Short-lived climate forcers (SLCF) in
the IPCC WGIII AR6: Input to the TFI
3rd Expert Meeting
Alaa Al Khourdajie, Senior Scientist, WGIII TSU
IPCC EXPERT MEETING ON SHORT-LIVED CLIMATE FORCERS, 11-14 APRIL 2022

WGIII Context• Trends and drivers• Co-benefits and trade offs
• Impacts on emission scenarios• Building sector: efficient cooling• Transport: aviation and shippingTrends and drivers
Example of assessed SLCFs and their sources• Aerosols, sulphur emissions or organic carbon (reduce forcing)• Black carbon, carbon monoxide or non-methane organic compounds (NMVOC) (contribute to warming)• These are co-emitted during combustion processes in power plants, cars, trucks, airplanes, but also
during wildfires and household activities such as traditional cooking with open biomass burning.

Trends and drivers• Conventional air pollutants that are
subject to significant emission controls
in many countries include: SO2, NOx,
BC and CO.
• 2015-2019, global SO2 and NOx
emissions have declined, mainly due toreductions in energy systems• Also, reductions in BC and COemissions appear to have occurred overthe same period, but trends are lesscertain (poorly quantified traditional
biofuel use).

Trends and drivers• Emissions of CH4, OC and NMVOC
have remained relatively stable in thepast five years.
• OC and NMVOC may have plateaued,although there is additional uncertainty
due to sources of NMVOCs that may
be missing in current inventories

Co-benefits and trade-offs
Co-benefits
SLCFs that contribute cooling: emission reduction thus leads to both air pollution and climate benefits.
• Reducing air-pollutants in the context of climate policies lead to substantial co-benefits of mitigation
efforts• Detrimental to human health, with estimates a total of 3.3 (1.6-4.8) million pre-mature deaths in 2010
from outdoor air pollution.
Trade-offs
SLCFs that contribute to warming: there is a possible trade-off• As aerosol emissions are mostly associated with fossil fuel combustion, the benefits of reducing CO2
could, in the short term, be reduced as a result of lower aerosol cooling.

Emission scenarios• As these co-emissions have implications for net warming, they are also considered in long-termemission reduction scenarios as covered in the literature• The smaller the residual Non-CO2 warming at the point of carbon neutrality, the larger the carbonbudget.
• Deep SLCF emission reductions also increase the remaining carbon budget for a specific temperaturegoal• Reducing SLCF emissions is critical to meet long-term climate goals and might help reduce the rate ofclimate change in the short term.
• A strong reduction in methane emissions is the most critical component in Non-CO2 mitigation tokeep the Paris climate goals in reachBuilding sector: efficient cooling• Synergies (HFCs): one possible synergy between SLCF and climate change mitigation is thesimultaneous improvement in energy efficiency in refrigeration and air-conditioning equipment in thebuilding sector during the hydrofluorocarbon (HFC) phase-down, as recognised in the Kigali
Amendment to the Montreal Protocol.
• The Kigali Amendment and related national and regional regulations are projected to reduce futureradiative forcing from HFCs by about half in 2050 compared to a scenario without any HFC controls,and to reduce future global average warming in 2100 from a baseline of 0.3-0.5°C to less than 0.1°C(WMO)• Extensive replacement of high-global warming potential (GWP) HFCs with commercially available lowGWP alternatives in refrigeration and air-conditioning equipment.

Transport: aviation• CO2 mitigation of aviation to ‘net zero’ levels, requires fundamental shifts in technology, fuel types, orchanges of behaviour or demand.
• In total, the net ERF from aviation’s non-CO2 SLCFs is estimated to be approximately 66% of aviation’s
current total forcing.
Mitigation options include:• Technology options for engine and airframe• Operational improvements for navigation• Alternative biofuels, synthetic fuels, and liquid Hydrogen• Technological and operational trade-offs between CO2 and non-CO2 effects• Modal shift to High-Speed Rail• Market-based offsetting measures

Transport: aviation

Transport: aviation

Transport: Shipping• Emissions of SLCF from shipping not only affects the climate, but also the environment, air quality, andhuman health. Maritime transport has been shown to be a major contributor to coastal air quality
degradation• Furthermore, increases in sulphur deposition on the oceans has also been shown to increase the flux of
CO2 from the oceans to the atmosphere
Mitigation• Pollution control is implemented to varying degrees in the modelling of the SSP scenarios; for example,
SSPs 1 and 5 assume that increasing concern for health and the environment result in more stringent
air pollution policies than today.
• There is a downward trend in SO2 and NOx emissions from shipping in all the SSPs, in compliance withregulations.

Thank you
Alaa Al Khourdajie
Senior Scientist, WGIII, TSU
a.alkhourdajie@ipcc-wg3.ac.uk@IPCC_CH#IPCReport
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