Not all greenhouse gases are equally potent at trapping heat. Global Warming Potential (GWP) is a standardised measure that allows different greenhouse gases to be compared and aggregated into a common unit for climate accounting.
GWP measures the total amount of energy a gas will absorb from infrared radiation over a specified time period (typically 100 years), relative to CO$_2$.
$$GWP_{100} = \frac{\text{Infrared energy absorbed by 1 kg of gas over 100 years}}{\text{Infrared energy absorbed by 1 kg of CO}_2 \text{ over 100 years}}$$
CO$_2$ is the reference gas: GWP$_{100}$ = 1
A gas with GWP$_{100}$ = 100 causes 100 times as much warming as the same mass of CO$_2$ over a 100-year period.
Two properties determine a gas’s GWP:
| Gas | Atmospheric Lifetime | GWP$_{100}$ |
|---|---|---|
| CO$_2$ | ~100–300 years (complex) | 1 |
| CH$_4$ (methane) | ~12 years | ~28–36 |
| N$_2$O (nitrous oxide) | ~114 years | ~265–273 |
| HFC-134a (refrigerant) | ~14 years | ~1,300 |
| SF$_6$ (sulfur hexafluoride) | ~3,200 years | ~23,500 |
| CF$_4$ (perfluorocarbon) | ~50,000 years | ~7,400 |
Methane has a high GWP${20}$ (~80) but lower GWP${100}$ (~28–36) because:
- It is very potent at absorbing IR radiation
- But it breaks down in the atmosphere after ~12 years (oxidised to CO$_2$ and water)
- Over 20 years, its cumulative warming effect per kg is ~80× CO$_2$
- Over 100 years, because it has mostly degraded, the ratio falls to ~28–36
SF$_6$ has an extremely high GWP (~23,500) because:
- Very high IR absorption capacity
- Essentially indestructible in the atmosphere (~3,200 year lifetime)
- Very small amounts cause large long-term warming
GWP values are used to convert all greenhouse gas emissions to a common unit:
$$\text{CO}2\text{e} = \text{mass of gas} \times GWP{100}$$
Example:
- 1 tonne of CH$_4$ = 1 × 28 = 28 tonnes CO$_2$e
- 1 tonne of N$_2$O = 1 × 273 = 273 tonnes CO$_2$e
National greenhouse inventories and carbon pricing schemes use CO$_2$e to aggregate all GHG emissions.
Example calculation: A farm emits 500 kg of CH$_4$ and 200 kg of N$_2$O.
- CH$_4$ contribution: 500 × 28 = 14,000 kg CO$_2$e
- N$_2$O contribution: 200 × 273 = 54,600 kg CO$_2$e
- Total: 68,600 kg CO$_2$e
Understanding GWP has practical policy implications:
- Methane reductions have a larger near-term benefit (GWP$_{20}$ ~80) than CO$_2$ reductions of the same mass
- Targeting agricultural methane and N$_2$O provides rapid climate benefits alongside CO$_2$ reductions
- F-gas phase-outs (under the Kigali Amendment to the Montreal Protocol) are highly cost-effective per tonne CO$_2$e avoided
- Fugitive methane emissions from natural gas infrastructure should be minimised — even small leaks can eliminate the climate advantage of switching from coal to gas
EXAM TIP: VCAA may ask you to explain why different greenhouse gases have different warming effects. Always address BOTH factors: IR absorption capacity and atmospheric lifetime. Do not assume that more abundant gases (CO$_2$) have the highest GWP — they do not; they are just emitted in much larger quantities.
