CLEAN AIR BRIEF 2020

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CO-CONTROL OF
AIR POLLUTION AND
SURFACE AIRTEMPERATURE
DEVELOPED BY PROF. TOSHIHIKO TAKEMURA
APCAP SCIENCE PANEL MEMBERRESEARCH INSTITUTE FOR APPLIEDMECHANICS, KYUSHU UNIVERSITY, JAPANSUMMARY
The IPCC Special Report: Global Warmingof 1.5ºC (IPCC, 2018) found that it isdifficult to reach the 1.5ºC target of theParis Agreement by only reducing long-lived
carbon dioxide (CO 2 ) emissions. There isgrowing international awareness of thepotential to mitigate climate change byreducing emissions of short-lived
climate forcers (SLCFs), many of whichcome from same CO 2 emission sources.
SLCFs include tropospheric ozone (O 3 )and its precursors, methane,hydrofluorocarbons, and suspended
particulate matter (i.e., aerosols) and theirprecursors. This policy brief describes thebasic scientific knowledge needed todevelop air pollution reduction policies in
the context of climate change mitigation.

SLCF gases with infrared absorption
properties warm the atmosphere through amechanism similar to that of CO 2 . SLCF
aerosols mainly scatter and absorbsunlight, contributes to climate change
through a different mechanism than thatcaused by greenhouse gases.
One of the key message in this policy briefis that reduction in majority of aerosol
species may accelerate global warming,
although emission reduction of SLCFs willbring direct and immediate benefits forhealth. It is important to develop
reduction pathways considering aerosol
species, rather than treating PM2.5 as awhole.
Another important message is that
estimates of temperature change due toSLCFs had been based solely on radiative
forcing, which is the change in the energybalance. Recent studies have shown thatthe surface air temperature change peraerosol radiative forcing (i.e. a climatesensitivity parameter) much depends on
aerosol components. Since climatechange research on SLCFs is a rapidly
progressing field, policymaking needs tobe based on the latest quantitativeevaluation, considering net impacts of allcomponents.

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CLEAN AIR POLICY BRIEF

PAGE 1INTRODUCTION
Climate change, like many other physicalphenomena, is primarily caused by animbalance in the energy budget. The Earth's
surface sphere (atmosphere, oceans, andland) receives energy from the sun andemits the same amount of received energy
as infrared radiation to maintain theequilibrium. As the concentration ofgreenhouse gases (GHGs) continues toincrease due to human activities and theabsorption of infrared radiation increases,
it is clear that the energy equilibrium has
been disrupted. As a result, heat energy
stored in oceans and land surface are beingtransported to the atmosphere, causing
temperatures to rise.

It is estimated that the current statedmitigation ambition in Nationally
Determined Contributions (NDCs) will stilllead to an increase in global mean surface
air temperature of about 3ºC in thiscentury relative to the preindustrial levels(IPCC 2018), which would clearly miss theParis Agreement target. At this rate, theloss of life and economy caused byflooding associated with heavy rainfall,heat waves, and other weather-related
disasters will only worsen. Reducing
emissions of short-lived climate forcers(SLCFs), most of which are air pollutants,
could provide additional means formitigating global warming in the next
several decades.

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CLEAN AIR POLICY BRIEF

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MECHANISM OF
CLIMATE CHANGE BYSLCFSThe main SLCFs are tropospheric ozone(O3 ) and its precursors, methane (CH 4 ),hydrofluorocarbons (HFCs), and suspended
particulate matter (i.e., aerosols including
sulfate, nitrate, organic matter, and blackcarbon) and its precursors. Smaller aerosol
particles less than 2.5 µm in diameter isPM2.5 . Their main characteristics andmechanisms of climate change aredescribed in this section. The definition ofshort-lived climate pollutants (SLCPs) by
the Climate and Clean Air Coalition (CCAC)
is a subset of SLCFs which absorb solar
and infrared radiation and then storesenergy in the Earth's surface andatmosphere, i.e., CH4 , tropospheric O3 ,HFCs, and black carbon (BC).
As shown in Figure 1, SCLFs have acomplicated relationship with each other,so it is necessary to carefully consider
and decide the pathway for optimizing theimpact mitigation by SLCFs as a whole.
Since O 3 , CH 4 , and HFCs are gases thatabsorb infrared radiation, the mechanismof climate change caused by them isthe same as that of CO 2 .

Figure 1: Relationship among SLCFs and their precursors and effects of SCLFs on climatechange, health, and crops. Note that this figure shows the effect of increasing SLCFs andtheir precursors, and therefore the effect is reversed when they are reduced.

