Editing IPCC:AR6/WGI/Chapter-6 (section)

== 6.1 Introduction ==

<div id="h1-2-siblings" class="h1-siblings"></div>

Short-lived climate forcers (SLCFs) are 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 their radiative forcing and indirect SLCFs being precursors of direct climate forcers. Direct SLCFs include methane (CH <sub>4</sub> ), ozone (O <sub>3</sub> ), short-lived halogenated compounds, such as hydrofluorocarbons (HFCs), hydrochlorofluorocarbons (HCFCs), and aerosols. Indirect SLCFs include nitrogen oxides (NO <sub>x</sub> ), carbon monoxide (CO), non-methane volatile organic compounds (NMVOCs), sulphur dioxide (SO <sub>2</sub> ), and ammonia (NH <sub>3</sub> ). Aerosols consist of sulphate (SO <sup>2</sup> <sub>4</sub> <sup>–</sup> ), nitrate (NO <sup>–</sup> <sub>3</sub> ), ammonium (NH <sup>+</sup> 4 ), carbonaceous aerosols (e.g., black carbon (BC), organic aerosols (OA)), mineral dust, and sea spray (see Table 6.1) and can be present as internal or external mixtures and at sizes from nano-meters to tens of micro-meters. SLCFs can be emitted directly from natural systems and anthropogenic sources (primary) or can be formed by reactions in the atmosphere (secondary; Figure 6.1).

<div id="_idContainer008" class="Basic-Text-Frame"></div>

[[File:c0b7725a80efeed207f370b34c119d78 IPCC_AR6_WGI_Figure_6_1.png]]

'''Figure 6.1 |''' '''Sources and processes leading to atmospheric short-lived climate forcer (SLCF) burden and their interactions with the climate system.''' Both direct and indirect SLCFs and the role of atmospheric processes for the lifetime of SLCFs are depicted. Anthropogenic emissions sectors illustrated are: fossil fuel exploration, distribution and use; biofuel production and use; waste; transport; industry; agricultural sources; and open biomass burning. Emissions from natural systems include those from open biomass burning, vegetation, soil, ocean, lightning and volcanoes. SLCFs interact with solar or terrestrial radiation, surface albedo, and cloud or precipitation systems. The radiative forcing due to individual SLCFs can be either positive or negative. Climate change induces changes in emissions from most natural systems as well as from some anthropogenic emissions sectors (e.g., agriculture) leading to a climate feedback (purple arrows). Climate change also influences atmospheric chemistry processes, such as chemical reaction rates or via circulation changes, thus affecting atmospheric composition leading to a climate feedback. Air pollutants influence emissions from terrestrial vegetation, including agriculture (grey arrow).

<div id="6.1.1" class="h2-container"></div>

<span id="importance-of-slcfs-for-climate-and-air-quality"></span>
=== 6.1.1 Importance of SLCFs for Climate and Air Quality ===

<div id="h2-7-siblings" class="h2-siblings"></div>

The atmospheric lifetime determines the spatial and temporal variability, with most SLCFs showing high variability, except methane and many HCFCs and HFCs that are also well-mixed (as a consequence methane is discussed together with other well-mixed greenhouse gases (GHGs) in Chapters 2, 5, and 7). In contrast to well-mixed GHGs, such as CO <sub>2</sub> , methane and some HFCs, the radiative forcing effects of most SLCFs are largest at regional scales and climate effects predominantly occur in the first two decades after their emissions or formation. However, changes in their emissions can also induce long-term climate effects, for instance by altering some biogeochemical cycles. Therefore, the temporal evolution of radiative effects of SLCFs follows that of emissions, that is, when SLCF emissions decline to zero their atmospheric abundance and radiative effects decline towards zero. The total influence of individual SLCF emissions on radiative forcing and climate includes their effects on the abundances of other forcers through chemistry (chemical adjustments).

