1. Methane Global Warming Potential (GWP)
accounting for coupled Stratosphere-Troposphere chemistry
Christopher D. Holmes (Florida State University)
Sebastian Eastham (Harvard)
100-yr GWP
GWP is a convenient, familiar metric for integrating all of the climate effects of CH4.
CH4
CO2
time
(IPCC 2013)

2. Methane GWP: direct and indirect warming
Effects included
GWP 100-y
CH4 direct
13.5
+ OH feedback 19
+ trop. O3 25
+ strat. H2O
28 (IPCC 2013)
+ strat. O3 and H2O
31.7? (one model only)
(Holmes et al., 2013)
+ Carbon cycle feedback
+6 (IPCC 2013)
Stratosphere
H2O
HOx O3
CO2 CH4
H2O
CH4
CH4 OH
The combined chemical effects have never been calculated in a consistent framework
UCX chemistry in GEOS-Chem enables that
SOLAR O3
INFRARED
Troposphere
CO2
CH4
Temperature

3. Experimental design O3 column variability O3 column, 2004-2010 mean
UCX (Stratosphere-troposphere) chemistry v (Eastham et al., 2014) (SOA for comparison)
GEOS-5 4°x5°
CH4 surface concentration (NOAA), perturb +5%
Updates:
H2 chemistry (JPL, NIST)
H2O prescribed up to tropopause/cold point
Detailed O3 P/L diagnostics
O3 column, mean
Bias: 0-20 DU (≲7%)
Polar O3 depletion (good variability)
QBO (magnitude good, shifted phase)
Obs: McPeters et al., 2013

4. Impact of +5% CH4 perturbation on O3
O3 % Difference
+0.1% in stratosphere
+0.2-1% in UTLS
+1% in troposphere
Ralph Cicerone once suggested that polar O3 depletion might be controlled by dumping Ethane in the stratosphere.
Why?
Troposphere & UTLS:
↑CH4 ⇒ ↑ P(Ox)
Upper Stratosphere:
↑CH4 ⇒ ↓[Cl]/[HCl] ⇒ ↓L(Ox)
CH4 increases stratospheric O3
This has been missing from CH4 GWP

5. Climate effects of CH4 with stratosphere & troposphere chemistry
UCX
Trop. only
CH4 lifetime, yr
8.9 (trop. OH)
8.8 (trop. OH)
7.6 (total)
7.5 (total)
𝝉 𝐂 𝐇 𝟒 feedback
1.43
1.37
∆O3, DU
0.76
0.32
troposphere
0.38
stratosphere --
∆H2O, %
1.1 (upper strat)
assumed
GWP-100 yr*
33.4
28.6
Literature
11.2 ± 1.3
(MCF constraint,
Prather et al., 2012)
9.1 ± 0.9
1.34 ± 0.06 (Holmes et al., 2013, IPCC 2013)
0.97
0.42
(Oslo CTM3; Holmes et al., 2013)
0.55
(RF efficiencies from Holmes et al., 2013)
EPA still uses 25
The NH/SH kMCF ratio is 1.19 (geographic)
28 (IPCC 2013)
*CC-feedbacks add 6 more (IPCC 2013)
Accounting for stratospheric chemistry increases the climate warming of CH4
through stratospheric O3 & lifetime feedback
UKCA & CTM3 are currently replicating this experiment (A. Archibald, A.Søvde)

6. GWP calculation
AGWP= 𝑅𝐹 𝑑𝑡 =𝛿 𝜏 CH 4 𝑓 𝐹 CH 4 + 𝑑 H 2 O strat 𝑑[ CH 4 𝐹 strat− H 2 O + 𝑑 O 3 trop 𝑑[ CH 4 𝐹 trop− O 3 + 𝑑 O 3 strat 𝑑[ CH 4 𝐹 strat− H 2 O
CH4
CO2 𝛿
Initial CH4 perturbation
0.364 ppb Tg(CH4)-1
𝜏 CH 4
Lifetime
9.1 yr 𝑓
Lifetime feedback
1.37
𝑑 O 3 trop 𝑑[ CH 4
Tropospheric O3 response
4.2 DU ppm(CH4)-1
𝑑 O 3 strat 𝑑[ CH 4
Stratospheric O3 response
𝑑 H 2 O strat 𝑑[ CH 4 𝐹 strat− H 2 O
Stratospheric H2O response
0.15 𝐹 CH 4 (Myhre et al., 2007)
𝐹 CH 4
RF efficiency of CH4
370 mW m-2 ppm(CH4)-1
𝐹 trop− O 3
RF efficiency of tropospheric O3
33.4 mW m-2 DU-1
𝐹 strat− O 3
RF efficiency of stratospheric O3
10.2 mW m-2 DU-1
AGWP (CH4, 100 yr)
2.91 mW yr m-2
AGWP (CO2, 100 yr)
0.087 mW yr m-2
time

7. O3 Evaluation: interannual variability
Obs: McPeters et al., 2013

8. H2O Evaluation GEOS-Chem: 2005-2008 Observations: 1998-2008
Interannual variability (QBO) degrades comparison
UTLS is too moist (GEOS-5)
UTLS is too moist (GEOS-5)
SPARC Data Initiative, 2017

9. GEOS-Chem UCX
Mesosphere too dry
Equator-pole gradients too weak, while LS-US gradient OK
Meridional mixing too fast?
Lower stratosphere too wet
SPARC 2017 fig
Extra-tropical UTLS too wet
GEOS-5 H2O
GEOS-5 matches observations because it assimilates them

10. Diagnosed Ox Production and Loss
NOx, ClOx, BrOx recycling
Ox Definition
Simple Ox,s = O3 + O
Extended Ox = O3 + O + NOx, ClOx, BrOx reservoirs

11. H2 chemical mechanism Production Notes O(1D) + H2O → H2 + O2
O(1D) + CH4 → H2 + CH2O
H + HO2 → H2 + O2
CH2O + hv → H2O + CO
CHOCHO + hv → H2 + 2CO
IAP, HC5OO + hv → H2 + products
Loss
OH + H2 → H2O + H
O(1D) + H2 → H + OH
O + H2 → OH + H T < 298 K rates from theory; not reviewed by JPL
Cl + H2 → H + HCl
Additional HOx change:
OH + CO (+M) → H + CO2 causes 5% O3 reduction in upper stratosphere, because 20% of H is lost through
H + O3 → OH + O2. Minor HO2 product when M=O2 is not in GC
Not included
H2O + hv → OH + H ~10% of P(HOx) in upper stratosphere; McCormack et al., 2008