This is an interesting white paper, Bruce-Iri et al 2021, METHANE Sources, Sinks and Uncertainties. Walter Jehne is one of the co-authors. I spoke with Walter at some length on the topic of monoterpenes and hydroxyl radical [OH] oxidation during a day long workshop prior to writing my blog, Ruminations- Methane math and context, back in 2018.
One of the main points of this Bruce-Iri et al white paper is that a lot of the current methane science is uncertain. The need for better methods for methane accounting was reflected in an excellent paper by Gasen et al 2019, Advancing scientific understanding of the global methane budget in support of the Paris agreement. Gasen et al, 2019 went into great detail as to the limitations of current bottom-up and top down accounting methods for methane emissions and methane sinks.
Thus another of the main points of the Bruce-Iri et al, 2021 white paper is that too many proposed policy narratives in response to ruminant enteric emissions are based on very reductive views derived from acontextual emission data. This is something I also argued at length in my 2018 Ruminations blog. But that reductive view has always been the argument for intensification. This is a reductive view of only enteric methane emissions that doesn’t account for soil and troposphere methane sinks or hydroxyl radical formation. So per this reductive view, offsets are excluded and enteric methane is only measured by masks, chambers or tracers to make the argument that the only way to reduce these GROSS (not net) emissions is through greater feed efficiency and shorter beef cattle lifespans. Almost all the feed additive (e.g. seaweed) science is based on this reductive methodology, even though reducing enteric CH4 as a by-product of rumen methanogenesis to improve feed efficiency is actually a very good thing.
One interesting and particular point that I hadn’t really looked at before was section 1.4 of Bruce-Iri et al 2021 as noted at the bottom of page 6 and continuing onto page 7 (and then further elaborated on in section 2.3). This section deals with quantifying the near-surface hydroxyl radical sink of methane and notes that there’s a pasture level green zone where surface level vegetation emit biological volatile organic compounds or BVOC (isoprene is noted… though I think it’s more likely pinene) which, in turn, create a hydroxyl radical oxidation zone to breakdown methane where cattle emit it. This intuitively makes sense because you need the monoterpenes (the BVOC) and water vapor transpired from plant stomata to interact with volatized nitrates (NO3) emitted from soil to form the surface level ozone to provide the excited oxygen needed to collide with water vapor to form hydroxyl radicals (Monk, P.S. et al. 2015) as follows: O3 + UV radiation–> O2 + O(+)–> O(+) +H2O–> 2OH. All of this can occur in what they’re calling the “vegetation interface”.
Thus what I like about this concept of a “vegetation interface” is that it reinforces the need to take an ecosystem approach to understanding methane emissions. Soils with methanotrophs are part of this total ecosystem view albeit a smaller one in regards to directly offsetting enteric emissions (Singh et al 2015). I’ve always made this argument, but I always abstracted hydroxyl radical oxidation as occurring somewhere up in the troposphere (see my 2020 blog, Methane: Accounting for both sides of the scale) …not down in a “green zone” or “vegetation interface”.
So this “green zone” strengthens my argument which has always recognized that desertification reduces hydroxyl radical formation and thus contributes to methane persistence due to less OH availability (plus more atmospheric CO2 due to less photosynthesis) as well as less rainfall since hydroxyl radicals are also needed to form ice nucleating particles (Wolf et al, 2020).
(Sidebar: I’ll discuss the connection between BVOC’s, hydroxyl radicals and rain cloud formation in another future blog post).
I think my mindset for hydroxyl oxidation happening somewhere “up” in the troposphere had a lot to do with the papers I read, including the ones referenced in this Bruce-Iri et al white paper, that look at monoterpene emissions more in the context of forests than grasslands. Trees emit isoprene, Grasslands emit pinene. I really haven’t seen many papers that look at these monoterpene (BVOC’s) emissions in the context of grasslands. I did though read an interesting paper (Brachmann et al. 2020) that tied increases of soil organic carbon [SOC] to lesser emissions of CH4 in alpine soils, which obviously makes sense due to better water dynamics through better soil aggregate formation and thus less anoxic conditions. So with more oxygen in the soil, the soil is less conducive to methanogenic archaea. Methanogenic archaea in anoxic places like compacted soils or the rumen of cattle are what produce biogenic methane.
Anyway, all sources of GHG emissions have a context..that context includes sinks….whether its tropical rain forests, beaver ponds, wetlands, termites, or cattle. Sadly, like I noted already, emissions of cattle have largely been taken out of context to rationalize feedlots. More recently and ironically, those rationalizations are also now being used to push ultra-processed “plant-based” foods and cell-Ag where large food corporations are trying to further consolidate and control the food supply through patent protected intellectual property. Many over zealous vegans are either the unwittingly pawns or willing prostitutes (e.g. George Monbiot) of these large food corporations.
Bruce-Iri, P. et al 2021, METHANE Sources, Sinks and Uncertainties.
Gasen, A.L. et al 2019, Advancing scientific understanding of the global methane budget in support of the Paris agreement
Monk, P.S. et all. 2015. Tropospheric ozone and its precursors from the urban to the global scale from air quality to short-lived climate forcer
Singh, J.S. et al. 2015. Methanotrophs and CH4 sink: Effect of human activity and ecological perturbations. Climate Change and Environmental Sustainability
Wolf, M. J . et al 2020, A biogenic secondary organic aerosol source of cirrus ice nucleating particles
Brachmann, C. G. et al. 2020. CH4 uptake along a successional gradient in temperate alpine soils