SLCFs can affect climate by interacting with radiation or by perturbing other components of the climate system (e.g., the cryosphere and carbon cycle through deposition, or the water cycle through modifications of cloud properties via cloud condensation nuclei or ice nuclei). SLCFs can have either net warming or net cooling effects on climate. In addition to altering the Earth’s radiative balance, many SLCFs are also air pollutants with adverse effects on human health and ecosystems. SLCFs are of interest for climate policies (e.g., methane, HFCs), and are regulated as air pollutants (e.g., aerosols, ozone) or because of their deleterious influence on stratospheric ozone (e.g., HCFCs). The list of SLCFs assessed in this chapter and their effects are provided in Table 6.1.

<div id="_idContainer009" class="_idGenObjectStyleOverride-1"></div>

'''Table 6.1 |''' '''Overview of SLCFs of interest for Chapter 6.''' For each SLCF, its source types, lifetime in the atmosphere, and associated radiatively active agent is given. Source type can be primary (emitted) and/or secondary (formed through multiple atmospheric mechanisms). Unless otherwise noted, the stated lifetime refers to tropospheric lifetime.* Climate effect of increased SLCFs is indicated as ‘+’ for warming and ‘–’ for cooling. ‘Direct’ is used for SLCFs exerting climate effects through their radiative forcing and ‘Indirect’ for SLCFs which are precursors affecting the atmospheric burden of other climatically active compounds. Other processes through which SLCFs affect climate are listed where applicable. The World Health Organization (WHO) guidelines for air quality (AQ) are given, where applicable, to show which SLCFs are regulated for air-quality purposes.

{| class="wikitable"
|-
| '''Compounds'''

| '''Source Type'''

| '''Lifetime'''

| '''Direct'''

| '''Indirect'''

| '''Climate Forcing'''

| '''Other Effects on Climate''' <sup>a</sup>

| '''WHO AQ Guidelines''' <sup>b</sup>

|-
| '''CH''' <sub>4</sub>

| Primary

| ~9 years

~12 years (perturbation time)

| CH <sub>4</sub>

| O <sub>3</sub> , H <sub>2</sub> O, CO <sub>2</sub>

| +

|
| No <sup>c</sup>

|-
| '''O''' <sub>3</sub>

| Secondary

| Hours to weeks

| O <sub>3</sub>

| CH <sub>4</sub> , secondary organic and sulphate aerosols

| +

| Ecosystems

| 100 μg m <sup>–3</sup>

<sup>8-hour mean</sup>

|-
| '''NO''' <sub>x</sub> '''(= NO + NO''' <sub>2</sub> ''')'''

| Primary

| Hours to days

|
| O <sub>3</sub> , nitrate aerosols, CH <sub>4</sub>

| +/–

| Ecosystems

| 40 μg m <sup>–3</sup>

<sup>annual mean</sup> 200 μg m <sup>–3</sup>

<sup>1-hour mean</sup>

|-
| '''CO'''

| Primary + Secondary

| 1 to 4 months

|
| O <sub>3</sub> , CH <sub>4</sub>

| +

|
| No

|-
| '''NMVOCs''' <sup>**</sup>

| Primary + Secondary

| Hours to months

|
| O <sub>3</sub> , CH <sub>4</sub> , organic aerosols

| +/–

|
| No

|-
| '''SO''' <sub>2</sub>

| Primary

| Days (trop.) to weeks (strat.)

|
| Sulphate and nitrate aerosols, <sub></sub> O <sub>3</sub>

| –

| Ecosystems

| 20 μg m <sup>–3</sup>

<sup>24-hour mean</sup> 500 μg m <sup>–3</sup>

<sup>10-minute mean</sup>

|-
| '''NH''' <sub>3</sub>

| Primary

| Hours

|
| Ammonium Sulphate, Ammonium Nitrate

| –

| Ecosystems

| No

|-
| '''HCFCs'''

| Primary

| Months to years

| HCFCs

| O <sub>3</sub>

| +/–

|
| No <sup>c</sup>

|-
| '''HFCs''' <sup></sup>

| Primary

| Days to years

| HFCs

|
| +

|
| No <sup>c</sup>

|-
| '''Halons and Methylbromide'''

