Risk of the hydrogen economy for atmospheric methane

Abstract

Hydrogen (H₂) is expected to play a crucial role in reducing greenhouse gas emissions. However, hydrogen losses to the atmosphere impact atmospheric chemistry, including positive feedback on methane (CH₄), the second most important greenhouse gas. Here we investigate through a minimalist model the response of atmospheric methane to fossil fuel displacement by hydrogen. We find that CH₄ concentration may increase or decrease depending on the amount of hydrogen lost to the atmosphere and the methane emissions associated with hydrogen production. Green H₂ can mitigate atmospheric methane if hydrogen losses throughout the value chain are below 9 ± 3%. Blue H₂ can reduce methane emissions only if methane losses are below 1%. We address and discuss the main uncertainties in our results and the implications for the decarbonization of the energy sector.

Fig. 1. Tangled hydrogen (H₂) and methane (CH₄) budgets.

Sketch of H₂ and CH₄ tropospheric budgets and their interconnections: (1) the competition for OH; (2) the production of H₂ from CH₄ oxidation; (3) the potential emissions [minimum-maximum] due to a more hydrogen-based energy system. Flux estimates (Tg/year) are from refs. ,. Arrows are scaled with mass flux intensity, CH₄ scale being 10 times narrower than H₂ scale. On a per-mole basis, H₂ consumes only around 3 times less OH than CH₄. ppq = part per quadrillon (10⁻¹⁵). ^a top-down estimate including also minor atmospheric sinks (<10%). ^b range obtained as a difference between total and fossil fuel emissions.

Fig. 2. Hydrogen replacement of fossil fuels.

a Changes in H₂ sources ($\mathrm{\Delta }{S}_{{\mathrm{H}}_2}$) as a function of fossil fuel replacement for different hydrogen emission intensity (HEI). b Changes in CH₄ sources ($\mathrm{\Delta }{S}_{{\mathrm{CH}}_4}$) as a function of fossil fuel replacement for different H₂ production pathways. Methane leak rates associated with blue H₂ production are 0.2, 1, and 2%. Bands for $\mathrm{\Delta }{S}_{{\mathrm{CH}}_4}$ account for different amounts of blue H₂ produced and lost. c Response of the tropospheric concentrations of H₂ and CH₄ for the emission scenarios of the previous panels. Symbols mark the different percentages of fossil fuel displacement. Only symbols for 100% fossil fuel replacement are reported for blue H₂ with 1% CH₄ leakage. Also reported is the difference in CO₂ concentration (Δ[CO₂e]) that would produce equivalent radiative forcing to the change in equilibrium CH₄ (upper axis).

Fig. 3. Methane response to increasing H₂ production.

a Changes in H₂ and CH₄ sources (ΔS) due to green and blue H₂ production (≈500 Tg yr⁻¹). HEI is the H₂ emission intensity. Gray lines mark the case for HEI = 0%. Blue bars for $\mathrm{\Delta }{S}_{{\mathrm{CH}}_4}$ are obtained with HEI = 10%. b Response of CH₄ atmospheric concentration. The right axis shows the Δ[CO₂e] that would produce equivalent radiative forcing to the change in equilibrium CH₄.

Fig. 4. Critical hydrogen emission intensity (HEI) for methane mitigation.

Critical HEI as a function of OH excess (E_OH) and hydrogen production method (green and blue H₂ with 0.2, 0.5, 1% CH₄ leak rates, respectively). Dashed (dotted) lines are obtained for a 20% increase (decrease) in the H₂ uptake rate by soil bacteria (k_d). Triangles mark the critical HEI for the best estimate of E_OH.

Fig. 5. Transient dynamics.

Tropospheric response to a pulse of H₂ (10% increase of its concentration). Temporal dynamics of H₂ (a), CH₄ (b), OH (c), and CO (d). Colors highlight the contributions of the different modes. When different modes superimpose, the faster-decaying mode is shown on top of the others.