Modeling Climate Impacts of Hydrogen Transition Pathways

Hydrogen has emerged as a key contender for decarbonizing hard-to-abate sectors, as it has the advantage of emitting no direct carbon dioxide emissions during combustion. However, modeled indirect climate warming impacts from additional hydrogen in the atmosphere have raised questions about its role in achieving net-zero energy transitions. Here we will present findings from two complementary modeling efforts that evaluated the climate implications of hydrogen emissions, and the life cycle impacts across various applications.

The first model effort evaluated emissions in 23 net-zero scenarios from prominent U.S. economy-wide studies, estimating the magnitude of hydrogen emissions relative to residual energy-related carbon dioxide and methane emissions. The model was used to evaluate the potential impact of hydrogen emissions relative to emissions reductions and carbon dioxide removal strategies needed for net-zero scenarios. Then the model was used to estimate energy-related hydrogen and methane emissions rates and global warming potentials with the best available data in literature. Modeling results indicated that hydrogen emissions ranged from 0.02–0.15 GtCO2e/year (using GWP100), with higher emissions in scenarios featuring increased hydrogen production. Despite these emissions, the calculated climate impacts represent less than 15% of combined hydrogen, methane, and carbon dioxide emissions in most scenarios. These impacts can be largely abated through reductions in residual CO₂ emissions or enhanced carbon dioxide removal. More specifically, residual CO2 emissions would need to be reduced by 1-25% in scenarios allowing fossil fuels and 32-98% in scenarios restricting fossil fuels to abate the warming effect of H2 emissions.

The second modeling effort involved a life cycle assessment (LCA) of electrolysis and steam methane reforming, highlighting that production methods and feedstock emissions are the dominant factors influencing life cycle emissions, rather than hydrogen leakage. Comparisons of hydrogen-based and fossil fuel-based systems revealed greenhouse gas emission reductions in steel production (800–1400 kgCO2e per tonne of steel) when hydrogen is used in direct reduction steel manufacturing (producing iron from iron ore without melting) rather than fossil fuels in blast furnaces, as well as in heavy-duty transportation (0.1–0.17 kgCO2e per tonne-km of cargo). Importantly, decarbonization potential of hydrogen varies by application, with steel production consistently showing emissions reductions, while benefits in heavy-duty transportation depend on the hydrogen production pathway.

These findings underscore the importance of advancing hydrogen emissions measurement, mitigation strategies, and tailored application areas to maximize its potential climate benefits while addressing indirect warming impacts.