Atmospheric Environment 43 (2009) 5674–5681
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Atmospheric Environment
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Radiative forcing from household fuel burning in Asiaq
Kristin Aunan a, *, Terje K. Berntsen a, b, Gunnar Myhre a, b, Kristin Rypdal a, David G. Streets c,
Jung-Hun Woo d, Kirk R. Smith e
a
Center for International Climate and Environmental Research – Oslo (CICERO), P.O. Box 1129, Blindern, N-0318 Oslo, Norway
Department of Geosciences, University of Oslo, 0315 Oslo, Norway
c
Decision and Information Sciences Division, Argonne National Laboratory, DIS/900 9700 South Cass Avenue Argonne, IL 60439, USA
d
Department of Advanced Technology Fusion, Konkuk University, Seoul 143-701, Korea
e
Global Environmental Health, School of Public Health, 747 University Hall, University of California, Berkeley, CA 94720-7360, USA
b
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 11 March 2009
Received in revised form
20 July 2009
Accepted 22 July 2009
Household fuel use in developing countries, particularly as biomass and coal, is a major source of carbonaceous aerosols and other air pollutants affecting health and climate. Using state-of-the-art emission inventories, a global three-dimensional photochemical tracer/transport model of the troposphere, and a global
radiative transfer model based on methods presented in the latest IPCC Assessment Report (2007-AR4), we
estimate the radiative forcing (RF) attributable to household fuel combustion in Asia in terms of current
global annual-mean RF and future global integrated RF for a one-year pulse of emissions (2000) over two
time horizons (100 and 20 years). Despite the significant emissions of black carbon (BC) aerosols, these
estimates indicate that shorter-lived (non-Kyoto) air pollutants from household fuel use in the region overall
seem to exert a small net negative RF because of the strong influence of reflective aerosols. There are,
however, major uncertainties in emission estimates for solid fuel burning, and about the sustainability of
household fuel wood harvesting in Asia (the carbon neutrality of harvesting). In addition, there is still
substantial uncertainty associated with the BC radiative forcing. As a result we find that the sign of the RF
from household biomass burning in the region cannot be established. While recognizing the value of integrating climate change and air pollution policies, we are concerned that for a ‘Kyoto style’ post-Kyoto treaty
(with global cap-and-trade and the Global Warming Potential as the metric) expanding the basket of
components with a selection of short-lived species without also including the wider range of co-emitted
species may lead to unintended consequences for global-scale climate. Additional measurement, modelling,
and policy research is urgently needed to reduce the uncertainties so that the net impact on climate of
emissions and mitigation measures in this sector can be accurately assessed.
Ó 2009 Elsevier Ltd. All rights reserved.
Keywords:
Solid fuel burning
Asia
Emissions
Radiative forcing
Air pollutants
1. Introduction
Solid fuel combustion in households in developing countries is
a major source of black carbon (BC), an aerosol that has been identified
as both a major threat to public health and an important global
warming agent. Mitigation policies targeted at household fuel
combustion are among the least expensive options for BC abatement,
thus making them especially appealing (Bond and Sun, 2005). Moreover, the atmospheric concentration of BC would respond quickly to
any emission reductions because of its short lifetime (few days in
contrast to over 100 years for CO2). However, BC is not currently
q The corrected proof posted online August 27 has significant changes from the
uncorrected proof posted online August 5.
* Corresponding author. Tel.: þ47 22 85 87 63/50; fax: þ47 22 85 87 51.
E-mail address: kristin.aunan@cicero.uio.no (K. Aunan).
URL: http://www.cicero.uio.no
1352-2310/$ – see front matter Ó 2009 Elsevier Ltd. All rights reserved.
doi:10.1016/j.atmosenv.2009.07.053
included in the basket of greenhouse species regulated through the
Kyoto Protocol to the United Nations Framework Convention on
Climate Change. Calls for widening the scope of post-Kyoto climate
treaties to include additional air pollutants such as aerosols and
tropospheric ozone are based on the increasing evidence that these air
pollutants play an important role in the climate system on a global as
well as on a regional scale (Haywood and Shine, 1995; Ackerman et al.,
2000; Ramanathan et al., 2001). Including components that are also
a direct threat to public health is also expected to increase the relevance of the climate regime to developing countries such as China and
India, presumably increasing the likelihood of their participation and
thus the overall effectiveness of the regime (Hansen et al., 2000;
Holloway et al., 2003; Streets and Aunan, 2005). The objective of the
present study is to estimate the importance of emissions from solid fuel
burning in Asia for current and future global warming, and discuss the
case for targeting these emissions from a global warming perspective.