| Primary

| Years

| Halons and Methylbromide

| Stratospheric O <sub>3</sub>

| +/–

|
| No <sup>c</sup>

|-
| '''Very Short-lived Halogenated Species (VSLSs)'''

| Primary

| Less than 6 months

|
| O <sub>3</sub>

| –

|
| No <sup>c</sup>

|-
| '''Sulphate aerosols'''

| Secondary

| Minutes to weeks

| Sulphate

|
| –

| Clouds

Ecosystems

| as part of PM <sup>d</sup>

|-
| '''Nitrate aerosols'''

| Secondary

| Minutes to weeks

| Nitrate

|
| –

| Clouds

Ecosystems

| as part of PM <sup>d</sup>

|-
| '''Carbonaceous Aerosols'''

| Primary + Secondary

| Minutes to Weeks

| BC, OA

|
| +/–

| Cryo, Clouds

Ecosystems

| as part of PM <sup>d</sup>

|-
| '''Sea spray'''

| Primary

| Day to week

| Sea spray

|
| –

| Clouds

Ecosystems

| as part of PM <sup>d</sup>

|-
| '''Mineral dust'''

| Primary

| Minutes to Weeks

| Mineral dust

|
| +/–

| Cryo Cloud

Ecosystems

| as part of PM <sup>d</sup>

|}

\* <sup></sup> for lifetimes reported in this table, it is assumed that the compounds are uniformly mixed throughout the troposphere, however, this assumption is unlikely for compounds with lifetimes &lt;1 year and, therefore, the reported values should be viewed as approximations ( [[#Prather--2001|Prather et al., 2001]] ).

\** Some NMVOCs are biogenic volatile organic compounds (BVOCs).

<sup>a</sup> Clouds: effect on clouds through aerosol–cloud interactions, Ecosystems: effect on ecosystems through changes in radiation and deposition, Cryo: effect on planetary albedo through deposition on snow and ice; <sup>b</sup> [[#Krzyzanowski--2008|Krzyzanowski and Cohen (2008)]] ; <sup>c</sup> regulated through Kyoto/Montreal protocols; <sup>d</sup> for Particulate Matter with diameter &lt;2.5 µm (PM <sub>2.5</sub> ): 10 µg m <sup>–3</sup> annual mean or 25 µg m <sup>–3</sup> 24-hour mean (99th percentile) and for Particulate Matter with diameter &lt;10 µm (PM <sub>10</sub> ): 20 µg m <sup>–3</sup> annual mean or 50 µg m <sup>–3</sup> 24-hour mean (99th percentile).

As depicted in Figure 6.1, emissions of SLCFs are governed by anthropogenic activities and sources from natural systems (see Section 6.2 for details). Atmospheric chemistry in this context is both a source and a sink of SLCFs. For instance, ozone and secondary aerosols are exclusively formed through atmospheric mechanisms (Sections 6.3.2 and 6.3.5 respectively). The hydroxyl (OH) radical, the most important oxidizing agent in the troposphere, acts as a sink for SLCFs by reacting with them and thereby influencing their lifetime (Section 6.3.6). Through SLCF radiative forcing and feedbacks (Section 6.4), key climate parameters, such as temperature, hydrological cycle and weather patterns are perturbed. Climate change also influences air quality (Section 6.5). As depicted in Figure 6.1, SLCFs affect both climate and air quality, hence SLCF mitigation has linkages to both issues (Section 6.6). Socio-economic narratives including air-quality policies determine future projections of SLCFs in the five core Shared Socio-economic Pathways (SSPs): SSP1-1.9, SSP1-2.6, SSP2-4.5, SSP3-7.0, and SSP5-8.5 (described in Chapter 1), and in addition, a subset of SSP3 scenarios make it possible to isolate the effect of various SLCF mitigation trajectories on climate and air quality (Section 6.7).

