Global warming potentials: AR5 or AR6?

What AR5 and AR6 are

A global warming potential (GWP) says how much warming a kilogram of a gas causes over 100 years compared with a kilogram of carbon dioxide. It is how methane, nitrous oxide and refrigerants become CO2e. The values come from the Intergovernmental Panel on Climate Change, and each of its assessment reports refines them:

• AR4, the Fourth Assessment Report (2007)
• AR5, the Fifth Assessment Report (2014)
• AR6, the Sixth Assessment Report, whose science volume was published in 2021

Between reports, the science on how long each gas lasts in the air and how it interacts with others improves, so the numbers move. AR6 also splits methane in two: fossil methane gets a slightly higher value because, once it breaks down, it leaves extra CO2 behind.

The values side by side

The AR4 and AR5 columns are the values in the UK government sets Carbon Recycling holds. The AR6 column is from the GHG Protocol's published GWP table. The UK sets moved from AR4 to AR5 with the 2023 release, which is why methane went up from 25 to 28 and nitrous oxide down from 298 to 265 that year.

What the UK factors use today

The DESNZ methodology paper for the 2026 factors sets it out:

• Most factors, including fuels, electricity, transport and water, use AR5 values, consistent with the UK greenhouse gas inventory and with reporting to the United Nations Framework Convention on Climate Change. DESNZ notes the IPCC has a newer version but its methods "have not yet been officially accepted" for use under the convention.
• Refrigerants use AR5 where the IPCC published an AR5 value, and AR6 (or AR4, or the EU F gas regulation) for the few gases AR5 doesn't cover.
• Bioenergy factors, for wood fuel and biofuels such as HVO, are still on AR4 values.
• Hotel stays are a mix, because the source data arrived already in CO2e.

So "AR5" is the honest one-word answer for a UK footprint built on government factors, with those small exceptions.

Does it matter for a small business?

Mostly not. CO2's GWP is 1 in every report, and for a typical office, shop or service business the footprint is mostly CO2 from gas, electricity and vehicle fuel. The methane and nitrous oxide inside those factors are small, so switching basis barely moves the total.

Refrigerants are the exception, because a leak is the gas itself. Take an engineer topping up 3 kg of R32:

• AR5, as in the UK 2026 factors: 3 kg × 677 = 2.03 tonnes CO2e
• AR6: 3 kg × 771 = 2.31 tonnes CO2e, about 14% more

Either figure is defensible if you say which you used. Our post on refrigerant leaks covers the data you need.

What to do

1. Use the government factors as published. PPN 006's technical standard asks for CO2e calculated with the UK government's conversion factors, and SECR reports are normally built on them too. That puts you on AR5.
2. Say so in your method note. One line is enough: "CO2e uses the DESNZ 2026 conversion factors, which apply IPCC AR5 100-year global warming potentials."
3. If a customer asks for AR6, answer the question they are really asking. Tell them your basis and that the difference for your footprint is small. If refrigerants are a large share, you can show that line both ways.
4. Keep every year on the same basis. If DESNZ moves to AR6 in a future set and the change is material for you, treat it as a method change and follow our guide to restating your baseline.

How Carbon Recycling helps

Carbon Recycling uses the DESNZ factors exactly as published for each reporting year, so your footprint is on the same GWP basis as the UK inventory and the factors PPN 006 asks for. Every line in your report records the factor's ID, year and value, which makes the method note simple to write and easy for a buyer to check. The guide to the 2026 conversion factors covers what else changed this year.