SIXTH ASSESSMENT REPORTSIXTHASSESSMENTWorking GroupI – The Physical ScienceREPORTBasis

Working Group I – The Physical Science Basis

Short-lived climate forcers (SLCF) in the IPCC WGI AR6:
Input to the TFI 3rd Expert Meeting
Jan Fuglestvedt, Vice Chair Working Group I
Sophie Szopa, CLA, Chapter 6
Vaishali Naik, CLA, Chapter 6
Sarah Connors, Head of Science Team, WGI TSU
IPCC Expert Meeting on Short-Lived Climate Forcers,
11-15 April 2022

SIXTH ASSESSMENT REPORTWorking Group I – The Physical Science Basis

What are Short-lived climate forcers (SLCF)?
A set of chemically and physically reactive compounds with atmospheric lifetimes typically shorter
than around two decades but differing in terms of physicochemical properties and environmentaleffects.
SLCFs can be classified as direct or indirect, with direct SLCFs exerting climate effects through theirradiative forcing and indirect SLCFs being precursors of direct climate forcers.
●●Direct SLCFs include methane (CH4), ozone (O3), short lived halogenated compounds, such ashydrofluorocarbons (HFCs), hydrochlorofluorocarbons (HCFCs), and aerosols.
Indirect SLCFs include nitrogen oxides (NOx), carbon monoxide (CO), non-methane volatileorganic compounds (NMVOCs), sulphur dioxide (SO2), and ammonia (NH3).

Aerosols consist of sulphate (SO42−), nitrate (NO3−), ammonium (NH4+), carbonaceous aerosols (e.g.,
black carbon (BC), organic aerosols (OA)), mineral dust, and sea spray.
Particulate matter (PM): A combination of all aerosol species

SIXTH ASSESSMENT REPORT
Working Group I – The Physical Science Basis

From emissions to climate effects

EmissionsConcentrations

Transformations

Radiative Forcing

Climate changes

Aerosols: climate impact of aerosols depend on shape, size,composition and optical properties● Therefore, for PM2.5 only characterizing the size thus does
not inform on climate effect. It is not possible to characterize
the optical properties of PM2.5 in a model to estimate theclimate effect● Therefore, PM2.5 has less of a priority within a climate
framing (but is still important within a health framing)SIXTH ASSESSMENT REPORT
Working Group I – The Physical Science Basis

Climate effect of Aerosols

SIXTH ASSESSMENT REPORT
Working Group I – The Physical Science Basis

Observed global warming is driven by emissions from human activities, withgreenhouse gas warming partly masked by aerosol cooling

Adapted from Figure SPM.2

SIXTH ASSESSMENT REPORT
Working Group I – The Physical Science Basis

Short-lived climate forcers (SLCFs) have modify global temperatures

SLCFs play an important rolein changing the climate

Observedwarming

Among non-CO2 emissions,methane contributes the mostto historical warming
SO2 emissions (via sulfateaerosols) have contributed themost to cooling

Adapted from Figure SPM.2

SIXTH ASSESSMENT REPORTWorking Group I – The Physical Science Basis

Short-lived climate forcers (SLCFs) modify global temperatures
Key SLCFs of importance, based on
current forcing on climate, are:

●
●
●
●
●
●
●
●
●Methane
Sulphur dioxide
Volatile organic carbon(VOCs)
Carbon monoxide
Nitrogen oxides
Black carbon
Halogenated gases
Organic carbon
Ammonia
Adapted from Figure SPM.2

SIXTH ASSESSMENT REPORT
Working Group I – The Physical Science Basis

For SLCFs, there are several
connections between climate
and AQ / health
IPCC’s main focus in onclimate

Figure FAQ 6.2

SIXTH ASSESSMENT REPORT
Working Group I – The Physical Science Basis

PM2.5 as an aggregate is less informative than individual species
PM2.5 mask the heterogeneity of composition - which is of importance for impact of aerosols (both AQ/health and climate)

Sulphate (SO2+)
Nitrate (NO3-)
Ammonia (NH4+)
Sodium (Na+)
Chloride (Cl-)
Organic Carbon (OC)Elemental carbon (EC)

