SIXTH ASSESSMENT REPORTSIXTHASSESSMENTWorking GroupI – The Physical ScienceREPORTBasis

Working Group I – The Physical Science Basis

Short-lived climate forcers (SLCF) in the IPCC WGIcontribution to the 6th Assessment Report
Jan Fuglestvedt, Vice chair Working Group I
Thanks to: Vaishali Naik (CLA Ch6), Sophie Szopa (CLA Ch6), Terje Berntsen (LA Ch6), Sarah Connors (Head of Science Team,TSU), Bill Collins (LA Ch7)

Joint 1st and 2nd IPCC Expert Meeting on Short-Lived Climate Forcers 11– 22 October 2021

IPCC Working Group I report outline
Chapter 1: Framing, context, methods
Large-scale
climatechange
Climateprocesses

Chapter 2: Changing state of the climate system
Chapter 3: Human influence on the climate system
Chapter 4: Future global climate: scenario-based projections and near-terminformation
Chapter 5: Global carbon and other biogeochemical cycles and feedbacks
Chapter 6: Short-lived climate forcers
Chapter 7: The Earth’s energy budget, climate feedbacks, and climate sensitivity
Chapter 8: Water cycle changes
Chapter 9: Ocean, cryosphere, and sea level change

Regional
climateinformation

Chapter 10: Linking global to regional climate changeChapter 11: Weather and climate extreme events in a changing climate
Chapter 12: Climate change information for regional impact and for risk assessment
Atlas of Regional Climate Information

IPCC Working Group I report outline
Chapter 1: Framing, context, methods
Large-scale
climatechange
Climateprocesses

Chapter 2: Changing state of the climate system
Chapter 3: Human influence on the climate system
Chapter 4: Future global climate: scenario-based projections and near-terminformation
Chapter 5: Global carbon and other biogeochemical cycles and feedbacks
Chapter 6: Short-lived climate forcers
Chapter 7: The Earth’s energy budget, climate feedbacks, and climate sensitivity
Chapter 8: Water cycle changes
Chapter 9: Ocean, cryosphere, and sea level change

Regionalclimateinformation

Chapter 10: Linking global to regional climate change
Chapter 11: Weather and climate extreme events in a changing climate
Chapter 12: Climate change information for regional impact and for risk assessment
Atlas of Regional Climate Information

SIXTH ASSESSMENT REPORT
Working Group I – The Physical Science Basis

What is SLCF?
A set of chemically and physically reactive compounds with atmospheric lifetimes typically shorter
than two decades but differing in terms of physiochemical properties and environmental effects.
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.

Fig 6.1

Table 6.1

Recent Evolution in SLCF Emissions and Abundances
Over the last decade, strong shifts in the geographical distribution of emissions
Observations show strong regional variations in trends of ozone (O3), aerosols and their precursors.
Tropospheric columns of nitrogen dioxide (NO2) and sulphur dioxide (SO2) continued to decline over North America and Europe and toincrease over South Asia, but have declined over East Asia.
Global carbon monoxide (CO) abundance has continued to decline.
The concentrations of hydrofluorocarbons (HFCs) are increasing.
Global carbonaceous aerosol budgets and trends remain poorly characterized due to limited observations, but sites representative of
background conditions have reported multi-year declines in black carbon (BC) over several regions of the Northern Hemisphere.

SIXTH ASSESSMENT REPORT
Working Group I – The Physical Science Basis“It is indisputable that humanactivities are causing climatechange, making extreme climate
events, including heat waves,heavy rainfall, and droughts,
more frequent and severe.

[Credit: Yoda Adaman | Unsplash]

SIXTH ASSESSMENT REPORT
Working Group I – The Physical Science Basis

Observed warming
is driven by emissions
from humanactivities, withgreenhouse gas
warming partly
masked by aerosol
cooling

Observed warming

warmingcooling

Total human influenceSIXTH ASSESSMENT REPORT
Working Group I – The Physical Science Basis

Observed warming
is driven by emissions
from humanactivities, withgreenhouse gas
warming partlymasked by aerosol
cooling

warmingcooling

Greenhouse gases

SIXTH ASSESSMENT REPORT
Working Group I – The Physical Science Basis

Observed warming
is driven by emissions
from humanactivities, withgreenhouse gas
warming partly
masked by aerosol
cooling

warmingcooling

Pollution particles (aerosols)

SIXTH ASSESSMENT REPORT
Working Group I – The Physical Science Basis

Observed warming
is driven by emissions
from humanactivities, withgreenhouse gas
warming partly
masked by aerosol
cooling

Observed warming

warmingcooling

Total human influenceSIXTH ASSESSMENT REPORT
Working Group I – The Physical Science BasisContributions by component to 2010-2019 warming relative to 1850-1900.

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

SPM.2cMore details
in chapter 6

SIXTH ASSESSMENT REPORT
Working Group I – The Physical Science Basis“

Unless there are immediate,
rapid, and large-scale
reductions in greenhouse gas
emissions, limiting warming to1.5°C will be beyond reach.

