Chapter 6 of WGI AR6:
Intention at the scoping
meeting and the outline
William Collins1 and Hong Liao2
Chapter 6 CLAs
1Lawrence Berkeley Laboratory and University of California, Berkeley

2Nanjing University of Information Science and Technology

“Near-Term Climate Forcer”
Definition from AR5 Glossary
Near-term climate forcers (NTCF) Near-term climateforcers (NTCF) refer to those compounds whose impact
on climate occurs primarily within the first decade after
their emission.
This set of compounds … has been sometimes referred
to as short lived climate forcers or short-lived climate
pollutants…
This set of compounds includes• methane, which is also a well-mixed greenhouse gas,
• ozone and aerosols, or their precursors, and• some halogenated species

Outline of WG1 AR6
Summary for Policy Makers
Technical Summary
Current state of the climate system, human influence on observed changes, and future projections
1.
2.
3.
4.

Framing, context, methods
Changing state of the climate system
Human influence on the climate system
Future global climate: scenario-based projections and near-term information

Global climate processes shaping global and regional climate
5.
6.
7.
8.

Carbon budgets, biogeochemical cycles and feedbacks
Short-lived climate forcers and air quality
The Earth’s energy budget, climate feedbacks, and climate sensitivity
Water cycle changes

Assessment of climate information at regional scales and regional climate change
9.
10.
11.
12.

Ocean, cryosphere, and sea level change
Linking global to regional climate change
Weather and climate extreme events in a changing climate
Climate change information for regional impact and risk assessment

Annexes• Regional Atlas• Glossary• Technical Annexes• List of Acronyms• List of Contributors• List of Reviewers

Linkages to other Working Groups
WGI-WGIII cross-cutting topics include• the assessment of carbon budgets compatible withclimate targets• land surface aspects (including land management
and climate feedbacks)• climate and air quality effects of short lived
climate forcers and their mitigation potential• greenhouse gas removal and solar radiation
management

as a dedicated chapter in WGI assessments, focused on land-climate interactions. For this reason,
the WGI AR6 outline does not include a specific chapter on land. The WGI report will build on andupdate the SRCCL assessment, in particular in terms of GHG fluxes, extremes, importance of landclimate interplays for regional climate, as well as through the assessment of CMIP6 simulations.

High level mapping of WG1 themes

Table 2: High level mapping of the treatment of WGI themes across the reports of the AR6cycle.

Connections to other WGI chapters• Chapter 2, Changing state of the climate system• Natural and anthropogenic forcings• Radiative forcing• Chapter 3, Human influence on the climate system• Natural variability versus anthropogenically-forced change• Chapter 4, Future Global Climate• Responses to short-lived forcers• Chapter 7, Earth’s energy budget, climate feedbacks, and climate
sensitivity• Radiative forcing: definitions, estimates, and its representation in models• Chapter 8, Water cycle changes• Cloud-aerosol processes• Technical Annex on Radiative Forcing and Climate Metrics

Chapter 6 Authors

Outline of Chapter 6 – Short-lived
Climate Forcers
Executive Summary• Key emissions: global overview, natural, anthropogenic,
historical and scenarios• Observed and reconstructed concentrations andradiative forcing• Direct and indirect aerosol forcing• Implications for greenhouse gas lifetimes• Implications of different shared socio-economic andemission pathways … for radiative forcing• Connections to air quality and atmospheric composition
Frequently Asked Questions

Key emissions: global overview, natural,
anthropogenic, historical and scenarios• Emissions governing short-lived climate forcers (SLCFs)
are unique to this chapter.
• These include primary and precursor emissions.
• Technological, socio-economic, and environmental
factors governing emission trends can be covered.
• Implications for the shared socio-economic pathway(SSP) scenarios can also be assessed.

Global anthropogenic andbiomass burning emissions

Fig. 8.2, AR5 WGIObserved and reconstructedconcentrations and radiative forcing• We will assess current and historical concentrations
and implied radiative forcing of SLCF gases.
• For most SLCFs and precursors there are limited
observations in time and space.
• We will assess reconstructed analysis of thespatial and temporal variability .
• Past radiative forcing by SLCFs supports estimatingclimate sensitivity from the historical records.
• This is a cross-chapter issue to be coordinated withChapter 7.

Total column ozone

Fig. 9.10, AR5 WGIDirect and indirect-aerosol forcing• This third bullet represents a major part of thechapter.
• Aerosol-radiation interactions and aerosol-cloudinteractions are still major sources of uncertainty inthe estimate of the net radiative forcing.
• Current understanding of these uncertainties will
be addressed in depth.
• New methods combining satellite observations andmodels would also be important.

Anthropogenic radiative forcing by species

Fig. 7.18, AR5 WGIGreenhouse gas lifetimes• Emissions governing SLCFs affect the oxidation capacityof the atmosphere.
• Models yield different simulations of oxidation capacitychanges due to complexity of photochemistry etc.
• The oxidation capacity in turns affects the lifetime of
key GHGs (e.g. CH4, HFCs, and HCFCs).
• Uncertainties affect relationship of GHG concentrations
to primary emission changes and indirect chemicaleffects.
• This has cross-chapter linkages with Chapters 2 and 7(on historical radiative forcing, global warming
potential, global temperature change and otheremissions metrics).

Impact of pollution controls on
specific emissions and climate impact

FAQ 8.2, AR5 WGIImplications of different sharedsocio-economic pathways• Shared socio-economic pathways (SSPs) determine
the future short-lived climate forcers
concentrations and radiative forcing.
• SSPs include projected population, urbanisation,etc., with implications for risks from air pollution.
• Assessment may consider how different SSPs affectair quality, for example, in urban areas• This connects to WGII and III assessments of howsocio-economic trends determine the susceptibilityof the population to harmful impacts of SLCFs.

Emissions for SSP5 baseline andmitigation cases

Kriegler et al, 2016

Connections to air quality andatmospheric composition• Primary SLCF and precursor emissions are also
sources of air pollution.
• Local and episodic changes in SLCFs are importantsince they contribute to air pollution.
• Air pollution affects health issues and agricultural
yields at local to regional scales.
• Different scenarios and climate feedbacks alter thelifecycles of short-lived climate forcers and hence
air quality.

Projected changes in annual meansurface O3

FAQ 11.23a, AR5 WGIDiscussion