Drawing upon our own results and those of other authors examining
K. Aunan et al. / Atmospheric Environment 43 (2009) 5674–5681
the effects on climate of the full range of emissions from specific sectors
(Fuglestvedt et al., 2008; Unger et al., 2008; Shindell et al., 2008), we
discuss the case for expanding the basket of components in post-Kyoto
climate regimes.
BC is claimed to be the second or third most important individual
anthropogenic warming agent after CO2 and possibly methane
(Jacobson, 2002; Hansen and Nazarenko, 2004). Household combustion probably represents the second largest major source category of
BC, after open biomass burning (of savanna, forests, and agricultural
residues), representing about one quarter of global BC emissions (Bond
et al., 2004). We argue in the following that it is highly uncertain,
however, whether targeting BC emissions from the combustion of solid
fuels in Asian households – which include coal and biomass fuels such
as wood, charcoal, crop residues, and dung – will in aggregate achieve
a net benefit in climate terms because most likely it will be accompanied by changes in other atmospheric constituents, such as organic
carbon (OC), sulfate, and ozone precursors, which also have impacts on
climate. While the direct effect of BC on climate is a warming of the
atmosphere through absorption of shortwave solar radiation, the
direct effect of OC and sulfate aerosols is a cooling due to scattering of
solar radiation. Aerosols also exert indirect and semi-direct aerosol
effects and impacts on snow and sea-ice albedo (Hansen and Nazarenko, 2004; Jacobson, 2004). Ozone precursors as NOx, CO, and
NMVOC (non-methane volatile organic components), contribute to
a warming because tropospheric ozone acts as a greenhouse gas. NOx,
however, also reduces the lifetime of the greenhouse gas methane, thus
NOx emissions will through this effect imply a cooling.
Here we estimate (with uncertainty ranges) the impact on the
global climate of household fuel use in the Asian region. We model
the RF exerted by the portfolio of air pollution constituents and
the greenhouse gases CO2 and CH4, in terms of current top-of-theatmosphere (TOA) global, annual-mean RF (measured in Wm2). The
estimated current RF values correspond to the global, annual-mean
radiative forcing values for the period from pre-industrial to present
as estimated, for instance, by the Intergovernmental Panel on
Climate Change IPCC (Forster et al., 2007). This metric is ‘backward
looking’, depicting the RF exerted today by present and historical
emissions. More relevant for policy makers, however, is the ‘forward
looking’ perspective of the integrated RF values, depicting the future
integrated impact of a one-year pulse of emissions over a given time
horizon. This is consistent with the perspective adopted in the Kyoto
Protocol through the application of GWPs (Fuglestvedt et al., 2008).
We quantify the impact of one-year’s emissions from Asian households in terms of the future global integrated RF over two time
horizon of, respectively, 100 and 20 years. The results can be
compared with corresponding estimates for global annual emissions
provided by IPCC (Forster et al., 2007, see its Fig. 2.22).
2. Materials and methods
2.1. Emissions from household fuel use in Asia
The study region stretches from Pakistan in the West to Japan in
the East and from Mongolia in the North to Indonesia in the South
(Streets et al., 2003). Solid fuels are the main household fuels used
by the rural populations in the region, but also continue to be
important for many poor people living in towns and cities. Biomass
is the dominant fuel, with some 70% of the total energy used by
households. Coal constitutes around 10%. The remaining share is
predominantly other fossil fuels such as liquefied petroleum gas
(LPG), kerosene, and natural gas. Nearly all (97%) of the household
coal burning in the region takes place in China, particularly in the
northern parts of the country (Streets et al., 2003).
The inventory for year 2000 includes BC, OC, SO2, NOx, CO,
NMVOC (17 species), NH3, CH4, and CO2. Monthly emissions are
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given on a 1 by 1 grid and were interpolated to the 1.875 by
1.875 grid in the photochemical tracer/transport model applied.
Emissions outside the Asia region are from the EDGAR database
(Olivier et al., 2001), except for BC and OC where we rely on Bond
et al. (2004). Regarding the fossil fuels, about 45% of the CO2
emissions from these fuels are from coal burning. Practically all
aerosols emissions from fossil fuels are from coal, and only a small
fraction (3–5%) of methane and NMVOC emissions are from noncoal fossil fuels.
The uncertainties in the emission estimates are large, particularly for OC and BC. Generally, emissions are known least well in
India, the rest of South Asia, and Southeast Asia, as described in
Streets et al. (2003). Measured as 95% confidence intervals, the
uncertainty in total emissions from all sources are 16% (SO2),
31% (CO2), 37% (NOx), 65% (CH4), 130% (NMVOC), 185% (CO),
360% (BC), and 450% (OC). Uncertainty in published estimates
for regional scale BC emissions is typically a factor of two to five
(Ramanathan and Carmichael, 2008) and emissions estimates are
regularly revised. Propagation-of-error analysis in Streets et al.
