculated numerical values for three different metrics or indices ofclimate change. All three metrics use the global and annual mean radiative forcingas the primary input parameter. First, we introduce a Warming Index (WI) overa time horizon (H) based on global and annual mean radiative forcing, followingthe concept of integrating global RF over time as in the global warming potential(GWP) index.

In addition to the calculations of the WI, we introduce the net RF followingemission reductions in each region into a simple climate model (SCM) to calculateglobal mean surface temperature change over the period from 2000 to 2100. Asa third index (Section 4.3) to study the characteristics of the regional variationsand the robustness of the conclusions with respect to choice of index we use asimple formulation that includes a non-linear temperature term and a discountingterm to weigh the effects over time. This formulation was introduced to estimateemission indices based on the economic damage of climate change caused byvarious emissions (e.g. Kandlikar, 1995; Hammit et al., 1996).

4.1. REGIONAL EFFECTS IN TERMS OF WI

The first metric we use to compare the effects of emissions in the different regionsis the Warming Index (WI) defined by Equation (1):

WIi,H =∫ 2010+H

2010�RFi (t) dt∫ 2010+H

2010�RFCO2

(t) dt(1)

The term �RFi refers to the difference in global radiative forcing for componenti between the reference case (the standard SRES A1B scenario) and the perturbationcases as defined in Table I. Note that the radiative forcing is not calculated per unitmass of a pulse emission (as for the GWP index), but for the actual emission changesgiven in Table I, applied for a period of N years starting in 2010 (N = 20 years inthe standard simulations). The definition of the standard emission metric GWP doesnot say anything about the background atmosphere or which gases the concept iswell suited for, but in the IPCC reports, GWPs are calculated for well-mixed gasesand for a constant background (IPCC, 1996, 2001). The WI-index as used hereis calculated with a background following the reference scenario, and short-livedsubstances that are not well mixed in the atmosphere are included. In addition,we use two time horizons: one for the implemented measure (N), and one forthe integration of radiative forcing (H). This is similar to an approach adopted byHarvey (1993).


For the RF of CO2 there are two effects of using a non-constant backgroundscenario. Due to the saturation effect of the absorption, the enhanced backgroundreduces the RF per unit increase in concentration. Compared to the impact of theCO2 perturbation (see Figure 4) put on top of a constant background level of 360ppmv, the integrated RF over 100 yr is reduced by 26%. Secondly, the non-linearresponse in the carbon cycle model we have used (see Section 3.1) enhances theRF perturbation from CO2 by 25–30% at the end of the period compared to linearcarbon models (Maier-Reimer and Hasselmann, 1987). The net effect of using anon-constant background on the integrated RF of CO2 is thus small, in agreementwith Caldeira and Kasting (1993) and IPCC (1996).

4.1.1. The Reference CaseTable V shows the calculated WI100 for the seven climate forcing agents consideredfor the different regions. Comparison of the total or net WI values in the bottomrow (the sum of individual WI values) indicates to which extent the location of theemissions does make a difference. For all regions the net effect is smaller than forCO2 alone, or for the sum of the Kyoto gases considered (CO2, N2O, and CH4).The main reason for this is the warming effect of reductions in sulphate aerosol,which can be (as for South America) as large as 60% of the CO2 effect, but withthe opposite sign. For South America the net effect is only about half of what iscounted for under the Kyoto Protocol. As discussed above, the photochemistry isquite different between the regions, with the RF from ozone changes being morethan six times larger for South America than for Europe. However, if the effectson the two gases affected by non-linear photochemistry (methane and ozone) areadded up, the net WI100 values of the two gases vary between 0.020 and 0.029 forthe four regions.

It should be noted that by adding up RF from short-lived species (ozone andaerosols,) and long-lived species (CO2, N2O and CH4), and also by adding up

TABLE V

WI100 for a 100-yr time horizon and 20 yr of emission reductions

WI100 (Europe) WI100 (China) WI100 (South Asia) WI100 (S. America)

CO2 1 1 1 1

N2O 0.031 0.031 0.031 0.031

CH4 0.009 0.010 −0.012 −0.041

O3 0.011 0.017 0.035 0.070

SO4 aerosols −0.35 (−0.75) −0.24 (−0.64) −0.55 (−0.95) −0.61 (−1.01)

BC aerosols 0.065 0.051 0.095 0.092

OC aerosols −0.016 −0.013 −0.029 −0.021

Total 0.75 (0.35) 0.86 (0.45) 0.57 (0.17) 0.53 (0.12)

The numbers in parenthesis for SO4 include the indirect effects on clouds.


negative and positive radiative forcings, we make some simplifications. Thesesimplifications are similar to calculating global mean temperature changes withreduced-form energy-balance climate models of the type used in IPCC (2001) andseveral other studies (Harvey et al., 1997; Hayhoe et al., 2002). Perturbations ofshort-lived species will lead to RF mainly during the period with emission reduc-tions, and large regional RF (positive or negative) might cause regional changesdue to circulation/precipitation changes that will not be cancelled by long-termglobally homogeneous positive RF. The integrated change in global RF, which canbe viewed as an approximation of a global temperature change, will not be able tocapture potential changes in regional aspects of climate changes (e.g. circulationand/or precipitations changes) due to strong regional RF.

In the design of our experiments we have made certain assumptions that have asignificant impact on the evaluation of the emission reductions in the different re-gions. To investigate the influence of some of these assumptions we have calculatedWI-numbers for three other cases:

• Increasing the length of the mitigation period (N) from 20 to 40 yr• Reducing the time horizon H to 50 yr in the calculation of WIH

• Enhancing the emission factors of SO2, BC and OC in China, South Asia, andSouth America corresponding to the maximum numbers given by Hayhoeet al. (2002)

4.1.2. Longer Mitigation Period (N)In our base case we have assumed that the emissions are reduced for 20 yr (seeSection 2). Extending the length of the emission reduction period will enhancethe relative effects of the short-lived components compared to that of the long-lived. Using the CTM calculations already performed for 2020 (see Figure 3), butassuming that the perturbations now last for 40 yr (i.e. extending the rectangularbox in Figure 3 to 2050), and running new simulations for CO2, N2O, and CH4, weget WI100 values for this case as given in Table VI.

Extending the emission reduction period to 40 yr reduces the net effects in allregions due to larger compensating effects of the short-lived cooling species (i.e.sulphate), but the changes are rather small (less than 0.2 even when indirect effectsare included) and it does not change the relative importance of the different regions.

4.1.3. Shorter Time Horizon (H)Applying a shorter time horizon (H) in the calculation of WIH , will also enhancethe contribution from the short-lived s pecies. Table VII shows the WIH values for atime horizon of 50 yr for the standard 20 yr of emission reductions. Now the directeffect of the reductions in sulphate aerosols becomes about equal to the CO2 effectfor the two tropical regions (South Asia and South America). The ranking of theregions does not change by using H = 50 yr.


TABLE VI

WI100 for 40 yr of emission reductions


CO2 1 1 1 1

N2O 0.032 0.032 0.032 0.032

CH4 0.011 0.011 −0.014 −0.048

O3 0.012 0.019 0.039 0.079

SO4 aerosols −0.39 (−0.84) −0.27 (−0.72) −0.61 (−1.07) −0.68 (−1.13)

BC aerosols 0.072 0.057 0.11 0.10

OC aerosols −0.018 −0.014 −0.032 −0.023

Total 0.72 (0.27) 0.84 (0.39) 0.52 (0.07) 0.47 (0.01)

TABLE VII

WI for a 50-yr time horizon and 20 yr of emission reductions


CO2 1 1 1 1

N2O 0.030 0.030 0.030 0.030

CH4 0.016 0.016 −0.019 −0.068

O3 0.019 0.029 0.061 0.12

SO4 aerosols −0.61 (−1.31) −0.42 (−1.12) −0.95 (−1.66) −1.05 (−1.76)

BC-aerosols 0.11 0.089 0.17 0.16

OC-aerosols −0.028 −0.022 −0.050 −0.036

Total 0.54 (−0.16) 0.73 (0.03) 0.24 (−0.47) 0.16 (−0.55)

4.1.4. Enhanced Aerosol EmissionsIn our experimental setup it is assumed that the mix of pollutants relative to CO2 isequal for all regions. This would of course not really be the case, but it is beyondthe scope of this study to estimate the relevant regional emission factors for a set ofemission reductions that would take place in a case of emissions trading. However,since the major uncertainty is in the emissions of aerosols and SO2, and since theemission to concentrations, and concentrations to radiative forcing relations forthe aerosols are reasonably linear, we can make a simple analysis of the possibleeffects of this assumption. We used the upper limit of the range of emission factorsfor SO2 and BC emissions from coal burning from Hayhoe et al. (2002) to scaleup the SO2, BC, and OC emissions (using the same scaling factor for OC as forBC) for the emissions in China, South Asia, and South America. Using the upperlimit for all three species can be justified as high sulphur-containing coal (ligniteor brown coal) has a low heat content that reduces the combustion temperature andthus increases the formation of carbonaceous aerosols. The corresponding radiativeforcing and WI100 values are given in Table VIII. Since the scaling factor is much


TABLE VIII

WI100 for 20-yr emission reductions, but maximum emission factors for SO2, BC and OC as

given by Hayhoe et al. (2002) for China, South Asia and South America


CO2 1 1.000 1.000 1.000

N2O 0.031 0.031 0.031 0.031

CH4 0.009 0.010 −0.012 −0.041

O3 0.011 0.017 0.035 0.070

SO4 aerosols −0.35 (−0.75) −0.68 (−1.8) −1.6 (−2.7) −1.7 (−2.9)

BC-aerosols 0.065 0.878 1.625 1.575

OC-aerosols −0.016 −0.218 −0.491 −0.357

Total 0.75 (0.35) 1.04 (−0.11) 0.63 (−0.52) 0.56 (−0.59)

Figure 11. Summary of total WI (only direct effects of aerosols included, see Tables V–VIII for the

indirect effects) for the reference case and the three sensitivity cases.

larger for BC (and thus for OC) than for SO2 (17.1 and 2.84 respectively), the effectof the carbonaceous aerosols becomes of similar magnitude to the direct effect ofsulphate, and for the tropical regions larger than for CO2.

Table VIII shows that even with a very large increase in the contribution from theaerosols, the net effect (i.e. the total WI), and the differences between the regionsdo not change significantly.

Summing up the results shown in Tables V–VIII, and Figure 11, we conclude that:

• The net effect of reducing emissions is always smallest in South America,followed by South Asia, Europe, and China.

• The maximum difference between the regions compared to the effect of CO2

reductions only ranges between 0.57 (i.e. 0.73–0.16 for H = 50 yr, andN = 20 yr) and 0.33 (i.e. 0.86–0.53) in the standard case.


4.2. TEMPERATURE RESPONSE

Based on the temporal evolution of the net radiative forcing we have calculated thechanges in global mean surface temperature with a simple climate model (SCM)which includes a carbon-cycle scheme from Joos et al. (1996), and an energy balanceclimate/upwelling-diffusion ocean model (Schlesinger et al., 1992; Fuglestvedt andBerntsen, 1999). The SCM uses a prescribed climate sensitivity of 0.80 K(Wm−2)−1

(3 K for a doubling of CO2).Figure 12 shows the temperature change in the background scenario (the SRES

A1B scenario), as well as the difference when the emission reductions are applied tothe four regions. At the beginning of the 20-yr mitigation period, the temperaturesincrease due to reductions in the cooling by sulphate aerosols (direct effects only, seeTable III). When the mitigation period ends, the effect of the short-lived componentsdisappears and the temperatures are reduced due to reduced radiative forcing bythe longer lived greenhouse gases, mainly CO2. The largest temperature reductionsoccur 20–40 yr after the end of the mitigation period. Since it is mainly the short-lived species that are regionally dependent, the difference between the regionsbecomes negligible towards the end of the century.

Including the indirect effects of sulphate in the simplified manner as has beendone in this study leads to larger temperature enhancements during the mitigationperiod (+0.0095 K for South America, and +0.0058K for Europe), and a smaller(0.001 K) reduction in warming around 2050. The final temperature change in 2100remains virtually unchanged by including indirect effects of aerosols.

Figure 12. Temporal development of global mean surface temperature in the reference case (direct

effects of sulphate only). Left axis shows the change due to the emissions given in the SRES A1B

scenario, while the right axis shows the temperature deviations compared to the A1B scenario for the

four regions.


Figure 13. Same as Figure 12, but with emission reductions sustained for the whole period.

To illustrate the effects of short term versus long term emission changes, we alsoshow the temperature changes given sustained emission changes until 2100. Radia-tive forcings for the short lived components are taken from Table III and kept con-stant until 2100, while for CO2, CH4, and N2O concentrations and radiative forcingsare calculated for the whole period with sustained emission changes as given in Ta-ble I. Figure 13 shows that the while the absolute temperature difference between theregions increases over time for sustained emission reductions, the differences rela-tive to the mean temperature reduction for the four regions decrease. This illustratesthat even if the regional differences in radiative forcing are sustained over a century,the long lived greenhouse gases are most important in the longer perspective.

4.3. REGIONAL EFFECTS USING A NON-LINEAR TEMPERATURE DEPENDENT METRIC

The Warming Index defined by Equation (1) and the temperature evolutions inFigures 12 and 13 represent two ways to evaluate and compare the potential climateimpact of an emission reduction package applied to different regions. We nowproceed a step further to show how the comparison behaves if the metric includes anon-linear temperature term to describe damages1 and a discounting term to weighthe effects over time. The metric given by Equation (2) is equal to the net presentvalue (NPV) metric of economic damages presented by Kandlikar (1995). The ideais that the NPV of the damage (Da, in monetary units) can be expressed by

Da =∫ th

0

k[�T (t)]n exp(−r t) dt (2)


where th is the time horizon, �T(t) is the change in global mean surface temperatureas a function of time (t), r is the discount rate, and k a scaling factor which convertsthe metric to monetary units. The exponent n is normally assumed to be greaterthan 1, causing the damage to increase super-linearly with temperature change. Thishighly simplified formulation of potential damage gives a conceptual illustrationof how the quantification and assessment of damages depend on the shape of thedamage function (i.e. the value of n) and the concern and weighting of long-termversus short-term damages (i.e. through the discount rate).

We have applied this metric definition in a similar way as we have done forthe WI metric, to calculate the relative effects (Dr ) of all emission reductions ina given region relative to a CO2 reduction only.2 This approach means that theability of Equation (2) to give the absolute economical damage (i.e. the calibrationof the constant k) is unimportant since we are only looking at the relative effects ofregional emission reductions. The results are directly comparable to the total WI100

and WI50 numbers given in Figure 11.Figure 14 shows the value of the total emission reductions in each region relative

to a CO2 only case, for different assumptions of n and choices of r (cf. Chapter 4in IPCC (1996b) for a review of the use of discounting in analysis of climatechange). The integral in Equation (2) is calculated for the period 2010 to 2110.Increasing n means that the value of temperature reductions increases when �T(t)in the reference A1B scenario is large, i.e. towards the end of the century. Thus, thewell-mixed gases become more important and the differences between the regions

Figure 14. Summary of net climate change for the reference case (direct effects of sulphate only)

evaluated with different metrics for climate change. All values are relative to the corresponding effect

of CO2 reductions only.


decrease. Increasing the discount rate r has the opposite effect and puts less valueon changes in the distant future and more on the short-term effects, thus enhancingthe differences between the regions.

Emphasis on the short-term impacts by applying a relatively high discount rateof 4% reduces the net effects of the reductions because the value of the heatingduring the first phase (2010–2030 in Figure 12) approaches the value of reducedheating after about 2030. The ranking of the regions does not change across allapplied metrics in this study. It should be noted that the apparent robustness of theresults across the applied metrics could be significantly reduced if more realisticregion-specific emission reductions were simulated.

