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

=== 6.3.1 Methane (CH <sub>4</sub> ) ===

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The global mean surface mixing ratio of methane has increased by 156% since 1750 ( [[IPCC:Wg1:Chapter:Chapter-2#2.2.3.4|Section 2.2.3.4]] and Annex III). Since AR5, the methane mixing ratio has increased by about 3.5% from 1803 ± 2 ppb in 2011 to 1866 ± 3 ppb in 2019 ( [[IPCC:Wg1:Chapter:Chapter-2#2.2.3.3.2|Section 2.2.3.3.2]] ) largely driven by anthropogenic activities as assessed in [[IPCC:Wg1:Chapter:Chapter-5|Chapter 5]] ( [[IPCC:Wg1:Chapter:Chapter-5#5.2.2|Section 5.2.2]] and Cross-Chapter Box 5.2).

An assessment of the global methane budget is provided in Chapter 5, while this section assesses methane atmospheric lifetime and perturbation time ( [[#Prather--2001|Prather et al., 2001]] ). The AR5 based its assessment of methane lifetime on [[#Prather--2012|Prather et al. (2012)]] . The methane chemical lifetime due to tropospheric OH, the primary sink of methane, was assessed to be 11.2 ± 1.3 years constrained by surface observations of methyl chloroform (MCF), and lifetimes due to stratospheric loss, <sup>[[#footnote-003|2]]</sup> tropospheric halogen loss and soil uptake were assessed to be 150 ± 50 years, 200 ± 100 years, and 120 ± 24 years, respectively ( [[#Myhre--2013|Myhre et al., 2013]] ). Considering the full range of individual lifetimes, the total methane lifetime was assessed in AR5 to be 9.25 ± 0.6 years.

The global chemical methane sink, essentially due to tropospheric OH, required to calculate the chemical lifetime is estimated by either bottom-up global CCMs and ESMs (BU) or top-down observational inversion methods (TD). BU global models represent the coupled chemical processes and feedbacks that determine the chemical sinks but show large diversity in their estimates, particularly the tropospheric OH sink ( [[#Zhao--2019|]] [[#Zhao--2019|]] [[#Zhao--2019|Zhao et al., 2019]] ; [[#Stevenson--2020|Stevenson et al., 2020]] ). TD inversion methods, on the contrary, provide independent observational constraints on the methane sink due to tropospheric OH over large spatio-temporal scales, but are prone to observational uncertainties and do not account for the chemical feedbacks on OH ( [[#Prather--2017|Prather and Holmes, 2017]] ; [[#Naus--2019|Naus et al., 2019]] ). The central estimate of mean chemical methane loss over the period 2008–2017 varied from 602 [minimum and maximum range of 507–803] Tg yr <sup>–1</sup> from BU chemistry–climate models in the Chemistry–Climate Modelling Initiative (CCMI) to 514 [474–529] Tg yr <sup>–1</sup> from TD inverse modelling ( [[IPCC:Wg1:Chapter:Chapter-5#5.2.2|Section 5.2.2]] and Table 5.2). The smaller range in the TD estimate (11%) results from the use of a common climatological mean OH distribution ( [[#Saunois--2020|Saunois et al., 2020]] ; [[#Zhao--2020a|Zhao et al., 2020a]] ), while the larger range in the BU estimate (49%) reflects the diversity in OH concentrations from different chemical

mechanisms implemented in the global models ( [[#Zhao--2019|]] [[#Zhao--2019|]] [[#Zhao--2019|Zhao et al., 2019]] ). See Section 6.3.6 for further discussion on the conflicting information on OH from CCMs/ESMs and TD inversion approaches. Further work is required to reconcile differences between BU and TD estimates of the chemical methane sink.

The present-day BU methane chemical lifetime shows a larger spread than that in the TD estimates (Table 6.2) in line with the spread seen in the sink estimates. The spread in the methane lifetime calculated by three CMIP6 ESMs is narrower and is enclosed within the spread of the BU CCMI model ensemble. Based on the consideration that the small imbalance in total methane sources versus sinks derived from TD estimates is close to the observed atmospheric methane growth rate (Table 5.2), the TD values are assessed to be the best estimates for this assessment. The relative uncertainty (± 1 standard deviation) is taken to be the same as that in AR5, that is, 11.8%, 33% and 10% for chemical, soil and total lifetime, respectively. The central estimate of the total atmospheric methane lifetime assessed here is the same as that in AR5.

