What do national greenhouse gas figures really tell us?

The National Atmospheric Emissions Inventory (also known as the National Inventory) is a record of greenhouse gas emissions in the UK from 1990 to present day, as well as some sequestration. It is used to monitor and evaluate the country’s progress towards net zero, as part of both national and international targets. It reports on both individual gases and emissions in terms of carbon dioxide equivalent (CO2e), using the Global Warming Potential 100 year methodology otherwise known as GWP100. It also splits these emissions into sectors e.g. manufacturing, transport, agriculture, etc. When sources say Agriculture produces around 10% of the UK’s greenhouse gas emissions, that figure is from the National Inventory.

Some points to note when looking at and using National Inventory data:

• The National Inventory reports on the UK’s production and manufacturing. If a product is imported to the UK, it won’t be included in the UK inventory, but would be on the national inventory for whichever country produced the product.
• We often don’t know the actual emissions, so averages, known as “factors,” are used. For example, emissions from cows will be calculated from the number of animals multiplied by the perceived emissions for an average cow.
• Farming is unique in that activities on farm can often sit across 4 different silos in the national inventory. In particular, sequestration and renewables do not offset the emissions from agriculture in the national inventory – they will be credited to other silos.
• All results are converted to carbon dioxide equivalent using GWP100.

Not all activities on farm are included in the agriculture category. The agriculture category is primarily made up of greenhouse gas emissions from enteric fermentation in an animal, animal manures, and the use of fertiliser, plus a few smaller categories.

Some farm activities will appear under other categories:

1. Land Use, Land Use Change and Forestry (LULUCF) - covers increases and losses of carbon from the land, including both converting land (e.g. cropland to forest) and maintaining land use (e.g. grassland remaining as grassland).
2. Energy - electricity taken from the grid for the farm business will be recorded under ‘Energy Supply’, as will renewable energy generated and used on a farm. 
3. Waste – Emissions from farm waste that goes into the municipal waste system (e.g. landfill) or sewage system would go here. Additionally, mitigation such as putting waste in an anaerobic digestion would also be captured here.

The table below gives some examples of farm activity and which category they would sit in:

The national inventory reports on both individual gases and emissions in terms of carbon dioxide equivalent (CO₂e), using GWP100 methodology. GWP100 is the internationally accepted standard, and is used by countries for their emissions accounting, as agreed at by IPCC of the United Nations. However, it is also recognised that it can overstate the impact of methane on Global Warming, as it is a short lived gas which breaks down after around 12 years. CO₂ and Nitrous Oxide are long lived gases and live in our atmosphere for more than 100 years, hence why GWP100 has been used to date.

The fact that GWP100 is used, is particularly significant for the agriculture sector, as its the source of about 50% of the country’s methane emissions.

GWP* is an alternative measure which by separating out methane and calculating its impact over twelve years, better models the warming impact of methane over time, accounting for that shorter lifespan. Click here to read more about GWP*.

If we used GWP* to report methane for the national inventory, then agriculture’s contribution would be 8% instead of the 10% measured under GWP100.

One further limitation on the data is that it is strictly domestic emissions. This means that the UK’s share of emissions from international aviation and shipping aren’t included. They do record these figures as separate ‘memo items’ that don’t go into the main total. If they were included, it would increase total UK emissions (2019) by a further 20% , and bring Agriculture’s share down to 9% based on GWP100

Similarly, the emissions from manufacturing are attributed to the country of manufacture. If those products are then exported, the destination country doesn’t take on the burden of the emissions. As mentioned above, emissions from applying mineral fertiliser come under ‘agriculture’. If it’s made in the UK, the emissions from making that fertiliser will go under ‘product manufacturing’ – but if it was imported, those emissions won’t be included in the UK inventory at all. This is different to when a farm does a carbon footprint, which will include the emissions from making the fertiliser regardless of origin (as part of the scope 3 emissions).

Lastly, the data is generally calculated by taking a standardised average for emissions from an activity (an “emissions factor”) and scaling it up e.g. emissions from 1 cow ruminating x number of cows. This is necessary as we are lacking baselining data in order to calculate things more accurately, but it comes with limitations. Emission factors can be based on broad averages, sometimes international ones, that don’t always reflect reality in present-day UK.

AHDB is advocating for baselining that would lead to improved emissions factors that reflect real UK farming activity.