Life Cycle Assessment of Hydrogen Production Technologies: A Systematic Review of Environmental Impacts and Policy Implications for the Green Energy Transition

Hydrogen is not intrinsically low-carbon; its environmental value depends on how, where, and with which energy system it is produced. This PRISMA 2020 systematic review synthesizes 28 peer-reviewed life cycle assessment (LCA) studies on major hydrogen production pathways, including steam methane reforming, electrolysis, biomass-based routes, thermochemical cycles, and emerging photoelectrochemical systems. Unlike reviews focused only on carbon intensity, this study jointly evaluates environmental performance, economic feasibility, and technology readiness to identify where apparent advantages remain robust and where they disappear under real deployment conditions. The evidence shows that renewable-powered electrolysis can deliver the lowest greenhouse gas emissions when supported by additional low-carbon electricity, but the same technology can lose its climate benefit in fossil-dominated grids. Biomass and emerging routes diversify supply options but introduce water, land, material, and maturity trade-offs that are often underrepresented in policy narratives. Regional conditions, especially grid carbon intensity and resource availability, explain much of the variation observed across studies. The review also identifies persistent methodological gaps, including inconsistent system boundaries, limited dynamic grid modelling, weak treatment of indirect land use effects, and insufficient accounting for system-level benefits from flexible electrolysis. Overall, the findings support performance-based carbon intensity standards, region-specific deployment strategies, and more transparent LCA methods capable of capturing hydrogen’s role in integrated energy systems.