Atmospheric Environment 45 (2011) 6103e6106
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Atmospheric Environment
journal homepage: www.elsevier.com/locate/atmosenv
Short communication
Mitigation of short-lived heating components may lead
to unwanted long-term consequences
Gunnar Myhre*, Jan S. Fuglestvedt, Terje K. Berntsen, Marianne T. Lund
Center for International Climate and Environmental Research-Oslo (CICERO), Oslo, Norway
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 19 April 2011
Received in revised form
27 July 2011
Accepted 3 August 2011
A mitigation strategy for reducing emissions of short-lived heating components such as black carbon
(BC) aerosols and ozone precursors to limit global warming has frequently been suggested (Bond, 2007;
Grieshop et al., 2009; Hansen et al., 2000; Jacobson, 2002; Molina et al., 2009; Nature Editorial, 2009).
BC emissions influence the radiative balance in several ways through direct and semi-direct aerosol
effects, as well as by impacting the surface albedo (Forster et al., 2007), and their net effect is likely
a warming that enhances the total man-made warming. However, the role that BC or other short-lived
heating components may play in future mitigation strategies must be formulated with caution to
avoid unforeseen and unwanted consequences. A near-term mitigation of short-lived heating components could lead to a delayed action on CO2 and other long-lived greenhouse gases and thus an increased
long-term warming. A key element is whether policies are designed as a consequence of predicted
warming or observed warming. Without a clear strategy, early BC or ozone reductions may even lead to
an unexpectedly larger temperature change.
Ó 2011 Elsevier Ltd. All rights reserved.
Keywords:
Black carbon
Mitigation strategy
Short-lived components
1. Introduction
Mitigation of climate compounds beyond the long-lived
greenhouse gases is frequently discussed in the scientific literature (Berntsen et al., 2010; Jackson, 2009; Jacobson, 2010; Kopp and
Mauzerall, 2010; Molina et al., 2009). However, the climate impact
of the short-lived compounds is much more uncertain than for the
long-lived greenhouse gases (Forster et al., 2007). The focus on the
short-lived compounds is mainly on the heating compounds such
as tropospheric ozone and black carbon.
The absorbing effect of BC leads to absorption of solar radiation
in the atmosphere (direct aerosol effect), atmospheric absorption
that may impact the cloud cover (semi-direct effect) and enhanced
solar absorption due to reduced surface albedo of snow (BC impact
on snow and ice). The total direct aerosol effect (i.e. of all aerosols) is
expected to have a negative radiative forcing and a cooling effect on
the climate (Forster et al., 2007; Myhre, 2009; Schulz et al., 2006).
This is due to the fact that scattering aerosols overwhelm the effect
of the absorbing BC aerosols, even with very high estimates of BC
from total sources of fossil fuel and biomass burning (Ramanathan
and Carmichael, 2008). Estimates of the radiative forcing due to
BC from fossil and biofuel differ significantly. IPCC AR4 (Forster et al.,
* Corresponding author.
E-mail address: gunnar.myhre@cicero.uio.no (G. Myhre).
1352-2310/$ e see front matter Ó 2011 Elsevier Ltd. All rights reserved.
doi:10.1016/j.atmosenv.2011.08.009
2007) best estimate for the radiative forcing of the direct effect of BC
through absorption is 0.2 Wm 2. However, this estimate was based
on a majority of studies with assumed externally mixed aerosols,
while observations show that a more realistic representation of
mixing of BC with other aerosols may lead to somewhat (up to 50%)
higher radiative forcing (Bond et al., 2006). BC is also emitted from
open biomass burning together with co-emitted scattering aerosols
(organic carbon) giving a net effect that is believed to be a weak
global net radiative forcing (less than 0.05 Wm 2 in magnitude)
(Forster et al., 2007). The semi-direct effect of BC (Ackerman et al.,
2000; Hansen et al., 1997) with heating of the atmosphere and its
impact on the cloud cover remains highly uncertain in terms of
radiative forcing and its sign may vary depending on the vertical
profile of BC (Johnson et al., 2004; Koch and Del Genio, 2010; Penner
et al., 2003). Recent estimates of the BC impact on snow and sea ice
albedo indicate weaker (at least a factor of 2) radiative forcing
(Flanner et al., 2007; Rypdal et al., 2009) than earlier estimates
(Hansen and Nazarenko, 2004), although studies show that this
radiative forcing mechanism is very efficient in altering the surface
temperature change relative to the radiative forcing, i.e. the climate
efficacy is high for this mechanism (Flanner et al., 2007; Hansen and
Nazarenko, 2004). The IPCC estimate (Forster et al., 2007) for this
radiative forcing mechanism is 0.1 Wm 2. BC may also act as cloud
and ice nuclei, giving an indirect aerosol effect with a highly
uncertain radiative forcing most likely to be negative (Bauer et al.,
2010; Koch et al., 2011; Liu et al., 2009; Penner et al., 2009).
