IN BRIEF: DIRECT AND INDIRECT GWPS FOR VERY LOW GWP SUBSTANCES
In simple terms, direct GWPs are the warming effect of emitting 1 kg of a substance compared to 1 kg of CO2 over a specific time period usually 100 years. Substances with very short atmospheric lifetimes typically have direct GWPs that are very low (eg <10, or >1). Indirect GWPs for some substances, notably hydrocarbons, are referenced in the F-Gas Regulation in Annex IV and for some very low GWP substances, indirect GWPs can be much larger than their direct GWPs.
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. [1]
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.
What contributes to indirect GWPs? For non-ozone depleting substances, such as the non-methane hydrocarbons (NMHCs) the main contributors to indirect GWPs are through tropospheric chemistry interactions, methane concentrations increase when the hydroxyl radicals are consumed by the more reactive organic compounds emitted during anthropogenic activities. The photochemical production of ozone, also a greenhouse gas, is also enhanced by an increased burden of NMHCs.[2]
The F-Gas Regulation 517/2014 in Annex IV references IPPC AR4 GWP values where available. The GWP values for hydrocarbons are the indirect GWP from AR4 (see table). A recent paper [3] has published revised indirect GWPs for some hydrocarbons and these are also shown in the table, with the indirect GWP component accounting for over 99% of the total GWP for propane and butane (over 95% for ethane).
For other short-lived substances, such as the very low GWP HFOs, indirect GWPs are not reported in 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 [4] “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.
Even taking into account indirect GWPs, for all these very low GWP substances, the RTOC 2018 report considers that GWPs<30 are ultra-low or negligible. [6]
Reproduced From RTOC 2018 page 41
References
[1] IPCC AR4 Chapter 2 Changes in Atmospheric Constituents and in Radiative Forcing 2.10.3 Indirect GWPs page 214
[2] IPCC/TEAP Special Report: Safeguarding the Ozone Layer and the Global Climate System page 163
[3] 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
[4] See the 2018 Scientific Assessment of Ozone Depletion Section 6.2.2
[5] IPCC AR4 Chapter 2 Changes in Atmospheric Constituents and in Radiative Forcing Table 2.15
[6] 2018 TOC Refrigeration, A/C and Heat Pumps Assessment Report Chapter 2 Refrigerants page 41