THE ENTERIC METHANE SAGA KEEPS ON COMING BACK!

Introduction: Methane of all greenhouse gases (GHG) has probably received the most attention in global environmental policy, and by some scientists, activists and the general public. Therefore, its perceived influence has been scrutinized by reputable scientists in research and commentary – the scientists covering the disciplines of environment, biology, chemistry, radiation and atmospheric physics. Methane’s influence in agriculture, in particular, has been centred around enteric emissions with cattle being regarded as the primary focus, to the extent that some outcries call for reduction in cattle numbers, even elimination in particular circumstances. This is because cattle are purportedly contributing about 10 - 15% of global methane (CH₄) emissions. Methane according to current regulatory acceptance has a global warming potential (GWP) of about 27 times that of carbon dioxide (CO₂). The question is how relevant is the figure of 27, given the research results and perspectives of scientists in the different disciplines – this being the purpose of this article.

The global accepted measure of GWP and suggested changes: All GHGs have a GWP value which expresses the ability of the gas to trap energy and heat in the atmosphere. Quantitatively water vapour is the most important of the GHGs, but being considered ‘’natural’’, it seldom enters the conversation, whereas CO₂, CH₄, N₂O (nitrous oxide), and chlorofluoro-hydrocarbons being often anthropogenic, and in that order, are of concern. Expressed in another way, GWP is an indicator of the potency of GHG emissions and the role they play in the warming of the atmosphere.

Since 1990, the standard metric used to indicate the heat-trapping ability of a specific GHG has been GWP100. This is the ratio of the absolute global warming potential (AGWP) of CH₄ (and the other GHG gases) to that of CO₂ at year 100 – with AGWP being the energy time-integrated radiative forcing after a single pulse of CH₄, and which represents the total energy trapped in the earth’s atmosphere by that pulse. GWP100 is the formally adopted standard under the Kyoto Protocol, the subsequent Paris Agreement and the Intergovernmental Panel on Climate Change (IPCC) reporting guidelines, and has been in operation for about 30 years. This means countries report to the IPCC their GHG loads, the targets set for reduction and the progress to that effect in the metrics relative to GWP100. It is important to note that the value of 27 of CH₄ is not a heat or energy factor per se, as is often assumed, but a dimensionless ratio between CH₄ and CO₂ at a particular point in time.

Since about 2018 some scientists began to challenge GWP 100 with respect to CH₄ (and the halo-GHGs, such as chlorofluoro-hydrocarbons) because they are short lived ‘’flow’’ gases, whereas CO₂ and N₂O are long lived ‘’stock’’ gases. Methane’s lifetime in the atmosphere is only 10-13.5 years, whereas the lifetime of N₂O is approximately 114-121 years and CO₂ can be more than a 1000 years. Thus, where GWP100 is appropriate for N₂O and CO₂, it is not for CH₄. The dynamics of CH₄ are also different as its decay follows a non-linear curve with rapid decay initially and slower tapering off after 12 years. The decay primarily results from oxidation by the hydroxyl (OH) radical which was the topic of an extensive discussion in last month’s Stock Farm, with the title: Fate of enteric methane in the atmosphere and biogenesis. As an alternative to GWP the scientists then proposed GWP* which estimates the warming of CH₄ over 20 years, rather than 100 years, to capture the non-linear nature and the more appropriate time frame. They could show that by doing so, the cumulative warming-equivalent of cattle emissions of CH₄ since 1981 were 28-44% lower when calculated with GWP* rather than GWP100. In addition, GWP* demonstrates that globally, net zero warming (no additional warming from cattle production) can be achieved by 2050 by reducing CH₄ emissions from cattle production at a sustained rate of 0.3% per year. This is equivalent to approximately 9% total reduction in CH₄ emissions from the cattle sector. From this knowledge, it is possible to construct hypothetical scenarios where the cattle sector can even go beyond net zero warming (i.e., declining temperatures) over a range of timelines. As a result, a plea by several countries and livestock institutions started, campaigning to report CH₄ separately from the stock gases using. Although being sympathetic this will be difficult, because GWP100 is the global standard, and to adopt GWP* would require coordinated changes across countries, industries and data systems. In addition, some groups have concerns that GWP* could be misused by high-emitting sectors and regions to appear climate-neutral with only minor reductions. Thus, one expects the implementation of GWP* will be delayed.

Apart from the delay, scientists recently started to ask whether GWP* is really that much of an improvement, as the 20 years, although an improvement, is still arbitrary. A measure must recognize the dynamics of CH₄ following emission: Methane when emitted is initially quickly oxidised by the OH radical in the troposphere in the biogenic cycle to CO₂ and H₂O, the CO₂ then returning to the earth through photosynthesis. Thus, accumulation does not really occur in contrast to the stock gases as CH₄ is continuously removed, provided the OH oxidation capacity is not exceeded, implying that what enters is balanced by what is removed. In practice though, biogenic CH₄ from natural sources (such as cattle) may increase if livestock numbers increase, and because this is supplemented by anthropogenic sources (fossil fuels, fires etc), the atmospheric CH₄ will steadily accumulate. As a side comment: If livestock numbers do not increase or decline as in South Africa, it can be calculated that the CH₄ influence on atmospheric warming is neutral or even declining. Moreover, it depends on how cattle is managed; grazing animals in extensive systems can even lead to the generation of new OH molecules that help offset the CH₄ emissions.