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G. Myhre et al. / Atmospheric Environment 45 (2011) 6103e6106
The increase in tropospheric ozone over the industrial era is
estimated to give a radiative forcing of 0.35 Wm 2 (Forster et al.,
2007). The main ozone precursors are nitrogen oxides (NOx),
carbon monoxide (CO) and hydrocarbons including methane (HC).
Mitigation of NOx emissions reduces tropospheric ozone, but it also
reduces the oxidation capacity of the troposphere, thereby enhancing
methane levels and giving a short-term cooling and a longer-term
warming effect (Shine et al., 2005; Wild et al., 2001). A decrease in
CO or HC emissions reduces ozone and increases OH, the latter leads
to reduced methane levels (Shindell et al., 2005).
2. Methods
In order to illustrate the potential role of short-lived heating
components (SLHC) in climate strategies we present some idealized
model experiments that focus on the important role of timing of
emission changes. We make use of two experiments modeling
reduction in BC emissions. Observed climate change is still relatively small and the rationale for international negotiations and
mitigation (e.g. the Kyoto Protocol) has to a large extent been based
on theoretical model results. This could be viewed as a precautionary approach, with the development of a comprehensive longterm mitigation strategy including both short-lived and long-lived
climate forcers. On the other hand, the lack of significant action
(e.g.. the failure of the COP-15 and COP-16 to reach a binding
agreement) may indicate a shift to a regime where significant
mitigation will only start when observed climate change reaches
a level where it becomes certain that the damages will be significant. Our two BC reduction experiments are designed to illustrate
how early SLHC mitigation could have very different effects in the
two regimes.
Due to the uncertainties in the radiative forcing of the short-lived
components as well as uncertainties in their co-emitted species
(Kopp and Mauzerall, 2010) we illustrate a mitigation strategy for
two highly different forcing estimates and we use BC as an example.
The simulations are performed for a total BC radiative forcing of both
0.2 Wm 2 and 0.8 Wm 2, with the latter value used in most of the
simulations presented. A radiative forcing of BC of 0.8 Wm 2 is
treated as an upper limit and close to the highest estimates where
BC from fossil fuel and biomass burning are treated together. Table 1
describes the three experiments and Fig. 1a shows the calculated
surface temperature changes for these experiments, which adopt
a radiative BC forcing of 0.8 Wm 2. The simulations are performed
with a simple energy balance climate/upwelling-diffusion ocean
model (Berntsen et al., 2006) with a climate sensitivity of
0.8 KWm 3. Additional tests are performed accounting for the
uncertainty in the climate sensitivity by including a probability
density function (pdf) of the climate sensitivity based on values
available in the literature (Meinshausen et al., 2009).
Table 1
Descriptions of cases shown in Fig. 1.
Reference case
A2 Scenario until 2050 and from 2060 a 70% reduction
in all the human emissions. Between 2050 and 2060
linear changes and constant 2060 levels there after.
Early BC-policy
Same as above, except 2/3 smaller BC emissions
case
between 2010 and 2020, and thereafter BC held
at constant 2020 level until 2050. After that,
BC same as above.
Observed temperature Same as the BC-policy case, except wait with
dependent case
the 70% reduction in all the anthropogenic
emissions until same temperature change as
in 2050 for the ‘Reference case’.
(This dT in 2050 is 2.75 K).
Fig. 1. Temperature change, a) for ‘reference’, ‘Early BC-policy case’, and the ‘observed
temperature dependent case’ with a radiative forcing of BC of 0.8 Wm 2, b) same as a)
but for a radiative forcing of BC of 0.2 Wm 2. The results are robust with respect to the
chosen climate sensitivity and timing of strong reductions in climate compounds.
The reference scenario follows the A2 SRES emissions until 2050
with a 70% reduction occurring linearly over a 10 year-period
between 2050 and 2060 in all the man-made emissions (longlived and short-lived species), illustrating an active climate policy.
We have formulated two idealized mitigation scenarios with
additional BC-only mitigation. In the first, named ‘early BC-policy
case’, 2/3 of the total BC emissions from fossil and biofuel sources
are reduced over the 10 year-period from 2010, illustrating a policy
with strong BC emphasis and early action for this component
compared to CO2 and other long-lived greenhouse gases. We
underscore that our scenarios are for illustrative purposes for the
SLHC and that the actual timing and magnitude of mitigation of BC
and other compounds do not change the overall results.
