ABATEMENT OF GREENHOUSE GASES: DOES LOCATION MATTER?
TERJE BERNTSEN1 , JAN FUGLESTVEDT1 , GUNNAR MYHRE2,3 ,
FRODE STORDAL2,3 and TORE F. BERGLEN3
1
CICERO, Center for International Climate and Environmental Research, P.O. Box 1129 Blindern,
N-0318 Oslo, Norway
E-mail: t.k.berntsen@cicero.uio.no
2
Norwegian Institute for Air Research (NILU), Norway
3
Department 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 location
of emissions reductions has no impact on the overall climate effect (e.g. Joint
Implementation, 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. Abatement
measures such as reduced fossil fuel consumption, biomass burning, or switching
from 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 organic
carbon (VOCs), which may indirectly cause significant radiative forcing of climate
through chemical processes in the atmosphere. These reductions combined with
a reduction in the emissions of carbonaceous aerosols (soot and organic carbon)
and SO2 giving sulphate particles, can affect the total change in radiative forcing
of a given abatement measure significantly. Changed emissions of such short-lived
species that occur alongside reductions of gases included in the Kyoto Protocol
through technological couplings may thus affect the total effect of emission
reductions. Previous studies have shown that the magnitude of these indirect
effects varies widely from region to region because of differences in chemical and
meteorological/physical key parameters and in emission ratios (Lin et al., 1988;
Johnson and Derwent, 1996; Fuglestvedt et al., 1999; Derwent et al., 2001; Wild
et al., 2001; Berntsen et al., 2002). Thus, a regional variation in the effectiveness
of abatement measures is expected depending on to which extent the measures
affect emissions of ozone precursors, SO2 or aerosols in addition to the well mixed
greenhouse gases. Studying the more realistic situation where several components
are reduced simultaneously because of coupled source strengths may provide new
information about possible regional variations in abatement effectiveness, which
will be important in the further development of international climate policy.
In this paper we explore whether reduction in other species (in particular gases
and aerosols not included in the Kyoto Protocol) as a consequence of CO2 abatement measures will lead to significant regional differences in the total efficiency
of such reductions. It is a well established fact that reductions in SO2 following
reductions in greenhouse cases can partly offset the impacts of the reductions
through 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 mitigation
measures have not been studied in detail previously. We use a global chemical
transport model (CTM) to study the chemical processes, and a radiative transfer
model to calculate the radiative forcing of ozone and aerosols. Concentration
changes for CO2 have been calculated by the model described by Joos et al. (1996),
while methane and N2 O responses to emission changes have been calculated
with a simple box model with variable lifetime of methane as given by the CTM.
Standard concentration-forcing relations from IPCC (2001) have been used for
CO2 , CH4 , and N2 O. A simple energy balance upwelling/diffusion climate model
has been used to estimate global mean temperature changes. We compare potential
future climate effects of the emission reduction including both short-lived and
long-lived components. The results of such comparisons can depend strongly on
the metric used (e.g. Smith, 2003). To emphasize this we perform the comparison
using 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 metric
that integrates a non-linear damage function with discounting (e.g. Kandlikar,
1995).
Previous studies using both simple and sophisticated models for atmospheric
processes have shown that the effects of emissions may depend on the location
(horizontal and vertical) and timing (season) of the emissions. For instance, emissions of NOx from aircraft in the upper troposphere and lower stratosphere have a
much stronger effect on local ozone (O3 ) concentrations than if the same amount
of 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, ozone
changes in these altitudes give a larger radiative forcing compared to an equal
ozone 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 efficiency
and radiative forcing can also be expected horizontally due to strong non-linear
relations 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) there
is a strong non-linear relation between levels of NOx and tropospheric ozone production. Due to its catalytic role in the production of ozone, NOx emitted in or transported to the remote troposphere is more efficient in producing ozone than if it were
introduced to the troposphere in a polluted region and oxidized there. In a study of effects of NOx reductions, Fuglestvedt et al. (1999) found a much higher sensitivity for
upper tropospheric O3 to reductions in NOx from ground sources in Southeast Asia
and Australia than in regions at middle and high latitudes like U.S.A., Europe and
Scandinavia. Differences in seasonal variation of O3 production efficiency were also
evident.
Emissions of ozone precursors also affect the oxidizing capacity of the
atmosphere mainly through perturbations of the hydroxyl (OH) radical. In terms
of 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 timescale
corresponding to the ‘primary mode’ of the tropospheric chemistry system (about
14 yr, Wild et al., 2001; Derwent et al., 2001). The effect of NOx emissions alone
on the levels of methane have been found to reduce its lifetime and to vary with
respect to location of emissions in a similar way as for ozone, i.e. with higher
sensitivities in low NOx backgrounds regions (Fuglestvedt et al., 1999; Wild et al.,
2001). Following pulse emissions of NOx from surface (different latitudes) as well
as free tropospheric sources (lightning and aircraft), Wild et al. (2001) find that
when integrated (for more than 50 yr) the net RF (ozone(+) and methane(−)) is
slightly negative (except for aircraft), but can be the difference between two large
numbers.
380
TERJE BERNTSEN ET AL.
2. Experimental Design
A large number of possible measures or combinations of measures to meet obligations under the Kyoto Protocol can be envisaged. The aim of this study is not
to explore the whole ‘measure space’, but to look more closely at one reasonably
realistic case to see whether other species affected by reduction measures will increase or decrease the total efficiency of emission reductions, and whether there are
significant regional variations in the efficiency of such reductions. We have chosen to study an idealized case where the reductions in emissions of other species
than CO2 do not vary between regions. In this way we can study the chemical and
physical effects of geographical location in isolation from other factors such as
differences in technology and emissions. Since reductions are likely to be carried
out through removal of the oldest and least efficient installations, a more complete
analysis would require a bottom-up analysis of potential measures in different regions. However, since we do not adopt very large emission changes and current
climate policy do not seem to indicate large emission changes, the results from our
simulations can be scaled within reasonable limits to represent the effects of other
more realistic combinations of measures.
The purpose of the experimental design was to simulate the transient total
effects (in terms of RF) of a realistic climate mitigation measure in response to
the 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 measures
aimed at large-scale sources only. These sources are based on the definition of
sectors by the European Environment Agency (EEA, 1999) and comprise public
power, cogeneration and district heating (sector 1 of EEA (1999)) and industrial
combustion (sector 3 of EEA (1999)). EEA (1999) gives the total emissions from
sectors 1 and 3 of CO2 , N2 O, CH4 , ozone precursors (NOx , CO, and VOCs),
and SO2 . Emissions of black- and organic carbon aerosols are based on emission
factors from Cooke et al. (1999). The emissions used are summarized in the next
section.
To evaluate the validity of the assumption that ‘location does not matter’ we have
selected four regions of the world (Figure 1), and performed separate calculations
with the Oslo-CTM2 model with equal emission reductions (in absolute terms, mass
units) 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 ,
N2 O, 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 make
the assumption that the reductions are obtained by investment in some kind of
new technical equipment with a specific lifetime. In our reference calculation
we 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
ABATEMENT OF GREENHOUSE GASES: DOES LOCATION MATTER?
381
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 last
for more than 20 yr. A discussion of how the length of this period affects the
results is therefore included in the Section 4. If sectors with a large number of
smaller sources (e.g. transportation or domestic sources) were targeted through for
example increased taxation, one would expect that the reductions in the emissions
would be reversed with a timescale of much less than 20 yr when the tax was lifted
again.
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 the
so-called flexible mechanisms of the Kyoto Protocol, we have chosen a baseline
scenario that assumes international cooperation. Of the IPCC SRES marker scenarios (IPCC, 2000), the A1B scenario is the one that assumes the most widespread
international cooperation. We have therefore used this emission scenario (it includes
all the gases of interest, but not the carbonaceous aerosols) as our baseline. Figure 2
shows a schematic illustration of how the emission perturbations are projected to
change over time.
For the long-lived gases (CO2 , N2 O and CH4 ) we perform transient calculations
of the concentrations following the 20-yr long reduction in emissions for the
period 1990–2110. Emissions are kept constant between 2100 and 2110. The
details of these calculations are given in Section 3.1. The perturbations induced
by the short lived gases and aerosols (tropospheric ozone, OH, sulphate and
carbonaceous aerosols) are assumed to appear instantaneously after the emission
reductions start, and vanish again immediately after 20 yr. Figure 3 shows a
schematic illustration of this assumption. Due to computational restrictions we
cannot perform transient CTM calculations of the effect of the short-lived gases
and aerosols. Instead we have performed time-slice calculations for 1990 and
382
TERJE BERNTSEN ET AL.
