Climate benefit of a future hydrogen economy

The study investigates how hydrogen emissions affect climate via indirect effects on methane, tropospheric and stratospheric ozone, and stratospheric water vapour. Although H2 is not a direct infrared absorber under atmospheric conditions, its chemistry alters lifetimes and abundances of potent greenhouse gases, notably extending methane lifetime by depleting OH and contributing to ozone and stratospheric water vapour formation. With increasing interest in large-scale deployment of hydrogen across power, transport, industry, and buildings to meet Paris Agreement goals, the research quantifies hydrogen’s climate impact using established metrics (GWP, GTP) over various time horizons and evaluates the net climate benefit of transitioning to hydrogen under different production pathways (green, blue, grey) and leakage rates. The central questions are: what are robust climate metrics for H2, and under what conditions does a hydrogen economy yield a net CO2 emissions abatement benefit or a climate penalty?

Prior work estimated relatively low hydrogen climate impacts (e.g., GWP100 ~3.3–5), but more recent studies suggest higher values, highlighting uncertainties in atmospheric chemistry and the hydrogen budget, particularly soil uptake and lifetimes. The climate outcome of a hydrogen economy depends strongly on production methods: blue hydrogen relies on fossil gas with CO2 capture and storage and is sensitive to upstream CH4 leakage and capture efficiency. There is also an ongoing debate about the suitability of GWP100 for short-lived climate forcers like H2, with arguments for alternative metrics (e.g., GTP) or shorter time horizons for policy relevance. Earlier modelling work explored air quality and stratospheric ozone implications of large-scale H2 use and found potential impacts depending on leakage rates. This study builds on these strands by using updated model-derived radiative efficiencies and decomposing H2’s indirect effects to refine climate metrics and assess scenario-based mitigation outcomes.

- Radiative forcing inputs: The study uses Effective Radiative Forcing (ERF) efficiencies for hydrogen from the GFDL-AM4.1 global model. The total H2 radiative efficiency is 0.13 mW m^-2 ppb^-1, decomposed into contributions from methane-mediated temperature response (~46%), stratospheric H2O (~28%), stratospheric O3 (~21%), and tropospheric O3 (~6%). - Metrics framework: Using analytical impulse response functions (IRFs), the authors compute Absolute GWP (AGWP) and Absolute GTP (AGTP) for H2 and CO2, then derive GWP and GTP as ratios. H2’s IRF uses an atmospheric lifetime on the order of two decades (global ~25 years; tropospheric ~21 years), while methane’s perturbation lifetime is 12.4 years. CO2’s IRF follows standard multi-timescale decay functions. They also define combined endpoint metrics (CGWP, CGTP) to capture responses to sustained emissions and to better reflect end-of-century temperature goals. - Decomposition: Metrics are decomposed into indirect components from CH4, tropospheric O3, stratospheric O3, and stratospheric H2O to elucidate different time-scale responses (short-lived ozone and stratospheric H2O vs. multi-decade CH4 component). - Scenarios and leakage: The climate implications of a hydrogen economy are quantified using global and European deployment roadmaps: HC2017 (Hydrogen Council), IEA2021, HCMK2021, and TYNDP2022 (EU). Hydrogen leakage rates spanning 0.1–10% (and sensitivity beyond) are considered. Production portfolios include: (1) green H2 only; (2) blue + green mixtures (e.g., 30% blue/70% green in some cases); and (3) grey + blue + green. - Emissions accounting: For green hydrogen, CO2e emissions are from hydrogen leakage effects only. For blue and grey, the analysis includes process CO2 emissions, methane emissions and leakage from natural gas supply chains and hydrogen production/transport, as well as hydrogen leakage. The total equivalent CO2 emissions are compared with CO2 emissions avoided by displacing fossil fuel use, producing ratios and cumulative abatement over 2030–2100. - Sensitivities: The study examines sensitivity to metric choice (GWP vs GTP), time horizon (e.g., 20 vs 100 years), and hydrogen energy basis (HHV vs LHV). Uncertainties are quantified following IPCC-style methods, propagating uncertainties in lifetimes, radiative efficiencies, and IRFs. - Outputs: Time-dependent AGWP/AGTP curves; GWP, GTP, CGWP, CGTP for H2 across time horizons; scenario-based CO2e/avoided-CO2 ratios and cumulative CO2 abatement (GtCO2e) under various leakage rates and production mixes.

