Abstract:As interest in hydrogen as an energy carrier has increased, the various ways that hydrogen is made are being categorized as “green,” “blue,” “gray,” and other colors in relation to their environmental impact. While these categorizations are somewhat useful to indicate the environmental and climate change impacts of different production pathways, they are not especially useful for policy making or industry decisionmaking purposes because they are subjective. For example, most definitions of green pathways for hydrogen production only include electrolysis from renewable electricity sources; however, Figure 1 indicates additional production pathways with some of these having near-zero or even negative greenhouse gas (GHG) emissions as well as low or no other emissions of concern. To help clarify the role of hydrogen in decarbonizing California, this brief summarizes the latest scientific findings from recent and in-progress research across the University of California Institute of Transportation Studies (UC ITS) concerning the relative carbon intensity (CI) of hydrogen production pathways. It also briefly covers the availability of biomass and biogas in California that could be applied to the production of low-CI hydrogen.