United States dairy farms and global warming

Several metrics have been developed for combining the warming effects of various GHG. The metric used can affect the life cycle assessment and comparison of dairy production systems due to the weighting placed on long- versus short-lived gases in the atmosphere. Global warming potential (GWP) with a time horizon of 100 years (GWP-100) has become the standard, but metrics are also available for other time horizons. Metrics for 20-, 100-, and 500-year horizons gave average farm-gate emission intensities of 2.08, 0.98, and 0.50 kg of CO₂ equivalents per kilogram of fat- and protein-corrected milk produced for current US dairy farms. Compared with the use of GWP metrics, which represent energy absorption, use of global temperature change potential (GTP), combined global temperature change potential (CGTP), or global warming potential star (GWP*) reduced the warming effect of methane relative to other GHG. These metrics representing temperature change reduced the warming potential of US dairy farms by 17% to 49% compared with the use of GWP-100. The metrics used also affected the comparison of individual production systems, providing different life cycle assessments of management practices. Use of GWP-100 metrics indicated that warming from GHG emissions of US dairy farms increased 11 % to 15% between 1971 and 2020, whereas the use of GTP, CGTP, and GWP* metrics showed little or no effect on global temperature change over the 50-year period. Use of GWP-100 metrics indicated that GHG emissions related to milk production on dairy farms represented 1.6% of all US GHG emissions in 2020 whereas use of other metrics ranged from 0.9% to 1.8%. Although all approaches for representing the integrated warming impact of GHG have benefits and challenges, approaches such as CGTP and GWP*, which account for the rate of methane emission relative to the oxidation rate in the atmosphere, provide a more process-based assessment of the long-term impact of dairy farms on global temperature and perhaps offer a more scientifically sound approach for assessing strategies to mitigate the warming effect of dairy farms.