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 by
the dashed line shows the perturbation of the concentrations of the short-lived
species.
ABATEMENT OF GREENHOUSE GASES: DOES LOCATION MATTER?
2.1.
383
EMISSIONS
The 1990 and 2020 base emissions are taken from the A1B marker scenario of
IPCC (2000). It includes the geographical distribution of the emissions of shortlived gases on a 1◦ × 1◦ grid. (sres.ciesin.org/final data.html). The 1990 emissions
of BC from fossil fuel (FF) sources are taken from Cooke et al. (1999), and BC from
biomass burning (BMB) are taken from the Global Emissions Inventory Activity
(GEIA, weather.engin.umich.edu/geia). Total emissions of BC from FF sources
are 5.12 Tg(C)/yr. Total (FF + BMB) monthly emissions for OC are taken from
Liousse et al. (1996). For each grid cell, the fraction of OC coming from BMB is
assumed 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 the
change in SO2 emissions.
Table I shows the annual emissions of gases and aerosols used in this study. The
last column shows the reductions following the 10% reduction in CO2 emissions
in Europe. EEA (1999) gives the total European (28 countries) emissions of CO2 ,
N2 O, CH4 , CO, NOx , VOC, and SO2 for 11 different sectors. Using these EEA
numbers we find that a 10% total reduction of CO2 emissions in Europe is equal
to a 19.3% reduction from sectors 1 and 3 (i.e. no reductions in other sectors). The
emission perturbations in Table I are then derived by reducing the total emissions
of CO2 , N2 O, 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, submicron emission factors (BC/CO2 and OC/CO2 ) for industrial combustion in semideveloped countries from Cooke et al., (1999) have been used. The emission factors
for semi-developed countries were derived by Cooke et al. by taking the maximum
in the range of the emission factors found for developed countries. Since it is likely
that older facilities with the highest emissions are closed down first as part of climate 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
CO2 (Pg(C)/yr)
CH4 (Tg/yr)
N2 O (Tg(N)/yr)
NOx (Tg(N)/yr
CO (Tg/yr)
NMVOC (Tg/yr)
SO2 (Tg(S)/yr)
BC (Tg(C)/yr)
OC (Tg(C)/yr)
Global 1990
Global 2020
Emission perturbations
7.10
506
12.9
43.1
895
247
73.1
5.91
81.1
12.6
632
13.7
58.2
1053
308
102
7.07
99.7
−0.13
−0.125
−0.025
−0.365
−2.06
−0.19
−2.2
−0.054
−0.123
−1.03%
−0.02%
−0.18%
−0.63%
−0.20%
−0.06%
−2.2%
−0.76%
−0.12%
384
TERJE BERNTSEN ET AL.
appropriate for this study. Generally the combustion in large-scale facilities is more
efficient (or includes technology to reduce emissions of e.g. NOx and N2 O) than
the 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 components except SO2 , the relative perturbations are smaller than for CO2 . In the case
of SO2 , the burning of sulphur-containing coal in large-scale combustion explains
this difference.
All 2020 simulations with the CTM have been carried out with a fixed concentration of methane of 2026 ppbv, taken from concentrations given for the A1B
scenario (IPCC, 2001).
2.2.
MODEL DESCRIPTION
Simple globally averaged models are used to calculate the atmospheric concentrations of the long-lived species (CO2 , N2 O and CH4 ), while a global 3-D
CTM is used to calculate the concentrations of the short-lived species (ozone,
and the sulphate and carbonaceous aerosols). The CTM is also used to calculate
atmospheric lifetimes for CH4 based on the distribution of OH radicals in the
CTM.
2.2.1. Models for CO2 , N2 O, and CH4
Transient simulations (1990–2110) of the concentrations and radiative forcing of
CO2 , N2 O and CH4 are performed using the modules of CICERO’s simple climate
model (Fuglestvedt and Berntsen, 1999; Fuglestvedt et al., 2000). The atmospheric
concentration of CO2 is calculated using an efficient and accurate representation of
the carbon cycle developed by Joos et al. (1996). The model uses an ocean mixedlayer pulse response function that characterizes the surface to deep ocean mixing
in combination with a separate equation describing the air-sea exchange based on
the HILDA model (Siegenthaler and Joos, 1992).
For N2 O, a fixed lifetime (e-folding time) of 120 yr is used to calculate the
concentrations. Methane concentrations are calculated using the lifetimes, including
the small changes driven by regional emissions of reactive gases, as calculated by
the full 3-D CTM. Between 1990 and 2020 a linear interpolation of the 1990 and
2020 lifetimes is used, while after 2020 the 2020 lifetimes are used. The global
scale feedback of methane on its own lifetime through its effects on OH (Berntsen
et al., 1992; Prather, 1996) is accounted for through the adjustment of the lifetime
proposed by Osborn and Wigley (1994)
N
C
0
τatm = τatm
C0
0
where τatm is the methane lifetime, and N = 0.3, C0 = 1700 ppbv, and τatm
is the
methane lifetime when C = C0 .
ABATEMENT OF GREENHOUSE GASES: DOES LOCATION MATTER?
385
The radiative forcing of the CO2 , CH4 , and N2 O perturbations are calculated
using the parameterizations of the relations between global mean concentrations
and RF given in IPCC (2001).
2.2.2. The Oslo CTM-2 Model
The OSLO-CTM2 is an off-line chemical transport model that uses pre-calculated
meteorological fields to drive the chemical turnover and distribution of tracers in
the troposphere (Sundet, 1997; Kraabøl et al., 2002). The horizontal resolution of
the model is determined by the input data and computer time available. The input
data set is based on ECMWF forecast data with a T63 (1.87◦ × 1.87◦ ) horizontal
resolution, truncated to T21 (5.6◦ × 5.6◦ ) for the simulations in this study. In the
vertical, the model has 19 levels from the surface up to 10 hPa. The meteorological
input data have been generated for year 1996 by running the Integrated Forecast
System (IFS) model at ECMWF in a series of forecasts starting from the analyzed
fields 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. Data
are sampled every 3 h. The advection of chemical species is calculated by the
second-order moment method, which is able to maintain large gradients in the
distribution of species (Prather, 1986). Vertical mixing by convection is based on
the Tiedtke mass flux scheme (Tiedtke, 1989). Turbulent mixing in the boundary
layer is treated according to the Holtslag K-profile scheme (Holtslag et al., 1990).
The chemical scheme includes 62 chemical compounds and 130 gas phase reactions in order to describe the photochemistry of the troposphere (Berntsen and
Isaksen, 1997; Berntsen and Isaksen, 1999). Recently, the scheme was extended to
include sulphur chemistry, which has been coupled to the photochemistry (Berglen
et al., 2004). The coupling of sulphur and the oxidant chemistry means that oxidation limitations (i.e. removal of H2 O2 before all of the SO2 oxidized) can be treated
properly.
The scheme is solved using the Quasi Steady State Approximation (Hesstvedt
et al., 1978). Photodissociation rates are calculated on-line, following the approach
described in Wild et al. (2000). NOx emissions from lightning are coupled on-line
to the convection in the model using the parameterisation proposed by Price and
Rind (1993) and the procedure given by Berntsen and Isaksen (1999).
Carbonaceous aerosols are implemented following Cooke et al. (1999). Both
BC 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 figure
is 50%. Hydrophobic aerosols are aged (oxidized or coated by a hydrophilic
compound) in the atmosphere and then become hydrophilic with an exponential
lifetime of 1.15 days. Dry deposition of hydrophilic carbonaceous aerosols is
calculated 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 of
0.025 cm/s is applied for all surfaces. The hydrophilic aerosols are also removed
by wet deposition. These aerosols are assumed to be 100% absorbed in the cloud
386
TERJE BERNTSEN ET AL.
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-month
spin-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 forcing
calculations. For the aerosols (sulphate, BC, and OC aerosols), three-dimensional
fields of concentrations were sampled every 3 h because of the effects of humidity
and clouds on radiative forcing. Data for humidity and clouds (for the RF calculations) were taken from the input data for the CTM described above.
