Edinburgh Research Explorer Perspectives on removal of atmospheric methane

Methane’s contribution to radiative forcing is second only to that of CO 2 . Though previously neglected, methane is now gaining increasing public attention as a GHG. At the recent COP26 in Glasgow, 105 countries signed “the methane pledge” committing to a 30% reduction in emissions from oil and gas by 2030 compared to 2020 levels. Removal methods are complementary to such reduction, as they can deal with other sources of anthropogenic emissions as well as legacy emissions already accumulated in the troposphere. They can also provide future insurance in case biogenic emissions start rising significantly. This article reviews proposed methods for atmospheric methane removal at a climatically significant scale. These methods include enhancement of natural hydroxyl and chlorine sinks, photocatalysis in solar updraft towers, zeolite catalyst in direct air capture devices, and methanotrophic bacteria. Though these are still at an early stage of development, a comparison is provided with some carbon dioxide removal methods in terms of expected costs. The cheapest method is potentially enhancement of the chlorine natural sink, costing as little as $1.6 per ton CO 2 -eq, but this should be carried out over remote areas to avoid endangering human health. Complementarity with methane emissions reduction is also discussed.


A spotlight on methane versus carbon dioxide
While the atmospheric stock of carbon dioxide (CO2) in the atmosphere has increased by about 50% since preindustrial time (417 vs 278 ppm), that of methane (CH4) has more than doubled (1879 vs 722 ppb) [1].Although the importance of CH4 as a greenhouse gas has been known about for many years, as reflected by the Kyoto protocol of 1997 [2], until recently public attention focused mainly on CO2.Most mitigation and remediation proposals targeted CO2.
Recently, however, more attention is being given to CH4.Thus, in November 2021, at the UN Climate Change Conference (COP 26) held in Glasgow, 105 participating countries signed "The Global Methane Pledge" committing to a 30% reduction in emissions from oil and gas by 2030 relative to 2020 [3].Moreover, the new contribution of Working Group 1 to the Sixth Assessment Report of the Intergovernmental Panel on Climate Change (IPCC WR1 AR6) released on August 2021 [4] highlights the need to quickly reduce global CH4 emissions to slow warming [5,6] and "buy us time" [7].
Previously, on July 2021, a U.S.-Russia Joint Statement Addressing the Climate Challenge expressed their intent to work together bilaterally to address climate change, including emissions reductions from non-CO2 greenhouse gases (GHGs), including CH4 [8].Methane featured in the agenda of the recent U.S. Leaders' Climate Summit.
Meanwhile, the Chinese 14th Five-Year Plan presented in March 2021 was expanded to include CH4 and other non-CO2 gases [9], and China's biggest gas and oil producer is targeting a 50% reduction in CH4 emission intensity by 2025 [10].The United Nations Environment Program (UNEP) and the Climate and Clean Air Coalition in their "Global Methane Assessment" [11] as well as the International Energy Agency [12] are calling for urgent action to cut CH4 emissions, and scientists and non-governmental organisations (NGOs) are calling for atmospheric CH4 removal [13] [14].
These calls and decisions are timely.In the absence of further climate action, by the end of the century, global-mean warming due only to CH4 emissions could contribute to about 0.9°C (±0.2°C), compared to a warming of about 0.5°C (±0.1°C) currently, due to historical CH4 emissions [15], and compared to the Paris agreement target of less than 2°C global warming including all GHGs.

The atmospheric concentration of CH4 is rising
In 2020, despite Covid-19 shutdowns, the annual increase in atmospheric CH4 was the largest recorded since 1983, when systematic measurements began [16].Since the preindustrial era, tropospheric concentrations of CO2 and CH4 have increased by 47% and 167%, respectively [1].Since 2007, tropospheric CH4 has been rising rapidly, with an average annual growth rate of 9.3 parts per billion (ppb) (approximately 0.4% year - 1 ) between 2014 and 2019 [1].Over a longer timescale, CH4 concentrations have multiplied 3.28 times from a minimum of 570 ppb, reached 5,000 years ago [17].The IPCC [18] predicts that, over the next 10 to 20 years, CH4 and CO2 will have similar global warming impacts, as measured by heat absorbed (global warming potential, GWP) and temperature rise (Global Temperature change Potential, GTP).See Figure 1.

Figure 1:
Equivalent emissions of the principal GHGs, on a GWP and GTP basis, compared over a time horizon of 10, 20, and 100 years, from the IPCC [18] Besides slowing global warming, CH4 removal also can help protect the ozone layer, because one of the indirect effects of the rising concentrations of CH4 is the increasing amounts of water in the stratosphere, which participates in ozone layer depletion [19].
Whereas the natural capacity of the atmosphere to remove GHGs remains roughly constant [20,21], CH4 emissions from the major natural and anthropogenic sources are increasing [16].The biogenic sources include tropical wetlands [22], lakes [23], ponds, hydroelectric reservoirs [24], and rivers [25]; and human-made sources include fossil fuels (coal mines, oil and gas wells); agriculture (livestock and rice cultivation); landfills; and some of the biomass burning due to intentional wildfires [26].
CH4 emissions from the fossil fuel industry are uncertain but recently shown to be approximately 40% higher than previously estimated [27].They include venting, flaring, and fugitive emissions of global diesel and gasoline [28], as well as leaks in gas distribution and use [29].
Meanwhile, increasing CH4 emissions due to warming of wetlands and landfills, eutrophication of lakes [24], and fossil fuel extraction [40] are rising.
Most anthropogenic CH4 emissions come from agriculture and waste management, which together constitute 60% of anthropogenic and 38% of total emissions.CH4 emissions from oil and gas industries represent about 33% of anthropogenic emissions and about 17-19% of total CH4 emissions, with extraction, processing and distribution accounting for about 2/3 and coal mining for 1/3 [26].According to the UNEP, by fixing leaks and reduce venting and flaring, many CH4 emissions reductions in the oil and gas industry can result in negative costs, as capturing CH4 adds revenue [11].
The rapid rise of atmospheric CH4 concentration requires action to try to limit global warming well below 2°C as targeted by the COP21 Paris Agreement, bearing in mind that the GWP of CH4 is nearly 28 times higher than that of CO2 on a 100-year basis, and 84 times higher on a 20-year basis [18].Moreover, because CH4 depletes the atmospheric reservoir of hydroxyl radicals responsible for removing CH4 itself, large additions of CH4 to the atmosphere extends the lifetime and GWP of the CH4 already in the atmosphere [41,42].
In the next section the importance of focusing in CH4 reduction is explained.The following sections review the infrastructure needed for large-scale CH4 removal with the methods and strategies proposed so far, summarizing the advantages and disadvantages of those methods, and their potential costs compared to some CO2 removal technologies.Then some expected co-benefits are discussed, before the concluding remarks.

2.
Actions are needed against methane emissions and its atmospheric stock Methods have been proposed to limit and reduce CH4 emissions from several anthropogenic sources [43][44][45]: examples include better management of landfill (separate biodegradable waste) and coal, oil and gas fields (reduce leakage, recovering instead of flaring, capturing instead of venting, etc.) [11,15], as well as dietary changes to reduce emissions from livestock populations [46].But few of these can be adapted to natural emissions, which are diffuse over large areas of thousands to millions of square kilometers and/or partly inaccessible.Unfortunately, most CH4 emissions come from such diffuse natural biogenic sources [44], which by top-down estimations represent about 40% of total emissions [26].Biogenic emissions come mainly from wetlands (31%) and a smaller part (7%) from freshwater systems, oceans, estuaries, permafrost, termites, wild animals and vegetation.Other estimates of the natural CH4 emissions are of about 50% [47], as well as 50% by bottom-up estimates [26].
Removal or remediation methods seem even more technologically challenging, because they need to deplete CH4 already released to the atmosphere, where it has been diluted about 0.5 million times in an air volume of some 1.4 billion cubic kilometers.In addition, the relatively short life cycle of atmospheric CH4 (about 10 years) means that removal would have to be ongoing to reduce the concentration by a target amount.This contrasts with CO2, for which a one-off removal achieves an almost permanent reduction.Further, CH4 molecules are about 200 times scarcer in the atmosphere than CO2 molecules.Unlike CO2, however, CH4 and other GHGs can be removed by in-situ oxidation, to products with no GWP or with much lower GWP than CH4; for instance, CO2 is obtained from CH4 and nitrogen and oxygen from nitrous oxide (N2O) without the need of capture, separation, or storage.The oxidation reaction is exothermic and exergonic, and as such requires no minimum energy input once the activation energy is overcome.This differs from CO2, which requires at least 18 kJ mol - 1 for its separation from the atmosphere [48], and more energy still if CO2 is to be reduced to carbon or organic compounds such as industrial polymers [49].Removal of CH4 may be considered an acceleration of the natural oxidation processes, as once in the atmosphere it finally ends-up as CO2.

Infrastructure needed for treating large volumes of air
As CH4 is a well-mixed GHG, and its life expectancy in the troposphere is approximately 10 years, in order to have an impact on global warming, some authors have proposed that at least one-tenth of the atmosphere has to be processed every year, to compete with natural sinks [43].In reality, any process of a portfolio of technologies allowing the atmospheric CH4 concentration to start decreasing (while the imbalance between sources and sinks is currently increasing) will reduce its direct global warming impacts, as well as its indirect impacts (due to tropospheric ozone generation) on heath, food production, and primary productivity [50,51].
Boucher et al. first proposed direct atmospheric CH4 removal in 2010 [52], but found that available technologies (zeolite minerals, adsorption filters, molecular sieves, and cryogenic separation) did not appear to be energetically or economically suitable for large scale CH4 capture from air.Consequently, instead of CH4 capture, they proposed to directly oxidize it in-situ by a variety of possible methods, including bio-inspired aqueous-phase catalytic oxidation, bio-reactors containing methanogens, enzymatic systems, and catalysts made of precious metals.But these authors did not provide any specific details about how to process the very large volume of air in the atmosphere, concluding that these ideas were speculative.
Lockley [53] proposed several additional mitigation or removal techniques, such as: ignition of CH4 at point sources, lake sealing with impermeable covers or with nonbiodegradable foaming agents, ducting CH4 bubble streams from underwater sources, and others.Again, in the absence of specific details, these ideas seem speculative.
Later, researchers proposed adding to the atmosphere CH4 depleting agents such as chlorine (Cl) atoms generated by iron salt aerosols in the air [43,54] or using human made infrastructure devoted to another use.These could include: • solar updraft chimneys (SUT) [55], which produce CO2-free renewable electricity and each unit can process on the order of 6,000 cubic kilometers (km 3 ) of air per year; • direct air capture (DAC) systems [56] that are being developed to capture CO2 from air [57,58].
Existing infrastructure could be used for CH4 removal.For example, titanium dioxide in self-cleaning windows or other photocatalytic coatings such as paintings on buildings could contribute to CH4 removal and attract carbon credits and certificates, thus helping to finance future removal infrastructure.Aircrafts, wind turbines, or other structures already in contact with large quantities of air might also provide reaction surfaces to oxidize CH4.

Enhancing the main natural CH4 sinks
Currently, in the troposphere, the principal natural CH4 sinks are the hydroxyl radicals (which remove nearly 90% of the CH4) [59], chlorine atoms (which remove about 2.5% of the CH4) [60], minerals in soils and dust [61], soil microbes, plants and trees.
Enhancing those natural sinks can be a strategy to increase atmospheric CH4 removal.

Enhancing the hydroxyl radical °OH and targeting in majority point sources
Atmospheric natural self-cleansing and volatile organic compound (VOC) removal is mainly due to hydroxyl radicals [62].Hydroxyl radical generators, as well as ozone generators, are commercially available, and indoor VOC pollution can be efficiently controlled by short-wave ultraviolet (UVC) light in closed systems [63].Some coauthors of this article are working on methods to enhance the °OH sink of CH4 and reduce its lifetime, although still more research is needed to be able to provide cost estimates and quantitative estimates of efficacy [64].Still, the increasing efficiency and lifetime and the decreasing costs of ultra-violet light-emitting diodes (UV-LEDs) are promising.
Research conducted at the University of Copenhagen has led to the creation of startups such as Infuser and AirLabs, which already apply this technology to point sources [65].Several other possible strategies to generate °OH radicals and apply them in the open atmosphere might be possible, based on the numerous and complex mechanisms by which they are produced [66].Knowing the intensity of sunlight UV, the °OH radical concentration can be predicted [67].But care has to be taken not to expose human beings, animals, and plants to dangerous UV radiation and to ozone.

Enhancing the natural chlorine sink of CH4-at the molecular level
In 2017, some of the coauthors of this article proposed to deplete the atmospheric CH4 directly in the lower troposphere with Cl atoms [54,57], mainly but not exclusively under the marine boundary layer.Recent scientific research from 2015-2017 proved that Cl atoms can be generated in large amounts from the sodium chloride (NaCl) content of natural sea-spray aerosols, thanks to an iron(III)/iron(II) sunlight photocatalyzed reaction [68,69].Acidity (pH<3) found over coastal areas is naturally generated from NaCl by acid displacement with biogenic sulfate and nitrate [70].
However, it also can be enhanced by anthropogenic pollution due to combustion sources, where nitrogen oxides and sulfur oxides are further oxidized in the atmosphere into nitric and sulfuric acids that react with sea salt to generate hydrochloric acid and sodium nitrate and sodium sulfate salts.Figure 2 illustrates a possible way by which CH4 is already being removed by enhancing its chlorine sink.
Over polluted coastal areas, the chlorine sink destroys up to 11% of CH4 [71].It is worth noting that authors in favor of this method do not propose enhancing acid air pollution, and do not target the enhancement of the Cl atom generation over populated areas or coasts [54,72].As CH4 is a well-mixed GHG, it can be removed anywhere at atmospheric concentrations (about 1.9 ppm), consequently they propose to do so in remote unpopulated areas.They also plan to address point sources of CH4, where it is more concentrated, to deplete it before it becomes diluted and mixed in the global atmosphere, and in this case the Cl atom generation will be carried out in closed systems such as the existing ventilation systems of coal mines, with very low risks.

Enhancing the natural mineral sink of CH4-on surfaces
In 2017, it was proposed to perform CH4 depletion by large scale photocatalysis [55] using solar updraft chimneys (SUT), which are structures able to process very large volumes of air, as illustrated by Figure 3.As an example, a hypothetical 400-MW SUT would process 38,000 km 3 of air yearly [75,76].
The proposed photocatalyst for CH4 depletion is a semiconductor metal oxide: a zinc oxide (ZnO) doped with 0.1% silver (Ag) [77].It is expected to remove 50% of the CH4 from the air processed thanks to sunlight at ambient temperatures [55].Then in 2018, DAC devices were proposed to remove atmospheric CH4 alongside CO2, using the same ZnO-0.1%Agphotocatalyst illustrated by Figure 4.The enormous energy costs of fan-driven DAC make it less attractive than passive generation of large airflows by the SUT devices proposed above.Nonetheless, it is anticipated that the DAC technology will develop rapidly to remove CO2 from the atmosphere.Once DAC plants exist, profiting from this existing infrastructure by upgrading it to also remove CH4 could enhance the capture yields in terms of CO2-eq by 20%.Depending on the CH4 oxidation yields and costs, it could be advantageous, especially since the removal of other GHGs also seems feasible [55,78,79] and does not introduce a pressure drop requiring more energy for the fans, and no additional CO2 capture capacity would be required [55].
In 2019, other scientists proposed the use of DAC devices dedicated to CH4 capture by zeolites and then removal of CH4 using a thermal catalyst [58].No published data was found on the pressure drop across the zeolite, making it difficult to know if the DAC device might be attractive to capture CO2 also.Lackner [80] commented on this proposal by pointing to the extreme dilution of CH4 in air (200 times more dilute than CO2) which (based on the Sherwood law) may cause a three-order of magnitude energy penalty in using fan-driven DAC systems -thus suggesting that approaches should take advantage of natural air flow and use passive methods.Both the comment and the response [81] mention that it would be more interesting to remove N2O, the third most important GHG by its radiative forcing, with an atmospheric lifetime estimated to 114 years and a GWP100 nearly 300 times higher than of CO2.Such a proposal was made in 2016 [78].Two proposals [55,57] also suggested removal of other GHGs (like N2O, as well as many halogenated GHG gases) with very high global warming potentials, which also damage the stratospheric ozone layer and are included in the Montreal Protocol.A long list of possible photocatalysts was proposed [78,79], mainly titanium dioxide derivatives, all acting at ambient temperature and activated by sunlight.Consequently, not only CH4, but almost all non-CO2 GHGs are targeted by the photocatalytic method [55].
In table 1, the principal methods of CH4 removal are summarized.[83] and as no capture, no purification, no compression, no transport and no long-term storage is required in the case of CH4 compared to CO2, then global costs might be reduced, with a target costrange of $100 ton -1 of CO2-eq [84].
In case of use of hybrid CO2 DAC plants upgraded to also capture and oxidize CH4, the costs might be lower, as almost all the infrastructure and the air-flow already exist.The estimated cost ton -1 CO2-eq is $166 by 2030 with a target of $100 by 2040 [84].For CH4 removal using chlorine atoms, directly in the troposphere, mimicking natural processes costs estimates range from $ 54 to as low as $1.7 ton -1 CO2-eq [86].A startup targets costs of about $1.6 ton -1 CO2-eq [87].
Those estimations of the costs look very The practice may not be generalizable.

Sewage sludge
Co-composting can reduce CH4 emissions by about 80% A better future strategy consists of removing all fermentable organic matter from new landfills.
[ 89,90] Leak repair Oil & gas industry The "methane pledge" signed at COP26 targets 30% reduction by 2030, from oil and gas industries.Usually applicable for CH4 concentration about 0.5-1% as the reaction is exothermic it can be self-sustained.If CH4 >1.5% energy generation is possible.
[92, 93] 1 Although it is yet too early in the development process to have accurate cost estimates for CH4 removal directly in the troposphere, initial estimations have been provided 2 Provided for illustration purposes only, as mitigation is out of the scope of this review and has been reviewed elsewhere [15,[43][44][45][46].

Enhancing the sinks by the use of plants, trees, and microbes
Mitigation of landfill CH4 emissions using soil amendments such as biochar [94], and microbial CH4 oxidation processes with bio-covers [90] or bio-trickling filters are wellestablished methods [95].Adding methanotrophs to flooded paddy soil also mitigates CH4 emissions [96].Enhancing methanotrophic activity is among the mitigation methods proposed to prevent CH4 from reaching the atmosphere [44,45].Similar methods also might be possible for greenhouse gas removal (GGR), as it has been shown that an important CH4 sink can be created by cropland reforestation [97].One can probably imagine that for afforestation projects and for the "one trillion trees" initiative, planting trees that absorb tropospheric CH4 [98] in addition to CO2 (instead of plants and trees that emit CH4 [98]), as recently observed in the seasonally flooded Amazon floodplain, is a good idea -especially if using local trees species, and if biodiversity is preserved or restored, without competing with agricultural land.
Airborne microbes are abundant in the atmosphere [99] and subject to long-range transport [100], so it might be possible to enhance the amount of CH4 consuming microorganisms (methanotrophs) [101] for instance by enhancing the amount of methanotrophs already present on the bark of tree trunks [102].Currently, plants, trees, and microbes represent 5%-6% of the sinks for atmospheric CH4, but to our knowledge apart a brief mention [52], no large-scale strategy has yet been proposed to take advantage of these sinks.

Discussion
The mail benefit of returning to CH4 pre-industrial levels will be to reduce global warming by up to 0.5°C [103], which can help reduce a temperature overshoot above 2°C by mid-century [104].
The expression "at a climatically significant scale" often appears in discussions about GHG removal, but lacks precise definition.Perhaps one benchmark could be the amount of GHG removal achieved up to now.According to the International Energy Agency [105], after 10 years of development, existing DAC installations captured just 9,000 tons CO2 yr -1 in 2019.The report of three U.S. ), many people would consider them significant.When "net-zero" is reached in the second half of this century, renewable energy will be preponderant.
Without knowing much about potential energy costs, and costs of scaling up, it is very difficult to understand and predict how feasible CH4 removal technologies can be.But in our opinion, even if during the first decade following their invention, a very smallscale effective efficiency is obtained at very high cost, technologies able to avoid GHG emissions or able to remove GHGs already in the atmosphere deserve attention as soon as their scalability and globalization seems possible.Otherwise, the criterion of "at a climatically significant scale" may lead to too many options being dismissed and too few remaining, while the scale of the global warming problem requires a large portfolio of methods and technologies to be developed.A significant scale might be achieved as the sum of many contributions that are not individually very significant.
There are several direct and indirect co-benefits to reduced CH4 atmospheric concentrations.The rapid climate benefits of reducing the concentration of CH4 in the atmosphere are significant [15] for agriculture and the economy [109,110] as the tropospheric ozone burden enhanced by CH4 will also be reduced [103].Lower surface ozone concentrations will increase crops yields and global photosynthesis, potentially allowing some CO2 removal [51].The co-benefits for human health are numerous [111], as it will reduce hospitalizations, asthma and pulmonary diseases and premature deaths due to the linked ozone pollution [11].Development of CH4 removal methods is still in its infancy and requires more research, development and funding [85].
Some of the CH4 enhanced oxidation methods proposed will have other co-benefits.
As °OH and Cl atoms are very reactive and not very selective, by enhancing their generation several other GHGs and atmospheric pollutants will be removed faster than CH4.This includes, for instance VOCs, whose removal will help reduce CH4 lifetime [112].Such VOCs include also organo-halogens, human made hydrofluorocarbons and hydrochlorofluorocarbons, as well as natural biogenic halogenated-methane compounds produced mainly by oceanic plankton and bacteria [113].The latter are not considered to be GHGs, but they participate in the stratospheric ozone layer natural cycle of destruction [114].By reducing faster the amount of natural biogenic halogenated-methane compounds in the lower troposphere, as well as by reducing the water content of the stratosphere due to CH4 oxidation [19], the ozone layer might recover faster.
Of course, the main benefit of returning to CH4 pre-industrial levels will be to reduce global warming by up to about 0.5°C [103], which can help reduce a temperature overshoot above 2°C by mid-century [104].

