Due to the large share of non-CO2 GHGs in emissions from livestock production, the choice of GHG metric used to compare emissions of different GHGs is crucial, both in order to assess the aggregate contribution of the livestock sector to climate change and for highlighting hot-spots in the animal food chain where emission reductions can be most cost-effectively made.
This paper discusses the rationale for using different metrics in different policy contexts, demonstrates the difference in short and long term consequences of applying various metrics in carbon footprint accounting of animal products, and discusses policy implications of the choice between metrics for the role of the livestock sector in climate mitigation.
The livestock sector is estimated to account for 15% of global greenhouse gas (GHG) emissions, 80% of which originate from ruminant animal systems due to high emissions of methane (CH4) from enteric fermentation and manure management. However, recent analyses have argued that the carbon footprint (CF) of ruminant meat and dairy products are substantially reduced if one adopts alternative metrics for comparing emissions of GHGs—e.g., the 100 year global temperature change potential (GTP100), instead of the commonly used 100 year global warming potential (GWP100)—due to a lower valuation of CH4 emissions. This raises the question of which metric to use. Ideally, the choice of metric should be related to a climate policy goal. Here, we argue that basing current GHG metrics solely on temperature impact 100 years into the future is inconsistent with the current global climate goal of limiting warming to 2 °C, a limit that is likely to be reached well within 100 years. A reasonable GTP value for CH4, accounting for current projections for when 2 °C warming will be reached, is about 18, leading to a current CF of 19 kg CO2-eq. per kilo beef (carcass weight, average European system), 20% lower than if evaluated using GWP100. Further, we show that an application of the GTP metric consistent with a 2 °C climate limit leads to the valuation of CH4 increasing rapidly over time as the temperature ceiling is approached. This means that the CF for beef would rise by around 2.5% per year in the coming decades, surpassing the GWP based footprint in only ten years. Consequently, the impact on the livestock sector of substituting GTPs for GWPs would be modest in the near term, but could potentially be very large in the future due to a much higher (>50%) and rapidly appreciating CF.
Persson U. M., Johansson, D. J., Cederberg, C. Hedenus F. Bryngelsson, D., 2015, Climate metrics and the carbon footprint of livestock products: where's the beef? Environ. Res. Lett. 10 034005 doi:10.1088/1748-9326/10/3/034005
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