Global warming potential

The global warming potential (GWP) depends on both the efficiency of the molecule as a greenhouse gas and its atmospheric lifetime. GWP is measured relative to the same mass of CO2 and evaluated for a specific timescale. Thus, if a gas has a high GWP on a short time scale (say 20 years) but has only a short lifetime, it will have a large GWP on a 20 year scale but a small one on a 100 year scale. Conversely, if a molecule has a longer atmospheric lifetime than CO2 its GWP will increase with the timescale considered.

Carbon dioxide has a variable atmospheric lifetime, and cannot be specified precisely. Recent work indicates that recovery from a large input of atmospheric CO2 from burning fossil fuels will result in an effective lifetime of tens of thousands of years.Carbon dioxide is defined to have a GWP of 1 over all time periods.

Methane has an atmospheric lifetime of 12 ± 3 years and a GWP of 72 over 20 years, 25 over 100 years and 7.6 over 500 years. The decrease in GWP at longer times is becausemethane is degraded to water and CO2 through chemical reactions in the atmosphere.

Examples of the atmospheric lifetime and GWP relative to CO2 for several greenhouse gases are given in the following table:

Atmospheric lifetime and GWP relative to CO2 at different time horizon for various greenhouse gases.
Gas name Chemical
formula
Lifetime
(years)
Global warming potential (GWP) for given time horizon
20-yr 100-yr 500-yr
Carbon dioxide CO2 See above 1 1 1
Methane CH4 12 72 25 7.6
Nitrous oxide N2O 114 289 298 153
CFC-12 CCl2F2 100 11 000 10 900 5 200
HCFC-22 CHClF2 12 5 160 1 810 549
Tetrafluoromethane CF4 50 000 5 210 7 390 11 200
Hexafluoroethane C2F6 10 000 8 630 12 200 18 200
Sulphur hexafluoride SF6 3 200 16 300 22 800 32 600
Nitrogen trifluoride NF3 740 12 300 17 200 20 700