Development of fully interactive hydrogen with methane in UKESM1.0

Abstract. Hydrogen is a potential candidate for an alternate energy source and carrier. As usage of hydrogen in industry rises, leakages into the atmosphere may occur, causing an increase in the global atmospheric hydrogen concentration. Hydrogen is an indirect greenhouse gas, known to increase methane, stratospheric water vapour, and tropospheric ozone. Methane and hydrogen are closely coupled, with the main atmospheric destructive pathway of both species being via reaction with the hydroxyl radical (OH). Currently, most earth system models (ESMs) simulate hydrogen or methane with a prescribed lower boundary condition, which suppresses chemical feedbacks at the surface. In this work, we implement hydrogen emissions and a hydrogen soil uptake scheme into an ESM with free-running methane to demonstrate the capability of a fully interactive hydrogen and methane emissions-driven ESM. We show that the model is able to capture long term trends and seasonal cycles of both species when compared to observations, and find that the inclusion of both fluxes does not impact other chemical species in the model, such as tropospheric ozone. We show that the model can be used under pre-industrial conditions and present day scenarios. The ESM with fully coupled hydrogen and methane chemistry has great potential to be used in future scenarios and to estimate a more accurate global warming potential of hydrogen.