4.4. CLIMATE SENSITIVITY AND REGIONAL CLIMATE CHANGE

In the discussion above we calculated global climate impact (either as WI, �Ts ,or economic damage) based on a global mean RF for emission reductions in thefour regions. This follows the assumption that the sensitivity of the global climatesystem is equal for all forcing mechanisms. Using three different GCMs forcedby idealized regional forcing through either CO2, solar or ozone changes, Joshiet al. (2003) found that the global climate sensitivity could vary by about ±30%compared to the sensitivity for a global CO2 perturbation. There are also indicationsfrom GCM studies (Lelieveld et al., 2002; Rotstayn and Penner, 2001; Menon et al.,2002; Kristjansson, 2002) that regionally heterogeneous RF can induce changes inthe large-scale circulation affecting the regional pattern of temperature changeas well as the hydrological cycle, thus affecting the regional patterns of floodingand droughts. These climate effects of regional heterogeneous forcings are stillquite uncertain and differ between the GCMs. For some forcing mechanisms andregions it might reduce the differences found above in the global impacts, while forother forcing mechanisms and regions the effects even on a global scale might beenhanced. However, the possibility of regional effects undoubtedly casts uncertaintyon the assumption that the climate impacts of measures aimed at the gases includedin the Kyoto Protocol are independent of location of the emissions.

5. Conclusions and Policy Implications

The results from this work show that it can not be assumed that identical emissionreductions will give equal climate effects if the reductions take place in different re-gions and if several gases and aerosols are affected. There are three main aspects (or“dimensions”) in these considerations: (i) geographical variations, (ii) the chemicalcomposition of the emission reduction and the characteristics of the species, and(iii) timescales both in terms of the duration of the measure implemented and thehorizon over which the effects are considered. In addition to the isolated effects


of these factors, there are also interactions between them, which may increase ordecrease the total effect.

Several previous studies have shown that the effects of emission changes mayvary widely between regions (Johnson and Derwent, 1996; Fuglestvedt et al., 1999;Wild et al., 2001, Berntsen et al., 2002). Thus the chemical, physical and meteo-rological environments in which abatement measures are implemented have to beconsidered. The current study takes into account an array of gases and aerosolsfollowing measures corresponding to a 10% reduction in European CO2 emissionsby reduced emissions from large sources, public power, cogeneration and districtheating, and industrial combustion, as opposed to earlier studies where one or twogases were considered. We find that the enhancement of the atmospheric burden ofthe aerosols (sulphate and carbonaceous aerosols) is about twice as large for the lowlatitude regions (South Asia and South America) due to longer lifetimes in the atmo-sphere. The effects through perturbations of tropospheric ozone and methane life-time through emissions of NOx, CO, and NMVOCs are significantly smaller (aboutan order of magnitude) than the direct sulphate effect. This should not be interpretedas a general result for all emission sources, since the chosen sources are the majorsource of sulphur (79% of the SO2 emissions in Europe), while they only account for35%, 1%, and 13% of the NOx, CO, and NMVOC emissions respectively. If a sectorlike road traffic had been chosen (3%, 44%, 31%, and 56% of the European SO2,NOx, CO, and NMVOC emissions, respectively) the relative importance of ozoneand methane would be significantly larger. However, for political and practical rea-sons we believe that large-scale sources, as we have focused on are more likely tobe affected by emission trading between regions than, for example, road traffic.

In terms of integrated radiative forcing, we find that the reductions in the emis-sions change the concentrations of non-CO2 gases and aerosols so that the totaldirect effect is always smaller than the effect of CO2 alone. Significant variationsbetween the regions are found (53–86% of the CO2 effect) for the net effect of thesame package of emission reductions. Inclusion of the indirect effects of sulphateaerosols reduces the net effect of measures towards zero when the weighting overtime is done as defined by the WI index. If the focus is on temperature changetowards the end of the century, the long-lived greenhouse gases dominates and thenet effect of the reductions is to decrease the warming. This shows that it is impor-tant that the changes that are initiated by a measure are quantified for the non-CO2

gases and assessed together with knowledge about their chemical and radiativebehaviour and the chemical, physical and meteorological conditions in the region.It should be kept in mind that it is a limitation with this study that semi-direct andindirect effects of aerosols are not taken into account. Partly because of the largeuncertainties in these effects on the regional level needed in our analysis and partlydue to the need for very costly GCM experiments to quantify them properly.

