By: Keith Shine
There are many possible avenues to reduce carbon dioxide emissions. One of these is a shift to using hydrogen (H2) as a fuel source; it could potentially be used for many current CO2-emitting activities, including industry, heating in the home and transport. There would be many challenges, but it is widely regarded as one component of pathways to reach “net zero”, which aims to stabilise human-induced climate change. A recent Royal Society briefing provides much information on the technological and economic challenges of a move to a hydrogen economy.
As with all potential climate-change solutions, it is necessary to assess their environmental impact. I played a small role in a modelling study on the “Atmospheric Implications of Increased Hydrogen Use”, led by the University of Cambridge (Warwick et al., 2022), funded by the Government’s Department for Business, Energy & Industrial Strategy. Studies of hydrogen’s climate impact go back about 20 years (e.g., Derwent et al. 2006; Schulz et al. 2003; Warwick et al. 2004) but there is now more urgency in understanding the issues (e.g., Derwent et al., 2020; Paulot et al., 2021).
The first issue is how hydrogen is generated. The “feedstock” is simply water. But it takes energy to split hydrogen from water, and it matters where that energy comes from. There are two low carbon methods. So-called “blue hydrogen” is generated using fossil fuels, but the CO2 produced is captured and stored rather than emitted into the atmosphere. “Green hydrogen” is generated using renewable energy sources. My focus is the impact of any hydrogen leakage during production, storage and distribution (e.g., from pipework and valves). (The use of the hydrogen just leads to the generation of water.)
Hydrogen itself is of little direct concern, from a climate point of view, although it can impact air quality; the Cambridge study focused on hydrogen’s role in altering the chemistry of the atmosphere, thereby changing concentrations of gases that can influence climate.
A major route to climate impact is via changes in concentrations of a very reactive molecule, the hydroxyl radical (OH), a gas present in tiny quantities but which plays a key role in atmospheric chemistry. It is sometimes referred to as an “atmospheric detergent” as it hastens the removal of many atmospheric pollutants. Leakage of hydrogen reduces OH concentrations, so reducing this cleansing capacity.
The effects of both the hydrogen itself and its impact on OH include increased concentrations of methane, tropospheric ozone and stratospheric water vapour; all these lead to climate warming. It is important to quantify these impacts, and identify uncertainties, to be clear that the climate advantages of reduced CO2 emissions far outweigh the impacts of increased hydrogen use.
My involvement in the Cambridge study was to help quantify the 100-year Global Warming Potential (GWP(100)), a metric to characterise the climate impact of emissions of a gas (relative to the emission of an equal mass of carbon dioxide). GWP(100) is just one possible metric to quantify climate impacts of emissions and in itself is quite contentious: see this blog post by my colleague Bill Collins. But contentious or not, it is widely used in policy applications, including national and international policy agreements.
Warwick et al. (2022) concluded that hydrogen’s GWP(100) was 11±5; about half came from its impact on methane and about a quarter each came from its impact on tropospheric ozone and stratospheric water vapour (some of which was due to a knock-on effect of methane changes). Clearly uncertainties are substantial, one of which is the atmospheric lifetime of hydrogen which is believed to be 2 to 3 years. As noted above, it is removed by reaction with OH but it is also removed by reactions with soil; the strength of this “soil sink” is particularly uncertain.
So hydrogen leakage does have a higher climate impact (as measured by GWP(100)) than CO2 per kg emitted. However, hydrogen emissions would be much smaller than the CO2 emissions that they would replace. For one illustrative future scenario, Warwick et al. (2022) estimate that hydrogen’s climate impact would be around 0.4 to 4% (for hydrogen leakage rates of 1 to 10% respectively) of the avoided “CO2-equivalent” emissions. This is all promising but nevertheless there can be no complacency. Leakage rates must be minimised. Remaining uncertainties in quantifying the climate impact must be reduced. The Natural Environment Research Council recently announced a funding opportunity “Environmental response to hydrogen emissions” to help reduce uncertainties.
Electric Car: BMW I Hydrogen Fuel Cell version of the X5 SUV (photo Marco Verch, Creative Commons 2.0)