A critical meta-survey of the lifecycle greenhouse gas emissions of hydrogen energy systems

Hydrogen energy systems are central to decarbonization strategies, yet their full climatic footprint remains contested. Lifecycle assessments reveal significant variability influenced by feedstock choices, energy inputs, and production processes. This paper conducts a meta-survey to examine assumptions behind varying estimations and identify improvement areas. We analyzed 653 academic studies, applying rigorous exclusion criteria to extract 906 estimations across ten lifecycle stages from 109 peer-reviewed studies covering 90 % of global hydrogen production over 2000–2024. We calculated emissions intensities based on grams of carbon dioxide equivalent per kilowatt-hour (gCO 2 e/kWh) and kilograms per kilogram of hydrogen produced (kgCO 2 e/kgH 2 ), disaggregating estimates by hydrogen “colors” (turquoise, blue, green, etc.) based on different energy inputs. Results indicate median carbon footprints for production alone of 164.3 gCO 2 e/kWh or 5.5 kgCO 2 e/kgH 2 . Full lifecycle accounting—including upstream sourcing, conversion, storage, distribution, end-use, and decommissioning—increases total median footprints to 435.4 gCO 2 e/kWh or 15.2 kgCO 2 e/kgH 2 . Mean values are significantly higher (1038.4 gCO 2 e/kWh and 34.5 kgCO 2 e/kgH 2 ), highlighting extreme outliers' impact. Key variability sources include process type, energy inputs, sectoral application, geographic location, leakage rates, and system capacity. We identify gaps in lifecycle methodologies, particularly system boundary definitions, reporting standards, and end-of-life infrastructure treatment. The findings challenge generic assumptions about hydrogen's climate benefits and emphasize the need for granular, pathway-specific analysis in policy, investment, and modeling decisions.