Figure 6.7

SIXTH ASSESSMENT REPORT
Working Group I – The Physical Science Basis

SLCFs are emitted fromboth anthropogenic andnatural sources

Figure 6.1

SIXTH ASSESSMENT REPORT
Working Group I – The Physical Science Basis

Regional Contribution

Anthropogenic emissions ofSLCFs vary significantly byregion and source sector
Chapter 6 provides assessments of how
SLCFs vary by region and sector.
Source Sector Contribution

Figure 6.3

SIXTH ASSESSMENT REPORT
Working Group I – The Physical Science Basis

More information in Chapter 6 of the WGI reportSIXTH ASSESSMENT REPORT
Working Group I – The Physical Science Basis

More information in Chapter 6 of the WGI report
Section 6.2 discussing emissions (and theiruncertainties)
Figure 6.3 for SLCF emissions
Figure 6.12 for effect on climate
Figures 6.18 and 6.19 for evolution ofemissions

Table 6.1

SIXTH ASSESSMENT REPORT
Working Group I – The Physical Science Basis

From the Executive Summary in report from the 2018 Expert Meeting“Reporting of SLCF and GHG inventories should be in mass units for each individual emitted compound. Some
SLCF species (e.g., VOC) comprise multiple different chemical compounds and thus mass-based emissions
must be carefully defined. It should be noted that the existing inventory methodology on GHGs (2006 IPCCGuidelines) does not require inventory compilers to calculate and report national total emissions in CO2
equivalent unit. The understanding of emission metrics and how they can be used, particularly in the context of
SLCF emissions, has advanced but there is currently no agreed recommendation. The meeting participants
concluded that SLCF emissions addressed in this meeting report should not be converted to CO2
equivalent units in the same way as done based on GWP100 in the inventory reporting under theUNFCCC. The meeting agreed that the issue of metrics and how they can be used may be further consideredbased on new scientific literature for coordination across Working Group reports, particularly those of Working
Group I and Working Group III, towards the synthesis report (SYR) of the sixth assessment report (AR6).”
The "key category analysis" (KCA) is an integral step in the national GHG inventory compilation cycle. In this analysis, all GHGs
are aggregated in terms of CO2-equivalents.

SIXTH ASSESSMENT REPORT
Working Group I – The Physical Science Basis

Which metrics to use for SLCFs? Very dependent on time horizon●No support in WGI for using CO2-eq for SLCFs, however there is no firm recommendation
for what alternative to use.

●

AR6 WGI did not provide emissions metrics for the SLCF addressed here.

●

Technical Summary: “As pointed out in AR5, ultimately, it is a matter for policymakers todecide which emission metric is most applicable to their needs. This Report does notrecommend the use of any specific emission metric as the most appropriate metricdepends on the policy goal and context.”
Figure 6.16

SIXTH ASSESSMENT REPORT
Working Group I – The Physical Science Basis

In Summary●Key SLCFs of importance, based on currentforcing on climate, are:
○
○
○
○
○
○
○

Sulphur dioxide
Volatile organic carbon (VOCs) & carbon
monoxide
Nitrogen oxides
Black carbon(Halogenated gases)
Organic carbon
Ammonia
overall warming
overall cooling●PM2.5 is important when considering air quality
and health effects - but not relevant as such forclimate●

The positive radiative forcing of Black carbon has
been reduced in AR6 compared to AR5●

SLCF emissions are evolving rapidly at theregional level due to implementation of AQ policies●Since AR5, the quality and completeness ofactivity and emission‐factor data and applied
methodology, have improved, raising confidence inmethods used to derive emissions.

●But emission uncertainties are still large,
particularly on the regional and sectoral level.
Improved emission inventories will also help toreduce uncertainties related to quantifications ofclimate effects

SIXTH ASSESSMENT REPORTSIXTHASSESSMENT
Working GroupI – The Physical ScienceREPORT
BasisWorking Group I – The Physical Science BasisThank you for your attention.
Information:
9 MoreAugust
2021

Follow Us:

IPCC: www.ipcc.chInteractive Atlas: interactive-atlas.ipcc.ch
IPCC Working Group I TSU:
IPCC Press Office: ipcc-media@wmo.int#ClimateReport

#IPCC

@IPCC
@IPCC_CH
linkedin.com/company/ipcc