Sensitivity of climate effects of hydrogen to leakage size, location, and chemical background

When hydrogen is used as an energy carrier, some hydrogen will leak into the atmosphere during production, storage, transport, and end use. Hydrogen itself is not a greenhouse gas, but via chemical reactions in the atmosphere, the leaked hydrogen will affect the atmospheric composition of methane, ozone, and stratospheric water vapor and hence radiation in the atmosphere. A recent multi-model study found the Global Warming Potential over a 100-year time horizon (GWP100) to be 11.6 ±2.8 (one standard deviation). In this study, a chemistry transport model (OsloCTM3) is used to investigate the sensitivity of the calculated GWP100 due to the size of the hydrogen perturbation, the location of the hydrogen perturbation as well as the chemical composition of the background atmosphere.

The hydrogen perturbation of an additional 0.1, 1, 10 and 100 Tg yr^-1 of anthropogenic hydrogen emissions gave GWP values that differed by only 0.4. To test the sensitivity of the location of the perturbation, 1 Tg yr^-1 was added to seven different sites around the world. Perturbations at sites that are further away from dry deposition areas (such as middle of the ocean and in Antarctica) resulted in feedback factor larger than one. The GWP values were enhanced compared to perturbations at sites influenced more by dry deposition where feedback factor was less than one. The difference in GWP100 between the two most extreme sites was 4, less than the width of the ± one standard deviation range from the multi-model GWP100 study.

The hydrogen economy is expected to grow, and in the future, the atmospheric composition might be different than the 2010 atmosphere used to calculate GWP100 in the multi-model study. To check the sensitivity to this the GWP100 is calculated with the perturbations on top of three different 2050 atmospheres using different SSP scenarios. The three different SSPs had different combinations of NOx to CO emission ratios and methane levels that both influence the atmospheric lifetime of hydrogen. The atmospheric lifetime increased in all the scenarios, and in SSP4-3.4 by as much as ~1 year. However, the dominant control on the total lifetime of hydrogen is the soil sink. Thus, future changes to the soil sink should be investigated, with a focus on how it influences the calculated GWP.