Ruminations: Methane math and context

“For me context is the key – from that comes the understanding of everything,”                  Kenneth Noland

Preface: This is pretty much a work in progress reflecting my evolving understanding of methane in the atmosphere. The following thoughts will be amended and many references will be added to this paper as my thinking further evolves on this topic.

Reductionism

1. the theory that every complex phenomenon, especially in biology or psychology, can be explained by analyzing the simplest, most basic physical mechanisms that are in operation during the phenomenon.
2. the practice of simplifying a complex idea, issue, condition, or the like, especially to the point of minimizing, obscuring, or distorting it.

Rain forests, peat bogs, wet lands, and wild ruminants all emit large amounts of methane. Using a reductive mindset, cutting down rain forests, draining wet lands/peat bogs, and extirpating huge herds of bison (and other wild ruminants) all reduce or reduced methane emissions. Rain forests are one of the largest sources of on going sources of atmospheric methane (Pangala et al. 2013), though like peat bogs, store a lot of carbon which more than offsets the methane emissions primarily from anaerobic decay of organic matter. (When the forests are cut down and burned, all that stored carbon is released all at once into the atmosphere primarily as CO and CO2, but to a lesser extent as CH4. Rain forests store almost all their carbon in their biomass, not the soil).

If methane over a short time exceeds carbon dioxide by eighty or hundred times (using a time frame of 20 years opposed to a longer time frame of 100 years), than all of these changes reducing methane from a reductive perspective should be a good thing. In North America we’ve drained ninety-five percent of all the wetlands, and in terms of numbers, replaced most of the wild ruminants (elk,  caribou, bison, pronghorn, moose, wild sheep, wild goat, and deer) with fewer domestic ones (cattle, sheep, goats). But atmospheric methane rates have gone up, not down. So something else must be going on. There’s always a larger context. With methane emissions, there are numerous sources of methane emissions including fossil fuel methane emissions. There are also methane sinks. There are other offsets from both greenhouse gases and indirect gases. Plus there are many ecosystem impacts that have to be accounted for in the methane math. (Not to mention a lot of the general methane math  or”equivalencies”  is false since methane is short-lived compared to carbon dioxide that can stay in the atmosphere for centuries and thus accumulate in much higher quantities (Allen et al. 2018). But that’s an entirely different story than the one I wish to convey here).

But cattle are typically looked at out of context. Only emissions are counted for but not sinks or other greenhouse gas offsets or ecosystem benefits. Emissions are easier to extrapolate than sinks, since sinks have more variability and are thus harder to measure. With methane emissions from cattle, enteric methane is the driver, since the amount of methane cow’s belch far exceeds methane from manure lagoons or dung dropped on fields.  So the primary methane math equation is a reductive one as follows:

Total methane emissions = number of animals x lifetime of animal x methane emissions per head per day.

In a prior entry, “A measure of make believe,” how methane emissions per head per day are calculated is discussed in some detail. Basically methane is measured at the source via masks, chambers or tracers. So cattle are essentially treated like tail pipes. Such math has been used to promote “intensification” by 2006 DEFRA and 2006 Long Shadow’s authors because the way to reduce methane emissions per this equation is to shorten the lifespan of the animals.  Therefore to increase gains over a shorter period of time, the logical conclusion per this mindset is to use feedlots, growth hormones, and genetics to grow larger cattle for slaughter in a shorter period of time. Thus total production is higher and total enteric emissions are less. Sounds simple, doesn’t it? This mindset is often used as an argument against grass finishing of cattle, since cattle finished on grass have to live longer to make harvest weight.

However, even within this mindset, higher quality feeds and supplements (e.g. seaweed) can reduce emissions per head per day quite significantly. On range lands or pasture, grazing management can also increase gains and feed quality significantly reducing methane emissions within just this reductive equation as demonstrate in recent range land research (Rowntree et al 2016, Teague et al 2016).

More recently, Rowntree and other researchers like Teague, Wang, Stanley have looked at methane emissions in the context of overall greenhouse gas emissions accounting for offsets from soil carbon sequestration and reduced use of nitrogen fertilizers. One recent study by Stanley and Rowntree that has gotten a lot of press. This study, Impacts of soil carbon sequestration on life cycle greenhouse gas emissions in Midwestern USA beef finishing systems (Stanley et al 2018), has shown during the finishing phase cattle left on pasture and managed with adaptive multi-paddock (AMP) management can offset enough carbon in the soil and reduce nitrous oxide emissions to be carbon negative meaning that the reductions from soil carbon sequestration and nitrous oxide emissions significantly exceeds the amount of enteric methane emitted.

