Information on SLCFs in“AR6 Climate Change 2022:
Mitigation of Climate Change”(The WGIII contribution to the Sixth Assessment Reportapproved/accepted at the IPCC-56/WGIII-14 session)
based on the version released on 4 April 2022
TFI TSUIntroduction• This file has been prepared by the IPCC TFI Technical Support Unit solely for the 3rd Expert Meetingon Short-lived Climate Forcers virtually held on 11-15 April 2022.
• The aim of this file is to help the meeting participants find most relevant information on SLCFs in the“AR6 Climate Change 2022: Mitigation of Climate Change” (the WGIII contribution to the Sixth
Assessment Report approved/accepted at the IPCC-56/WGIII-14 session) that is of potential relevanceto discussion at the Expert Meeting.
• “AR6 Climate Change 2022: Mitigation of Climate Change” (the version which is subject to final copyedit and layout, released on 4 April 2022) is available at:• https://www.ipcc.ch/report/ar6/wg3/•MetricChapter 2: Emissions Trend and Drivers• Recently developed step/pulse metrics such as the CGTP (Combined Global Temperature Change
Potential; Collins et al. 2019) and GWP* (referred to as GWP-star; Allen et al. 2018; Cain et al. 2019)recognise that a sustained increase/decrease in the rate of SLCF emissions has a similar effect on
global surface temperature over multiple decades as a one-off pulse emission/removal of CO2. [Ch2p2-17]• These metrics use this relationship to calculate the CO2 emissions or removals that would result inroughly the same temperature change as a sustained change in the rate of SLCF emissions (CGTP)
over a given time period, or as a varying time series of CH4 emissions (GWP*). [Ch2 p2-17]• The ability of these metrics to relate changes in emission rates of short-lived gases to cumulative CO2emissions makes them well-suited, in principle, to estimating the effect on the remaining carbon
budget from more, or less, ambitious SLCF mitigation over multiple decades compared to a givenreference scenario (high confidence; Collins et al. 2019; Forster et al. 2021). [Ch2 p2-17/18]

non-methane SLCFs
Chapter 2: Emissions Trend and Drivers• There are other emissions with shorter atmospheric lifetimes that contribute to climate changes. Someof them like aerosols, sulphur emissions or organic carbon reduce forcing, while others like blackcarbon, carbon monoxide or non-methane organic compounds (NMVOC) contribute to warming (alsosee Figure 2.4) as assessed in Working Group I (Forster et al., 2021c; Naik et al., 2021a). [Ch2 p2-24]• Many of these other short-lived climate forcers (SLCFs) are co-emitted during combustion processes inpower plants, cars, trucks, airplanes, but also during wildfires and household activities such as
traditional cooking with open biomass burning. [Ch2 p2-24][Figure 2.4 - Ch2 p2-15]

Global Warming
Chapter 3: Mitigation Pathways compatible with long-term goals – [p 3-31]• In the case of the first group, emission reduction thus leads to both air pollution and climate benefits.
• For the second, group there is a possible trade-off (Shindell and Smith 2019; Lund et al. 2020). Asaerosol 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. [Ch3 p3-31][Figure 2.4 - Ch2 p2-15]

Aviation
Chapter 10: Transport• Aviation’s net warming effect results from its historical and current emissions of CO2, and non-CO2
emissions of water vapour, soot, sulphur dioxide (from sulphur in the fuel), and nitrogen oxides (NOX, =NO + NO2) (Penner et al. 1999; Lee et al. 2021; Naik et al. 2021). [Ch10 p10-59]• Emissions of NOX currently result in net positive warming from the formation of short-term ozone(warming) and the destruction of ambient methane (cooling). If the conditions are suitable, emissionsof soot and water vapour can trigger the formation of contrails (Kärcher 2018), which can spread toform extensive contrail-cirrus cloud coverage. [Ch10 p10-59]• In total, the net ERF from aviation’s non-CO2 SLCFs is estimated to be approximately 66% ofaviation’s current total forcing. [Ch10 p10-59]• For example, improved fuel efficiency has resulted from high overall pressure ratio engines with large
bypass ratios. This improvement has increased pressure and temperature at the combustor inlet, witha resultant tendency to increase thermal NOX formation in the combustor. Combustor technology aims
to reduce this increase, but it represents a potential technology trade-off whereby NOX control may beat the expense of extra fuel efficiency. [Ch10 p10-62]
ERF: Effective Radiative Forcing

