Editing IPCC:AR6/WGI/Chapter-2 (section)

=== 2.2.4 Halogenated Greenhouse Gases (CFCs, HCFCs, HFCs, PFCs, SF6 and others) ===

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This category includes ozone depleting substances (ODS), their replacements, and gases used industrially or produced as by-products. Some have natural sources (Section 6.2.2.4). The AR5 reported that atmospheric abundances of chlorofluorocarbons (CFCs) were decreasing in response to controls on production and consumption mandated by the Montreal Protocol on Substances that Deplete the Ozone Layer and its amendments. In contrast, abundances of both hydrochlorofluorocarbons (HCFCs, replacements for CFCs) and hydrofluorocarbons (HFCs, replacements for HCFCs) were increasing. Atmospheric abundances of perfluorocarbons (PFCs), SF <sub>6</sub> , and NF <sub>3</sub> were also increasing.

Further details on ODS and other minor greenhouse gases can be found in the Scientific Assessment of Ozone Depletion: 2018 ( [[#Engel--2018|Engel et al., 2018]] ; [[#Montzka--2018b|Montzka et al., 2018b]] ). Updated mixing ratios of the most radiatively important gases (ERF &gt;0.001 W m <sup>–2</sup> ) are reported in Table 2.2, and additional gases (ERF &lt;0.001 W m <sup>–2</sup> ) are shown in Annex III.

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==== 2.2.4.1 Chlorofluorocarbons (CFCs) ====

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Atmospheric abundances of most CFCs have continued to decline since 2011 (AR5). The globally-averaged abundance of CFC-12 decreased by 25 ppt (4.8%) from 2011 to 2019, while CFC-11 decreased by about 11 ppt (4.7%) over the same period (Table 2.2 and Figure 2.6). Atmospheric abundances of some minor CFCs (CFC-13, CFC-115, CFC-113a) have increased since 2011 (Annex III), possibly related to use of HFCs ( [[#Laube--2014|Laube et al., 2014]] ). Overall, as of 2019 the ERF from CFCs has declined by 9 ± 0.5% from its maximum in 2000, and 4.7 ± 0.6% since 2011 (Table 7.5).

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[[File:d1f994b46cfc8ab9668df2eb9ef8c20c IPCC_AR6_WGI_Figure_2_6.png]]

'''Figure 2.6''' '''|''' '''Global mean atmospheric mixing ratios of select ozone-depleting''' '''substances and other greenhouse gases.''' Data shown are based on the CMIP6 historical dataset and data from NOAA and AGAGE global networks. PFCs include CF <sub>4</sub> , C <sub>2</sub> f <sub>6</sub> , and C <sub>3</sub> F <sub>8</sub> , and ''c'' -C <sub>4</sub> F <sub>8</sub> ; Halons include halon-1211, halon-1301, and halon-2402; other HFCs include HFC-23, HFC-32, HFC-125, HFC-143a, HFC-152a, HFC-227ea, HFC-236fa, HFC-245fa, and HFC-365mfc, and HFC-43-10mee. Note that the y-axis range is different for '''(a)''' , '''(b)''' and '''(c)''' and a 25 parts per trillion (ppt) yardstick is given next to each panel to aid interpretation. Further data are in [[IPCC:Wg1:Chapter:Annex-iii|Annex III]] and details on data sources and processing are available in the chapter data table (Table 2.SM.1).

While global reporting indicated that CFC-11 production had essentially ceased by 2010, and the atmospheric abundance of CFC-11 is still decreasing, emissions inferred from atmospheric observations began increasing in 2013–2014 and remained elevated for 5–6 years, suggesting renewed and unreported production ( [[#Montzka--2018a|Montzka et al., 2018a]] , 2021; [[#Rigby--2019|Rigby et al., 2019]] ; [[#Park--2021|Park et al., 2021]] ). The global lifetimes of several ozone-depleting substances have been updated (SPARC, 2013), in particular for CFC-11 from 45 to 52 years.

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==== 2.2.4.2 Hydrochlorofluorocarbons (HCFCs) ====

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The atmospheric abundances of the major HCFCs (HCFC-22, HCFC-141b, HCFC-142b), primarily used in refrigeration and foam blowing, are increasing, but rates of increase have slowed in recent years (Figure 2.6). Global mean mixing ratios (Table 2.2) showed good concordance at the time of AR5 for the period 2005–2011. For the period 2011–2019, the UCI network detected larger increases in HCFC-22, HCFC-141b, and HCFC-142b compared to the NOAA and AGAGE networks. Reasons for the discrepancy are presently unverified, but could be related to differences in sampling locations in the networks ( [[#Simpson--2012|Simpson et al., 2012]] ). Emissions of HCFC-22, derived from atmospheric data, have remained relatively stable since 2012, while those of HCFC-141b and HCFC-142b have declined ( [[#Engel--2018|Engel et al., 2018]] ). Minor HCFCs, HCFC-133a and HCFC-31, have been detected in the atmosphere (currently less than 1 ppt) and may be unintentional by-products of HFC production ( [[#Engel--2018|Engel et al., 2018]] ).