Furthermore, since the multitude of gases involved show a wide range of atmo-spheric lifetimes/adjustments times, the timescales of the implemented measure (i.e.N) as well as for the assessment of effects (i.e. H, or discount rate in Equation (2))


are crucial. Due to short adjustment times, the changes in aerosols and ozone domi-nate during the first parts of the period, causing a temperature enhancement mainlydue to reduced cooling by sulphate aerosols (see Figure 12). The changes in longerlived gases are more important later, causing a reduced temperature increase afterthe end of the mitigation period (N). Thus, a realistic evaluation of the expectedlifetime of the measure and a conscious choice of the horizon for the assessmentof effects are necessary.

Although the importance of the short-lived components (compared to CO2 onlyor the Kyoto gases) varies considerable between the regions and depends on themetric applied (WIH for different H and N, or the damage defined in Equation (2)),we find the ranking of the regions is a robust result. In all cases (except the WI100

for high aerosol emission factors, Section 4.1.4), reducing the emissions in Chinastands out as the most effective, followed by reductions of emissions in Europe,South Asia, and South America. The main reason for this is the regional differencesin the perturbations of the sulphate burden (see Figure 8), where in the CTM theremoval of sulphate is most efficient for sulphate originating from emissions inChina. Also the normalized RF of sulphate for the experiment with reduction inChina is lower than for the other regions (Table IV). It should be kept in mind thatthe conclusion that reductions in China are most effective is based on equal emissionreductions in all regions. This may not be a very realistic assumption, but it hasallowed us to study the isolated effects of regional differences in chemical, physicaland meteorological conditions and how they determine the climate response tochanges in emissions. An important result of this study is that it has allowed us toderive species specific emission indices relative to CO2 (i.e. WI numbers in TablesV–VIII). These indices can be used in the same way as the GWP numbers (asdiscussed in Section 4.1.4) to analyze more realistic and complex sets of regionalmitigation options including short-lived species.

A large part of the emission reductions to meet the targets of the Kyoto Protocol(or any alternative regime) will probably be implemented through the flexibilitymechanisms such as Joint Implementation, international emission trading, and theClean Development Mechanism (CDM). Strictly speaking, the problems relatedto the location of the implementation of measures and array of gases affected arenot only related to the flexibility mechanisms. The same logic or reasoning can beapplied to the initial distribution of emission reductions to the parties of a climateagreement (i.e. without flexibility mechanisms), since the same amount of CO2

reductions in one country may cause a different net change in radiative forcing ifseveral non-CO2 gases are affected when the atmospheric conditions are different.

Obviously, inclusions of such considerations will complicate the political ne-gotiations and the implementation of the flexibility mechanisms during the nextcommitment period of the Kyoto Protocol. The main question is, then, how muchscientific knowledge can be taken into account in the process of developing newprotocols for the period after 2012. Geographical variations, timescales and oppos-ing effects on radiative forcing are scientific issues with no simple policy responses.


For instance, there is still nothing that indicates consensus on the crucial questionof how negative forcings should be treated in the context of a climate agreement.

Our results indicate a need for a regional differentiation in the evaluation ofmitigation measures. However, we believe that there is a need for further scientificstudies which go one step further from what is done in this paper and start with aregional bottom-up analysis of potential climate mitigation options before effects ofshort-lived species can be treated successfully in climate negotiations. This analysisshould include all relevant sectors and account for important regional differencesin emission reductions following realistic mitigation options. Identification of themost likely combination of emission reductions on a regional level would also makeit possible to assess co-benefits in terms of reduced air pollution. Such effects couldbe important in enhancing the political feasibility of including short-lived speciesin climate agreements.

Acknowledgments

This work has been supported by the Norwegian Research Council, project number142128/720, and from the European Union (m e τ r i ◦C project, EVK2-CT-1999-00021). We thank Lynn Nygaard and Michele Twena for help with editing themanuscript.

Notes

1Note that we refer to climate impacts as “damage” following the normal usage, but recognize

that impacts can be positive or negative.2 Dr = (Da(p) − Da(ref))/(Da(CO2) − Da(ref)) where ref is the reference case (the standard A1

scenario) while p is the perturbation case i.e. with all reductions in one of the regions.

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(Received 23 July 2003; in revised form 23 November 2004)

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