Methane - sources and sinks

 

Global warming potential GWP of methane

We've seen how methane is a far worse greenhouse gas than CO2. We don't want it in our atmosphere!

It has a GWP of 86 over 20 years, 34 over 100 years and 7.6 over 500 years. The decrease in GWP at longer times is because methane is degraded to water and CO2 through chemical reactions in the atmosphere.

(GWP = Greenhouse Warming potential)

Lifetime of methane in the atmosphere

The atmospheric lifetime of CH4 is 8.4 years. 

Attributing individual lifetimes to the different components of CH4 loss results in: 

9.6 years for loss due to tropospheric OH-

120 years for stratospheric loss, and 

160 years for the soil sink 

(i.e., 1/8.4 yr = 1/9.6 yr + 1/120 yr + 1/160 yr).   IPPC report 2001

Other sources claim 12 +or- 3 years.

Sinks

Methane is removed from the atmosphere by a variety of processes, frequently called "sinks". The dominant sink is oxidation by chemical reaction with hydroxyl radicals OH-. Methane reacts with OHto produce CH3- and H2O in the tropospheric layer of the atmosphere. Stratospheric oxidation plays a minor role in removing methane from the atmosphere.

Similar to tropospheric oxidation, minor amounts of methane are destroyed by reacting with OH- in the stratosphere. These two OH- reactions account for almost 90% of methane removals (IPCC, 2001c). 

Microbial uptake of methane in soils contribute 7% = 38 Mt/y

Methane’s reaction with chlorine (Cl) atoms in the marine boundary layer, 2% of total methane removal.              Source: US EPA

OH- + CH4  CH3- + H2O

 

 

This graph shows all emission as CO2 equivalent. Methane is taken as 21 times CO2 for greenhouse warming potential. A coal mine will give off both CO2 and CH4 (methane).

Underground coal mines are increasingly using CH4 as a fuel. Open cut mines tend to have less CH4 emissions as much of the coal has dried out long ago, and CH4 held to the coal has escaped in the process.

 

Sources and sinks of methane 

Table 4.2: Estimates of the global methane budget (in Tg(CH4)/yr) from different sources compared with the values adopted for this report (TAR).
Reference: Fung et al. Hein et al. Lelieveldet al. Houwelinget al. Mosier et al. Olivier et al. Cao et al. SAR TARa
  (1991) (1997) (1998) (1999) (1998a) (1999) (1998)    
Base year: 1980s - 1992 - 1994 1990 - 1980s 1998
Natural sources
 
 
 
 
 
 
 
 
 
Wetlands
115
237
225c
145
 
 
92
 
 
Termites
20
-
20
20
 
 
 
 
 
Ocean
10
-
15
15
 
 
 
 
 
Hydrates
5
-
10
-
 
 
 
 
 
Anthropogenic sources
 
 
 
 
 
 
 
 
 
Energy
75
97
110
89
 
109
 
 
 
Landfills
40
35
40
73
 
36
 
 
 
Ruminants
80
90b
115
93
80
93b
 
 
 
Waste treatment
-
b
25
-
14
b
 
 
 
Rice agriculture
100
88
c
-
25-54
60
53
 
 
Biomass burning
55
40
40
40
34
23
 
 
 
Other
-
-
-
20
15
 
 
 
 
Total source
500
587
600
 
 
 
 
597
598
Imbalance (trend)
 
 
 
 
 
 
 
+37
+22
Sinks
 
 
 
 
 
 
 
 
 
Soils
10
-
30
30
44
 
 
30
30
Tropospheric OH
450
489
510
 
 
 
 
490
506
Stratospheric loss
-
46
40
 
 
 
 
40
40
Total sink
460
535
580
 
 
 
 
560
576
a TAR budget based on 1,745 ppb, 2.78 Tg/ppb, lifetime of 8.4 yr, and an imbalance of +8 ppb/yr.
b Waste treatment included under ruminants.
c Rice included under wetlands.

TAR = IPCC Third Assessment Report - Climate Change 2001

Tg    = Teragrams = Mega tonnes = Total burden  

Burden = The total mass of a gaseous substance of concern in the atmosphere.

Source IPPC report

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