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GWP* of U.S. beef and dairy systems


Global warming potential (GWP) is used to quantify the impact that greenhouse gases (GHG) have on the warming of the Earth's atmosphere relative to carbon dioxide (CO2). GWP* is a metric that is used to better quantify short-lived climate pollutants (SLCP) such as methane, hydrofluorocarbons, and sulfur dioxide. GWP* allows SLCP to be more consistently expressed by equating a change in the emission of the SLCP to a one-off pulse emission of CO2. Therefore, GWP* can be positive or negative. The objective of this work was to compare the GWP* and GWP100 for U.S. beef and dairy systems using livestock methane emissions data from the Food and Agriculture Organization (FAO) and the Environmental Protection Agency (EPA). Total methane emissions for this study are the sum of enteric and manure methane emissions. GWP100 was greater than GWP* for both beef and dairy systems using both datasets, with the exception GWP* for dairy using the EPA data. Dairy GWP* calculated using the EPA data was lower than GWP100 from 1990–2000, after which point on it became greater than GWP100 and continued increasing annually, because the emission factors used by the EPA increased annually, and the difference between weighted emissions from that year and the weighted emissions from 20 years prior surpassed the current emissions used in GWP100. Overall, the GWP* of EPA dairy increased by 507% from 1990–2020. The primary drivers of the differences in GWP* and GWP100 with the EPA dataset are the use of methane emission factors for manure methane, which increase yearly, and the use of a larger dairy population estimate than FAO. The EPA emission factors increase yearly based on the trend towards larger farm sizes managing more liquid manure, therefore produce more manure methane emissions. The dairy GWP* using EPA data was greater than the beef GWP* every year, despite greater total methane emissions for beef than for dairy, because the average rate of change for dairy (29.8 kt of CH4/yr) was greater than the average rate of change for beef (9.4 kt of CH4/yr). Accounting methods play a key role in the amount of methane emissions that are calculated, and thus how GWP100 and GWP* are calculated. The EPA larger population estimate and annual increase in manure methane emission factors led to greater GWP* and GWP100 values for the EPA data than for the FAO data for both beef and dairy systems. Data source is critical to the policy implications of GWP* and GWP100 for livestock systems, as evidenced by the differences in GWP* and GWP100 results between datasets.


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