Well, there’s plenty of debate on how much carbon soil can be sequestered and for how long until soil becomes “saturated.” This debate is discussed in another recent entry, It’s the Soil Biology Stupid. In short enhancing soil biology increases the capacity of the soil to store more carbon. A lot of older research doesn’t account for soil biology as a parameter in the soil carbon accounting analysis. Plus the notion of soil becoming “saturated” may also be a bit obsolete given new soil science research demonstrating how soil organic matter is built and how the microbial carbon pump works. Or, in other words, as long as photosynthesis is occurring with diverse plant covers, more carbon is being pumped into the soils building more soil through accumulated microbial necromass and waste. New soil can be built within years, not hundreds of years as was previously thought. Though many hang onto old beliefs.

Before discussing a broader methane context for enteric methane math, here are some abbreviations for reference:

CH4 = methane; CO  = carbon monoxide; O2  = oxygen; O3 = ozone; UV = ultra violet light; OH = hydroxyl radicals; CO2 = carbon dioxide ; N2O = nitrous oxide; NOx = Nitrogen oxides; HO2 = Hydroperoxyl radical   

Methane is emitted in large quantities from numerous sources including natural resources like peat bogs, wetlands, rain forests as well as via methanogenesis (enteric methane) from various insects (cockroaches, termites, arthropods), mollusks and wild ruminants. Methane is also emitted in large quantities from manmade, anthropogenic sources like landfills, burning of biomass, compost piles, fossil fuels, rice cultivation, manure lagoons, and domesticated ruminants. There are also methane hydrates which are trapped sources of methane.  Some of these sources are easier to measure with more certainty than others as to how much methane is emitted. For example, counting head of cattle is much easier to do than counting cockroaches or termites. Counting head of cattle is also much easier to do than determining how much methane is lost via leaky pipes from old natural gas fuel infrastructure.

Methane is accounted for two ways: bottom up and top down. Bottom up takes existing data for various sources (inventories) and sums up all the amounts using process models. Again with data from some sources there’s a lot of variability and uncertainty. Therefore the data from more certain sources (e.g. cattle) tends to get more heavily weighted in such bottom-up analysis. “Bottom-up methods suffer from uncertainties and potential biases in the available activity data or emissions factors or the extrapolation to large scales of a relatively small number of observations. Furthermore, there is no constraint on the global total emissions from bottom-up techniques.” (Rigby et al 2017). Top down analysis is where amounts of methane are inferred from atmospheric chemical transport models. The problem with top down analysis is that it is hard to ascertain where exactly the methane in the atmosphere is coming from, though different sources of methane do have slightly different signatures. Another larger problem is that bottom-up and top down analysis result in a significant mismatch between the two methods (Kirschke et al 2013).

(From Prinn et al., 1995)

The way methane CH4 is discussed in the media and many institutions, one could easily  get the misimpression that all the various sources of CH4 just goes up into the atmosphere unabated and continue to accumulate into a massive cloud causing heat to be trapped. The reality is a bit different. Most methane is short lived. Most methane after being emitted is oxidized in soil, water or in the atmosphere. Oxidization reduces CH4 to water and CH3+, which in turn reverts to CO2 and becomes part of the carbon cycle. Sources of CH4 emitted from numerous sources is oxidized in the ocean or soil before it gets into atmosphere. This is done via “low affinity oxidation” and is not considered part of the atmospheric methane sink. Soil can oxidize methane that gets into the atmosphere but only to a small extent. However, when soil doesn’t oxidize methane before it gets into the atmosphere, more methane is released into the atmosphere. Soil methane oxidation is largely done by methanotrophs, a form of bacteria.

Many industrial and conventional agricultural practices (tillage, bare fallows, and syn N inputs) undermine or destroy methanotrophic communities. Many industrial and conventional agriculture practices (again tillage, bare fallows and syn N inputs) also become large sources of other greenhouse gas (CO2, and N2O) emissions. So certain forms of agriculture and land management (e.g. grazing and conservation Ag) that reduce tillage, bare fallows, and syn N inputs can reduce the amount of CH4 (from numerous sources), CO2 and N2O into the atmosphere.

Another hypothesis floated out there in this video, GFE 2017 – Russ Conser ‘Cattle & Methane,  based on a review of methane emission intensity maps, is that organic matter decomposing in anaerobic compacted soils is also a significant source over a large land area. This really hasn’t been researched yet. Though if indeed correct, soils that should be approximately five or so percent of the CH4 atmospheric sink instead become a source of CH4 emission. Say such soils emit around three percent of atmospheric emissions, then the net swing would be an increase in emissions of around eight percent. This swing may not be enough to offset all cattle enteric emissions, but as will later be demonstrated is enough to reduce enough CH4 to make overall CH4 emissions reach a steady state (balance between emissions and sinks) or even better, decline due to the short lived nature of methane..

So some cow burps may be directly offset in the atmosphere by more effective methanotrophic soil sinks, but since most of these enteric emissions from cattle are already in the atmosphere, the soil sink only provides a small direct offset to these enteric emissions. The larger direct benefit of grazing range lands, cover crops, and crop residues is the reduction of emissions CH4 from other natural sources of CH4 as well as the reduction of emissions from other greenhouse gases. There’s also a significant reduction in the amount of CO2 used to produce synthetic nitrogen for synthetic fertilizer to grow feed/ethanol/seed oil crops. This reduction also has to be accounted for in the overall GHG math (Stanley et al 2018).

 Layers of the atmosphere. [Credit Bredk]

Though once methane gets into the atmosphere, including from cow burps (enteric methane), that methane isn’t free to accumulate. Most methane (and a number of other trace gases) emitted is oxidized in the lowest level of the atmosphere, the troposphere, by hydroxyl radicals [OH] (Prinn 2014) and to a lesser extent in the next level of the atmosphere, the stratosphere. So between the soil sink, troposphere and stratosphere sink, most methane is oxidized (see figure below). What isn’t oxidized is the small amount that accumulates and doesn’t break down for a longer period of time.

(from Methane UK report 2000)

So levels of atmospheric methane are directly related to the amount of OH that’s available in the troposphere to oxidize methane (Rigby et al 2017). OH availability though is impacted by how much water vapor, CO and NOx is also in the atmosphere. To balance out the sources of emissions with the sinks, one has to reduce methane emissions by approximately 3 to 4% (so the theoretical soil sink figures noted above would more than suffice). This can be done through reducing emissions from a number of sources and increasing OH availability. Such a realization puts enteric methane into a broader context. Well managed cattle can increase OH availability in two ways, by increasing water vapor from transpiration and decreasing CO, and CH4 from burning biomass.

Let’s look at these two latter points in more detail.

Well managed cattle and other ruminants, by enhancing soil biology via their saliva/urine/manure in a diverse pasture or cover crop system, increase carbon sequestration. This in turn increases the water retention of the soil since in some soils per the NRCS, an increase of one percent of soil carbon increases the water holding capacity of the soil in the top six inches of soil per acre up to 26,000 gallons of water. Thus there’s greater drought resistance and greater plant growth. More plant growth, in turn, leads to more transpiration from plants and thus more water vapor. When ozone is zapped by UV light, ozone become oxygen and an excited oxygen atom as follows:  O3 + UV –> 02 + O*. The O2 becomes part of the atmosphere while the excited O* atom bonds with water vapor as follows to form hydroxyl radicals: O* + H2O —> 2OH. This is the primary pathway for forming OH. OH again is what interacts with methane to oxidize methane OH+ CH4—> H20 + CH3+. It’s a much more extensive process as shown in this figure below. (Another way to forming OH as shown in this diagram is via NOx as follows: NO + HO2 –> NO2 + OH).

(From Appendix 1 of Methane UK)

So the end result of an improved soil ecosystem with more transpiration from the stomata of plants is the creation of hydroxyl radicals that then are available to oxidize CH4 and other trace gases. As noted, in a recent seminar with Australian soil scientist Walter Jehne, this increased transpiration includes bacteria within the stomata that rise up into the clouds and become precipitation nuclei causing water droplets in clouds to become rain fall. Rain follows vegetation since water vapors helps build clouds (Sheil, D. 2014). So the whole hydrologic cycle improves.

Now without grazing livestock, un-grazed grasses continue to grow until they die off for the season. When the grasses die off, they emit all the CO2 they stored in the above ground biomass. Certain species of grasses also become dominate and shade out other species. As show in this video, “What we are learning about grazing”  this eventually leads to less overall plant diversity and less plant life to transpire water vapor. There’s less soil carbon and soil life below ground as well. This is a downward cycle leading to more arid environments with compacted soils that don’t retain water. Thus even less plant growth and less transpiration. Such observations further the understanding that grasslands co-evolved with grazers (Retallack 2013, Stebbins 1981) including large herds of ruminants like auroch, bison, elk, and wildebeest. Such observations also demonstrate the important role that grazers play in carbon sequestration, water vapor and hydroxyl radical formation, and thus methane oxidation as well.

Cattle and other domestic/wild ruminants also are mobile bio-digesters, so in arid environments, where other moisture isn’t as readily available, they break down plants into soil microbial digestible material. This reduces plants as a source of fuel for the burning of biomass. As noted previously biomass is a large source of CH4. It’s also a large source of CO and CO2. Thus burning biomass not only emits CH4 but it also reduces OH’s availability for oxidation two ways- first via more methane and second via CO which binds with OH to produce CO2 as per this reactions CO + 2OH —> CO2 + H2O.

In prior measurements, massive fires in Indonesian Russian and North America have decreased OH levels. Fires alone in Indonesia in 1997 alone lowered OH levels by 6% (Prinn et al. 2005).  Again the less OH there is, the less methane gets oxidized. So reducing the frequency and  intensity of fires is one way to significantly reduce CH4 (and CO2) levels directly and indirectly.

As reported by  Boise’s 7KTVB Dean Johnson in his report, BLM using cattle to decrease wildfire risk, recently to help prevent more catastrophic fires in Idaho, the Bureau of Land Management [BLM] has teamed up with ranchers on a new three to five year experiment called the Targeted Grazing Project to see if cattle grazing can be used to provide fuel breaks for fires. If successful, the fuel breaks will help prevent fires from spreading and make fires smaller. Elsewhere other ruminants, like goats in California, have been used to clear out undergrowth which again reduces fuel for fires.

Hydroxyl radicals [OH] are short lived and their chemical reactions occur rapidly.  Thus measuring OH levels directly in the atmosphere is difficult.  So when doing methane math, the accounting for the efficiency of sinks isn’t simple or easy to do. Many of these variables that can function as offsets may be negated by different production methods, inappropriate land use and or poor livestock management.  So when looking at methane emissions in a broader context as an equation by themselves or as part of total greenhouse gas emissions, as in the below equations, there’s a lot more variability and uncertainty with the results.  The +/- in the equations below indicate that the following variables may either add to or subtract from the overall methane or GHG emissions.

Total methane emissions = (number of animals x lifetime of animal x methane emissions per head per day)  +/- soil sink offset +/- water vapor/improve hydrology +/- fire prevention

Total GHG emissions = (number of animals x lifetime of animal x methane emissions per head per day)  +/- soil sink offset +/- water vapor/improve hydrology +/- fire prevention +/- N2O production +/- soil carbon sequestration

As noted previously, what’s frequently measured and looked at is what’s most easily quantifiable. The math then is simply reduced to the original equation presented above where cattle are pretty much treated like tail pipes without any context.  Whether these “tail pipe” emissions  add to or offset emissions has to do with context and management.  Rain forests slashed and burned to continuously graze cattle is a completely different context than grasslands or oak savanna restored and protected  and grazed by AMP managed cattle. In the first destructive scenario, water vapor is reduced and CO/CH4 is increased through fire. While in the second scenario, methanotrophic sinks and OH availability are increased through increased water vapor and decreased fire emissions. So context is pretty much everything.

Everything is also interconnected: soil carbon impacts water retention which impacts plant growth. Plant growth, in turn, impacts transpiration with water vapor impacting OH formation and availability, and thus methane oxidation. So carbon, hydrologic and methane cycles are all interconnected with system interactions. Due to these interactions, a reductive approach looking at these cycles independently doesn’t provide a full understanding of what actually is occurring with green house gases in the atmosphere. 

When man tries to improve and intensify production with feedlots, other GHG emissions (N2O, CO2) from crop production offset the direct reductions of enteric emissions. Though even with feedlot systems, there’s room for improvement with grazing management on cow/calf and stocker operations. There’s also room for improvement with how grains are grown. Regenerative systems with inter-cropping and covers can replace industrial monocrops without covers.  Cover crops and grazers (or crimpers) reduce the need for synthetic nitrogen and eliminate  bare fallows and tillage (significant sources of atmospheric CO2). So there are various points within different production methods for dramatic improvements that increase carbon capture, enhance OH availability, while decreasing CO2 and N2O emissions from tillage, bare fallows, and synthetic N usage.  Though once the cattle end up in a feedlot to finish on bare dirt any other ecosystem benefits are lost. With feedlot cattle, and CAFO’s in general, methane emissions from manure lagoons also becomes a much larger and significant concern.

If cattle, in the proper context, aren’t to blame for huge rises in atmospheric CH4 levels, what sectors or other sources of CH4 are to blame?

Looking at a graph of rising methane levels over the past 260 years (see graph above), most methane levels started to accumulate significantly during the industrial age and then rise rapidly from the 1950’s to the 1980’s before leveling off for a brief time between 1998 and 2007 (see graph below). CH4 levels started rising again in the past ten years. Prior to the use of coal, there were plenty of wild ruminants in North America as well as considerably much larger areas of wetlands, so there were plenty of natural occurring sources methane emissions. Though it’s hard to get exact numbers, estimates of 30 to 70 million bison, 30 million elk, 20 million pronghorn, etc exceed current domestic populations of ruminant in the United States (Hristov, A 2011).  Globally wetlands have been significantly reduced. In California alone, 95% of wetland areas have been drained (Drexler et al. 2009). These naturally occurring sources of CH4 haven’t changed much. If anything emissions from wetlands and peat bogs have gone down.

What occurred in from 1950 to 1980 was also not a huge rise globally in cattle population. Global inventory of cattle today is the same as it was in 1970 while US inventories are significantly lower than they were in the 1970’s (see chart below ). What occurred was a huge increase of natural gas as a fuel source, especially to generate electricity.  After the initial boom in the 60’s and 70’s, better management of excess natural gas leaks during extraction and transportation led to less excess accumulation of CH4 in the atmosphere, so levels of accumulating CH4 started to slow  down in the 1980’s before plateauing in 1998.

(Source: Food Safety News)

Recent increases of atmospheric methane (post 2007) also appear not to be due to increase burning of biomass, since emissions of methane decreased by 3.7 (±1.4) Tg CH4 per year from the 2001–2007 to the 2008–2014 time periods. Wetter years associated with La Nina during the 2008 through 2014 time periods likely contributed to the observed decrease in fire emissions in South America and Indonesia (Worden, et al 2017).

Dr. Robert Howarth, an earth systems scientist and professor at Cornell University, in his presentation at UN Climate Change COP23,  Is the Global Spike in Methane Emissions Caused by the Natural Gas Industry or Animal Agriculture? Reconciling the Conflicting Views as well in this recorded online webinar,  Methane and Global Warming in the 21st Century clearly demonstrated that recent rises in atmospheric methane since 2007 are primarily due to the shale gas extraction and old leaking natural gas infrastructure, not cattle.  When the shale gas industry started and increased production (approximately 2006) in the US was when methane levels started to rise again. This was at the same time when US cattle numbers were decreasing.  So, in the US and Canada, there’s no correlation between recent rises of CH4 and cattle.

In the webinar, Howarth demonstrated how other recent studies (specifically Schaefer et al. 2016), that got a lot of media attention blaming cattle, wrongly attributed CH4 emissions to cattle that should have been attributed to shale gas. In these studies, Howarth stated these other researchers mistakenly believed the signature carbon 13 isotope in emitted CH4  for shale is the same as that for other fossil fuels. In reality, this carbon 13 isotope from shale sometimes looks like a biogenic source (e.g. from cattle). Thus the wrong attribution.

Howarth continued to note, shale gas obtained via high precision directional drilling and  high volume hydraulic fracturing produces up to three times higher the amount of methane as prior conventional methods of natural gas extraction (Howarth et al. 2011, Caulton et al 2014). Plus globally most recent increases since 2007 in CH4 emissions have come from the United States and Canada. This is where most shale gas extraction is occurring.  Unfortunately, he also adds that the natural gas industry has exerted pressure on the EPA to under assess the impacts of their production and distribution of natural gas.  EPA emission numbers have repeatedly under assessed emission numbers for both conventional and shale natural gas production.

In this below slide from Howarth’s presentation, he provided 2015 estimates of methane levels and sources showing where he believes most of the increases have come from since the last major inventory of CH4 was taken in 2000. These estimates are his red marks.

Howarth in his webinar presentation distinguishes between two types of methane:   geologically ancient methane (released in obtaining fossil fuels) and recently formed biological decomposition in absence of oxygen. The ancient form of methane formed over million of years. The biological form of methane (including enteric methane from cattle) is formed in real time by bacteria decomposing organic matter in anaerobic conditions.

As the 260+ year historic graph shows, the release of ancient sources of methane overwhelmed and continues to overwhelm any semblance of CH4 balance. Ancient forms of methane far exceed the capacity of sinks to oxidize this excess methane.  So is the solution to reestablish balance between emissions and sinks to reduce the number of head of cattle? Certainly better grazing management in the appropriate context that increases the quality of the forage as well as OH availability can be part of a solution to reduce methane along with the possible use of supplements to further reduce methanogenesis. Better management in the appropriate context, as demonstrated above, with improved soil sinks,  increased water vapor, and decreased fire intensity by increasing OH availability helps to offset enteric methane and other greenhouse gas emissions.

However, any real or lasting solution to methane imbalance has to include a reduction of natural gas use. This would entail transitioning more quickly to renewable sources of energy for electricity along with better battery technology.  So rather than using natural gas for cooking and heating, electricity derived from wind, solar, waves and hydroelectric power should provide the power for induction cook tops and furnaces. Better batteries will allow for such power to also be decentralized and continuous power rather than centralized and intermittent power.

Or, in other words, when  methane is put into a broader, rather than a reductive context, we all have to stop blaming cattle (“cows”) for climate change.

References:

Pangala, S.P. et al. 2013. Trees are major conduits for methane egress from tropical forested wetlands

Allen, M.R. et al. 2018. A solution to the misrepresentations of CO2-equivalent emissions of short-lived climate pollutants under ambitious mitigation.

Rowntree, J et al 2016, Potential mitigation of Midwest grass-finished beef production emissions with soil carbon sequestration in the United States of America

Teague, W.R. et al 2016 The role of ruminants in reducing agriculture’s carbon footprint in North America

Stanley et al. 2018 Impacts of soil carbon sequestration on life cycle greenhouse gas emissions in Midwestern USA beef finishing systems

Lakhani, N et al. 2017.  Methanogenesis: Are ruminants only responsible: A review

Tiwari et al. 2015. Methanotrophs and CH4 sink: Effect of human activity and ecological perturbations

Rigby et al. 2017. Role of atmospheric oxidation in recent methane growth.

Prinn, R.G.  2014 Ozone, hydroxyl radical, and oxidative capacity. Massachusetts Institute of Technology

Sheil, D. 2014. How plants water our planet: advances and imperatives.

Retallack, G.J. 2013. Global cooling by grassland soils of the geological past and near future.

Stebbins, G. 1981 . Coevolution of Grasses and Herbivores

Hristov, A et al. 2017. Discrepancies and Uncertainties in Bottom-up Gridded Inventories of Livestock Methane Emissions for the Contiguous United States

Schwietzke, S et al 2016. Upward revision of global fossil fuel methane emissions based on isotope database

Prinn, R.G. et al 2005, Evidence for variability of atmospheric hydroxyl radicals over the past quarter century

Worden, J. R. et al 2017 Reduced biomass burning emissions reconcile conflicting estimates of the post-2006 atmospheric methane budget

Hristov, Alexander 2011. Wild Ruminants Burp Methane, too. Penn State University

Drexler, J.Z. et al. 2009. The legacy of wetland drainage on the remaining peat in the Sacramento — San Joaquin Delta, California, USA

Schaefer et al. 2016 A 21st-century shift from fossil fuel to methane emissions indicated by 13CH4 .

Howarth, R. et al 2011. Methane and greenhouse-gas footprint of natural gas from shale formations.

Caulton, D et al 2014. Toward a better understanding and quantification of methane emissions from shale gas development

Jardine, CN et al. Methane UK. Environmental Change Institute, University of Oxford