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==== 2.2.4.3 Hydrofluorocarbons (HFCs), Perfluorocarbons (PFCs), Sulphur Hexafluoride (SF <sub>6</sub> ) and Other Radiatively Important Halogenated Gases ====

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Hydrofluorocarbons (HFCs) are replacements for CFCs and HCFCs. The atmospheric abundances of many HFCs increased between 2011 and 2019. HFC-134a (mobile air conditioning, foam blowing, and domestic refrigerators) increased by 71% from 63 ppt in 2011 to 107.6 ppt in 2019 (Table 2.2). The UCI network detected a slightly smaller relative increase (53%). HFC-23, which is emitted as a by-product of HCFC-22 production, increased by 8.4 ppt (35%) over 2011–2019. HFC-32 used as a substitute for HCFC-22, increased at least by 300%, and HFC-143a and HFC-125 showed increases of 100% and 187%, respectively. While the ERF of HFC-245fa is currently &lt;0.001 W m <sup>–2</sup> , its atmospheric abundance doubled since 2011 to 3.1 ppt in 2019 (Annex III). In contrast, HFC-152a is showing signs of stable (steady-state) abundance.

Other radiatively important gases with predominantly anthropogenic sources also continue to increase in abundance. SF <sub>6</sub> , used in electrical distribution systems, magnesium production, and semi-conductor manufacturing, increased from 7.3 ppt in 2011 to 10.0 ppt in 2019 (+36%). Alternatives to SF <sub>6</sub> or SF <sub>6</sub> -free equipment for electrical systems have become available in recent years, but SF <sub>6</sub> is still widely in use in electrical switch gear ( [[#Simmonds--2020|Simmonds et al., 2020]] ). The global lifetime of SF <sub>6</sub> has been revised from 3200 years to about 1000 years ( [[#Kovács--2017|Kovács et al., 2017]] ; [[#Ray--2017|Ray et al., 2017]] ) with implications for climate emissions metrics (Section 7.6.2). NF <sub>3</sub> , which is used in the semi-conductor industry, increased 147% over the same period to 2.05 ppt in 2019. Its contribution to ERF remains small, however, at 0.0004 W m <sup>–2</sup> . The atmospheric abundance of SO <sub>2</sub> f <sub>2</sub> , which is used as a fumigant in place of ozone-depleting methyl bromide, reached 2.5 ppt in 2019, a 46% increase from 2011. Its ERF also remains small at 0.0005 W m <sup>–2</sup> .

The global abundance of CCl <sub>4</sub> continues to decline, down about 9.6% since 2011. Following a revision of the global lifetime from 26 to 32 years, and discovery of previously unknown sources (e.g., biproducts of industrial emissions), knowledge of the CCl <sub>4</sub> budget has improved. There is now better agreement between top-down emissions estimates (based on atmospheric measurements) and industry-based estimates ( [[#Engel--2018|Engel et al., 2018]] ). Halon-1211, mainly used for fire suppression, is also declining, and its ERF dropped below 0.001 W m <sup>–2</sup> in 2019. While CH <sub>2</sub> cl <sub>2</sub> has a short atmospheric lifetime (6 months), and is not well-mixed, its abundance is increasing and its ERF is approaching 0.001 W m <sup>–2</sup> .

Perfluorocarbons CF <sub>4</sub> and C <sub>2</sub> f <sub>6</sub> , which have exceedingly long global lifetimes, showed modest increases from 2011 to 2019. CF <sub>4</sub> , which has both natural and anthropogenic sources, increased 8.2% to 85.5 ppt, and C <sub>2</sub> f <sub>6</sub> increased 16.3% to 4.85 ppt. ''c'' <sup>–</sup> C <sub>4</sub> F <sub>8</sub> , which is used in the electronics industry and may also be generated during the production of polytetrafluoroethylene (PTFE, also known as ‘Teflon’) and other fluoropolymers ( [[#Mühle--2019|Mühle et al., 2019]] ), has increased 34% since 2011 to 1.75 ppt, although its ERF remains below 0.001 W m <sup>–2</sup> . Other PFCs, present at mixing ratios &lt;1 ppt, have also been quantified ( [[#Droste--2020|Droste et al., 2020]] ; see Annex III).

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[[File:c7b7cf9bf1bbed375a5977b87d763289 IPCC_AR6_WGI_Figure_2_37.png]]

'''Figure 2.37 |''' '''Indices of interannual climate variability from 1950–2019 based upon several sea surface temperature data products.''' Shown are the following indices from top to bottom: IOB mode, IOD, Niño3.4, AMM and AZM. All indices are based on area-averaged annual data (see Annex IV). Further details on data sources and processing are available in the chapter data table (Table 2.SM.1).

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==== 2.2.4.4 Summary of Changes in Halogenated Gases ====

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In summary, by 2019 the ERF of halogenated GHGs had increased by 3.5% since 2011, reflecting predominantly a decrease in the atmospheric mixing ratios of CFCs and an increase in their replacements. However, average annual ERF growth rates associated with halogenated gases since 2011 are a factor of seven lower than in the 1970s and 1980s. Direct radiative forcings from CFCs, HCFCs, HFCs, and other halogenated greenhouse gases were 0.28, 0.06, 0.04, and 0.03 W m <sup>–2</sup> respectively, totalling 0.41 ± 0.07 W m <sup>–2</sup> in 2019 (see Table 7.5).

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