[Credit: Peter John Maridable]

SIXTH ASSESSMENT REPORT
Working Group I – The Physical Science Basis

Scenarios used in IPCC AR6 WGI
SSP5-8.5⨯Climate change mitigation✔✔Air pollution controls

SSP3-7.0⨯ Climate change mitigation⨯Air pollution controls

SSP1-1.9 & SSP1-2.6✔✔ Climate change mitigation✔✔Air pollution controls

Feasibility or likelihood of individual scenarios is not part of the assessment

Wider range
between SSP
scenarios than RCPs
Rapid evolution of emissions,
hard to catch in inventories

Figure 6.18. Global anthropogenic and biomass burning short-lived climate forcer (SLCF) and CO2 emissions from 1850 to 2100 and HFC emissions from1990 to 2100. Emissions for the Coupled Model Intercomparison Project Phase 6 (CMIP6) for the period 1850-2014 are based on Hoesly et al. (2018)

SIXTH ASSESSMENT REPORT
Working Group I – The Physical Science Basis

Global surface temperature will continue to increase until at least
the mid-century under all emission scenarios considered
CO2 emissions
Very highHigh
Intermediate

2 °C
1.5 °C

Low
Very lowSIXTH ASSESSMENT REPORT
Working Group I – The Physical Science Basis“[Credit: evgeny-nelmin.]To limit global warming, strong,rapid, and sustained reductions in
CO2, methane, and othergreenhouse gases are necessary.
This would not only reduce theconsequences of climate change
but also improve air quality.

SIXTH ASSESSMENT REPORT
Working Group I – The Physical Science Basis

Links between
actions aiming tolimit climatechange and actions
to improve airquality

Figure FAQ 6.2

Impacts on air quality. PM2.5 and ozone

Figure 6.17 Emission source-sector attribution of regional population weighted mean concentrations of PM2.5 and ozone for present day
emissions (year 2014). Regional concentrations and source apportionment calculated with the TM5-FASST model (Van Dingenen et al., 2018) for the2014 emission data from the Community Emissions Data System (CEDS) (Hoesly et al., 2018) and van Marle et al.(2017) for open biomass burning.

Effective Radiative Forcing, 1750 to 2019

Reducedfrom AR5

Global mean temperature change, 1750 to 2019SIXTH ASSESSMENT REPORTWorking Group I – The Physical Science Basis

Implications of COVID-19 Restrictions for Emissions, Air Quality and Climate•Emissions reductions in 2020 associated with measures to reduce the spread of COVID-19 led totemporary but detectible effects on air pollution (high confidence), and an associated small,
temporary increase in total radiative forcing, primarily due to reductions in cooling caused byaerosols arising from human activities (medium confidence).

•Global and regional climate responses to this temporary forcing are, however, undetectable above
natural variability (high confidence).

•Atmospheric CO2 concentrations continued to rise in 2020, with no detectable decrease in theobserved CO2 growth rate (medium confidence).

Figure 6.3. Relative regional and sectoral
contributions to the present day (year 2014)
anthropogenic emissions of Short Lived ClimateForcers (SLCFs).

Emission basedtemperaturechange for- Regions- Sectors

Figure 6.16. Global-mean temperature response 10 and 100 years following one year of present-day(year 2014) emissions. The temperature response is broken down by individual species and shown for total
anthropogenic emissions (top), sectoral emissions (left) and regional emissions (right). Sectors and regions
are sorted by (high-to-low) net temperature effect on the 10-year time scale. Error bars in the top panel
show uncertainty (5-95% interval) in net temperature effect due to uncertainty in radiative forcing

Evolution of the effects ofshort-lived climate forcers(SLCFs) and HFCs on global
temperature across thecore Shared SocioEconomic Pathways (SSPs).
Effects of net aerosols,
methane, troposphericozone andhydrofluorocarbons (HFCs)(with lifetimes <50years),
relative to year 2019 andto year 1750.

Remaining carbon budgets
Estimates may vary by ±220
GtCO2 depending on the level of
non-CO2 emissions at the time
global anthropogenic CO2emissions reach net zero levels.
This variation is referred to as nonCO2 scenario uncertainty and will
be further assessed in the AR6Working Group III Contribution.
Geophysical uncertainties
surrounding the climate response tothese non-CO2 emissions (such as
CH4, N2O, and aerosols) result inan additional uncertainty of at least±220 GtCO2

SIXTH ASSESSMENT REPORT
Working Group I – The Physical Science Basis

Impacts, Adaptation and Vulnerability

The structureof the AR6
WGI Report

Mitigation of Climate Change

SIXTH ASSESSMENT REPORTSIXTHASSESSMENT
Working GroupI – The Physical ScienceREPORTBasis

Working Group I – The Physical Science BasisThank you.
Information:
9 MoreAugust2021

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