Global warming potentials (GWPs) are used to convert emission contributions from specific greenhouse gases, such as methane, into units of carbon dioxide equivalent in order to facilitate the comparison of their environmental impact. Each greenhouse gas has its own global warming potential; for example, the GWP of methane is 25 and the GWP of nitrous oxide is 298. The source of the GWP you apply should be in line with that required by the particular standard or scheme you are reporting to, so be sure that you have chosen GWPs that are in compliance with your specific reporting goals.
The IPCC assessment reports – in addition to publishing GWPs for the frequently used carbon dioxide, methane and nitrous oxide – contain a large number of GWPs for various other gases, including Montreal Protocol substances, perfluorocarbons (PFCs), hydrofluorocarbons (HFCs), and fluorinated ethers. Standard greenhouse gases, such as methane or HFC-125, have a single global warming potential. However, many combinations of PFCs and HFCs, colloquially known as ‘refrigerant gases’, are actually mixtures of several different gases in set proportions. While the IPCC reports don’t publish GWPs for blends, Defra and DECC present a selection of common refrigerant blends in their “Guidelines to GHG Conversion Factors for Company Reporting”. This year’s guidance was recently released (access the document here) and Annex 5 contains the tables you’ll need.
However, it is possible that the GWP for a particular refrigerant gas mixture that you require might not be among those listed by Defra/DECC – don’t worry. The global warming potential of a blended refrigerant gas can be easily calculated by using the blend composition and the known GWPs for each gas contributing to the blend. An example of this calculation is shown below using R410A, a common refrigerant in air conditioning systems that has replaced the phased-out R22 in residences and businesses in many places, including Europe and the USA. The GWPs shown are from IPCC Fourth Assessment Report (AR4), as opposed to those published in the Second Assessment Report (SAR).
- HFC-32: 50%
- HFC-125: 50%
- HFC-32: 675
- HFC-125: 3,500
Refrigerant Mix GWP Calculation
- (675 * 50%) + (3,500 * 50%) = 2,087.5
- R410A: 2,087.5
As is evident, this calculation is not difficult, but it does require that you know both the composition of the particular refrigerant gas and the GWPs of its components.
 Using IPCC Fourth Assessment Report GWPs
 The Montreal Protocol, an international treaty enacted January 1, 1989, caused the phase-out of substances believed to be responsible for ozone depletion including various CFCs, HCFCs, halons, carbon tetrachloride, methyl bromide and methyl chloroform