Hydrocarbon GWPs and Indirect GWPs

The hydrocarbon GWPs listed in Annex IV of the F-Gas Regulation are IPCC AR4 values. A recent paper reports revised GWPs for these hydrocarbons.

GWPs for some hydrocarbons
F-Gas Regulation 517/2014 Annex IV (from AR4) From 2018 Publication [1]
ethane R-170 6 10.2
Propane R-290 3 9.5
butane R-600 4 6.5

 

[1] Reference: Lifetimes, direct and indirect radiative forcing, and global warming potentials of ethane (C2H6), propane (C3H8), and butane (C4H10), Atmos. Sci. Lett. 2018;19:e804. wileyonlinelibrary.com/journal/asl2 https://doi.org/10.1002/asl.804  Øivind Hodnebrog,  Stig B. Dalsøren, Gunnar Myhre, Center for International Climate and Environmental Research-Oslo (CICERO), Oslo, Norway

The hydrocarbon GWPs (from 2018 paper) have an indirect GWP component accounting for over 99% of the total GWP for propane and butane and over 95% for ethane.

Indirect GWPs

In simple terms, direct GWPs are the warming effect over a specific time period usually 100 years for a substance

What are indirect GWPs? “Indirect radiative effects include the direct effects of degradation products or the radiative effects of changes in concentrations of greenhouse gases caused by the presence of the emitted gas or its degradation products.” From IPCC AR4 (Fourth Assessment Report).

What is excluded from indirect GWPs? “Following the approach taken by the SAR and the TAR, the CO2 produced from oxidation of CH4, CO and NMVOCs of fossil origin is not included in the GWP estimates since this carbon has been included in the national CO2 inventories. This issue may need to be reconsidered as inventory guidelines are revised.” From IPCC AR4.

What is the magnitude of indirect GWPs? Direct effects of degradation products for the greenhouse gases are not considered to be significant. The indirect effects are linked to ozone formation or destruction, enhancement of stratospheric water vapour, changes in concentrations of the OH radical with the main effect of changing the lifetime of CH4, and secondary aerosol formation.” From IPCC AR4.

For other short-lived substances, such as the very low GWP HFOs and HCFOs, indirect GWPs are not reported in IPCC AR4 or AR5. Their potential for formation of photochemical ozone at ground level is low for these compounds (compared to the HCs), which might be expected to influence their indirect GWPs. The recent Scientific Assessment Report for Ozone stated “The photochemical ozone creation potentials of HCFCs, HFCs, and HFOs are very small, and tropospheric ozone formation resulting from their degradation is of negligible importance”. There does not appear to be any published literature on any possible indirect GWP methane concentration effect.