Perspective: Enteric methane mitigation and its impact on livestock hydrogen emissions.

In a hydrogen-based economy future, hydrogen leakage is becoming an environmental concern. Ruminants naturally produce small amounts of hydrogen, which is emitted in the environment along with other fermentation gases, such as the GHG methane and carbon dioxide. Here, for the first time, we estimated hydrogen emissions from the global ruminant livestock at 527 kt/yr (95% CI: 399, 654), or about 3.5% (95% CI; 2.7, 4.4) of the global anthropogenic hydrogen emissions. When methanogenesis is decreased by various methane-mitigation practices, hydrogen emissions increase, raising questions regarding net environmental impacts because hydrogen is an indirect greenhouse gas. Therefore, we estimated the potential contribution of hydrogen from ruminant livestock under 3 enteric methane reduction scenarios. At the highest methane reduction (75%) scenario, the percentage increase in global ruminant hydrogen emissions over baseline emissions (as kilotons per year) due to the reduction in enteric methanogenesis was 5.95% (95% CI: 4.52, 7.39) and yielded a 0.48% (95% CI: 0.37, 0.60) increase in CO2-equivalents (CO2-eq), when the benefit of reduced methane was accounted for. Our study suggests that the net climate benefit of reduced methane emissions from ruminants is decreased by less than 1%, when expected increases in hydrogen emissions are considered as an offset to the CO2-eq emissions avoided.