(2003) showed that uncertainties in emissions from the residential
sector are generally higher than from other sectors. The range
depends on how well we know the amounts of fuel used and how
well we know the combustion characteristics that determine the
emission factors. A major source of uncertainty particularly in
emissions of products of incomplete combustion (PIC) is the large
variations in emission factors reported in laboratory studies, and
the lack of realistic measurements in field studies (emission factors
applied are mainly based on standard cooking protocols in simulated kitchens (Smith et al., 2000)). Few field studies in representative homes and regions of Asia are available. Recent work in
Mexico and Honduras, however, indicates that emission factors
based on tests in controlled settings, such as those used here, may
substantially underestimate actual emissions under field conditions (Johnson et al., 2007; Roden et al., 2009).
Household fuel burning, particularly as biomass, is a dominating
source of several air pollutants, especially for PIC, such as BC, OC,
NMVOCs, and CO. According to the emission inventory applied,
household emissions overall contribute to 66% and 65%, respectively, of the Asian region’s total anthropogenic emissions of BC and
OC. Using the global estimates for 1996 from Bond et al. (2004) and
for 2000 from Dentener et al. (2006), BC and OC emissions from
households in the Asian region represent approximately one-fifth
of the total global emissions of BC and OC. Moreover, Asian
households accounts for about 11% of the SO2 emissions in the
region, and the sector’s share of emissions of ozone precursors in
the region is 35% for NMVOC, 37% for CO, and 7% for NOx. Finally,
the sector is responsible for 9% of the emissions of methane and
27% of the CO2 emissions. Keeping the issue of land use change and
forestry outside the scope of this paper, we in the modeling assume
zero CO2 emissions from household burning of biomass, i.e., a 100%
renewable harvesting of fuel wood. With this assumption, the
sector’s contribution to CO2 emissions in the region is reduced to
7%. According to Streets et al. (2003) much of the harvesting of
biomass fuels in Asia is non-sustainable. The real value of net CO2
emissions from the use of firewood and charcoal is unknown (crop
residues and dung can be considered CO2 neutral), however, mainly
because reliable statistical data for harvesting practices in rural
areas are lacking. A study by FAO (2007) in seven Asian countries
(India not included and only Yunnan province of China) indicates
that about 50% of the rural population has some degree of supply/
demand deficit when it comes to fuel wood, with large variability
between countries. Below we report sensitivity estimates for the
assumption that 10% of the harvesting of biomass fuels was nonsustainable in 2000, which is the share assumed in the EDGAR
emission inventory (Olivier et al., 2005). 10% non-sustainable
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K. Aunan et al. / Atmospheric Environment 43 (2009) 5674–5681
harvesting implies that assumed CO2 emissions from biomass fuels
increase from zero to 210 Tg and the total CO2 emission from the
household sector in the region increases with about 37%.
2.2. Radiative forcing
We use two global models in our calculation – one for modeling
the atmospheric burden of the constituents and one for modeling
the RF exerted by this burden. We estimate the RF exerted by aerosols of BC, OC, and sulfate (the direct effect related to absorption or
scattering of shortwave solar radiation by the aerosols), tropospheric
ozone, and the RF from changes in the CH4 lifetime and concentrations induced by ozone precursors. The experimental design was
to model the RF metrics from emissions in the Asian region with
and without the household emissions included. The difference in RF
metrics obtained in the two model runs gives estimates of the
contribution from the household sector emissions. The uncertainty
analysis assumes uncertainty in emissions as cited above for the total
emissions in the region, uncertainty in modeling of atmospheric
dispersal and removal, and RF. The procedure of the Monte Carlo
simulation and the uncertainties assumed for modeling parameters
and RF are given in Fuglestvedt et al. (2008), except for CH4 where we
rely on Prather et al. (2009). For CO2 we include uncertainty in
today’s emission, not in history (see separate sensitivity analysis
for assumption about non-renewable harvesting). For BC and OC
emissions we draw from a lognormal distribution.
Air pollutants have a short atmospheric lifetime and emissions
during one year will have no impact on RF the next year. When
calculating current RF from household sector emissions of air
pollutants we therefore need not consider historical emissions and
simply model the RF exerted by one-year’s emissions (for 2000).