2.2.3. Radiative Transfer Models
The radiative transfer calculations of radiative forcing from ozone changes
were made using schemes for thermal infrared radiation and solar radiation as
described in Berntsen et al. (1997) and Myhre et al. (2000). The thermal infrared
scheme is an absorptivity/emissivity broadband model and the solar scheme is
a multi-stream model using the discrete ordinate method (Stamnes et al., 1988).
Radiative transfer calculations of aerosols are performed with a multi-stream
model using the discrete ordinate method (Stamnes et al., 1988; Myhre et al.,
2002). The solar scheme treats the gas absorption with the exponential sum fitting
method.
The optical properties of the aerosols are calculated with Mie Theory. For
sulphate aerosols the optical properties are taken from Myhre et al. (2002) and
for 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 of
pure scattering aerosols in the calculation of the optical properties. No hygroscopic
growth is taken into account for the organic carbon aerosols.
Aerosols are known to cause indirect radiative effects through modifications
of clouds. It is not unlikely that the magnitude of these effects is different for the
different regions we consider in our analysis. To estimate the indirect RF of sulphate
aerosols 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 aerosols
and the simple order-of-magnitude estimates of the indirect forcing is only added to
illustrate the large uncertainties. Organic carbon aerosols can give a negative indirect
radiative forcing through the same mechanism as sulphate aerosols, but most GCM
studies indicate that it is smaller than for sulphate (Kristjansson, 2002). Black carbon
are less hydrophilic so their indirect effects are probably smaller and linked to the
amount of sulphate available, however, it has been suggested that they may cause
a semi direct effect through absorption of solar radiation leading to evaporation of
cloud droplets and inhibiting formation of clouds (Hansen et al., 1997; Ackerman
et al., 2000; Lohmann and Feichter, 2001). Due to the large uncertainties and the
need to perform very costly GCM calculation to do these calculations properly (our
focus is on regional differences which we believe are even more uncertain than
ABATEMENT OF GREENHOUSE GASES: DOES LOCATION MATTER?
387
the global numbers) we have not made separate calculations of indirect effects of
aerosols on radiative forcing.
3. Results
The following section describes how applying the same package of absolute emission reductions (column 4 of Table I) to the four regions defined above affects
perturbations of the chemical composition and radiative forcing. All differences,
including regional differences, shown in Figures 4–14 refer to the impact of these
emission reductions.
3.1.
CHEMICAL PERTURBATIONS
3.1.1. Long-Lived Species
The transient behavior of the atmospheric CO2 abundances given in Figure 4, closely
resembles 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 give
717 and 703 ppmv respectively. In response to the reduced emissions between
2010 and 2030 the concentration is reduced by 0.88 ppmv (−0.19%) by the end
of this period. However, even if the emissions after 2030 are equal to the reference
scenario, this initial perturbation does not recover, but stabilizes at a reduction of
about 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. After
80 yr (2110), 72% of the maximum perturbation (0.63 ppmv out of 0.88 ppmv)
Figure 4. Calculated concentration of CO2 and N2 O for the SRES A1B scenario (thick solid line, left
axis) and deviations from the reference scenario for the perturbation (right axis).
388
TERJE BERNTSEN ET AL.
still remains in the atmosphere. For comparison, using the two different linear
atmospheric 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 N2 O increases from 310 to 383 ppbv from 1990 to 2110
as 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 levels
and 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 fixed
natural source of 300 Tg/yr the concentration peaks around 2050 at a level of
2460 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 the
changes in the emissions of NOx , CO, and VOCs. The atmospheric lifetimes given
in Table II are calculated by the 3-D CTM, and used together with a fixed lifetime
of 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
τatm (years)
τatm (years)
1990
2020
Europe
China
South Asia
South America
7.9056
7.9272
7.92619
−0.00110
7.92616
−0.00114
7.9319
0.0046
7.93981
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.
ABATEMENT OF GREENHOUSE GASES: DOES LOCATION MATTER?
389
The abundance of the OH radical that determines the lifetime of methane is
governed by a complex photochemical system (Crutzen, 1987; Berntsen et al.,
1992; Poppe et al., 1993; Karlsdóttir and Isaksen, 2000). Equal reductions in the
emissions 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 South
America) due to differences in the background levels of primarily NOx , differences
in humidity, and differences in amounts of sunlight to drive the photochemistry. An
increase 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 emission
reductions in Europe and China, the reductions in the lifetime and emissions of
CH4 both contribute to reducing the concentrations. For South Asia and South
America on the other hand, the increase in the lifetime dominates, giving a positive
perturbation of methane.
3.1.2. Ozone
Figure 6 shows the zonally and annually averaged change in ozone concentrations
for the four regions. It can be clearly seen that the sensitivity increases as one goes
south 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 reductions in emissions of ozone precursors in the four different regions.
390
TERJE BERNTSEN ET AL.
America. For Europe and China the background levels of pollutants are sufficiently
high that reduced emissions of ozone precursors cause an increase in ozone close
to the ground. The sensitivities, defined as the change in total tropospheric ozone
burden 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 using
more simplified emission perturbations, added to 1990 emission estimates (Berntsen
et 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. The
lower sensitivity found in our current study is due to higher NOx background due
to enhanced emissions of NOx in South East Asia in the A1B scenario for 2020 and
the fact that our China region includes northern China, while the South East Asia
region 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 (September
for Europe, November for South Asia). The smallest perturbation occurs during
February and March. For South America the maximum change is found in May and
June, with a minimum in September; however, the seasonal cycle is less pronounced
than for the other regions.
Several studies have shown that the radiative forcing of ozone is particularly
strong 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 mixing
by deep convection in the tropics gives rise to significant enhancements in the
12–16 km regions for emission perturbations in South Asia and South America.
3.1.3. Sulphate Aerosols
The changes in annual mean column burden of sulphate for emission perturbations
in the four regions are shown in Figure 7. In the CTM the emission reductions (−2.2
Tg(S)/yr in all regions) caused the largest local reduction in the annual sulphate
burden when the reductions were carried out for South Asia, reaching more than
1 mg(S)/m2 over north eastern parts of the region. For all regions the mid-latitude
westerlies cause a plume extending eastwards from the source regions. The plume
is more extensive for the subtropical regions (South Asia and South America) due to
decreasing precipitation as the air mass approaches the subtropical high-pressure
systems. For all regions even for Europe, there are also well identified plumes
moving west in the tropical easterlies.
Figure 8 shows the annual cycle in the reductions of the global sulphate burden caused by the emission reductions in each region. The annual mean change
is largest for South Asia with −13.2 Gg(S) compared to the other regions where
the reductions were 8.9, 6.9 and 12.2 Gg(S) for Europe, China, and South America
respectively. The seasonal cycle is strongest for Europe for three reasons: Less efficient oxidation of SO2 to sulphate during winter (less chemical oxidant production
in the atmosphere), less ventilation out of the planetary boundary layer which increases the dry deposition of SO2 , and more efficient wet scavenging of SO4 during
ABATEMENT OF GREENHOUSE GASES: DOES LOCATION MATTER?
391
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.
392
TERJE BERNTSEN ET AL.
winter by large-scale frontal precipitation. For South Asia the perturbations are
particularly large during the winter monsoon period from November to May. During these months the reductions in the plume extending south westwards towards
Africa, which can also be seen in the annual average in Figure 7, are particularly
pronounced. This is consistent with the measurements from the INDOEX experiment (Lelieveld et al., 2001). Changes in burden during summer are about 50% of
the winter changes due to shorter lifetimes of the water soluble sulphate aerosols
during the rainy season.
3.1.4. Carbonaceous Aerosols
The changes in the annual mean burdens of BC aerosols and OC aerosols given
in Figures 9 and 10, respectively, show many features similar to those of sulphate
aerosols (Figure 7), with plumes at mid-latitude to the east, and less pronounced
westerly plumes in the tropics. For both types of carbonaceous aerosols, reducing
the emissions in South Asia causes the largest local reduction in burden (152 and
344 μg(C)/m2 for BC and OC, respectively). In terms of contribution to the global
annual 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 .
ABATEMENT OF GREENHOUSE GASES: DOES LOCATION MATTER?