Aviation (Liquid Hydrogen fuel [LH2])
Chapter 10: Transport• The non-CO2 impacts of LH2-powered aircrafts remain poorly understood. The emission index of water
vapour would be much larger (estimated to be 2.6 times greater by Ström and Gierens (2002)) than forconventional fuels, and the occurrence of contrails may increase but have lower ERF because of thelower optical depth (Marquart et al. 2005). Moreover, contrails primarily form on soot particles fromkerosene-powered aircraft, which would be absent from LH2 exhaust (Kärcher 2018). The overall effectis currently unknown as there are no measurements. Potentially, NOX emissions could be lower withcombustor redesign (Khandelwal et al. 2013). [Ch10 p10-62]

Shipping
Chapter 10: Transport• Like aviation, shipping is also a source of emissions of the SLCFs described in Section 10.5, including
nitrogen oxides (NOX), sulphur oxides (SO2 and SO4), carbon monoxide (CO), black carbon (BC), andnon-methane volatile organic carbons (NMVOCs) (Naik et al. 2021). Though SLCF have a shorter
lifetime than the associated CO2 emissions, these can have both a cooling effect (e.g., SOX) or a
warming effect (e.g., ozone from NOX). [Ch10 p10-68]• Furthermore, increases in sulphur deposition on the oceans has also been shown to increase the fluxof CO2 from the oceans to the atmosphere (Hassellöv et al. 2013). [Ch10 p10-68]• Changing the location of the emissions adds complexity to the assessment of the climatic impacts ofArctic shipping, as the local conditions are different and the SLCF may have a different impact on
clouds, precipitation, albedo and local environment (Marelle et al. 2016; Fuglestvedt et al. 2014;
Dalsøren et al. 2013). [Ch10 p10-69]

Ammonia as fuelChapter 10: Transport• If produced from green hydrogen or coupled with CCS, ammonia could reduce short lived climate
forcers and particulate matter precursors including black carbon and SO2. However, the combustion ofammonia could lead to elevated levels of nitrogen oxides and ammonia emissions. [Ch10 p10-165]• These fuels have their own unique transport and storage challenges as Ammonia requires a pilot fueldue to difficulty in combustion, and Ammonia combustion could lead to elevated levels of NOX, N2O, or
NH3 emissions depending on engine technology used (DNV GL 2020). [Ch10 p10-70]
Chapter 6: Energy Systems• Ammonia can also be used in low and high temperature fuel cells (Lan and Tao 2014), whereby bothelectricity and hydrogen can be produced without any NOX emissions. A key challenge in use ofammonia is related to significant amount of NOX emissions, which is released from nitrogen andoxygen combustion, and unburned ammonia. [Ch6 p6-60]

Carbon Capture & Storage (CCS)
Chapter 17: Accelerating the Transition in the context of sustainable development• CCS applied in industry is assessed as having synergies in terms of the control of non-CO2 pollutants(such as sulphur dioxide), but increases in non-CO2 pollutants (such as particulate matter, nitrogenoxide and ammonia). [Ch17 p17-54]Chapter 11: Industry• Methane based syngas (hydrogen and carbon monoxide) direct reduced iron (DRI) with CCS.
Most DRI facilities currently use a methane-based syngas of H2 and CO as both reductant and fuel(some use coal). A syngas DRI-EAF steel making facility has been operating in Abu Dhabi since 2016that captures carbon emitted from the DRI furnace (where it is a co-reductant with hydrogen) andsends it to a nearby oil field for enhanced oil recovery. [Ch11 p11-44]

EAF: Electric Arc Furnace

Solar Radiation Modification Schemes (SRM)
Stratospheric Aerosol Intervention (SAI)
Chapter 14: International Cooperation• In addition, the effects of proposed SRM options would only last as long as a deployment ismaintained— e.g. requiring ca. yearly injection of aerosols in the case of SAI as the lifetime of aerosolsin the stratosphere is 1-3 years (Niemeier et al. 2011) or continuous spraying of sea salt in the case of
MCB as the lifetime of sea salt aerosols in the atmosphere is only about 10 days—which contrasts
with the long lifetime of CO2 and its climate effects, with global warming resulting from CO2 emissions
likely remaining at a similar level for a hundred years or more (MacDougall et al. 2020) and long-termclimate effects of emitted CO2 remaining for several hundreds to thousands of years (Solomon et al.
2009). [Ch14 p14-56]• SAI - Injection of reflective aerosol particles directly into the stratosphere or a gas which then converts
to aerosols that reflect sunlight. [Ch14 p14-57]