Regarding CH4, which is relatively short-lived (about 12 years), the
approach in principle is the same, but since CH4 has an atmospheric
live-time longer than one year, we approximate by calculating
the steady-state RF from one-year’s emissions (for 2000). Due to its
long atmospheric lifetime, historical emissions of CO2, on the
other hand, need to be included in order to depict how the sector
contributes to climate forcing today. RF from CO2 was estimated by
assuming that the CO2 emissions from the household sector have
been proportional to world emissions since pre-industrial time
(1750). The simulation of CO2 was performed using the modules of
a simple climate model (Fuglestvedt et al., 2000; Fuglestvedt and
Berntsen, 1999). The atmospheric concentration of CO2 was calculated using an efficient and accurate representation of the carbon
cycle developed by Joos et al. (1996).
A global, three-dimensional (19-layer) photochemical tracer/
transport model of the troposphere, the Oslo CTM2 (Berntsen et al.,
2006), was used to model the contribution from household fuels to
concentrations of the different constituents in different atmospheric
layers and the total column burden. A global radiative transfer model
was applied to estimate the RF associated with the modeled changes
in atmospheric burden. The radiative forcing model is documented
in Myhre et al. (2007, 2003). The Oslo CTM2 has aerosol abundances
close to the average of 16 models in the intercomparison exercise
AeroCom (Textor et al., 2006). In the AeroCom exercise 9 global
aerosol models using identical emission data provided RF results
(Schulz et al., 2006). For the aerosol components of fossil fuel origin,
the results from the models used in the present study were close to
the average (within 15–20%) of the 9 models. For the biomassburning aerosols, the results from the models used here gave more
negative RF values than the other models, which is likely linked to
the large variation in the vertical profile of the aerosols in the
AeroCom models (Textor et al., 2006; Schulz et al., 2006). Treatment
of hygroscopic effects, size distributions, and assumptions of mixing
of the aerosols in the model used here are documented in Myhre
et al. (2007). The model is compared to aircraft measurements of
the aerosol vertical profile (also in regions with high BC content),
showing that Oslo CTM2 reproduces the main characteristics of the
vertical profile (Myhre et al., 2008, 2003). A crucial parameter for the
magnitude and even the sign of the radiative forcing due to aerosols
is the single scattering albedo. In Table 1 the values for single scattering albedo in the radiative forcing model applied here is shown
for various aerosol components. For OC a fraction has been suggested
to contain an absorbing component (Andreae and Gelencser, 2007;
Lukacs et al., 2007), especially at short wavelengths (Jacobson, 1999;
Sun et al., 2007). However, it has also been measured that for the
main chemical components of water soluble OC pure scattering is
more likely (Myhre and Nielsen, 2004). Internal versus external
mixing of BC has been shown to be important for the RF and a simple
method is recently suggested to account for the internal mixture
(Bond et al., 2006). This method proposes to increase the absorption
by 50% for the hydrophilic BC particles (but with no change for the
hydrophobic BC particles). Using this approach in a global sensitivity
test with the Oslo CTM2 resulted in an increased RF for BC of
28%. Below we report the estimates obtained by applying such an
increase for the RF from BC. Note, however, that secondary organic
aerosols (SOA), which are not included in our study, have a fraction of
the total OC that is non-negligible in this region (Hoyle et al., 2007).
Human activity has increased the amount of SOA mainly due to
larger abundance of organic aerosol to partition at as well as an
increase in the oxidation gases (Hoyle et al., 2009). Inclusion of SOA
would have strengthened the negative RF due to aerosols. In Myhre
et al. (2007) the aerosol optical depth, radiative effect, and absorption by the aerosols are compared to satellite-derived products.
The model results showed good agreement with the satellite data,
including over the oceanic regions around China and India.
3. Results
3.1. Net global radiative forcing
The modeled current RF attributable to the household sector is
shown in Fig. 1. Best estimates and uncertainty ranges for current RF
and integrated RF over two time horizons are given in Tables 2 and 3.
The current global annual-mean RF from the sector as a whole is
estimated at 34 mWm2. If we include only the Kyoto gases (CO2 and
methane), the estimated forcing from the sector is 46 mW m2,
which corresponds to about 2% of the global annual-mean RF from
greenhouse gases included in the Kyoto Protocol for the period from
pre-industrial (1750) to present (about 2000). The relatively small
difference (12 mW m2) between the RF from the Kyoto gases only
and the estimated RF from all components is the sum of a relatively
large negative forcing (56 mW m2) from scattering aerosols and
shortening of methane’s atmospheric lifetime from NOx and other
ozone precursors and a relatively large positive forcing (45 mW m2)
from BC and tropospheric ozone. Aerosols alone (BC, OC and sulfates)
is estimated to exert an RF of 17 mW m2. The negative aerosol
forcing is in accordance with a study using nine global aerosol
models (Schulz et al., 2006). Note that the GISS model used in the
study by Koch et al. (2007), which reports a net positive RF from
Table 1
Single scattering albedo (550 nm) for anthropogenic aerosols used in this study.
