United States dairy farms and global warming

Authors: Rotz, Clarence; MITLOEHNER, FRANK - University Of California, Davis

Submitted to: Journal of Dairy Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 11/15/2024
Publication Date: 3/3/2025
Citation: Rotz, C.A., Mitloehner, F. 2025. United States dairy farms and global warming. Journal of Dairy Science. 108(3):2610-2619. https://doi.org/10.3168/jds.2024-25360.
DOI: https://doi.org/10.3168/jds.2024-25360

Interpretive Summary: Several metrics have developed to integrate the warming effects of different greenhouse gases and the metric used affects the life cycle comparison of milk carbon footprints of dairy production systems. Global warming potential with a 100-year time horizon has become the standard but this metric does not accurately represent the long-term warming effect of methane which only remains in the atmosphere for a relatively short time. Compared to this metric, use of combined global temperature potential (CGTP) or global warming potential star (GWP*) allows less emphasis on methane relative to carbon dioxide which can remain in the atmosphere for centuries. Therefore, carbon footprints for dairy farms with greater methane emission are reduced the most by using these alternative metrics that consider the rate of emission relative to the rate of oxidation in the atmosphere.While all approaches for representing the warming impact of dairy farms have benefits and challenges, approaches such as CGTP and GWP* provide a more realistic assessment of the long-term impact of dairy farms on global climate and a better approach for assessing strategies to mitigate the warming effect of dairy farms.

Technical Abstract: Several metrics have developed for combining the warming effects of different greenhouse gases (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. Life cycle emission data generated by simulating US dairy farms in 1971 and 2020 provide a basis for comparing GHG emissions using various metrics for quantifying global warming based upon both amount and rate of emission. Use of global warming potential (GWP) with time horizons of 20, 100 and 500 years gave carbon footprints for US milk of 2.18, 0.97 and 0.48 kg CO2e/kg of fat and protein corrected milk. Compared to the use of GWP with a 100-year time horizon (GWP-100), the use of global temperature potential (GTP), combined global temperature potential (CGTP) and models for GWP* reduced the warming effect of methane relative to other GHG, which reduced the carbon footprint of US milk production by 38 to 55%. The warming calculation used affected the comparison of individual production systems providing different recommendations for best management practices. Use of GWP-100 metrics indicated that warming from GHG emissions of US dairy farms increased 10-15% between 1971 and 2020, while the use of GTP metrics indicated little or no effect on global temperature change over the 50-year period. Use of GWP-100 metrics indicated that the life cycle GHG emission related to dairy farms represented 1.6% of all GHG emissions of the US in 2020. Use of GTP, CGTP and GWP* approaches decreased this portion to between 0.8 and 1.2%. While all approaches for representing the warming impact of dairy farms have benefits and challenges, approaches such as CGTP and GWP* that account for the rate of methane emission relative to the oxidation rate in the atmosphere provide a more realistic assessment of the long-term impact of dairy farms on global climate and a better approach for assessing strategies to mitigate the warming effect of dairy farms.