- Hydrogen climate metrics: - GWP100 = 12.8 ± 5.2; GWP20 = 40.1 ± 24.1 (H2 relative to CO2, per kg). - Time-resolved AGWP shows a peak ~20 years after emission; indirect ozone and stratospheric H2O contributions peak ~8 years; the CH4-mediated component builds over ~30 years. - Metric uncertainties are substantial (e.g., ~40% on GTP100, ~60% on GTP20), reflecting uncertainties in lifetimes, radiative efficiencies, and atmospheric processes. - Green hydrogen transition: - For a global transition scenario (HC2017), a green H2 economy is beneficial across policy-relevant time horizons and leakage rates. Using GWP100, cumulative CO2 abatement over 2030–2100 is ~327 GtCO2 for 1% leakage, decreasing to ~289 GtCO2 for 10% leakage. Similar abatement magnitudes are obtained with CGTP and across other global scenarios (e.g., IEA2021, HCMK2021) and an EU scenario (TYNDP2022), albeit with smaller totals for the EU case. - Ratios of CO2e from H2 to avoided CO2 remain modest for green H2 at low leakage (e.g., at 10% leakage, ~13% of abated CO2 at 100-year horizon; ~40% at 20-year horizon). - Blue + green mixes: - Incorporating blue hydrogen reduces the net climate benefit and can produce a climate penalty at higher leakage rates or higher blue shares. - Example thresholds: With GWP20, benefit is lost if blue share exceeds ~30% and leakage exceeds ~3%. At higher leakage rates, CO2e from the H2 system can reach a large fraction of abated CO2 at 100-year horizons and exceed 100% at shorter horizons (e.g., around 20 years), implying a net penalty. - Metric choice impacts: - GTP (endpoint temperature) tends to ascribe greater importance to longer-lived components; using GTP instead of GWP for green H2 can increase calculated CO2e up to a factor ~6.2 at certain horizons (e.g., ~25 years). Nevertheless, abatement totals computed with CGTP are similar to those with GWP around a ~40-year horizon, suggesting this horizon is informative for end-of-century goals. - Sensitivity to energy basis: - Using LHV instead of HHV slightly increases CO2e fractions and reduces net abatement, but does not alter the qualitative conclusions about the superiority of green H2 and the risks associated with blue H2 at higher leakage rates.

The study quantifies hydrogen’s climate impact with updated metrics that resolve its indirect effects on CH4, O3, and stratospheric H2O. By decomposing responses and applying scenario analyses, it directly addresses whether and when a hydrogen economy yields net climate benefits. Results show that a green hydrogen pathway robustly delivers CO2 abatement across leakage rates and time horizons, thereby supporting long-term climate objectives. Conversely, blue hydrogen’s associated CO2 and CH4 emissions erode the mitigation gains and can cause net warming, especially under higher leakage and larger blue shares, aligning with concerns about fossil-based hydrogen pathways. The analysis also clarifies how metric choice affects perceived benefits: GWP can overemphasize short-lived forcing relative to long-lived CO2, whereas endpoint metrics like GTP provide insight into effects on end-of-century temperature. Around a 40-year horizon, both approaches yield similar abatement estimates, indicating a practical timeframe for policy assessment. Overall, reducing hydrogen (and, for blue hydrogen, methane) leakage and prioritizing green production are critical levers to ensure a clear climate benefit from hydrogen deployment.

- The paper provides updated hydrogen climate metrics (e.g., GWP100 = 12.8 ± 5.2; GWP20 = 40.1 ± 24.1) based on state-of-the-art modelling and a decomposition of indirect effects. - Scenario analyses demonstrate that transitioning to a green hydrogen economy yields substantial cumulative CO2 abatement through 2100 across plausible leakage rates and time horizons. - Inclusion of blue hydrogen significantly reduces net benefits and can produce a climate penalty at elevated leakage or high blue shares; minimizing leakage and maximizing green hydrogen are essential to secure climate gains. - Endpoint metrics and a ~40-year horizon provide useful perspectives for end-of-century goals, complementing traditional GWP. - Future work should better constrain hydrogen and methane leakage across the value chain, improve understanding of soil uptake and atmospheric lifetimes, and refine radiative efficiencies to reduce metric uncertainty and strengthen policy guidance.

- Substantial uncertainties in key parameters (e.g., H2 and CH4 lifetimes, soil uptake, radiative efficiencies) propagate into GWP/GTP estimates, with metric uncertainties up to ~60–70% for some horizons. - Leakage rates for H2 (and CH4 in blue hydrogen) are poorly constrained and scenario-dependent, affecting conclusions about net benefits versus penalties. - Results rely on specific model-derived radiative efficiencies (GFDL-AM4.1) and chosen IRF frameworks, which may differ in other modelling systems. - Scenario inputs (HC2017, IEA2021, HCMK2021, TYNDP2022) carry their own assumptions about technology deployment, energy demand, and production mixes, which may evolve. - Metrics are sensitive to time-horizon selection; policy interpretation requires clarity about goals (near-term vs end-of-century) and appropriate metric choice (GWP vs GTP/combined metrics).