393
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 the
global annual mean burden by 3.2, 2.6, 1.9, and 1.6 μg(C)/m2 for South Asia, South
America, Europe, and China respectively. For OC the corresponding numbers are
6.6, 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 from
the different gases and aerosols caused by a particular climate mitigation measure
will vary considerably over time. Table III shows the radiative forcing in 2030 at
the end of the 20-yr mitigation period (see Figure 2), when the contribution from
the short-lived species is at a maximum. The effects of the temporal evolution of the
radiative forcing are discussed in the next section. There is a clear difference in the
effect of reduction in well-mixed and short-lived components, with large regional
differences in the radiative forcing for the short-lived species. Methane is a wellmixed gas in the atmosphere. However, the loss of methane through reaction with
the OH radical is controlled by short-lived components and thus causes different
394
TERJE BERNTSEN ET AL.
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
CO2
N2 O
Methane
Ozone
Sulphate aerosols
Black carbon aerosols
Organic carbon aerosols
−9.8
−0.28
−0.22
−0.35
11 (24)
−2.0
0.51
China
−9.8
−0.28
−0.22
−0.52
7.5 (20)
−1.6
0.40
South Asia
−9.8
−0.28
0.28
−1.1
17 (30)
−3.0
0.90
South America
−9.8
−0.28
0.97
−2.2
19 (32)
−2.9
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 only
component 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 are
the two dominating components. They are of the same order of magnitude, but
of opposite sign. For the short-lived components, the magnitude of the radiative
forcing 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 importance
are the regional differences in the change in the burden of the components as
discussed in Section 3.1. In addition, there are different mechanisms that influence
the magnitude of the radiative forcing of the short-lived components. The radiative
forcing of tropospheric ozone depends strongly on the vertical distribution (Lacis
et al., 1990) and the horizontal distribution (Berntsen et al., 1997) in the change
in the abundance, as well as the background ozone concentration. The radiative
forcing of sulphate depends significantly on the distribution of relative humidity
and clouds. Clouds reduce the magnitude of the radiative forcing, while uptake
of water with increasing relative humidity strengthens the radiative forcing due to
sulphate (Haywood et al., 1997; Myhre et al., 2002). At mid and high latitudes the
seasonal cycle in perturbations (see Figure 8 for sulphate) is also important. At
higher latitudes (Europe and to a lesser extent in China), the larger changes during
summer increase the radiative forcing for these regions. For OC, clouds have a
strong effect on reducing the magnitude of the radiative forcing and may cause
significant spatial inhomogenities. The radiative forcing due to BC is also very
dependent on the cloud distribution: BC above the cloud layer has a much greater
ABATEMENT OF GREENHOUSE GASES: DOES LOCATION MATTER?
395
forcing than BC in a clear sky, and BC below the cloud layer has a reduced forcing
compared to BC in a clear sky. For all the aerosol components, the surface albedo
is also important because the magnitude of the forcing increases for scattering
aerosols (sulphate and OC) in regions with low surface albedo and increases for
absorbing aerosols (BC) in regions with high surface albedo.
To isolate the other effects from the differences in burden we show in Table IV the
radiative 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 off
the west coast of South America with high relative humidity and a small amount
of low stratocumulus clouds that largely explains the high normalized values for
the aerosols. Indirect effects on clouds could offset this since it would probably be
below average in this region.
The normalized forcings given in Table IV agree well with the numbers given
in IPCC (2001). The mean normalized forcing for tropospheric ozone for nine
studies is 0.042 Wm−2 /DU, with a range from 0.033 to 0.056 Wm−2 /DU. For the
normalized direct forcing of sulphate aerosols the 19 studies referenced in IPCC
give 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 from
four 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, only
three 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 the
change in global burden appear to be more important than the differences in the
normalized radiative forcings given in Table IV.
4. Potential for Climate Change
Comparing the potential climate effects of the mitigation measures applied to the
various 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
RF/dO3 (Wm−2 /DU)
RF/dSO4 (W/g)
RF/dBC (W/g)
RF/dOC (W/g)
0.035
−213
1090
−152
China
0.040
−186
1010
−122
South Asia
0.038
−224
930
−136
South America
0.037
−268
1120
−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.
396
TERJE BERNTSEN ET AL.
differs between the regions, and for the different forcing agents it differs over
time. Fuglestvedt et al. (2003) thoroughly reviews the complex questions related
to the assumptions behind the choice of indices to compare emissions of different
climate forcing agents.
To compare the potential climate effects of the emission reductions studied in this
work, we have calculated numerical values for three different metrics or indices of
climate change. All three metrics use the global and annual mean radiative forcing
as the primary input parameter. First, we introduce a Warming Index (WI) over
a time horizon (H) based on global and annual mean radiative forcing, following
the 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 following
emission reductions in each region into a simple climate model (SCM) to calculate
global mean surface temperature change over the period from 2000 to 2100. As
a third index (Section 4.3) to study the characteristics of the regional variations
and the robustness of the conclusions with respect to choice of index we use a
simple formulation that includes a non-linear temperature term and a discounting
term to weigh the effects over time. This formulation was introduced to estimate
emission indices based on the economic damage of climate change caused by
various 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 regions
is the Warming Index (WI) defined by Equation (1):
2010+H
RFi (t) dt
2010
WIi,H = 2010+H
(1)
RFCO2 (t) dt
2010
The term RFi refers to the difference in global radiative forcing for component
i between the reference case (the standard SRES A1B scenario) and the perturbation
cases as defined in Table I. Note that the radiative forcing is not calculated per unit
mass of a pulse emission (as for the GWP index), but for the actual emission changes
given in Table I, applied for a period of N years starting in 2010 (N = 20 years in
the standard simulations). The definition of the standard emission metric GWP does
not say anything about the background atmosphere or which gases the concept is
well suited for, but in the IPCC reports, GWPs are calculated for well-mixed gases
and for a constant background (IPCC, 1996, 2001). The WI-index as used here
is calculated with a background following the reference scenario, and short-lived
substances 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 for
the integration of radiative forcing (H). This is similar to an approach adopted by
Harvey (1993).
ABATEMENT OF GREENHOUSE GASES: DOES LOCATION MATTER?
397
For the RF of CO2 there are two effects of using a non-constant background
scenario. Due to the saturation effect of the absorption, the enhanced background
reduces the RF per unit increase in concentration. Compared to the impact of the
CO2 perturbation (see Figure 4) put on top of a constant background level of 360
ppmv, the integrated RF over 100 yr is reduced by 26%. Secondly, the non-linear
response in the carbon cycle model we have used (see Section 3.1) enhances the
RF perturbation from CO2 by 25–30% at the end of the period compared to linear
carbon models (Maier-Reimer and Hasselmann, 1987). The net effect of using a
non-constant background on the integrated RF of CO2 is thus small, in agreement
with Caldeira and Kasting (1993) and IPCC (1996).
4.1.1. The Reference Case
Table V shows the calculated WI100 for the seven climate forcing agents considered
for the different regions. Comparison of the total or net WI values in the bottom
row (the sum of individual WI values) indicates to which extent the location of the
emissions does make a difference. For all regions the net effect is smaller than for
CO2 alone, or for the sum of the Kyoto gases considered (CO2 , N2 O, 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 with
the opposite sign. For South America the net effect is only about half of what is
counted for under the Kyoto Protocol. As discussed above, the photochemistry is
quite different between the regions, with the RF from ozone changes being more
than six times larger for South America than for Europe. However, if the effects
on the two gases affected by non-linear photochemistry (methane and ozone) are
added up, the net WI100 values of the two gases vary between 0.020 and 0.029 for
the four regions.
It should be noted that by adding up RF from short-lived species (ozone and
aerosols,) and long-lived species (CO2 , N2 O and CH4 ), and also by adding up
TABLE V
WI100 for a 100-yr time horizon and 20 yr of emission reductions
CO2
N2 O
CH4
O3
SO4 aerosols
BC aerosols
OC aerosols
Total
WI100 (Europe)
WI100 (China)
WI100 (South Asia)
WI100 (S. America)
1
0.031
0.009
0.011
−0.35 (−0.75)
0.065
−0.016
0.75 (0.35)
1
0.031
0.010
0.017
−0.24 (−0.64)
0.051
−0.013
0.86 (0.45)
1
0.031
−0.012
0.035
−0.55 (−0.95)
0.095
−0.029
0.57 (0.17)
1
0.031
−0.041
0.070
−0.61 (−1.01)
0.092
−0.021
0.53 (0.12)
The numbers in parenthesis for SO4 include the indirect effects on clouds.
