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.
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 OH- to 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
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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