Topic A. Hydrogen Emissions: Constraining The Earth system Response (HECTER)

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A global hydrogen economy is growing rapidly. As hydrogen usage increases, leakage to the atmosphere is inevitable, and atmospheric hydrogen levels will rise. Many aspects of hydrogen's atmospheric life cycle are poorly understood, placing large uncertainties on the environmental consequences of this shift to hydrogen. Soil microbes remove a large but uncertain proportion (50-80%) of hydrogen from the atmosphere. Atmospheric chemistry removes the rest, through reaction with the hydroxyl radical (OH). Rising levels of hydrogen thus deplete OH, lengthening methane's lifetime. Hydrogen oxidation also generates tropospheric ozone and stratospheric water vapour. In this way, hydrogen acts as an indirect greenhouse gas (GHG). There are further impacts on stratospheric ozone and changes in oxidants that will affect aerosols and clouds. The representation of how hydrogen emissions will affect all these processes in models is in its infancy. This project will improve our global modelling capabilities, assess future impacts, and identify and reduce uncertainties associated with hydrogen use. Most current global atmospheric hydrogen models prescribe surface layer mixing ratios of hydrogen and methane, rather than adding emissions. This project will develop versions of the UKESM model (already with methane emissions) to include surface fluxes (emissions and deposition) of hydrogen that will be tuned and evaluated with observations from surface sites, aircraft data, and firn ice records. We will use two chemistry schemes - a standard scheme and another with a more comprehensive description of oxidants - in order to explore how important the representation of chemistry is for quantifying hydrogen's impacts. We will also develop another UK model (STOCHEM), which additionally represents the isotopomers of hydrogen, adding further constraints on process evaluation. We will co-ordinate our modelling efforts with several other modelling groups from around the world in order to explore model diversity. We will analyse simulations with different hydrogen leakage amounts and quantify in detail how this affects the global hydrogen budget, and the resultant impacts on methane, ozone and stratospheric water vapour. Analysis of the range of model budget terms and impacts will allow us to identify commonality and differences between models, and hence identify uncertain processes, such as processes that lead to different hydrogen lifetimes. Further model experiments will explore how impacts depend upon the location and season of hydrogen leakage - we expect there to be important differences related to the proportion of hydrogen deposited to soils (e.g., dependence on hemisphere, proportion of land/ocean, and soil properties) and levels of oxidants (e.g., tropics/high-latitudes, summer/winter). We will synthesize our results and analysis of uncertainty to produce a comprehensive quantitative assessment of climate metrics (e.g., Global Warming Potential, Global Temperature Potential, and Effective Radiative Forcing) associated with hydrogen. We will incorporate this new knowledge about hydrogen into the FaIR model, which is a policy tool used for analysing a range of future scenarios. This will allow us (and policymakers) to explore a wide range of future hydrogen scenarios, including for example: (i) the extent to which hydrogen use offsets other GHG emissions; (ii) different levels of hydrogen leakage, from different world locations; (iii) differences is the representation of atmospheric chemistry; and (iv) differences in hydrogen end usage (e.g., hydrogen combustion may be accompanied by NOx emissions, which also affect oxidants). As well as being a medium to simply communicate the implications of our new modelling results to the policy community, FaIR will also allow us to co-ordinate rapidly with the other funded projects within this call, i.e. Topic B (different representations of hydrogen's soil sink) and Topic C (future scenarios).