The question is now: if the GWP time frame calculations are too arbitrary, why don’t scientists investigate direct measurements of warming by the GHGs? The metric used is radiative forcing. This is discussed in the next section.

Radiative forcing: Radiative forcing (RF) is a measure of the imbalance between the energy entering the earth’s atmosphere from the sun as measured by the difference between incoming solar energy absorbed by the planet and the thermal energy radiated back into space. It is measured in Watts (W) per square meter; that of CH₄ being 1W/m². Greenhouse gas RFs arise due to absorption of thermal infrared (‘’long wave’’) radiation, but GHGs also absorb solar radiation. Methane has significant absorption bands between wavelengths 1.7 to 7.6 μm which absorb incoming solar ‘’shortwave’’ (SW) radiation and contribute to RF.

There is a considerable amount of variation in RF measurements because of methods used, cloud cover, season, whether measured over land or sea, and interpretation due to the scientist’s specific discipline. For example, although the absorption bands of CH₄ at the wavelengths above are agreed, since water vapour has similar SW absorption bands and water vapour is much more abundant than CH₄, water vapour absorbs vastly more energy. Hence, any SW radiation that CH₄ might absorb has already been absorbed by the water vapour. The ratio of the percentages of water vapour to CH₄ is such that the effects of CH₄ are completely masked by the water vapour. The amount of CH₄ must increase 100-fold to make it comparable to water vapour. The small effect in the SW region is supported by some other studies, which are based on calculations that include satellite measurements of CH₄distribution and spectrally varying surface albedo (the measure of how much sunlight is reflected by a surface or body), as well as absorption of solar mid-infrared radiation by methane's 7.6 μm band. These factors substantially influence methane's SW effect. For a 750 to 1,800 ppb increase, the all-sky top-of-atmosphere SW instantaneous RF is 0.082 W/ m²; at the troposphere-stratosphere interface it is 0.002 W/m², considerably smaller than previous estimates.

Effects on change in RF and temperature: The arguments presented on absorption and wave band evidence, are supported by minute RF and temperature increases in the atmosphere: (1) One study showed that CH₄ levels in the atmosphere are slowly increasing, If the current rate of increase, about 0.0076 ppm/year, were to continue unchanged, it would take about 270 years to double the 2020 concentration. Methane concentrations may never double, but if they do, this would only increase the RF by 0.8 W/m². This is a tiny fraction of representative total RFs at mid-latitudes of about 140 W/m²at the troposphere-stratosphere interface and 120 W/m²at the top of the atmosphere. (2) The IPCC uses a scenario where GHG emissions peak at 2040 and then begin to decline (RCP 4.5), and a stringent low-emissions scenario (RCP 2.6) to model future climate change. Modelling showed that by bringing down the RCP 4.5 final CH₄ emissions to RCP 2.6 levels, there would only be a 0.22◦C temperature difference at 2300. (3) Sabotage of two underwater pipelines in the Baltic Sea happened in 2022. Massive quantities of natural gas, primarily CH₄, were released into the atmosphere, which lasted for about one week. Using multiple methods and datasets, a recent study reported a relatively accurate magnitude of the leaked CH₄ at 0.25 million tons. A negligible increase in global surface air temperature of 1.8 × 10^{-5 o}C in a 20-year time horizon caused by the CH₄ leaks was calculated.  

Conclusions and implications: The global proposals to place harsh restrictions on CH₄ emissions because of warming fears are clearly not justified by the facts presented. This is not synonymous to denying atmospheric warming as the stock gases, in particular CO₂, plays a major role, if not overwhelming, and should be reduced urgently. What is interesting though is that many recent observations support an emerging picture that halo-GHGs made the dominant contribution to global warming in the late 20th century and that a gradual reversal in warming has occurred since ~2005 due to the phasing out of halo-GHGs. For CH₄, although of diminutive importance in this debate, there remains the distinction between biogenic and anthropogenic sources. For the biogenic, the OH radical driven oxidation and carbon sequestration, as discussed in last month’s Stock Farm, with the title: Fate of enteric methane in the atmosphere and biogenesis, needs to be maximised and research to that effect should be maximised. For the anthropogenic, fossil fuels in industrialization and otherwise should be phased out as far as feasible. It is interesting to observe in the map below that where livestock dominate such as in Brazil and Argentina with a joint cattle population of about 390 million and the rural areas in southern Africa and Australia, the atmospheric CH₄ content is 1650-1700 ppb, versus 1800-1850 ppb in industrialised (or polluted) areas in China, India, New York etc. This probably support the lower influence of livestock and the positive influence of biogenesis.

Globally, and in South Africa, there is a thrust to emphasize reduction in enteric CH₄ in livestock by means of additives in feed and genomic and other selection methods of low CH₄ animals. This is clearly not justifiable. The selection and feed additive research should rather emphasize efficiency of production, and then if it so happens that enteric CH₄ is reduced, then it is a bonus. Some additives tested do not improve efficiency and then the added cost of the product doesn’t make it feasible. In selection, cognisance should be taken that the goal in production systems differs. In more intensive systems animals with lower enteric fermentation will likely either have lower intakes for a particular production level, or faster exit of digesta from the rumen; either way this will lower CH₄ production and increase efficiency. In extensive systems and drought conditions though, which dominate in southern Africa, the more efficient animals will produce or survive because they either eat more, or digest better or both, and because the diet is more fibrous, CH₄ production in the rumen will be higher. Thus, this is a survival mechanism under more adverse conditions, and the reason why wild herbivores over millennia and across most continents have survived.