Single scattering albedo (550 nm)
Sulfate
Black carbon, fossil and bio-fuel
Organic carbon, fossil and bio-fuel
Aerosols from biomass burning
1.0
0.21
1.0
0.91
K. Aunan et al. / Atmospheric Environment 43 (2009) 5674–5681
40
30
20
mW/m2
10
0
-10
CO2
CH4 Sulfate
BC
-20
OC
O3
Total
net RF
CH4
(NOX)
Total net RF
-30
Household fossil fuels
-40
Household biomass
-50
60
CH4
CH4 (NOX)
40
O3
OC
BC
mW/m2
20
Sulfate
CO2
0
Household biomass
-20
Household fossil fuels
5677
household fuel burning to be positive (Unger et al., 2008). As the
balance between ozone precursors affects the resulting estimate, the
differing results may be related to the CO/NOx ratio being approximately 40% lower in the current paper than the ratio assumed for
household fuels in 2030 in Unger et al. (2008).
The best estimate for current RF from household biomass burning
is close to zero, since the positive and negative RF values essentially
cancel each other out. The RF from air pollutants alone (non-Kyoto
components) is estimated to be 12 mW m2 for biomass fuel
burning. The total RF from fossil fuels (35 mW m2) closely resembles the RF exerted by the Kyoto gases CO2 and methane alone for
this emission source, as the net RF from all non-Kyoto components is
close to zero (Fig. 1 and Table 2).
Applying the calculated 28% increase in the BC RF as described
above (to account for a potentially higher absorption by hydrophilic
BC particles), would imply that net RF from biomass fuels is positive, but still the RF from all non-Kyoto components and for aerosols would be negative (for current RF 3 mW m2 for non-Kyoto
components and 8 mW m2 for aerosols).
Moving on to the integrated RFs as calculated for the two time
horizons of 100 and 20 years the pattern remains the same as for
the current RF values both when it comes to the net effect of the
two fuel types, the net effect of the subgroups of components, and
the uncertainty ranges (Table 3). Assuming a 10% non-sustainable
harvesting practice for biomass fuels gives RF values for CO2 of 18
and 5 mW m2 yr for a 100 and 20 years time horizon, respectively,
hence the net RF from biomass gets positive.
The uncertainty ranges calculated in the Monte Carlo simulation
were, as expected, broad (Tables 2 and 3). The large uncertainties in
RF from particularly BC and OC imply that the net RF from biomass
burning and, as a consequence, for the whole sector in total, cannot
be reliably established, mainly due to uncertainties in the emissions.
-40
3.2. Regional radiative forcing due to the direct effect
of absorbing and scattering aerosols
-60
40
30
20
mW/m2
10
0
-10
Biomass Kyoto
-20
-30
-40
-50
Fossil fuels
Kyoto
Biomass nonKyoto
Fossil fuels nonKyoto
CH4 (NOX)
O3
Sulfate
OC
CH4
BC
CO2
-60
Fig. 1. Modeled global annual-mean RF since pre-industrial times in 2000 from emissions from household fuel combustion in Asia (mW m2) shown for each component
(upper), for the two main types of fuels (middle), and for Kyoto components (long-lived)
versus non-Kyoto components (short-lived) (lower).
residential sector emissions of aerosols in the Asian region, is one of
the models in Schulz et al. (2006) which, contrary to the majority of
the models included, estimates a positive RF from anthropogenic
aerosols over significant areas in Asia, and, as a consequence, estimates the least negative RF from the Asian region as a whole. Other
researchers have found the RF from the indirect methane effect from
Fig. 2 shows the spatial distribution of modeled annual average RF
from BC, OC, and sulfate aerosols, as well as their net value in the Asian
region. According to our model results, the positive forcing caused by
BC emissions in the most densely populated regions in China and
India is heavily counteracted by the negative forcing exerted by OC
and sulfate. Whereas both OC and sulfate aerosols are important over
China, the negative forcing over India is almost exclusively caused by
OC because biomass fuels dominate in this region.
The direct effect on climate of absorbing and scattering aerosols is
highly dependent on cloud distribution and the surface albedo.
Moreover, season and location of emissions influence RF due to
changing meteorology and solar radiation (Berntsen et al., 2006).
