Global warming potentials (GWPs) are values that allow direct comparison of the impact of different greenhouse gases in the atmosphere by comparing how much energy one tonne of a gas will absorb compared to one tonne of carbon dioxide.
Updates to these values are periodically undertaken by Intergovernmental Panel on Climate Change (IPCC) to take into account an improved scientific understanding of the physical properties of these gases. The
Parties to the Paris Agreement, which includes Australia, adopted the GWPs provided by the in its
Fifth Assessment Report (AR5) for the purpose of international greenhouse gas inventory reporting. Previously used GWPs were set in the
Second Assessment Report (AR2) and
Fourth Assessment Report (AR4).
From 1 July 2020, amendments to the
National Greenhouse and Energy Reporting Regulations 2008 and the
National Greenhouse and Energy Reporting (Measurement) Determination 2008 for updates to GWPs mean that the for all greenhouse gases reorted under NGER except CO2 have changed. This means that for the 2020–21 reporting year, several emissions factors used for calculating non-CO2 gases used in National Greenhouse and Energy Reporting (NGER) have been revised.
The NGER legislation is updated with these GWP changes to maintain the accuracy and comparability of Australia's national emissions estimates with the global community. This is the second time GWPs have been changed since NGER was established. The below table shows a summary of the GWPs for different greenhouse gases that apply to specific NGER reporting years.
Data reported under NGER will not be changed to account for the GWP changes because they reflect the requirements of the NGER legislation that were in force at the time. Therefore, in order to compare data that was reported using different GWPs, you will need to adjust one of the years to match the other. This is needed to ensure consistency through the time series and enables identification of real changes in emissions rather than changes due to varying emissions estimation methods.
The agency publishes NGER data on our website by 28 February each year, including comparisons with data from previous years. Please note that data which is published by the Clean Energy Regulator that compares emissions between years has already been adjusted to accommodate changes in GWPs.
If you wish to convert emissions from one GWP to another, you first divide the tonnes of carbon dioxide equivalent emissions by the GWP of the gas that was used for calculating those emissions. This reverts the tonnes of carbon dioxide equivalent emissions back to tonnes of greenhouse gases. The greenhouse gas is then multiplied by the new GWP.
If a facility reported 100,000 t CO2-e of methane emissions in 2019–20 and you want to compare it to 120,000 t CO2-e of methane emissions reported by that same facility in 2020–21. Without converting them to the same GWP, at first glance it looks like there was a 20% increase in methane emissions at this facility.
From the table above, the GWP used for methane in 2019–20 is 25. Therefore, 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 the table above, the GWP used for methane in 2020–21 is 28. Therefore, 4,000 tonnes of methane multiplied by 28 results in 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.
explanatory statement to the National Greenhouse and Energy Reporting Amendment (2020 Measures No.1) Regulations 2020 and the
explanatory statement to the National Greenhouse and Energy Reporting (Measurement) Amendment (2020 Update) Determination 2020 for more information about the legislative changes that were made in 2020 and apply from the 2020–21 NGER reporting period online.
Contact us at
firstname.lastname@example.org if you need any assistance with converting emissions to apply different GWPs.
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