Values
Carbon dioxide has a GWP of exactly 1 (since it is the baseline unit to which all other greenhouse gases are compared).
| GWP values and lifetimes from 2007 IPCC AR4 p212 (2001 IPCC TAR in parentheses) |
Lifetime (years) | GWP time horizon | ||
|---|---|---|---|---|
|
|
|
|
||
| Methane | 12 (12) | 72 (62) | 25 (23) | 7.6 (7) |
| Nitrous oxide | 114 (114) | 289 (275) | 298 (296) | 153 (156) |
| HFC-23 (hydrofluorocarbon) | 270 (260) | 12,000 (9400) | 14,800 (12,000) | 12,200 (10,000) |
| HFC-134a (hydrofluorocarbon) | 14 (13.8) | 3,830 (3,300) | 1,430 (1,300) | 435 (400) |
| Sulfur hexafluoride | 3200 (3,200) | 16,300 (15,100) | 22,800 (22,200) | 32,600 (32,400) |
Although water vapour has a significant influence with regard to absorbing infrared radiation (which is the green house effect; see greenhouse gas), its GWP is not calculated. Its concentration in the atmosphere mainly depends on air temperature. There is no possibility to directly influence atmospheric water vapour concentration.
The values given in the table assume the same mass of compound is released. This must not be confused with chemical reactions in which masses change from reactants to products. For instance, burning methane to carbon dioxide would indeed reduce the global warming impact, but by a smaller factor than the one given in the table because the mass of methane burning is lesser than the mass of carbon dioxide released (ratio 1:2.75). If you started with 1 tonne of methane which has a GWP of 25, after combustion you have 2.75 tonnes of CO2, each tonne of which has a GWP of 1. The effect of this burning is to reduce the Global warming effect of the gas released in the ratio 25:2.75 or by about 9.
Read more about this topic: Global-warming Potential
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