398
TERJE BERNTSEN ET AL.
negative and positive radiative forcings, we make some simplifications. These
simplifications are similar to calculating global mean temperature changes with
reduced-form energy-balance climate models of the type used in IPCC (2001) and
several other studies (Harvey et al., 1997; Hayhoe et al., 2002). Perturbations of
short-lived species will lead to RF mainly during the period with emission reductions, and large regional RF (positive or negative) might cause regional changes
due to circulation/precipitation changes that will not be cancelled by long-term
globally homogeneous positive RF. The integrated change in global RF, which can
be viewed as an approximation of a global temperature change, will not be able to
capture potential changes in regional aspects of climate changes (e.g. circulation
and/or precipitations changes) due to strong regional RF.
In the design of our experiments we have made certain assumptions that have a
significant impact on the evaluation of the emission reductions in the different regions. To investigate the influence of some of these assumptions we have calculated
WI-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 WI H
• Enhancing the emission factors of SO2 , BC and OC in China, South Asia, and
South America corresponding to the maximum numbers given by Hayhoe
et al. (2002)
4.1.2. Longer Mitigation Period (N)
In our base case we have assumed that the emissions are reduced for 20 yr (see
Section 2). Extending the length of the emission reduction period will enhance
the relative effects of the short-lived components compared to that of the longlived. Using the CTM calculations already performed for 2020 (see Figure 3), but
assuming that the perturbations now last for 40 yr (i.e. extending the rectangular
box in Figure 3 to 2050), and running new simulations for CO2 , N2 O, and CH4 , we
get WI100 values for this case as given in Table VI.
Extending the emission reduction period to 40 yr reduces the net effects in all
regions 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 effects
are 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 WI H , will also enhance
the contribution from the short-lived s pecies. Table VII shows the WI H values for a
time horizon of 50 yr for the standard 20 yr of emission reductions. Now the direct
effect of the reductions in sulphate aerosols becomes about equal to the CO2 effect
for the two tropical regions (South Asia and South America). The ranking of the
regions does not change by using H = 50 yr.
ABATEMENT OF GREENHOUSE GASES: DOES LOCATION MATTER?
399
TABLE VI
WI100 for 40 yr of emission reductions
CO2
N2 O
CH4
O3
SO4 aerosols
BC aerosols
OC aerosols
Total
WI100 (Europe)
WI100 (China)
WI100 (South Asia)
WI100 (S. America)
1
0.032
0.011
0.012
−0.39 (−0.84)
0.072
−0.018
0.72 (0.27)
1
0.032
0.011
0.019
−0.27 (−0.72)
0.057
−0.014
0.84 (0.39)
1
0.032
−0.014
0.039
−0.61 (−1.07)
0.11
−0.032
0.52 (0.07)
1
0.032
−0.048
0.079
−0.68 (−1.13)
0.10
−0.023
0.47 (0.01)
TABLE VII
WI for a 50-yr time horizon and 20 yr of emission reductions
CO2
N2 O
CH4
O3
SO4 aerosols
BC-aerosols
OC-aerosols
Total
WI50 (Europe)
WI50 (China)
WI50 (South Asia)
WI50 (S. America)
1
0.030
0.016
0.019
−0.61 (−1.31)
0.11
−0.028
0.54 (−0.16)
1
0.030
0.016
0.029
−0.42 (−1.12)
0.089
−0.022
0.73 (0.03)
1
0.030
−0.019
0.061
−0.95 (−1.66)
0.17
−0.050
0.24 (−0.47)
1
0.030
−0.068
0.12
−1.05 (−1.76)
0.16
−0.036
0.16 (−0.55)
4.1.4. Enhanced Aerosol Emissions
In our experimental setup it is assumed that the mix of pollutants relative to CO2 is
equal for all regions. This would of course not really be the case, but it is beyond
the scope of this study to estimate the relevant regional emission factors for a set of
emission 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 the
emission to concentrations, and concentrations to radiative forcing relations for
the aerosols are reasonably linear, we can make a simple analysis of the possible
effects of this assumption. We used the upper limit of the range of emission factors
for SO2 and BC emissions from coal burning from Hayhoe et al. (2002) to scale
up the SO2 , BC, and OC emissions (using the same scaling factor for OC as for
BC) for the emissions in China, South Asia, and South America. Using the upper
limit for all three species can be justified as high sulphur-containing coal (lignite
or brown coal) has a low heat content that reduces the combustion temperature and
thus increases the formation of carbonaceous aerosols. The corresponding radiative
forcing and WI100 values are given in Table VIII. Since the scaling factor is much
400
TERJE BERNTSEN ET AL.
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
N2 O
CH4
O3
SO4 aerosols
BC-aerosols
OC-aerosols
Total
WI100 (Europe)
WI100 (China)
WI100 (South Asia)
WI100 (S. America)
1
0.031
0.009
0.011
−0.35 (−0.75)
0.065
−0.016
0.75 (0.35)
1.000
0.031
0.010
0.017
−0.68 (−1.8)
0.878
−0.218
1.04 (−0.11)
1.000
0.031
−0.012
0.035
−1.6 (−2.7)
1.625
−0.491
0.63 (−0.52)
1.000
0.031
−0.041
0.070
−1.7 (−2.9)
1.575
−0.357
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 effect
of the carbonaceous aerosols becomes of similar magnitude to the direct effect of
sulphate, and for the tropical regions larger than for CO2 .
Table VIII shows that even with a very large increase in the contribution from the
aerosols, the net effect (i.e. the total WI), and the differences between the regions
do 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, and
N = 20 yr) and 0.33 (i.e. 0.86–0.53) in the standard case.
ABATEMENT OF GREENHOUSE GASES: DOES LOCATION MATTER?
4.2.
401
TEMPERATURE RESPONSE
Based on the temporal evolution of the net radiative forcing we have calculated the
changes 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 balance
climate/upwelling-diffusion ocean model (Schlesinger et al., 1992; Fuglestvedt and
Berntsen, 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 to
the four regions. At the beginning of the 20-yr mitigation period, the temperatures
increase due to reductions in the cooling by sulphate aerosols (direct effects only, see
Table III). When the mitigation period ends, the effect of the short-lived components
disappears and the temperatures are reduced due to reduced radiative forcing by
the longer lived greenhouse gases, mainly CO2 . The largest temperature reductions
occur 20–40 yr after the end of the mitigation period. Since it is mainly the shortlived species that are regionally dependent, the difference between the regions
becomes negligible towards the end of the century.
Including the indirect effects of sulphate in the simplified manner as has been
done in this study leads to larger temperature enhancements during the mitigation
period (+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 2100
remains 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.
402
TERJE BERNTSEN ET AL.
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 also
show the temperature changes given sustained emission changes until 2100. Radiative forcings for the short lived components are taken from Table III and kept constant until 2100, while for CO2 , CH4 , and N2 O concentrations and radiative forcings
are calculated for the whole period with sustained emission changes as given in Table I. Figure 13 shows that the while the absolute temperature difference between the
regions increases over time for sustained emission reductions, the differences relative to the mean temperature reduction for the four regions decrease. This illustrates
that 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 in
Figures 12 and 13 represent two ways to evaluate and compare the potential climate
impact of an emission reduction package applied to different regions. We now
proceed a step further to show how the comparison behaves if the metric includes a
non-linear temperature term to describe damages1 and a discounting term to weigh
the effects over time. The metric given by Equation (2) is equal to the net present
value (NPV) metric of economic damages presented by Kandlikar (1995). The idea
is that the NPV of the damage (Da , in monetary units) can be expressed by
th
Da =
0
k[T (t)]n exp(−r t) dt
(2)
ABATEMENT OF GREENHOUSE GASES: DOES LOCATION MATTER?
403
where th is the time horizon, T(t) is the change in global mean surface temperature
as a function of time (t), r is the discount rate, and k a scaling factor which converts
the metric to monetary units. The exponent n is normally assumed to be greater
than 1, causing the damage to increase super-linearly with temperature change. This
highly simplified formulation of potential damage gives a conceptual illustration
of how the quantification and assessment of damages depend on the shape of the
damage function (i.e. the value of n) and the concern and weighting of long-term
versus short-term damages (i.e. through the discount rate).