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'''Table 6.2 |''' '''Methane lifetime due to chemical losses, soil uptake and total atmospheric lifetime based on CMIP6 multi-model analysis, and bottom-up and top-down methane budget estimates in Table 5.2.''' Bottom-up and top-down methane lifetimes are calculated using the central estimates of the respective sinks for the mean 2008–2017 period in Table 5.2 together with the mean 2008–2017 global methane concentration of 1815 ppb (see Annex III) converted to methane burden using a fill-factor of 2.75 Tg/ppb from [[#Prather--2012|Prather et al. (2012)]] . Values in parentheses show the minimum and maximum range.

{| class="wikitable"
|-
| Study

| Total Chemical Lifetime (years)

| Soil Lifetime

(years)

| Total Atmospheric Lifetime

(years)

| Number of Models/ Inversions

|-
| [[#Stevenson--2020|Stevenson et al. (2020)]] <sup>a</sup>

| 8.3 (8.1–8.6) <sup>b</sup>

| 160

| 8.0 (7.7–8.2)

| 3 CMIP6 ESMs

|-
| Bottom-up (based on Table 5.2)

| 8.3 (6.2–9.8)

| 166 (102–453)

| 8.0 (6.3–10.0)

| 7 CCMI CCMs/CTMs

|-
| Top-down (based on Table 5.2)

| 9.7 (9.4–10.5)

| 135 (116–185)

| 9.1 (8.7–10.0)

| 7 inversion systems

|-
| AR6 assessed value <sup>c</sup>

| 9.7 ± 1.1

| 135 ± 44

| 9.1 ± 0.9

| Based on top-down with uncertainty estimate from AR5

|}

<sup>a</sup> Mean over 2005–2014

<sup>b</sup> Does not include lifetime due to tropospheric halogen loss

<sup>c</sup> Uncertainties indicate ±1 standard deviation

The methane perturbation lifetime ( τ <sub>pert</sub> ) is defined as the e-folding time it takes for the methane burden to decay back to its initial value after being perturbed by a change in methane emissions. Perturbation lifetime is longer than the total atmospheric lifetime of methane, as an increase in methane emissions decreases tropospheric OH, which in turn increases the lifetime and therefore the methane burden ( [[#Prather--1994|Prather, 1994]] ; [[#Fuglestvedt--1996|Fuglestvedt et al., 1996]] ; [[#Holmes--2013|Holmes et al., 2013]] ; [[#Holmes--2018|Holmes, 2018]] ). Since perturbation lifetime relates changes in emissions to changes in burden, it is used to determine the emissions metrics assessed in [[IPCC:Wg1:Chapter:Chapter-7|Chapter 7]] ( [[IPCC:Wg1:Chapter:Chapter-7#7.6|Section 7.6]] ). The perturbation lifetime is related to the atmospheric lifetime as τ <sub>pert</sub> = f * τ <sub>total</sub> where f is the feedback factor and is calculated as f = 1/(1-s), where s = δ (ln τ <sub>total</sub> )/ δ (ln[CH <sub>4</sub> ]) ( [[#Prather--2001|Prather et al., 2001]] ). Since there are no observational constraints for either τ <sub>pert</sub> or f, these quantities are derived from CCMs or ESMs. AR5 used f = 1.34 ± 0.06 based on a combination of multi-model (mostly CTMs and a few CCMs) estimates ( [[#Holmes--2013|Holmes et al., 2013]] ). A recent model study explored new aspects of methane feedbacks finding that the strength of the feedback, typically treated as a constant, varies in space and time but will in all likelihood remain within 10% over the 21st century ( [[#Holmes--2018|Holmes, 2018]] ). For this Assessment, the value of f is assessed to be 1.30 ± 0.07 based on a six-member ensemble of AerChemMIP ESMs ( [[#Thornhill--2021b|Thornhill et al., 2021b]] ). This f value is slightly smaller but within the range of the AR5 value. This results in an overall perturbation methane lifetime of 11.8 ± 1.8 years, within the range of the AR5 value of 12.4 ± 1.4 years. The methane perturbation lifetime assessed here is used in the calculation of emissions metrics in [[IPCC:Wg1:Chapter:Chapter-7#7.6|Section 7.6]] .

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