In the second BC scenario, named ‘observed temperature
dependent case’, BC emissions are reduced as in the ‘early BC-policy
case’, while the 70% reduction in the other anthropogenic emissions does not start until the same temperature change equals that
of 2050 in the reference case (2.75 K). The motivation for this
scenario is to illustrate how the climate outcome depends on what
is the necessary basis for implementing a climate mitigation policy.
Thus, if observed temperature change is a necessary basis for regulations of emissions then this case is relevant.
G. Myhre et al. / Atmospheric Environment 45 (2011) 6103e6106
The main results are shown for the A2 SRES scenario, but recent
emission scenarios indicate the possibility for earlier phase out of
short-lived components. Therefore, the results are also shown for the
representative concentration pathway (RCP) 8.5 (Riahi et al., 2007).
3. Results
If regulations of CO2 and other anthropogenic components
except BC are based on long-term scientific policy as in the ‘early
BC-policy case’ the surface temperature change is lower than in the
‘reference case’ over the whole century. In the reference case the
temperature is reduced a few years after the 70% reduction in the
anthropogenic emissions is achieved due to the fact that this
involves reduction in greenhouses gases, scattering aerosols, as
well as a strong reduction in the warming from BC. However, in the
‘early BC-policy case’ most of the BC is already reduced, and
a reduction in the temperature after the strong reduction of
anthropogenic components is therefore not observed since reduction of short-lived cooling components plays a significant role.
6105
In the ‘observed temperature dependent case’ a higher temperature than in the reference case is reached since a longer time period
with emissions of CO2 and other greenhouse gases is allowed. In
addition, the temperature increases after the strong emission
reductions are identical to the ‘early BC-policy case’ and differ from
the reference case. If dramatic observed climate changes are
necessary for implementation of strong reductions in the greenhouse gas emissions, the ‘observed temperature dependent case’
shows that early BC mitigation delays the timing of this reduction
and that a strong masking effect by short-lived scattering aerosols
results in a higher long-term temperature change.
It is possible that short-lived cooling species are reduced earlier
than assumed in our scenarios due to regulations based on air
quality control policy. We therefore repeat the model runs with
a second set of the scenarios where we reduce SO2 and OC emissions earlier, between 2030 and 2040, while keeping everything
else the same. Removing the cooling species earlier leads to
a higher temperature change by mid-century because the masking
effect is reduced. However, the main features of the temperature
developments remain the same.
The results in Fig. 2a using the RCP8.5 scenario show the same
pattern as in Fig. 1a for the A2 SRES scenario, but smaller magnitude
(about 50% smaller) on the temperature effect of BC mitigation.
Fig. 2b shows that the results and the magnitude of the mitigation
of short-lived heating components are sensitive to the climate
sensitivity, but the overall pattern does not change.
The net radiative forcing of BC is uncertain as described above
and with a weaker radiative forcing of BC of 0.2 Wm 2, Fig. 1b
shows that a mitigation strategy on BC has a very small impact on
the global surface temperature. Therefore, a mitigation strategy
related to BC has either a small impact on the surface temperature
or enhances the cooling effect of aerosols which increase the
committed warming to the future under a scenario with no mitigation on long-lived greenhouse gases before a certain temperature
threshold is reached. In the alternative scenario in Hansen et al.
(2000) mitigation strategy on BC was suggested in combination
with mitigation on long-lived greenhouse gases, which seems not
to be the case in the near future.
4. Summary
A potential mitigation of SLHC should be viewed in connection
to mitigations of CO2 emissions, otherwise unwanted long term
consequences may occur. The potential role of BC reductions in
future climate mitigation strategies depends on how climate policy
is developed; whether it is formulated and implemented prior to
unwanted global warming, or if it is formulated and implemented
in response to global warming. In other words, what type of
knowledge and information e observations and ‘hard facts’ or
model based predictions e will trigger action. Therefore, actions
regarding BC and other short lived heating compounds should not
be viewed in isolation, but in a holistic context of a climate mitigation policy that addresses both short-term and long-term effects.
Acknowledgement
Fig. 2. Temperature change, a) for ‘reference’, ‘BC-policy case’, and the ‘observed
temperature dependent case’ with a radiative forcing of BC of 0.8 Wm 2 and the
RCP8.5 scenario and a fixed climate sensitivity of 0.8 KWm 3, b) same as a) for the
‘reference’ and ‘temperature dependent case’ but with a pdf of the climate sensitivity.
The ‘reference’ is shown in gray shading with black line as the median, and the 50, 80
and 90 percentiles in dark gray, gray and light gray, respectively. A thick red line is the
median for the ‘observed temperature dependent case’ and the red, orange and yellow
lines give the 50, 80 and 90 percentiles.
This work has received funding from the Norwegian Research
Council within the project ‘Climate and health impacts of ShortLived Atmospheric Components (SLAC)’.
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