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\(_2\).
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:
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.
