The importance of using multiple metrics for calculating the climate impact of foods

Wednesday, 13 September 2023

Calculating greenhouse gas (GHG) emissions is becoming increasingly prevalent in agriculture. It has been acknowledged for some time that the standard methods for these calculations over-simplify important nuances in agricultural systems, particularly livestock. In July 2023, a new study by authors across eight UK institutions further highlighted the need to consider multiple metrics when assessing the climate impact of foods. As an evidence-based organisation, AHDB welcomes this development in scientific and academic debate of our industry.

Overall, the study highlights the necessity of reporting the climate impacts of food under multiple measures, over multiple time horizons, and on individual gases as well as in CO2 equivalents. This complexity is needed to allow decision makers to be fully informed. Additionally, more consideration needs to be given to broader sustainability issues e.g. human health, agricultural resilience, nutritional complexities, and global food security.

Life Cycle Assessments (LCAs) assess the environmental impact of a product, including its associated GHG emissions. These assessments often model the impact based on broad average figures for different production systems. Converting non-carbon dioxide (CO2) GHGs into CO2 equivalents (CO2-eq) using GWP100 factors, which do not well represent the impact of short-lived gases like methane (CH4).

McAuliffe et al’s study assessed the environmental impact of a British pasture-based beef system using a number of different emissions factors, scenarios and global warming potential (GWP) metrics. This varied assessment approach included a range of values for CH4 production and nitrous oxide (N2O) losses from inorganic and organic fertilisers under several emissions factor scenarios. The study went on to analyse the impact that different metrics for calculating GHG’s had on the carbon footprint of products. It focused on three different metrics for converting gases to CO2 equivalents:

  • GWP100 – Global Warming Potential averaged over 100 years, the most commonly used measure.
  • GTP100 – Global Temperature change Potential over 100 years
  • GWP* - Global Warming Potential, adjusted to account for methane’s short lifespan.

The study showed that the choice of emissions factors used and the GWP metrics can both impact on the results. Under GWP100, the average emissions intensity from the scenarios was 23.1 kg CO2-eq per kg liveweight (LW), and the highest results was 29.8 kg CO2-eq/kg LW. Under GTP100, the average was 12.3 kg CO2-eq/kg LW with a maximum of 23.1 kg CO2-eq/kg LW. Additionally, under GWP100, CH4 made up 39.9% of total emissions, whilst under GTP100 it was considerably lower at 9.14% (using IPCC default factors). This means that depending on the GWP metric used, either CH4 or N2O could be flagged as the key gas to work on reducing.

For GWP*, the study’s models reflected the stronger impact CH4 has in its first ~20 years, followed by a much lower impact after this point. Its models indicated that after the first 20 years of CH4 emissions, the longer-lived N2O emissions become the dominant GHG in terms of total CO2-w.e. emissions associated with the lowland permanent pasture-based beef system. Following this, the authors acknowledge that GWP* is a more accurate way of measuring the warming impact of CH4, as opposed the currently internationally accepted and used GWP100.

Altogether, this analysis demonstrates the importance of using multiple metrics for calculating the climate impact of foods, as using just one metric doesn’t give the whole picture.

What is GWP*? Click here for our explainer