<div id="6.1.2" class="h2-container"></div>

<span id="treatment-of-slcfs-in-previous-assessments"></span>
=== 6.1.2 Treatment of SLCFs in Previous Assessments ===

<div id="h2-8-siblings" class="h2-siblings"></div>

Although ozone, aerosols and their precursors have been considered in previous IPCC assessment reports, AR5 considered SLCFs as a specific category of climate-relevant compounds but referred to them as near-term climate forcers (NTCFs; Myhre et al. , 2013) . In AR5, the linkages between air quality and climate change were also considered in a more detailed and quantitative way than in previous reports ( [[#Kirtman--2013|Kirtman et al., 2013]] ; [[#Myhre--2013|Myhre et al., 2013]] ).

The AR5 WGI assessed radiative forcings for short-lived gases, aerosols, aerosol precursors and aerosol–cloud interactions as well as the evolution of confidence levels in the forcing mechanisms from SAR to AR5. Whereas the forcing mechanisms for ozone and aerosol–radiation interactions were estimated to be characterized with ''high confidence'' , the ones induced by aerosols through other processes remained of ''very low'' to ''low confidence'' . The AR5 also reported that forcing agents such as aerosols and ozone are highly heterogeneous spatially and temporally, and these patterns affect global and regional temperature responses as well as other aspects of climate response such as the hydrologic cycle (Myhre et al. , 2013) .

The AR5 WGI also evaluated the air quality-climate interaction through the projected trends of surface ozone and PM <sub>2.5</sub> . [[#Kirtman--2013|Kirtman et al. (2013)]] concluded with ''high confidence'' that the response of air quality to climate-driven changes is more uncertain than the response to emissions-driven changes, and also that locally higher surface temperatures in polluted regions will trigger regional feedbacks in chemistry and local emissions that will increase peak levels of ozone and PM <sub>2.5</sub> ( ''medium confidence'' ).

In the IPCC Special Report on Global Warming of 1.5°C (SR1.5; [[#Allen--2018a|Allen et al., 2018a]] ), [[#Rogelj--2018a|Rogelj et al. (2018a)]] state that the evolution of methane and SO <sub>2</sub> emissions strongly influences the chances of limiting warming to 1.5°C, and that, considering mitigation scenarios to limit warming to 1.5°C or 2°C, a weakening of aerosol cooling would add to future warming in the near term, but can be tempered by reductions in methane emissions ( ''high confidence'' ). In addition, as some SLCFs are co-emitted alongside CO <sub>2</sub> , especially in the energy and transport sectors, low CO <sub>2</sub> scenarios, relying on decline of fossil fuel use, can result in strong abatement of some cooling and warming SLCFs ( [[#Rogelj--2018a|Rogelj et al., 2018a]] ). On the other hand, specific reductions of the warming SLCFs (methane and BC) would, in the short term, contribute significantly to the efforts of limiting warming to 1.5°C. Reductions of BC and methane would have substantial co-benefits, improving air quality and therefore limiting effects on human health and agricultural yields. This would, in turn, enhance the institutional and socio-cultural feasibility of such actions in line with the United Nations’ Sustainable Development Goals (SDGs; [[#Coninck--2018|Coninck et al., 2018]] ).

Following SR1.5, the IPCC Special Report on Climate Change and Land (SRCCL; [[#IPCC--2019a|IPCC, 2019a]] ) took into consideration the emissions on land of three major SLCFs: mineral dust, carbonaceous aerosols (BC and OA) and biogenic volatile compounds (BVOCs) ( [[#Jia--2019|Jia et al., 2019]] ). The SRCCL concluded that: (i) there is no agreement about the direction of future changes in mineral dust emissions; (ii) fossil fuel and biomass burning, and secondary organic aerosols (SOA) from natural BVOC emissions are the main global sources of carbonaceous aerosols whose emissions are expected to increase in the near future due to possible increases in open biomass burning and increase in SOA from oxidation of BVOCs ( ''medium confidence'' ); and (iii) BVOCs are emitted in large amounts by forests and they are rapidly oxidized in the atmosphere to form less volatile compounds that can condense and form SOA, and in a warming planet, BVOC emissions are expected to increase but magnitude is unknown and will depend on future land-use change, in addition to climate ( ''limited evidence'' , ''medium agreement'' ).

