Editing IPCC:AR6/WGI/Chapter-7 (section)

==== 7.3.2.4 Halogenated Species ====

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The stratospheric-temperature-adjusted radiative efficiencies (SARF per ppb increase in concentration) for halogenated compounds are reviewed extensively in [[#Hodnebrog--2020a|Hodnebrog et al. (2020a)]] , an update to those used in AR5. Many halogenated compounds have lifetimes short enough that they can be considered short-lived climate forcers (SLCFs; Table 6.1). As such, they are not completely ‘well-mixed’ and their vertical distributions are taken into account when determining their radiative efficiencies. The World Meteorological Organization ( [[#WMO--2018|WMO, 2018]] ) updated the lifetimes of many halogenated compounds and these were used in [[#Hodnebrog--2020a|Hodnebrog et al. (2020a)]] .

The tropospheric adjustments to chlorofluorocarbons (CFCs), specifically CFC-11 and CFC-12, have been quantified as 13% ± 10% and 12% ± 14% of the SARF, respectively ( [[#Hodnebrog--2020b|Hodnebrog et al., 2020b]] ). The assessed adjustment to CFCs is therefore 12% ± 13% with ''low confidence'' due to the lack of corroborating studies. There have been no calculations for other halogenated species so for these the tropospheric adjustments are therefore assumed to be 0 ± 13% with ''low confidence.'' The radiative modelling uncertainties are 14% and 24% for compounds with lifetimes greater than and less than five years, respectively ( [[#Hodnebrog--2020a|Hodnebrog et al., 2020a]] ). The overall uncertainty in the ERFs of halogenated compounds is therefore assessed to be 19% and 26% depending on the lifetime. The ERF from CFCs is slowly decreasing, but this is compensated for by the increased forcing from the replacement species (HCFCs and HFCs). The ERF from HFCs has increased by 0.028 ± 0.05 W m <sup>–2</sup> . Thus, the concentration changes mean that the total ERF from halogenated compounds has increased since AR5 from 0.360 ± 0.036 W m <sup>–2</sup> to 0.408 ± 0.078 W m <sup>–2</sup> (Table 7.5). Of this, 0.034 W m <sup>–2</sup> is due to increased radiative efficiencies and tropospheric adjustments, and 0.014 W m <sup>–2</sup> is due to increases in concentrations. As the adjustments are assessed to be small there remains ''high confidence'' in the overall assessment.

Halogenated compounds containing chlorine and bromine lead to ozone depletion in the stratosphere which will reduce the associated ERF ( [[#Morgenstern--2020|Morgenstern et al., 2020]] ). [[IPCC:Wg1:Chapter:Chapter-6|Chapter 6]] (Section 6.4 and Figure 6.12) assesses the ERF contributions due to the chemical effects of reactive gases.

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