We have applied this metric definition in a similar way as we have done for
the WI metric, to calculate the relative effects (Dr ) of all emission reductions in
a given region relative to a CO2 reduction only.2 This approach means that the
ability of Equation (2) to give the absolute economical damage (i.e. the calibration
of the constant k) is unimportant since we are only looking at the relative effects of
regional 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 4
in IPCC (1996b) for a review of the use of discounting in analysis of climate
change). 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, the
well-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.
404
TERJE BERNTSEN ET AL.
decrease. Increasing the discount rate r has the opposite effect and puts less value
on changes in the distant future and more on the short-term effects, thus enhancing
the differences between the regions.
Emphasis on the short-term impacts by applying a relatively high discount rate
of 4% reduces the net effects of the reductions because the value of the heating
during the first phase (2010–2030 in Figure 12) approaches the value of reduced
heating after about 2030. The ranking of the regions does not change across all
applied metrics in this study. It should be noted that the apparent robustness of the
results across the applied metrics could be significantly reduced if more realistic
region-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 the
four regions. This follows the assumption that the sensitivity of the global climate
system is equal for all forcing mechanisms. Using three different GCMs forced
by idealized regional forcing through either CO2 , solar or ozone changes, Joshi
et 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 indications
from GCM studies (Lelieveld et al., 2002; Rotstayn and Penner, 2001; Menon et al.,
2002; Kristjansson, 2002) that regionally heterogeneous RF can induce changes in
the large-scale circulation affecting the regional pattern of temperature change
as well as the hydrological cycle, thus affecting the regional patterns of flooding
and droughts. These climate effects of regional heterogeneous forcings are still
quite uncertain and differ between the GCMs. For some forcing mechanisms and
regions it might reduce the differences found above in the global impacts, while for
other forcing mechanisms and regions the effects even on a global scale might be
enhanced. However, the possibility of regional effects undoubtedly casts uncertainty
on the assumption that the climate impacts of measures aimed at the gases included
in 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 emission
reductions will give equal climate effects if the reductions take place in different regions and if several gases and aerosols are affected. There are three main aspects (or
“dimensions”) in these considerations: (i) geographical variations, (ii) the chemical
composition of the emission reduction and the characteristics of the species, and
(iii) timescales both in terms of the duration of the measure implemented and the
horizon over which the effects are considered. In addition to the isolated effects
ABATEMENT OF GREENHOUSE GASES: DOES LOCATION MATTER?
405
of these factors, there are also interactions between them, which may increase or
decrease the total effect.
Several previous studies have shown that the effects of emission changes may
vary widely between regions (Johnson and Derwent, 1996; Fuglestvedt et al., 1999;
Wild et al., 2001, Berntsen et al., 2002). Thus the chemical, physical and meteorological environments in which abatement measures are implemented have to be
considered. The current study takes into account an array of gases and aerosols
following measures corresponding to a 10% reduction in European CO2 emissions
by reduced emissions from large sources, public power, cogeneration and district
heating, and industrial combustion, as opposed to earlier studies where one or two
gases were considered. We find that the enhancement of the atmospheric burden of
the aerosols (sulphate and carbonaceous aerosols) is about twice as large for the low
latitude regions (South Asia and South America) due to longer lifetimes in the atmosphere. The effects through perturbations of tropospheric ozone and methane lifetime through emissions of NOx , CO, and NMVOCs are significantly smaller (about
an order of magnitude) than the direct sulphate effect. This should not be interpreted
as a general result for all emission sources, since the chosen sources are the major
source of sulphur (79% of the SO2 emissions in Europe), while they only account for
35%, 1%, and 13% of the NOx , CO, and NMVOC emissions respectively. If a sector
like road traffic had been chosen (3%, 44%, 31%, and 56% of the European SO2 ,
NOx , CO, and NMVOC emissions, respectively) the relative importance of ozone
and methane would be significantly larger. However, for political and practical reasons we believe that large-scale sources, as we have focused on are more likely to
be affected by emission trading between regions than, for example, road traffic.
In terms of integrated radiative forcing, we find that the reductions in the emissions change the concentrations of non-CO2 gases and aerosols so that the total
direct effect is always smaller than the effect of CO2 alone. Significant variations
between the regions are found (53–86% of the CO2 effect) for the net effect of the
same package of emission reductions. Inclusion of the indirect effects of sulphate
aerosols reduces the net effect of measures towards zero when the weighting over
time is done as defined by the WI index. If the focus is on temperature change
towards the end of the century, the long-lived greenhouse gases dominates and the
net effect of the reductions is to decrease the warming. This shows that it is important 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 radiative
behaviour 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 and
indirect effects of aerosols are not taken into account. Partly because of the large
uncertainties in these effects on the regional level needed in our analysis and partly
due to the need for very costly GCM experiments to quantify them properly.
Furthermore, since the multitude of gases involved show a wide range of atmospheric 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))
406
TERJE BERNTSEN ET AL.
are crucial. Due to short adjustment times, the changes in aerosols and ozone dominate during the first parts of the period, causing a temperature enhancement mainly
due to reduced cooling by sulphate aerosols (see Figure 12). The changes in longer
lived gases are more important later, causing a reduced temperature increase after
the end of the mitigation period (N). Thus, a realistic evaluation of the expected
lifetime of the measure and a conscious choice of the horizon for the assessment
of effects are necessary.
Although the importance of the short-lived components (compared to CO2 only
or the Kyoto gases) varies considerable between the regions and depends on the
metric applied (WI H 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 China
stands 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 differences
in the perturbations of the sulphate burden (see Figure 8), where in the CTM the
removal of sulphate is most efficient for sulphate originating from emissions in
China. Also the normalized RF of sulphate for the experiment with reduction in
China is lower than for the other regions (Table IV). It should be kept in mind that
the conclusion that reductions in China are most effective is based on equal emission
reductions in all regions. This may not be a very realistic assumption, but it has
allowed us to study the isolated effects of regional differences in chemical, physical
and meteorological conditions and how they determine the climate response to
changes in emissions. An important result of this study is that it has allowed us to
derive species specific emission indices relative to CO2 (i.e. WI numbers in Tables
V–VIII). These indices can be used in the same way as the GWP numbers (as
discussed in Section 4.1.4) to analyze more realistic and complex sets of regional
mitigation 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 flexibility
mechanisms such as Joint Implementation, international emission trading, and the
Clean Development Mechanism (CDM). Strictly speaking, the problems related
to the location of the implementation of measures and array of gases affected are
not only related to the flexibility mechanisms. The same logic or reasoning can be
applied to the initial distribution of emission reductions to the parties of a climate
agreement (i.e. without flexibility mechanisms), since the same amount of CO2
reductions in one country may cause a different net change in radiative forcing if
several non-CO2 gases are affected when the atmospheric conditions are different.
Obviously, inclusions of such considerations will complicate the political negotiations and the implementation of the flexibility mechanisms during the next
commitment period of the Kyoto Protocol. The main question is, then, how much
scientific knowledge can be taken into account in the process of developing new
protocols for the period after 2012. Geographical variations, timescales and opposing effects on radiative forcing are scientific issues with no simple policy responses.
ABATEMENT OF GREENHOUSE GASES: DOES LOCATION MATTER?
407
For instance, there is still nothing that indicates consensus on the crucial question
of 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 of
mitigation measures. However, we believe that there is a need for further scientific
studies which go one step further from what is done in this paper and start with a
regional bottom-up analysis of potential climate mitigation options before effects of
short-lived species can be treated successfully in climate negotiations. This analysis
should include all relevant sectors and account for important regional differences
in emission reductions following realistic mitigation options. Identification of the
most likely combination of emission reductions on a regional level would also make
it possible to assess co-benefits in terms of reduced air pollution. Such effects could
be important in enhancing the political feasibility of including short-lived species
in climate agreements.
Acknowledgments
This work has been supported by the Norwegian Research Council, project number
142128/720, and from the European Union (m e τ r i ◦ C project, EVK2-CT-199900021). We thank Lynn Nygaard and Michele Twena for help with editing the
manuscript.
Notes
1
Note 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.
References
Ackerman, A. S., Toon, O. B., Stevens, D. E., Heymsfield, A. J., Ramanathan, V., and Welton, E. J.:
2000, ‘Reductions of tropical cloudiness by soot’, Science 288, 1042–1047.
