Applying GWP* to UK national GHG emissions

AHDB, with others, have been working hard to demonstrate the need for dual reporting when it comes to greenhouse gas (GHG) emissions. Using GWP* as well as the standard GWP100 will better reflect the warming impact of short-lived gases like methane. This in turn will help better identify where efforts to mitigate emissions should be focused. For instance, based on analysis of GWP*, no additional warming has arisen from UK methane emissions over the last 20 years.

Here we will explore what dual reporting could look like at a national level. The GHG National Inventory shows us UK emissions (and some sequestration) by gas and by sector. This is reported using GWP100 conversions to CO₂ equivalent[1]:

Under this reporting method (GWP100), methane would appear to be a key source of emissions for both agriculture and waste management. However, because methane breaks down quickly, it’s not necessarily a source of additional warming. To understand the additional warming we need to look at how current emissions compare to past emissions. That is where GWP* comes in.

If we apply GWP* to the methane emissions, the change in emissions over time is taken into account, and the picture changes:

Under GWP* methane emissions in the UK show a negative CO₂ warming equivalent. In other words, no additional warming arose from UK methane emissions in 2019. That is because methane emissions have fallen and old methane has broken down faster than new methane has replaced it. In fact, because UK (and agricultural) methane emissions have been steadily falling for some time, no additional warming has arisen from UK methane emissions over the last 20 years.

We currently use 2019 data as it’s the most recent non-pandemic year available, but once 2022 is available we will likely see a more neutral result for Ag methane. The graph above is just a snapshot, and GWP* works best when modelling impact over many years, not just looking at one specific year.

The table below shows the overall emissions in the UK, and the emissions attributed to agriculture. Agriculture’s share of emissions does reduce under GWP*, but because total emissions are also lower under GWP*, Agriculture’s share is still put at 8%. However, we should remember the number of limitations of the GHG National Inventory remain for this calculation. Namely, that sequestration is not included against agriculture’s silo, and neither are energy or waste mitigations.

What the GWP* calculation does highlight, however, is the need to address N₂O emissions. These can be reduced by improving practices around application of mineral and organic fertilisers, and manure management.

Table: UK greenhouse gas emissions:

Source: Data from UK Gov (DESNZ), 1990-2021 dataset. GWP* calculation AHDB.

While our focus is on the UK’s emissions, the picture may not be the same worldwide. Under GWP* stable emissions cause no additional warming, but increased emissions have a stronger warming impact than shown by GWP100. In the UK, most of our herd expansion was in the 20th century. GWP* considers changes in the last 20 years, a period where UK cattle and sheep numbers have reduced overall, thereby reducing our agricultural methane emissions. In contrast, developing countries that are expanding their herds would find using GWP* increases the significance of methane in their emissions.

Dual reporting using GWP* as well as GWP100 is a useful tool for businesses to understand the warming potential from their activities. As shown here, it can highlight the impact of emissions and where action should be targeted.

[1] Note that data is from Final UK greenhouse gas emissions national statistics: 1990 to 2021 - GOV.UK (www.gov.uk), published February 2023. However, 2019 is used as the reference year as this is more representative of a ‘normal’ year than those during the pandemic.