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

==== 7.6.1.1 Radiative Properties andLifetimes ====

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The radiative properties and lifetimes of compounds are the fundamental component of all emissions metrics. Since AR5, there have been advances in the understanding of the radiative properties of various compounds (see Sections 7.3.1, 7.3.2 and 7.3.3), and hence their effective radiative efficiencies (ERFs per unit change in concentration). For CO <sub>2</sub> , CH <sub>4</sub> and N <sub>2</sub> O, better accounting of the spectral properties of these gases has led to re-evaluation of their stratospheric-temperature-adjusted radiative forcing (SARF) radiative efficiencies and their dependence on the background gas concentrations ( [[#7.3.2|Section 7.3.2]] ). For CO <sub>2</sub> , CH <sub>4</sub> , N <sub>2</sub> O, CFC-11 and CFC-12 the tropospheric adjustments (Sections 7.3.1 and 7.3.2) are assessed to make a non-zero contribution to ERF. There is insufficient evidence to include tropospheric adjustments for other halogenated compounds. The re-evaluated effective radiative efficiency for CO <sub>2</sub> will affect all emissions metrics relative to CO <sub>2</sub> .

The effective radiative efficiencies (including adjustments from ( [[#7.3.2|Section 7.3.2]] ) for 2019 background concentrations for CO <sub>2</sub> , CH <sub>4</sub> and N <sub>2</sub> O are assessed to be 1.33×10 <sup>–5</sup> , 3.89×10 <sup>–4</sup> and 3.19×10 <sup>–3</sup> W m <sup>–2</sup> ppb <sup>–1</sup> respectively (see Table 7.15 for uncertainties), compared to AR5 assessments of 1.37×10 <sup>–5</sup> , 3.63×10 <sup>–4</sup> and 3.00×10 <sup>–3</sup> W m <sup>–2</sup> ppb <sup>–1</sup> . For CO <sub>2</sub> , increases due to the adjustments do not quite balance the decreases due to the increasing background concentration. For CH <sub>4</sub> , increases due to the re-evaluated radiative properties more than offset the decreases due to the increasing background concentration. For N <sub>2</sub> O the addition of tropospheric adjustments increases the effective radiative efficiency. Radiative efficiencies of halogenated species have been revised slightly ( [[#7.3.2.4|Section 7.3.2.4]] ) and for CFCs include tropospheric adjustments.

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'''Table 7.15''' '''|''' '''Emissions metrics for selected species: global warming potential (GWP), global temperature-change potential (GTP).''' All values include carbon cycle responses as described in ( [[#7.6.1.3|Section 7.6.1.3]] . Combined GTPs (CGTPs) are shown only for species with a lifetime less than 20 years ( [[#7.6.1.4|Section 7.6.1.4]] ). Note CGTP has units of years and is applied to a change in emissions rate rather than a change in emissions amount. The radiative efficiencies are as described in ( [[#7.3.2|Section 7.3.2]] and include tropospheric adjustments where assessed to be non-zero in ( [[#7.6.1.1|Section 7.6.1.1]] . The climate response function is from Supplementary Material 7.SM.5.2. Uncertainty calculations are presented in Supplementary Tables 7.SM.8 to 7.SM.13. Chemical effects of CH <sub>4</sub> and N <sub>2</sub> O are included ( [[#7.6.1.3|Section 7.6.1.3]] ). Contributions from stratospheric ozone depletion to halogenated species metrics are not included. Supplementary Table 7.SM.7 presents the full table.

{| class="wikitable"
|-
| Species

| Lifetime

(Years)

| Radiative Efficiency (W m <sup>–2</sup> ppb <sup>–1</sup> )

| GWP-20

| GWP-100

| GWP-500

| GTP-50

| GTP-100

| CGTP-50 (years)

| CGTP-100 (years)

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

| Multiple

| 1.33 ± 0.16 ×10 <sup>–5</sup>

| 1.

| 1.000

| 1.000

| 1.000

| 1.000

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

| 11.8 ± 1.8

| 5.7 ± 1.4 ×10 <sup>–4</sup>

| 82.5 ± 25.8

| 29.8 ± 11

| 10.0 ± 3.8

| 13.2 ± 6.1

| 7.5 ± 2.9

| 2823 ± 1060

| 3531 ± 1385

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

| 11.8 ± 1.8

| 5.7 ± 1.4 ×10 <sup>–4</sup>

| 79.7 ± 25.8

| 27.0 ± 11

| 7.2 ± 3.8

| 10.4 ± 6.1

| 4.7 ± 2.9

| 2675 ± 1057

| 3228 ± 1364

|-
| '''N''' <sub>2</sub> '''O'''

| 109 ± 10

| 2.8 ± 1.1 ×10 <sup>–3</sup>

| 273 ± 118

| 273 ± 130

| 130 ± 64

| 290 ± 140

| 233 ± 110

|
|-
| '''HFC-32'''

| 5.4 ± 1.1

| 1.1 ± 0.2 ×10 <sup>–1</sup>

| 2693 ± 842

| 771 ± 292

| 220 ± 87

| 181 ± 83

| 142 ± 51

| 78,175 ± 29,402

| 92,888 ± 36,534

|-
| '''HFC-134a'''

| 14.0 ± 2.8

| 1.67 ± 0.32 ×10 <sup>–1</sup>

| 4144 ± 1160

| 1526 ± 577

| 436 ± 173

| 733 ± 410

| 306 ± 119

| 146,670 ± 53,318

| 181,408 ± 71,365

|-
| '''CFC-11'''

| 52.0 ± 10.4

| 2.91 ± 0.65 ×10 <sup>–1</sup>

| 8321 ± 2419

| 6226 ± 2297

| 2093 ± 865

| 6351 ± 2342

| 3536 ± 1511

|
|-
| '''PFC-14'''

| 50,000

| 9.89 ± 0.19 ×10 <sup>–2</sup>

| 5301 ± 1395

| 7380 ± 2430

| 10,587 ± 3692

| 7660 ± 2464

| 9055 ± 3128

|
|}

The perturbation lifetimes of CH <sub>4</sub> (Section 6.3.1). and N <sub>2</sub> o ( [[IPCC:Wg1:Chapter:Chapter-5#5.2.3.1|Section 5.2.3.1]] ) have been slightly revised since AR5 to be 11.8 ± 1.8 years and 109 ± 10 years, respectively (Table 7.15). The lifetimes of halogenated compounds have also been slightly revised ( [[#Hodnebrog--2020a|Hodnebrog et al., 2020a]] ).

Although there has been greater understanding since AR5 of the carbon cycle responses to CO <sub>2</sub> emissions (Sections 5.4 and 5.5), there has been no new quantification of the response of the carbon cycle to an instantaneous pulse of CO <sub>2</sub> emission since [[#Joos--2013|Joos et al. (2013)]] .

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