Emissions from households show a clear seasonal pattern in the
higher latitudes, including most of China, Mongolia and North and
South Korea, with considerably larger emissions during winter due to
the need for heating. The seasonal pattern becomes weaker as one
moves southwards, and is absent in the southernmost areas (Fig. 3
and Fig. S1). The RF exerted by short-lived air pollutants from
household fuel use in higher latitudes is affected by the fact that these
emissions are at their highest in winter when less sunlight is available
for absorption and scattering and the solar angle is low. Moreover, the
chemical features prevailing in winter affect the burden of pollutants
in the atmosphere. For instance, formation of O3, being a photochemical oxidant, is less effective in winter. Although SO2 concentrations from household emissions are significantly higher in winter
than in summer at high latitudes, this pattern is less clear for sulfate
aerosol concentrations attributable to these emissions. This is due to
the suppressed formation of sulfate in winter (Fig. S1).
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K. Aunan et al. / Atmospheric Environment 43 (2009) 5674–5681
Table 2
Best estimates with uncertainty ranges (68% confidence intervals) of global annual-mean RF since pre-industrial times in 2000 from household fuel combustion in Asia
(mW m2).
CO2
Biomass fuels
Fossil fuelsc
Total
a
b
c
a
–
33 (27–39)
33 (27–39)
CH4
Sulfate
BC
OC
O3
CH4 (indirect)b
Sum all
12 (10–14)
1 (0.8–1.2)
13 (11–15)
2 (3 to 1)
6 (9 to 3)
8 (12 to 5)
26 (9–72)
6 (2–15)
32 (11–89)
39 (128 to 12)
1 (4 to 0)
40 (131 to 12)
8 (5–8)
6 (4–7)
13 (9–17)
5 (8 to 2)
4 (6 to 1)
8 (13 to 3)
0 (83 to 48)
35 (26–47)
34 (51 to 92)
Assuming 100% renewable harvesting of biomass fuels.
RF from NOx, CO, and NMVOC-induced changes in the methane lifetime and concentrations.
For all except CO2 coal is the dominant contributor to each of the calculated RF values (see text).
4. Major limitations and uncertainties
The global mean TOA RF has been generally accepted as
a reasonably adequate metric for assessing how emissions affect
mean temperatures on a global scale. There are still, however,
unresolved questions related to the adequacy of comparing RF values
for the short-lived components with those of long-lived. Moreover, as
pointed out by others (Bond and Sun, 2005), the metric of global
mean TOA RF is not sufficient for capturing regional climatic impacts
related to the short-lived components. Neither the impacts related to
changes in regional climate nor ‘cloud burning’ (Ackerman et al.,
2000), nor the indirect impact related to cloud enhancement, are
captured in our estimates. We note that it is extremely difficult to
simulate the effects on clouds in climate models. The negative forcing
related to the indirect effect through cloud enhancement is, however,
likely to be larger than the potential net positive forcing from cloud
burning, thus an attempt to include clouds effects would likely have
pushed the RF estimates from aerosols in a negative direction.
The large variation in emission factors due to different fuel/stove
combinations and burning conditions and a lack of realistic measurements in the field are among the large sources of uncertainties in
a study like ours. Another major source of uncertainty relates to the fuel
use data, particularly for rural populations. Together these two sources
of uncertainty give broad uncertainty ranges for emission estimates,
and subsequently for RF estimates. Of particular importance is the
balance of absorbing and scattering aerosols. While the BC/OC ratio in
household fuels use emissions in Streets et al. (2003) is around 0.25,
this ratio is higher in comparable inventories, e.g., around 0.31 in
Shindell et al. (2008) for 2001 and 0.39 in Zhang et al. (2009) for 2001.
Based on in-field measurements in rural homes in China, Li et al. (2009)
report higher BC/OC ratios for fuel wood combustion than most
previous studies, while the ratios for crop waste combustion are
comparable to previous studies. The overall estimated BC/OC ratio for
biomass combustion in Chinese households is 0.47, compared to 0.20
for biomass fuels in Asian households in Streets et al. (2003). A ratio
higher that 0.3 would result in the net RF from BC and OC from biomass
in Tables 2 and 3 becoming positive. SO2 emissions (the precursor of
scattering sulfate aerosols), on the other hand, are lower in Streets et al.
compared to the EDGAR inventory (Olivier et al., 2005) and in Dentener
et al. (2006), but higher than in Shindell et al. (2008) and Zhang et al.
(2009). An evaluation of the Streets et al. (2003) inventory by Tan et al.
