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

==== 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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