Global warming potential (GWP) values allow us to compare the impact of different greenhouse gases in the atmosphere.
GWP values have changed over time. This affects how you interpret NGER data from different years.
What is a global warming potential (GWP)?
We use carbon dioxide as a benchmark to compare how different greenhouse gases trap heat in the atmosphere.
The heat-trapping ability of a gas measured against carbon dioxide is its GWP.
For example, methane has a GWP of 28, meaning it is 28 times more potent than carbon dioxide. Emitting one tonne of methane into the atmosphere is equivalent to emitting 28 tonnes of carbon dioxide.
Updates to GWP values
The Intergovernmental Panel on Climate Change (IPCC) updates GWP values based on new scientific knowledge of gases.
The Parties to the Paris Agreement, which includes Australia, currently use the GWP values provided in the Fifth Assessment Report (AR5) for international greenhouse gas reporting. Previously used GWPs were set in the Second Assessment Report (AR2) and Fourth Assessment Report (AR4).
Summary of updates to GWP values
Table 1 provides a summary of the GWPs for different greenhouse gases that apply to specific NGER reporting years.
Table 1: Summary of the GWPs for different greenhouse gases by year.
| Greenhouse gas | AR2 GWPs (2008–09 to 2014–15) | AR4 GWPs (2015–16 to 2019–20) | AR5 GWPs (2020–21 onwards) | 2020–21 GWPs / 2019–20 GWPs |
|---|---|---|---|---|
| Carbon dioxide | 1 | 1 | 1 | 0% |
| Methane | 21 | 25 | 28 | 12% |
| Nitrous oxide | 310 | 298 | 265 | –11% |
| Perfluoromethane (tetrafluoromethane) | 6,500 | 7,390 | 6,630 | –10% |
| Perfluoroethane (hexafluoroethane) | 9,200 | 12,200 | 11,100 | –9% |
| Sulphur hexafluoride | 23,900 | 22,800 | 23,500 | 3% |
| Hydrofluorocarbons(HFCs) | dependent on HFC type | dependent on HFC type | dependent on HFC type | dependent on HFC type |
Comparing NGER data across years
Data reported under NGER will not be adjusted to account for changes in GWP values because they reflect the requirements of the NGER legislation in force at the time.
To compare data reported using different GWPs, you will need to adjust one of the years to match the other. This allows you to identify actual changes in emissions, rather than changes caused by different methods used for estimating emissions.
Converting historical emissions data to the current GWPs
You may want to convert historical emissions data to the current GWP for trend analysis.
To convert emissions from one GWP to another, divide the tonnes of carbon dioxide equivalent emissions by the GWP of the gas used for calculating those emissions.
This changes the tonnes of carbon dioxide equivalent emissions back to tonnes of greenhouse gases. The greenhouse gas is then multiplied by the new GWP.
If you need help converting emissions, you can also contact us.
Example on how to convert emissions
A facility reported 100,000 t CO2-e of methane emissions in 2019–20.
The same facility reported 120,000 t CO2-e of methane emissions in 2020–21.
Without converting them to the same GWP, you can see there was a 20% increase in methane emissions at this facility.
Converting emissions
From table 1, the GWP used for methane in 2019–20 is 25, so the first step is to divide 100,000 by 25.
This has converted 100,000 t CO2-e of methane into 4,000 tonnes of methane.
The second step is to multiply the tonnes of methane by the GWP for 2020–21.
From table 1, the GWP used for methane in 2020–21 is 28, so you multiply 4,000 by 28.
This has converted 4000 tonnes of methane into 112,000 t CO2-e of methane.
When you compare this converted value of 112,000 t CO2-e of methane, instead of the original 100,000 t CO2-e of methane, there was actually a 7.1% increase between 2019–20 and 2020–21.
The original 20% increase was overstated due to the effect of the changing GWPs.
Legislative changes to GWPs
NGER legislation was amended in 2020 to update GWP values. Read the explanatory statement to the: