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https://assets.publishing.service.gov.uk/government/uploads/system/uploads/attachment_data/file/1067144/atmospheric-implications-of-increased-hydrogen-use.pdfUse of hydrogen (H2) as a substitute for carbon-containing fossil fuels such as natural gas
would prevent emissions of carbon dioxide into the atmosphere, with significant climate
benefits. Nevertheless, any leakage of hydrogen will affect atmospheric composition (with
implications for air quality) and have an indirect warming effect on climate, partially offsetting
some of the climate benefits of the reduction in carbon dioxide.
We have explored the possible impact of hydrogen leakage using a chemistry/climate model of
the atmosphere. Uncertainties in hydrogen leakage rates, the extent to which hydrogen might
replace fossil fuels in the future and future hydrogen uptake rates by soils, make it difficult to
predict accurately changes to atmospheric hydrogen mixing ratios in a hydrogen economy. We
have considered a range of possible hydrogen future global scenarios, with surface mixing
ratio increases ranging from 0.25 parts per million (50% increase) by volume (ppm or
millimoles/mole) to 1.5 ppm (300% increase) above the current background mixing ratio of
about 0.5 ppm. We believe these scenarios span much of the uncertainty in potential changes
to the atmospheric mixing ratios of hydrogen associated with the ultimate size of the hydrogen
economy, the hydrogen soil sink and hydrogen leakage rates. For example, a 1.5 ppm
increase in the surface hydrogen mixing ratio would be consistent with a high leakage rate of
20% in a global hydrogen economy providing 23% of present day global energy consumption,
where the magnitude of the soil sink of hydrogen increases linearly with hydrogen mixing
ratios; it would also be consistent, for the same energy consumption, with a constant
magnitude, fixed soil sink and an hydrogen leakage rate of about 4%. Use of this range of
global scenarios provides a clear signal in the atmospheric response to increased hydrogen
mixing ratios relative to interannual variability and also allows us to explore the linearity of the
atmospheric response to increasing hydrogen. We have also considered the impact of
changes in emissions of gases other than carbon dioxide (CO2) which could follow increased
adoption of Hydrogen as an energy source; these gases, emitted alongside carbon dioxide and
called hereafter ‘co-emitted species’, include carbon monoxide (CO), methane (CH4), volatile
organic compounds (VOCs) and the oxides of nitrogen (NOx)