CLEAN AIR POLICY BRIEF

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1. Tropospheric ozone (O 3 )Tropospheric or ground level ozone is amajor component of photochemical
oxidants. It is not directly emitted, but isproduced by chemical reactions of nitrogenoxides (NOx) and volatile organic
compounds (VOCs) in the presence ofsunlight. Increases in CH4 and carbon
monoxide (CO) also contribute to theformation of O 3 . It has direct negative
impacts on health and crops.
2. Methane (CH 4 )Increase in CH 4 is due to direct emissions
from anthropogenic sources fromagriculture, coal mining, oil and gas
production and distribution, biomassburning, and municipal waste landfilling. It
is also due to reduction of hydroxyl (OH)radicals due to the presence of CO andVOCs, because CH 4 is eliminated by OH
radicals. On the other hand, CH4 decreases
with an increase in NOx because OH
radicals are generated in the chemical
reaction that produces tropospheric O 3from NOx.
3. Hydrofluorocarbons (HFCs)
HFCs are used as a major refrigerant andtheir atmospheric concentrations continueto rise. The CCAC aims to reduce HFCs

CLEAN AIR POLICY BRIEF

emissions under the Efficient Cooling
Initiative initiated in 2019.
4. Aerosols
Aerosols are a general term for fineparticles with a radius of approximately
1 nm to 100 µm. Most aerosols originatingfrom human activities are classified as
PM 2.5 , a smaller category of aerosols. The
largest source of anthropogenic aerosols
is the combustion of fossil fuels, which
produces sulfur dioxide (SO2 ) (a precursorof sulfate aerosol), NOx (a precursor of
nitrate aerosol and O3 ), organic aerosols,and black carbon (BC). Most of these arealso produced by biofuel consumption andforest fires.
The mechanism of aerosols’ effects on theclimate system can be generally divided
into two categories. First, because ofaerosols’ light scattering and absorbing
properties, their energy balance isdisrupted when aerosol concentrations
change. This is called “aerosol-radiationinteraction”. Sulfate, nitrate, and organic
aerosols, which are transparent or whiteaerosols, cool the atmosphere because
they scatter sunlight back into space,
while black carbon heats the atmospherebecause it absorbs sunlight morethan scattering.

PAGE 4

The aerosol-radiation interaction is visually
recognizable as a phenomenon that causes
the atmosphere to appear hazy at highconcentrations of PM 2.5 .
Another mechanism is the change in cloudproperties as aerosol particles become
cloud condensation nuclei for water
cloud or ice nuclei for ice cloud. This isknown as “aerosol-cloud interaction”.
Clouds effectively scatter sunlight to create
shade and absorb infrared radiation toprevent radiative cooling at night. In otherwords, a change in clouds is a significant
factor in climate change because theycause a change in the energy balance.
Anthropogenic aerosols also have a direct
negative impact on human health. In
addition, while crop yields may be reduced
as temperature decreases due to aerosolradiation and aerosol-cloud interactions,yield may increase due to enhanced
photosynthesis with increased scatteredsunlight by the aerosol-radiationinteraction.

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CO-CONTROL OF AIR
POLLUTION AND SURFACE
AIR TEMPERATURE CHANGE
Figure 2 shows the annual and global meanvalues of the energy imbalance “radiativeforcing” due to anthropogenic SLCFs as of2011 by emitted components. Considering
the estimated radiative forcing due to CO2of +1.68 W m –2, it is clear that the overall
impact of the SLCFs must be considered.
Although the positive radiative forcing fromCH 4 emissions is significant, alternativeemission reduction measures are needed
because methane is largely derived fromsources other than fossil fuels.
Figure 2 indicates that, if emission
reductions related to SOx and NOx, which

are mostly emitted from fossil fuelcombustion, are implemented by end ofpipe measures only withoutaccompanying CO 2 emission reduction
measures, global warming will beaccelerated. This indicates that emission
reduction of long-lived GHGs and SLCFs
must be implemented simultaneously.
While there are still limited studies on
short-term and long-term costs of climatechange adaptation and mitigation
measures on society, the combined
implementation of SLCFs measures withlong-lived GHGs could lead to cost savingsfor society as a whole in the long term.

Figure 2: Global and annual mean radiative forcing due to anthropogenic SLCFs as of 2011
partitioned according to the emitted components assessed by IPCC (2013). Resulting
drivers shown by color mean the components in the atmosphere after chemical reaction.