Berglen, T. F., Berntsen, T., Isaksen, I. S. A., and Sundet, J.: 2004, ‘A global model of the coupled sulfur/oxidant chemistry in the troposphere: The sulfur cycle’, J. Geophys. Res. 109(D19), D19310,
doi:10.1029/2003JD003948.
Berntsen, T., Fuglestvedt, J. S., and Isaksen, I. S. A.: 1992, ‘Chemical-dynamical modelling of the
atmosphere with emphasis on methane oxidation’, Ber. Bunsen Ges. Phys. Chem. 96, 241–251.
Berntsen, T. and Isaksen, I. S. A.: 1997, ‘A global 3-D chemical transport model for the troposphere;
1. Model description and CO and ozone results’, J. Geophys. Res. 102, 21,239–21,280.
Berntsen, T., Isaksen, I. S. A., Fuglestvedt, J. S., Alsvik Larsen, T., Stordal, F., Freckleton, R. S., and
Shine, K. P.: 1997, ‘Effects of anthropogenic emissions on tropospheric ozone and its radiative
forcing’, J. Geophys. Res. 102, 28,101–281.
Berntsen, T. and Isaksen, I. S. A.: 1999, ‘Effects of lightning and convection on changes in tropospheric
ozone due to NOx emissions from aircraft’, Tellus 51B, 766–788.
408
TERJE BERNTSEN ET AL.
Berntsen, T., Fuglestvedt, J. S., Joshi, M., Shine, K. P., Ponater, M., Sausen, R., and Hauglustaine, D.
A.: 2002, ‘Indirect forcing from emissions of NOx and CO: Is the location of emissions important?’
Proceedings from the Third International Symposium on Non-CO2 Greenhouse Gases. Millpress,
Rotterdam, pp. 363–369.
Brasseur, G. P., Cox, R. A., Hauglustaine, D. A., Isaksen, I. S. A., Lelieveld, J., Lister, D. H., Sausen,
R., Shumann, U., Wahner, A., and Wiesen, P.: 1998, ‘European scientific assessment of the
atmospheric effects of aircraft emissions’, Atmos. Environ. 32, 2329–2418.
Caldeira, K. and Kasting, J. F.: 1993, ‘Insensitivity of global warming potentials to carbon dioxide
emission scenarios’, Nature, 366, 251–253.
Cooke, W. F., Liousse, C., Cachier, H., and Feichter, J.: 1999, ‘Construction of a 1◦ × 1◦ fossil fuel
emission data set for carbonaceous aerosol and implementation and radiative forcing impact in
the ECHAM4 model’, J. Geophys. Res. 104, 22137–22162.
Crutzen, P. J.: 1987, ‘Role of the tropics in atmospheric chemistry’, in Dickinson, R. E. (ed.)The
Geophysiology of Amazonia, Wiley, New York, pp. 107–130.
Derwent, R. G., Collins, W. J., Johnson, C. E., and Stevenson, D. S.: 2001, ‘Transient behaviour of
tropospheric ozone precursors in a global 3-D CTM and their indirect greenhouse effects’, Clim.
Change, 25, 1–25.
European Environment Agency (EEA): 1999, ‘Joint EMEP/CORINAIR Atmospheric emission inventory guidebook’, Technical report No. 30, 2nd edn. European Environmental Agency, Copenhagen,
Denmark.
Forster, P. M. de F. and Shine, K. P.: 1997, ‘Radiative forcing and temperature trends from stratospheric
ozone depletion’, J. Geophys. Res. 102, 10,841–10,855.
Fuglestvedt, J. S., Berntsen, T. K., Godal, O., and Skodvin, T.: 2000, ‘Climate implications of GWPbased reductions in greenhouse gas emissions’, Geophys. Res. Lett. 27, 409–412.
Fuglestvedt, J. S. and Berntsen, T. K.: 1999, ‘A simple model for scenario studies of changes in global
climate: Version 1.0’, Working Paper 1999-02. CICERO, Oslo, Norway.
Fuglestvedt, J. S., Berntsen, T. K., Isaksen, I. S. A., Mao, H., Liang, X.-Z., and Wang, W.-C.: 1999,
‘Climatic effects of NOx emissions through changes in tropospheric O3 and CH4 – A global 3-D
model study’, Atmos. Environ. 33, 961–977.
Fuglestvedt, J. S., Isaksen, I. S. A., and Wang, W. -C.: 1996, ‘Estimates of indirect global warming
potential for CH4 , CO and NOx ’, Clim. Change 34, 404–437.
Fuglestvedt, J. S., Berntsen, T., Godal, O., Sausen, R., Shine, K. P., and Skodvin, T.: 2003, ‘Metrics
of climate change: Assessing radiative forcing and emission indices’, Clim. Change 58, 267–
331.
Hammit, J. K., Jain, A. K., Adams, J. L., and Wuebbels, D. J.: 1996, ‘A welfare-based index for
assessing environmental effects of greenhouse-gas emissions’, Nature 381, 301–303.
Hansen, J., Sato, M., and Ruedy, R.: 1997, ‘Radiative forcing and climate response’, J. Geophys. Res.
102, 6831–6864.
Harvey, L. D. D.: 1993, ‘A guide to global warming potentials (GWPs)’, Energy Pol. 21, 24–34.
Harvey, L. D. D., Gregory, J., Hoffert, M., Jain, A., Lal, M., Leemans, R., Raper, S. B. C., Wigley,
T. M. L., and Wolde, J. de.: 1997, ‘An introduction to simple climate models used in the IPCC
Second Assessment Report:’, IPCC Technical Paper 2, IPCC, Geneva, 50 p.
Hasselmann, K., Hasselmann, S., Ocana, V., and Storch, H. V.: 1997, ‘Sensitivity study of optimal
CO2 emission paths using a simplified structural integrated assessment model (SIAM)’, Clim.
Change 37,345–386.
Hayhoe, K., Kheshgi, H. S., Jain, A. T., and Wuebbles, D. J.: 2002, ‘Substitution of natural gas for
coal: Climate effects of utility sector emissions’, Clim. Change 54, 107–139.
Haywood, J. M., Ramaswamy, V., and Donner, L. J.: 1997, ‘A limited-area-model case study of the
effects of sub-grid scale variations in relative humidity and cloud upon the direct radiative forcing
of sulphate aerosol’, Geophys. Res. Lett. 24, 143–146.
ABATEMENT OF GREENHOUSE GASES: DOES LOCATION MATTER?
409
Hesstvedt, E., Hov, Ø., and Isaksen, I. S. A.: 1978, ‘Quasi steady-state approximation in air pollution
modelling: Comparison of two numerical schemes for oxidant prediction’, Int. J. Chem. Kinet.
X, 971–994.
Holtslag, A. A. M., DrBruijn, E. I. F., and Pan, H. -L.: 1990, ‘A High resolution air mass transformation
model for short-range weather forecasting’, Mon. Wea. Rev. 118, 1561–1575.
Intergovernmental Panel on Climate Change (IPCC): 1990, Climate Change. The Scientific Assessment, UNEP/WMO, Cambridge University Press, Cambridge, U.K.
Intergovernmental Panel on Climate Change (IPCC): 1996, Climate Change 1995. The Science of
Climate Change, Cambridge University Press, Cambridge, U.K.
Intergovernmental Panel on Climate Change (IPCC): 1996b, Climate Change 1995. Economic and
Social Dimensions of Climate Change, Cambridge University Press, Cambridge, U.K.
Intergovernmental Panel on Climate Change (IPCC): 1997. An introduction to simple climate models
used in the IPCC second assessment report. IPCC Technical Paper II, Cambridge University
Press, Cambridge, U.K.
Intergovernmental Panel on Climate Change (IPCC): 1999, Aviation and the Global Atmosphere – A
Special Report of IPCC Working Groups I and III, Cambridge University Press, Cambridge, U.K.
Intergovernmental Panel on Climate Change (IPCC): 2000, Special Report on Emission Scenarios,
Cambridge University Press, Cambridge, U.K.
Intergovernmental Panel on Climate Change (IPCC): 2001, Climate Change 2001 – The Scientific
Basis, Cambridge University Press, Cambridge, U.K.
Isaksen, I. S. A., Hov, Ø., and Hesstvedt, E.: 1978, ‘Ozone generation over rural areas’, Environ. Sci.