(2004) indicates that especially CO and carbonaceous particulates (all
sources) may be severely underestimated (the underestimation was
within the uncertainties ranges in Streets et al., however). Finally,
a third major source of uncertainty relates to the parameterization of
(partly not well understood) processes important for radiative forcing
calculations in the models used. For instance, results from Ramanathan
and Carmichael (2008) indicate that models being applied in the IPCC
assessment reports (ours being one of them) generally underestimate
the heating from BC aerosols. The differing estimates for BC heating in
Ramanathan and Carmichael (2008) and in IPCC models is partly
a result of the manner in which BC from biomass burning (including
both open burning and contained combustion) and fossil fuel burning
is treated. While IPCC models separate the BC from these two sources,
in Ramanathan and Carmichael (2008) contributions from fossil fuel
and biomass burning are added together. IPCC models separate the BC
from these two sources since BC from biomass burning are associated
with co-emissions of large amount of scattering organic aerosols, while
this is not the case to the same extent for fossil fuel BC. The models
applied in the IPCC assessment models differ when it comes to the
vertical distribution of aerosols and assumptions about how aerosols
are internally or externally mixed, which affects their heating estimates for BC. Above, we used a simplified methodology to tentatively
estimate how our estimates change when internal mixing is taken into
account.
5. Discussion and conclusions
5.1. Short-term and long-terms effects of emission reductions
Household solid fuel burning is among the most promising
targets for reduction of carbonaceous particles (Streets, 2007).
However, the BC/OC ratio signature of emission reductions is of
importance for their net impact on global RF. A detailed bottom-up
analysis addressing, e.g., stove technologies and fuel properties is
needed to draw conclusions about the climate impacts of individual
control options within the sector at given points of time in the future.
Control options, like improving stove efficiencies and switching to
Table 3
Best estimates with uncertainty ranges (68% confidence intervals) of global integrated RF from emissions in 2000 from household fuel combustion in Asia (mW m2 yr).
CH4
Sulfate
BC
OC
O3
CH4 (indirect)b
Sum all
Time horizon 100 years
Biomass fuels
–a
48 (39–57)
Fossil fuelsc
Total
48 (39–57)
13 (11–15)
1 (0.8–1.2)
14 (11–17)
2 (3 to 1)
6 (9 to 3)
8 (12 to 5)
26 (9–72)
6 (2–15)
32 (11–89)
39 (128 to 12)
1 (4 to 0)
40 (131 to 12)
6 (4–6)
5 (3–6)
10 (7–13)
4 (7 to 1)
3 (5 to 1)
7 (12 to 3)
1 (83 to 48)
49 (38–63)
48 (37 to 106)
Time horizon 20 years
Biomass fuels
–a
14 (11–17)
Fossil fuelsc
Total
14 (11–17)
11 (9–13)
1 (0.8–1.2)
12 (10–14)
2 (3 to 1)
6 (9 to 3)
8 (12 to 5)
26 (9–72)
6 (2–15)
32 (11–89)
39 (128 to 12)
1 (4 to 0)
40 (131 to 12)
6 (4–9)
5 (4–6)
11 (7–15)
3 (6 to 1)
3 (4 to 1)
6 (10 to 2)
2 (84 to 47)
15 (9–26)
13 (70 to 72)
CO2
a
b
c
Assuming 100% renewable harvesting of biomass fuels (see text for sensitivity estimate).
RF from NOx, CO, and NMVOC-induced changes in the methane lifetime and concentrations.
For all except CO2 coal is the dominant contributor to each of the calculated RF values (see text).
K. Aunan et al. / Atmospheric Environment 43 (2009) 5674–5681
5679
Fig. 2. Geographical distribution of annual-mean RF exerted by aerosols from household fuels in Asia. A: Total net RF from aerosols. B: RF from black carbon aerosols. C: RF from
organic carbon aerosols. D: RF from sulfate aerosols.
cleaner fuel types (e.g., coal briquettes to replace raw coal and pellets
to replace crop residues) may alter the emission factors and the ratio
between emitted species (Edwards et al., 2007), thus detailed
information about the specific emission factors associated with
alternative fuel/stove combinations and -improvements is needed.
