Green hydrogen: The only oxygen and water balanced
The use of any fuel depletes the oxygen content of the atmosphere, with one exception: hydrogen produced from water. Water electrolysis liberates oxygen from water in the precise stoichiometric ratio required to oxidise (and hence release energy from) the co-produced hydrogen. As a commercial fuel production process, electrolysis is unique in providing the oxidant as well as the fuel; electrolytic oxygen can thereby replenish the consumption of atmospheric oxygen due to hydrogen use. Furthermore, the amount of water consumed during electrolysis is reproduced when the hydrogen is oxidised. So the use of electrolysers and electrolytic hydrogen does not affect global oxygen and water resources: ‘green’ hydrogen may thus be described as the only oxygen and water balanced fuel. Conversely, the use of hydrogen derived from fossil fuels (with or without carbon capture and storage, CCS) depletes the oxygen resource and increases water vapour emissions to the atmosphere, which enhances the rate of global warming. Therefore, a worldwide multi-TW deployment of electrolysers could provide very substantial amounts of hydrogen for the energy system, and oxygen for the global ecosystem. This should be done in combination with other measures for combatting oxygen depletion (such as reducing combustion, increasing forestation, and reducing nutrient inputs to the ocean from sewage and agriculture). In this way the long-term objective should be to stabilise, or even increase slightly, the concentrations of atmospheric and aquatic oxygen, and possibly speed up the decay of atmospheric methane. Clearly the production and use of hydrogen derived from fossil fuels contravenes this objective, and should cease without delay.
Oxygen depletion and water vapour addition
Oxygen is the second most abundant gas on Earth after nitrogen. It is produced primarily by photosynthesis and consumed mainly by combustion, respiration and fire (e.g. it has been estimated that fossil fuel combustion consumes over eight times more oxygen per annum than human respiration). There are also several industrial processes which of themselves consume oxygen (e.g. steel production, oil refining and wastewater treatment). Based on measurements taken since 1989, the atmospheric oxygen concentration has been decreasing slowly at an annual rate of about 19 molecules per million.This trend will continue, and for the business-as-usual RCP8.5 (Representative Concentration Pathway, 8.5 W/m2) global warming scenario,the concentration is predicted to decrease from 20.946% to 20.825% by 2100, or on average by about 0.0015% per annum. The overall rate of change (e.g. see Figure 1), which is influenced both by changes in oxygen production and consumption, may be parabolic rather than linear, and so result in the concentration falling to zero in about 4400 years. In the short term, the change is too gradual to impact human health; this will only occur when the concentration falls below about 19.5%. However, the current rate of oxygen depletion is sufficient to influence global warming.