CLEAN AIR POLICY BRIEF

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It has been common practice to estimate
temperature changes from the values ofradiative forcing. For example, thesurface air temperature change perradiative forcing (i.e., a climate sensitivityparameter) was assumed to be the same asthat of CO 2 when estimating the effect ofreducing CH 4 and BC on mitigating globalwarming (Figure 6.1 in UNEP and WMO
2011). However, recent studies withdetailed analysis using coupledatmosphere-ocean climate models havefound that the climate sensitivity
parameter for BC is very small (Samset etal. 2016; Stjern et al. 2017; Takemura andSuzuki 2019). This is because, unlike
GHGs, there is a mechanism to counteract
the positive radiative forcing created by BC
due to absorbing sunlight (Suzuki andTakemura 2019).

CONCLUSION
Since fossil fuel consumption is a primarycommon anthropogenic source of CO 2 andmost SLCFs, a comprehensive
understanding of climate change caused byboth is essential. Measures to reduce
SLCFs for climate benefits should be basedon the latest quantitative assessments
which considers multiple impacts such as
climate extremes (e.g., heat wave andheavy rain), floods, health, and crop yields.
For example, in Japan, a recent integrated
study was conducted to assess SLCFs
impacts and to identify the optimal
reduction pathway for SLCFs emissions(Nakajima et al. 2020), and more detailed
studies are planned.

CLEAN AIR POLICY BRIEF

The IPCC Task Force on National
Greenhouse Gas Inventories (TFI) has
started process to develop new guidelines
for estimating emissions of SLCFs. Thismeans that countries may be required toreport SLCFs emissions in the near
future in addition to Kyoto-GHGs. Optimal
air pollutant reductions are becoming
increasingly important in the context ofclimate change mitigation as
simultaneous mitigation of SLCFs andlong-lived GHGs is needed to help us
achieve the Paris Agreement’s targets.

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REFERENCES
IPCC (2013). Climate Change 2013: The Physical Science Basis. Cambridge University Press, Cambridge,
United Kingdom and New York, NY, USA, 1535 pp.
IPCC (2018). Global Warming of 1.5°C: An IPCC Special Report on the impacts of global warming of1.5°C above pre-industrial levels and related global greenhouse gas emission pathways, in thecontext of strengthening the global response to the threat of climate change, sustainable
development, and efforts to eradicate poverty. 616 pp.
Nakajima, T., Ohara, T., Masui, T., Takemura, T., Yoshimura, K., Goto, D., Hanaoka, T., Itahashi, S.,
Kurata, G., Kurokawa, J., Maki, T., Masutomi, Y., Nakata, M., Nitta, T., Seposo, X., Sudo, K., Suzuki, C.,
Suzuki, K., Tsuruta, H., Ueda, K., Watanabe, S., Yu, Y., Yumimoto, K. and Zhao, S. (2020). A
development of reduction scenarios of the short-lived climate pollutants (SLCPs) for mitigating global
warming and environmental problems. Progress in Earth and Planetary Science 7, 33,
doi:10.1186/s40645-020-00351-1.
Samset, B. H., Myhre, G., Forster, P. M., Hodnebrog, Ø., Andrews, T., Faluvegi, G., Fläschner, D., Kasoar,M., Kharin, V., Kirkevåg, A., Lamarque, J.-F., Olivié, D., Richardson, T., Shindell, D., Shine, K. P.,
Takemura, T. and Voulgarakis, A. (2016). Fast and slow precipitation responses to individual climate
forcers: A PDRMIP multi-model study. Geophysical Research Letters 43, 2782–2791,
doi:10.1002/2016GL068064.
Stjern, C. W., Samset, B. H., Myhre, G., Forster, P. M., Hodnebrog, Ø., Andrews, T., Boucher, O., Faluvegi, G.,
Iversen, T., Kasoar, M., Kharin, V., Kirkevåg, A., Lamarque, J.-F., Olivié, D., Richardson, T., Shawki, D.,
Shindell, D., Smith, C. J., Takemura, T. and Voulgarakis, A. (2017). Rapid adjustments cause weaksurface temperature response to increased black carbon concentrations. Journal of GeophysicalResearch Atmospheres 122, 11462–11481, doi:10.1002/2017JD027326.
Suzuki, K. and Takemura, T. (2019). Perturbations to global energy budget due to absorbing andscattering aerosols. Journal of Geophysical Research Atmospheres 124, 2194–2209,
doi:10.1029/2018JD029808.
Takemura, T. and Suzuki, K. (2019). Weak global warming mitigation by reducing black carbon
emissions. Scientific Reports 9, 4419, doi:10.1038/s41598-019-41181-6.
UNEP and WMO (2011). Integrated Assessment of Black Carbon and Tropospheric Ozone. 285 pp.

Asia Pacific Clean Air Partnership (APCAP)
UN Environment, Asia Pacific Office
Email: apcap@un.org