Using the modeled integrated RF from the Asian household
sector emissions as estimated for two different time horizons, and
acknowledging the limitations of the concept of global mean RF and
the large uncertainties in the estimates, we draw tentative conclusions about the case for targeting household solid fuel use in a global
warming context. On an aggregate level it seems that abatement of
these emissions will have only a modest impact on global warming,
as their net contribution to integrated RF seems not to be large for
either time horizon. Biomass burning appears to be approximately
climate neutral on a global aggregate level when a range of components are considered (and when renewability of harvesting is not
accounted for, see above). The integrated RF is modeled to be close to
zero (on the negative side) for both a 100 and a 20 years time horizon
(Table 3). Overall reduced household burning of biomass seen in
isolation, i.e., without considering how the energy will be replaced,
could entail a short-term warming due to reductions in short-lived
species exerting a net negative RF. However, as the response time of
CH4 is also relatively short (about 12 years as compared to up to a few
months for air pollutants), the cooling resulting from reduced CH4
could relatively soon cancel out the positive RF from the removed air
pollutants. Of course, the energy source substituted for biomass will
determine the magnitude of the climate effect of a fuel switch. An
overall switch from biomass to coal in households could increase the
net integrated RF considerably, primarily due to the increase in longlived components (CO2 and CH4). As long as sulfur is not removed
from the coal, the negative forcing resulting from increased sulfate,
OC and reduced lifetime of CH4 from NOx and other ozone precursors
might balance the positive forcing from increased BC and O3 associated with a switch from biomass to coal. As technologies to reduce
sulfur in coals are being employed to an increasing extent, however,
a short-term warming is likely to result if a large-scale switch from
14 %
South (Latitude 10-20)
12 %
Middle (Latitude 20-30)
10 %
North (Latitude 30-45)
8%
6%
4%
2%
0%
Jan
Feb
Mar
Apr
Mai
Jun
Jul
Aug
Sep
Okt
Nov
Des
Fig. 3. Monthly distribution of percentage contribution of submicron PM (BC, OC, and sulfate) from household sector emissions to the total regional surface concentration of
submicron PM (BC, OC, and sulfate) in Asia.
5680
K. Aunan et al. / Atmospheric Environment 43 (2009) 5674–5681
biomass to coal in households takes place (because the positive RF
from air pollutants then may exceed the negative RF). Leapfrogging
the coal rung on the ‘household energy ladder’ (Smith et al., 1994) to
advanced combustion renewable biomass technologies for example
is thus valuable from a global warming perspective.
Reduction in residential coal burning seen in isolation, again
without considering the fuel alternatives, contributes unambiguously to mitigating global warming for both time horizons according
to our results. In contrast to emissions reductions in coal-fired power
plants in this part of the world, which imply a substantial short-term
global warming due to reduced cooling from sulfate aerosols
(Berntsen et al., 2006), a corresponding reduction in coal use in
households would not have such an effect. We draw this conclusion
because the net integrated forcing from non-Kyoto air pollutants is
modeled to be around zero for fossil fuel burning in household stoves
for both time horizons (Table 3).
5.2. Broadening the ‘Kyoto basket’
Broader participation and deeper emissions cuts are essential for
any post-Kyoto climate treaty to have a significant effect. Agreements to reduce the shorter-lived climate-active pollutants potentially provide attractive initial incentives for such engagement
because reductions tend to pose less economic, technical, and
political difficulties than CO2 controls, as well as providing significant local co-benefits in health and ecosystem protection. The
current study and previous studies, e.g., by Fuglestvedt et al. (2008),
Unger et al. (2008), and Shindell et al. (2008), however, demonstrate
that the estimated global climate impact of abatement policies for
short-lived species using models based on methods presented in the
latest IPCC Assessment Report is heavily influenced by the selection
of radiatively active species being taken into account. Based on these
findings we conclude that for a ‘Kyoto style’ post-Kyoto treaty (with
global cap-and-trade and GWPs as the metric) expanding the
‘‘Kyoto-basket’’ of components with a selection of short-lived species
without also including the wider range of co-emitted species may
send the wrong signal about the net impact on global climate of
planned interventions. To include the full range of components and
types of effects would, on the other hand, pose serious challenges in
choosing appropriate metrics for measurement and comparison. An
unresolved matter, for example, is how to deal with components
exerting a negative forcing. Currently, the large uncertainties and
methodological complexity regarding metrics for measuring climate
effects of an extended number of components would complicate
verification and compliance, and thus potentially compromise
feasibility (Rypdal et al., 2004). Thus, there is urgent need for additional measurement, modeling, and policy research to pave the way
for a closer integration of health-damaging air pollutants into
climate change policy regimes.
Acknowledgements
We thank Hans Martin Seip, Jan S. Fuglestvedt, and Lynn P. Nygaard
for discussions and helpful suggestions during the preparation of
the manuscript, and Ragnhild Bieltvedt Skeie for running the Monte
Carlo simulation. We also thank two anonymous referees for valuable
comments and suggestions. This work was supported by the Research
Council of Norway and the Norwegian Ministry of Foreign Affairs.
Appendix. Supplementary material
Supplementary data associated with this article can be found in
the online version at doi:10.1016/j.atmosenv.2009.07.053.
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