Technol. 12, 1279–1284.
Johnson, C. E. and Derwent, R. G.: 1996, ‘Relative radiative forcing consequences of global emissions of hydrocarbons, carbon monoxide and NOx from human activities estimated with zonallyaveraged two-dimensional model’, Climate Change 34, 439–462.
Joos, F., Bruno, M., Fink, R., Stocker, T. F., Siegenthaler, U., Le Quéré, C., and Sarmiento, J. L.:
1996, ‘An efficient and accurate representation of complex oceanic and biospheric models of
anthropogenic carbon uptake’, Tellus 48B, 397–417.
Joshi, M., Shine, K. P., Ponater, M., Stuber, N., Sausen, R., and Li, L.: 2003, ‘A comparison of climate
response to different radiative forcings in three general circulation models: Towards an improved
metric of climate change’, Clim. Dyn. 20, 843–854.
Kandlikar, M.: 1995, ‘The relative role of trace gas emissions in greenhouse abatement policies’,
Energy Pol. 23, 879–883.
Karlsdóttir, S., and Isaksen, I. S. A.: 2000, ‘Changing methane lifetime: Possible cause for reduced
growth’, Geophys. Res. Lett. 27, 93–96.
Kraabøl, A. G., Stordal, F. Berntsen, T., and Sundet, J.: 2002, ‘Impacts of NOx emissions from
subsonic aircraft in a global 3-D CTM including plume processes’, J. Geophys. Res. 107,
doi:10.1029/2001JD001019.
Kristjansson, J. E.: 2002, ‘Studies of the aerosol indirect effect of sulphate and black carbon aerosols’,
J. Geophys. Res. 107(D15), doi:10.1029/2001JD000887.
Lacis, A. A., Wuebbles, D. J., and Logan, J. A.: 1990, ‘Radiative forcing of climate by changes in the
vertical distribution of ozone’, J. Geophys. Res. 95, 9971–9981.
Lelieveld, J. et al.: 2001, ‘The Indian Ocean experiment: Widespread air pollution from south and
southeast Asia’, Science 291, 1031–1036.
Lelieveld, J., Berresheim, H., Borrmann, S., Crutzen, P. J., Dentener, F. J., Fischer, H., Feichter, J.,
Flatau, P. J., Heland, J., Holzinger, R., Korrmann, R., Lawrence, M. G., Levin, Z., Markowicz,
K. M., Mihalopoulos, N., Minikin, A., Ramanathan, V., de Reus, M., Roelofs, G. J., Scheeren,
H. A., Sciare, J., Schlager, H., Schultz, M., Siegmund, P., Steil, B., Stephanou, E. G., Stier, P.,
Traub, M., Warneke, C., Williams, J., and Ziereis, H.: 2002. Global air pollution crossroads over
the Mediterranean, Science, 298, 794–799.
410
TERJE BERNTSEN ET AL.
Lin, X., Trainer, M., and Liu, S. C.: 1988, ‘On the non-linearity of the tropospheric ozone production’,
J. Geophys. Res. 93, 15,879–15,888.
Liousse, C., Penner, J. E., Chuang, C., Walton, J. J., Edelmann, H., and Cachier, H.: 1996, ‘A global
three-dimensional model study of carbonaceous aerosols’, J. Geophys. Res. 101, 19411–19432.
Lohmann, U. and Feichter, J.: 2001, ‘Can the direct and semi-direct aerosol effect compete with the
indirect effect on a global scale?’, Geophys. Res. Lett. 28, 159–161.
Maier-Reimer, E. and Hasselmann, K.: 1987, ‘Transport and storage of CO2 in the ocean – An
inorganic ocean-circulation carbon cycle model,’ Clim. Dyn. 2, 63–90.
Menon, S., Hansen, J., Nazarenco, L., and Lao, Y.: 2002, ‘Climate effects of black carbon aerosols
in India and China’, Science 297, 2250–2253.
Myhre, G., Stordal, F., Restad, K., and Isaksen, I. S. A.: 1998, ‘Estimates of the direct radiative forcing
due to sulfate and soot aerosols’, Tellus 50B, 463–477.
Myhre, G., Karlsdóttir, S., Isaksen, I. S. A., and Stordal, F.: 2000, ‘Radiative forcing due to changes
in tropospheric ozone in the period 1980 to 1996’, J. Geophys. Res. 105, 28,935–28,942.
Myhre, G., Jonson, J. E., Bartnicki, J., Stordal, F., and Shine, K. P.: 2002, ‘Role of spatial and
temporal variations in the computation of radiative forcing due to sulphate aerosols: A regional
study’, Quart. J. Roy. Met. Soc. 128, 973–989.
Osborn, T. J. and Wigley, T. M. L.: 1994, ‘A simple model for estimating methane concentrations and
lifetime variations’, Clim. Dyn. 9, 181–193.
Penner, J. E., Chuang, C. C., and Grant, K.: 1998, ‘Climate forcing by carbonaceous and sulphate
aerosols’, Clim. Dyn. 14, 839–851.
Poppe, D., Wallasch, M., and Zimmermann, J.: 1993, ‘The dependence of the concentrations of OH
on its precursors under moderately polluted conditions: A model study’, J. Atmos. Chem. 16,
61–78.
Prather, M. J.: 1986, ‘Numerical advection by conservation of second-order moments’, J. Geophys.
Res. 91, 6671–6681.
Prather, M. J.: 1996, ‘Natural modes and timescales in atmospheric chemistry: Theory, GWPs for
CH4 and CO, and runaway growth’, Geophys. Res. Lett. 23, 2597–2600.
Price, C. and Rind, D.: 1993, ‘What determines the cloud-to-ground fraction in thunderstorms. Geophys. Res. Lett. 20, 463–466.
Rotstayn, L. D. and Penner, J. E.: 2001, ‘Indirect aerosol forcing, quasi-forcing and climate response.
J. Climate 14, 2960–2975.
Schlesinger, M. E., Jiang, X., and Charlson, R. J.: 1992, ‘Implications of anthropogenic atmospheric
sulphate for the sensitivity of the climate system’, Reprinted from Climate Change and Energy
Policy. American Institute of Physics, New York.
Siegenthaler, U. and Joos, F.: 1992, ‘Use of a simple model for studying oceanic tracer distributions
and the global carbon cycle’, Tellus 44B, 186–207.
Smith, S.: 2003, ‘The evaluation of greenhouse gas indices: An Editorial Comment’, Climate Change
58, 261–265.
Stamnes, K., Tsay, S.-C., Wiscombe, W., and Jayaweera, K.: 1988, ‘A numerically stable algorithm
for discrete-ordinate-method radiative transfer in multiple scattering and emitting layered media’,
Appl. Opt. 27, 2502–2509.
Sundet, J. K.: 1997, ‘Model Studies with a 3-d Global CTM using ECMWF data’. Ph.D. Thesis,
Department of Geophysics, University of Oslo, Norway.
Tiedtke, M.: 1989, ‘A comprehensive mass flux scheme for cumulus parameterisation on large scale
models’, Mon. Wea., Rev. 117, 1779–1800.
Wang, W.-C., Pinto, J. P., and Yung, Y. L.: 1980, ‘Climatic effects due to halogenated compounds in
the earth’s atmosphere’, J. Atmos. Sciences 37, 247–256.
Wang, W.-C. and Sze, N. D.: 1980, ‘Coupled effects of atmospheric N2 O and O3 on the Earth’s
climate’, Nature 286, 589–590.
ABATEMENT OF GREENHOUSE GASES: DOES LOCATION MATTER?
411
West, J. J., Hope, C., and Lane, S. N.: 1997, ‘Climate change and energy policy, the impacts and
implications of aerosols’, Energy Policy 25, 923–939.
Wild, O., Zhu, X., and Prather, M. J.: 2000, ‘Fast-J: Accurate simulation of in- and below cloud
photolysis in tropospheric chemical models’, J. Atmos. Chem. 37, 245–282.
Wild, O., Prather, M. J., and Akimoto, H.: 2001, ‘Indirect long-term global radiative cooling from
NOx emissions’, Geophys. Res. Lett. 28, 1719–1722.
Wigley, T. M. L.: 1991, ‘Could reducing fossil-fuel emissions cause global warming?’, Nature 349,
503–506.
(Received 23 July 2003; in revised form 23 November 2004)