Concluding remarks
This perspective article has discussed different strategies (some already proposed and several new ones) to accelerate the removal of already emitted CH4, reducing its radiative forcing by direct and indirect effects.A reduction of the atmospheric CH4 burden might help the ozone layer to recover faster and will have rapid climate benefits together with significant co-benefits for agriculture, human health and the economy.
Unlike CO2 removal methods, the CH4 removal methods described here do not require capture and long-term geological sequestration as for CO2, as they only accelerate the natural oxidation processes that will anyway occur with the products remaining in the atmosphere.By returning to CH4 pre-industrial levels, the increase of atmospheric CO2 resulting from CH4 oxidation is small compared to global CO2 annual emissions, while the reduction of the radiative forcing could be significant.Net warming could be reduced by about 0.5°C.
Although CH4 enhanced oxidation methods can be applied both to global tropospheric CH4 and to some local concentrated sources, removal and mitigation strategies and methods do not necessarily target the same sources and are complementary.In case of an abrupt acceleration of CH4 emissions from natural sources (e.g.submarine methane-hydrates, or wetlands), the availability of effective and proven techniques would constitute an assurance to avoid a rapid acceleration of global warming.
Those innovative methods deserve more attention from the scientific community to help evaluate their potential risks, costs, public acceptability, and societal appropriation.Together with CO2 and CH4 mitigation and with CO2 removal, CH4 removal methods can help fight climate change, win time by slowing down warming and thus meet the targets of the Paris Agreement with limited temperature overshoot.

Figure 2 :
Figure 2: A container ship powered by bunker fuel mixed with commercially availableiron additives[73,74] which are sold to reduce black carbon and carbon monoxide (CO) emissions and to reduce fuel consumption.In the exhaust plume, the iron compounds react with sea salt to produce iron chloride (FeCl3), which under sunlight generates Cl atoms[69] that oxidize CH4 16 times faster than °OH radicals[71].
Photocatalysis on surfaces favorable but are uncertain prior to demonstration, waiting for field trails which cannot start before a full-scale environmental assessment has been conducted.For CH4 mitigation at point sources, for instance in ventilation systems of coal mines where CH4 is more concentrated, costs might even be lower as the infrastructure and the air-flow already exist and the generation of Cl atoms can be made by photolysis of Cl2 gas, produced [54, 72] by the well-established chlor-alkali industrial process.Possible co-benefit: iron salt aerosols provide iron to depleted oceans, with possible CO2 capture in the oceans at costs about $1 ton -1 of CO2 [72] based on the "Redfield ratio" of oceanic C-N-P-Fe stoichiometry and assuming 10% sequestration in the bottom of the oceans.Methanotrophic bacteria Point sources (see next section) Spraying methanotrophs cultures on point sources such as trees which transfer CH4 from underground to the atmosphere, or over large thawing permafrost areas and wetlands effective feed additives for beef: 3nitrooxypropanol and nitrates (respectively 22% and 14% CH4 reduction) Many other feeds are effective, but less: chestnut, coconut, grape pomace, linseed, red seaweed… bio-covers exist, oxygenation of the soil is necessary.

Table 1 :
Summary of the methane removal methods, technologies and estimated costs.For comparison, some methane mitigation strategies are also briefly described.