Uncertainty analysis and future changes in the soil uptake of hydrogen

We present results from the EU HYDRA Project (Hydrogen Economy Benefits and Risks: Tools Development and Policies Implementation to Mitigate Possible Climate Impacts), focusing on the soil sink of hydrogen, and its associated historical and future variations. We calculate the dry deposition velocity of hydrogen to soil with and without consideration of soil carbon content and simulate atmospheric hydrogen concentrations in 2010 using the FRSGC/UCI global chemical transport model (CTM). Surface concentrations and their seasonal variation compare better with observations when soil carbon content is accounted for. The contribution of each soil parameter to the overall uncertainty in hydrogen dry deposition was quantified using Gaussian process (GP) emulation and Sobol sensitivity analysis. We find that soil moisture is the key factor influencing soil uptake, and we identify regions with differing responses to soil moisture using the Expectation-Maximization (EM) method. We then explore the influence of dry deposition on the interannual variation in surface hydrogen concentration from 2010 to 2022 using the CTM and find that the interannual variation is driven principally by variations in dry deposition, especially in years with a large soil moisture anomaly relative to the decadal mean. The impact of climate change on future hydrogen dry deposition was then estimated using soil information output from 11 CMIP6 models, and we find a change in soil uptake between 2015 and 2100 of -1.5% to +7.8% following the SSP1-2.6 and +4.6% to +22.2% following the SSP5-8.5 pathway.