Hydrogen is a potential low-carbon energy carrier to replace fossil fuels, especially in industrial and transportation applications where decarbonization is particularly challenging. Hydrogen can be generated via several feedstocks and technology combinations (pathways) that result in different life cycle greenhouse gas emissions intensities, thus policies and investments intended to deploy hydrogen as a climate solution must differentiate among pathways. To collect and analyze current estimates of the life cycle greenhouse gas intensity of hydrogen pathways, a systematic scholarly literature review was conducted capturing article published between 2018 and 2022.

The review yielded 85 studies with 492 reported greenhouse gas intensity values. Steam reforming, fossil fuel gasification, biomass gasification, and water electrolysis were the most common pathways. Analysis of the reported intensity values shows large variability among pathways due to the choice of feedstock, energy sources used, and conversion method. However, estimates within pathways also show significant variability, in some cases driven by real-world spatiotemporal or system design differences, but also arising from life cycle assessment practitioner choices regarding methods and data sources.

Despite significant variability, clear differences between hydrogen pathways can be observed. The lowest intensity pathways (∼20 gCO2e/MJ H2) correspond to those using low carbon or renewable energy. Steam methane reforming, the most common pathway, shows high emissions at nearly 110 gCO2e/MJ H2. Other promoted alternatives like electrolysis with carbon-intensive grid electricity can have even higher emissions, highlighting the need to assess pathways individually, considering feedstock, energy source, and conversion technology.

Key words: life cycle assessment, carbon intensity, greenhouse gas, sustainability, hydrogen, steam methane reforming, electrolysis