The climate impact of a future hydrogen economy

Abstract

Inflammable air, known today as hydrogen, was first identified and produced in 1766 by the British chemist and physicist Henry Cavendish. Today, hydrogen can be produced by splitting the liquid water molecules. The water electrolysis producing hydrogen can be powered by renewable energy in the case of "green" hydrogen. Hydrogen is also produced from fossil fuels by steam reforming of methane in natural gas in conjunction with carbon sequestration in the case of "blue" hydrogen, or without carbon sequestration in the case of "grey" hydrogen. The use of hydrogen enables energy conversion and storage, and can provide a way to decarbonize sectors of the economy where decarbonization has no alternative or is hard to reach, such as long-distance transport by truck, train or airplane, heavy industries, or for domestic use in mixture with natural gas. Hydrogen has no direct greenhouse effect but is an indirect climate gas which induces perturbations of atmospheric methane, ozone and water vapour, three powerful greenhouse gases. The budget of atmospheric molecular hydrogen will be presented and the main sources and sinks will be briefly discussed. Based on the results of state-of-the-art global numerical climate and chemistry models, we derive various indicators intended to quantify the climate impact of hydrogen and in particular derive its Global Warming Potential (GWP). All the scenarios considered in this study for a future transition towards a hydrogen economy in Europe or in the world clearly suggest that a "green" hydrogen economy is beneficial in terms of CO2 emissions mitigation for the relevant time horizons and leakage rates considered. In contrast, the results suggest that carbon dioxide (CO2) and methane (CH4) emissions associated with the production and transport of "blue" (and "grey") hydrogen reduce the climate benefit of such a transition and even introduce a climate penalty in the event of a very high leakage rate or strong penetration of "blue" hydrogen on the market. Various assumptions will be illustrated for future “blue” hydrogen production carbon intensity. Reducing the leakage rate of H2 (and CH4 in the case of "blue" hydrogen production) and increasing the "green" hydrogen production sector appear to be the key levers towards maximum mitigation of CO2 emissions from a large-scale structural transition to a hydrogen economy. In addition, in the specific case of aviation, the use of liquid hydrogen powered aircraft induces additional climate forcings from water vapour emissions in the upper atmosphere and from impact on contrail formation. In the case of an hydrogen powered fleet, the forcings from NOx and from contrails are still subject to large uncertainties. These effects will be illustrated based on various assumptions for future aircraft using hydrogen fuel.