At a Glance
Hydrogen (H2 ) plays a crucial role in global energy scenarios aimed at achieving net-zero. Because it is not a greenhouse gas, H2 has been touted as an alternative to fossil fuels in certain energy sectors. But the environmental consequences of perturbing the global hydrogen cycle are still largely unknown. Specifically, there are concerns around hydrogen interference with the methane (CH4) atmospheric sink by the hydroxyl radical (OH). To sharpen future H2 projections, the Porporato group has been quantitively addressing the major sink of tropospheric H2 , namely the soil uptake by bacteria, and the methane feedback of H2 fugitive emissions. This research informs bp’s aims of developing the H2 economy in a manner that minimizes adverse climate impacts.
Hydrogen will play a crucial role in the decarbonization of energy systems and may provide a cost-effective option to replace fossil fuels in applications where emission reductions are difficult, such as in heavy transport. However, large scale H2 production with its consequent fugitive emissions may increase the H2 atmospheric concentration, which currently hovers around 530 ppb (Novelli et al., 1999). The environmental implications of this are not yet clear.
Hydrogen has an indirect global warming effect due to its interactions with other greenhouse gases (GHG) in the atmosphere, such as, for example, ozone, water vapor, and methane. Indirect radiative forcing of H2 is expected to be small compared to that of fossil fuels. However, recent global climate models (Paulot et al., 2021) have raised concerns about some possible consequences of an increasing concentration of hydrogen on tropospheric methane (CH4), the second most important GHG.
To improve our understanding of the global hydrogen cycle and the consequences of its possible perturbation, Porporato’s research group has refined the modeling of the major sink of atmospheric H2, namely the uptake by soil bacteria, and quantitatively addressed the methane feedback of H2 fugitive emissions.
H2 uptake by soil bacteria currently accounts for nearly 80% of tropospheric removal (Ehhalt and Roher, 2009) and is widespread in all ecosystems worldwide, including extreme environments (Ji et al., 2017). The main abiotic driver of the uptake is soil moisture, which influences both the biotic (the bacteria metabolism) and the abiotic (H2 diffusion through the soil) processes that govern the H2 uptake.
Bertagni et al. (2021) have improved the mechanistic representation of H2 uptake as a function of soil moisture and highlighted the influence of rainfall-driven moisture variability on the H2 biotic consumption (Figure 5.1). Results show that limitations to the uptake are ecosystem dependent. H2 diffusion through the soil generally limits the H2 uptake in humid temperate and tropical regions, while biotic limitations tend to occur in arid or cold regions.
An increase in global average temperature is expected to slightly favor the uptake on a global scale, while shifts in rainfall regimes can be important drivers of H2 uptake changes at the local scale (Figure 5.1). Results also suggest that, although there is a greater potential of H2 consumption by soil bacteria, worldwide-spread diffusive limitations will likely impede the complete offset of additional H2 emissions.
It is crucial to accurately evaluate the feedback on the methane burden because H2 fugitive emissions are likely to increase the H2 atmospheric concentration. Porporato’s group has been addressing this problem using a box model for the coupled atmospheric system CH4– H2 -OH (Bertagni et al., in preparation). The tropospheric budgets of H2 and CH4 are, in fact, deeply interconnected. Methane is a primary precursor of hydrogen, and the two gases share the sink by the hydroxyl radical OH. Furthermore, H2 and CH4 are linked at the industrial level because most of current and near-term future H2 production comes from steam methane reforming.
Model results show that H2 emission pulses cause a small, transient growth of tropospheric CH4 that slowly decays in a few decades. Moreover, the replacement of fossil-fuel energy with renewable or low-carbon hydrogen can have very different consequences for tropospheric CH4. This depends on the H2 production method and the amount of H2 lost to the atmosphere (Figure 5.2). For renewable H2 , tropospheric CH4 would decrease due to the fossil-fuel displacement only if the rate of H2 leakages is kept below a critical rate. This critical rate is around 8%, but with an uncertainty interval of between 5 and 11% related to how OH consumption is partitioned among the tropospheric gases and how much of atmospheric H2 is consumed by soil bacteria. For blue H2 , not only would the CH4 emissions increase, but the combination of CH4 and H2 leakages may have undesired consequences for the tropospheric burden of CH4 (Figure 5.2).
These preliminary results call for more precise estimates of future H2 leakage rates and additional analyses with high resolution three-dimensional atmospheric chemistry models.