Hydrogen Emissions from a Hydrogen Economy and their Potential Global Warming Impact

Hydrogen (H2) is expected to be a key instrument to meet the European Union (EU) Green Deal main objective: i.e., climate neutrality by 2050. Renewable hydrogen deployment is expected to significantly reduce EU greenhouse gas (GHG) emissions by displacing carbon-intensive sources of energy. However, concerns have been raised recently regarding the potential global warming impact caused by hydrogen emissions. Although hydrogen is neither intentionally emitted to the atmosphere when used nor a direct greenhouse gas, hydrogen losses affect atmospheric chemistry, indirectly contributing to global warming. To better understand the potential environmental impact of a hydrogen economy and to assess the need for action in this respect, the Clean Hydrogen Joint Undertaking and the U.S. Department of Energy jointly organised, with the support of the European Commission, Hydrogen Europe, Hydrogen Europe Research, the Hydrogen Council, and the International Partnership for Hydrogen and Fuel Cells in the Economy, a 2-day expert workshop. Experts agreed that a low-carbon and, in particular, a renewable hydrogen economy would significantly reduce the global warming impact compared to a fossil fuel economy. However, hydrogen losses to the atmosphere will impact the lifetime of other greenhouse gases, namely methane, ozone, and water vapour, indirectly contributing to the increase of the Earth’s temperature in the near-term. To minimise the climate impact of a hydrogen economy, losses should therefore be minimised, prevented, and monitored. Unfortunately, current loss rates along the hydrogen supply chain are not well constrained and are currently estimated to go from few percents for compressed hydrogen (1-4%), up to 10-20% for liquefied hydrogen. Both the global warming impact of hydrogen emissions and the leakage rates from a developed hydrogen economy are subject to a high level of uncertainty. It is therefore of paramount importance to invest in developing the ability to accurately quantify hydrogen emissions, as well as engage in more research on hydrogen leakage prevention and monitoring systems. More data from the hydrogen industry and improved observational capacity are needed to improve the accuracy of the global hydrogen budget. Finally, it is recommended to always report the amount and location of hydrogen emissions when environmental assessments are performed. There is a range of emission metrics and time scales that are designed to evaluate the climate impacts of short-lived GHG emissions compared to CO2 (i.e., CO2 equivalents). The metric choice must depend on the specific policy goal, as they can provide very different perspectives on the relative importance of H2 emissions on the climate depending on the time horizon of concern. These differences need to be viewed in the context of the specific policy objectives.