Finally, the IPCC Special Report on the Ocean and Cryosphere in a Changing Climate (SROCC; [[#IPCC--2019b|IPCC, 2019b]] ) discussed the effects of BC deposition on snow and glaciers, concluding that there is ''high confidence'' that darkening of snow through the deposition of BC and other light-absorbing particles enhances snowmelt in the Arctic ( [[#Meredith--2019|Meredith et al., 2019]] ), but that there is ''limited evidence'' and ''low agreement'' that long-term changes in glacier mass of high mountain areas are linked to light-absorbing particles ( [[#Hock--2019|Hock et al., 2019]] ).

<div id="6.1.3" class="h2-container"></div>

<span id="chapter-roadmap"></span>
=== 6.1.3 Chapter Roadmap ===

<div id="h2-9-siblings" class="h2-siblings"></div>

Figure 6.2 presents the Chapter 6 roadmap.

<div id="_idContainer011" class="Basic-Text-Frame"></div>

[[File:d929029f2179b8b06e91b3538da3a74b IPCC_AR6_WGI_Figure_6_2.png]]

'''Figure 6.2 |''' '''Visual guide to Chapter 6.''' See Section 6.1.3 for additional description of the chapter.

Specific aspects of SLCFs can also be found in other chapters of this report: the evolution of ozone, HFCs and aerosols, as well as the long-term evolution of methane, dust and volcanic aerosols are discussed in Chapter 2; near-term climate projections and SLCFs are discussed in Chapter 4; the global budget of methane is addressed in Chapter 5; aerosol–cloud and aerosol–precipitation interactions are treated in Chapters 7 and 8, respectively; the global radiative forcing of SLCFs is assessed in Chapter 7; some aspects of downscaling methodology in climate modelling concerning SLCFs are discussed in Chapter 10. The WGII report assesses how climate change affects air pollution and its impacts on human health and the WGIII report assesses the role of SLCFs in abatement strategies and their cost-effectiveness, the implications of mitigation efforts on air pollution as well as the articulation between air pollution policies and GHG mitigation.

This chapter discusses air quality from a global point of view with a focus on surface ozone and particulate matter concentrations. Local and indoor air pollution, as well as the effect of air pollution on health, are beyond the scope of this chapter. This Assessment is mainly based on results and studies relying on global models or observation datasets operated through global networks or from satellites. Global chemistry-climate models enable the quantification of changes in background concentrations, such as changes in surface ozone due to large-scale changes in climate or methane, by considering comprehensively the physiochemical processes (Box 6.1). In addition, climate effects are often non-linear responses to concentrations which already respond non-linearly to emissions, with per-mass unit effects often larger in pristine than in polluted regions, justifying the relevance of global models. However, specific aspects of urban air quality cannot be captured by global models and require high-resolution models that reproduce the temporal and spatial variability of emissions and abundances necessary to precisely account for the non-linearity of the chemistry and the sensitivity of local air pollution to its drivers. Consequently, the sectoral analysis in Section 6.6 and the mitigation effects in Section 6.7 cannot be directly applied for local air-quality planning.

Due to their short lifetimes, SLCF trends and effects are strongly related to the localization and evolution of the emissions sources. To better link the drivers of emissions evolution and SLCFs, Chapter 6 makes use of regions defined by the WGIII in most of the analysis. An exception is made for the effect of SLCFs on the climate, for which analysis relies on WGI [[IPCC:Wg1:Chapter:Atlas|Atlas]] regions.

<div id="6.2" class="h1-container"></div>

<span id="global-and-regional-temporal-evolution-of-slcf-emissions"></span>