Incorporating methane into ecological footprint analysis: A case study of Ireland

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Incorporating methane into ecological footprint analysis:A case study of Ireland

Conor Walsh⁎, Bernadette O'Regan, Richard MolesCentre for Environmental Research, Chemical and Environmental Sciences Department, University of Limerick, Ireland

A R T I C L E I N F O

⁎ Corresponding author. Room LG007, LonsdDepartment, University of Limerick, Co. Lime

E-mail address: [email protected] (C. Wals

0921-8009/$ – see front matter © 2008 Elsevidoi:10.1016/j.ecolecon.2008.07.008

A B S T R A C T

Article history:Received 14 September 2007Received in revised form 11 July 2008Accepted 13 July 2008Available online 15 August 2008

Carbondioxide (CO2) accounting is important to global ecological footprint analysis. Howevermethane (CH4), with a global warming potential (GWP) 25 times that of CO2, should not beneglected as an environmental indicator for informed environmental management. Whilethis is a significant component, the CH4 associated with imported embodied energy shouldalso be included in national greenhouse gas (GHG) inventories. This study proposes an initialmethod for incorporating methane into ecological footprint analyses and hopes to informfuture debate on its inclusion. In order to account for differences inmethane intensities fromexporting countries, methane intensities for OECD countries were calculated using emissionand energy consumption estimates taken directly from National Inventory Reports (NIR),published in conjunction with the Intergovernmental Panel on Climate Change (IPCC). Forother countries the methane intensities were estimated using energy balances published bythe International Energy Association (IEA) and IPCC default emission factors. In order toestimate embodied organic methane, material imports and exports were translated intounits (such as live animals) capable of conversion into methane emissions. A significantincrease in Ireland's footprint results from the inclusion of the GWP of methane is includedwithin the footprint calculation.

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Keywords:MethaneEcological footprintImports and exports

1. Introduction

Global warming and its probable consequences has replacedozone depletion as perhaps the most publicly recognised en-vironmental issue. There is a general consensus that anthro-pogenic carbon contributes to increasing global temperatures.The Intergovernmental Panel on Climate Change (IPCC) FourthAssessment Report summary for policy makers demonstratesa “very high confidence” (IPCC, 2007) that the net effect of humanactivity since industrialisation has been one of global warm-ing. In order to satisfy the requirements of the Kyoto protocol,signatories are required to compile GHG inventories. Nationalinventories are primarily concerned with CO2 emissions, itselfwidely used as an environmental indicator. While inclusion ofthe effects of trade in CO2 accounting is vital in order to satisfy

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the responsibility principle (allocating the environmental costto consumers), its role has rarely been considered in methane(CH4) accounting. The application of the responsibility princi-ple suggests that GHG inventories should incorporate theeffect of trade. Once the methane embodied in trade has beencalculated, it can be incorporated into domestic emission asreported within the GHG inventory.

This paper reports on an attempt to calculate national me-thane consumption and estimate the extent towhich Ireland'sfootprint increases when methane is included in its calcula-tion (2003 is the data year). This is important given thatmethane has an associated global warming potential 25 timesthat of CO2. The method adopted here follows on from workpublished in Subak (1995). The main purpose of this paper isto address the fact that to date, methane has not been

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incorporated into national footprint accounts. As methane isnot directly tied to bio-productivity in the same manner asCO2, it is not as readily converted into global hectares. How-ever this paper evaluates possible methods to calculate anational methane footprint for Ireland as a contribution tofurther methodological debate on the issue.

2. Calculation of the ecological footprint

The ecological footprint concept (Wackernagel and Rees, 1996)provided one of the first examples of adopting land-use as ametric for sustainability. Essentially this provides a ratio be-tween human demand and the ecosystem's regenerative ca-pacity and determines the extent to which natural capital isbeing degraded to satisfy human consumption. The originalmodel divided consumption into five main categories: food,housing, transportation, consumer goods, and services. Theseare quantified by trade corrected consumption data. Oncesufficient data have been collected, the model can calculate anational footprint and scale it down to an approximate percapita figure. This method can be applied to a range of cate-gories, for example, arable land to produce food, energy landto sequester the CO2 emissions from industry, transport, andso on. A national or regional ecological footprint can be com-pared against locally or globally available bio-capacity. Themost prominent example of national scale footprinting can beseen in the national footprint accounts as demonstrated in theLiving Planet Report (WWF, 2006). Regional footprinting hasbeen applied in a number of diverse areas, including Australia(McDonald and Patterson, 2004) or as an attempt to value en-vironmental impacts of human activity (Knaus et al., 2006).

When expressed using global yields and averages, a percapita footprint can be compared to global average per capitabio-capacity, as a means of illustrating the finite nature of na-tural capital. To summarise, footprinting challenges the viewthat sustainable development is not quantifiable, and mayallow for the practical application of the sustainability precept“think globally and act locally”. Footprinting methodology hasdeveloped in the years since its inception. The originalmethodhas been described as compound footprinting, as aggregatednational trade data are used in calculations. The compoundmethod, and its development is discussed in detail by Mon-freda et al. (2004) and more recently by Kitzes et al. (2007a).While the datasets, constants and methodologies are con-stantly being re-evaluated, the core function of the ecologicalfootprint remains the same.

While a detailed appraisal of the end use of products is notrequired for accurate compound footprinting, the samecannot be said for component-based footprinting. Componentmethods are “bottom up” and account for various productsand practices but may comprise a greater degree of uncer-tainty. This is particularly relevant given the complexity ofmodern industrial processes and the difficulty in accuratelyallocating resource demand to each stage in the industrialprocess. Despite this issue component footprinting is seen asbeing worthwhile, not just for its potentially greater regionalrelevance, but its applicability to a wide variety of subjects. Intheory, if calculated correctly a compound footprint shouldinclude all the information contained within the component

footprint. Simmons et al. (2000) argue that neither methodshould be regarded in isolation, as both have advantages anddisadvantages. Monfreda et al. (2004) argue for calibratingcomponentmethodologywithmethods and constants appliedin compound footprinting.

Other methods include the calculation of a footprint usingactual hectares (Erb, 2004), which dispenses with the use ofglobal yields to calculate how much land area is being ap-propriated by a given economy, as opposed to calculating howmuch area is needed for a given economy to be sustainable.

In order to compensate for complex interdependencies be-tween economic sectors, and their impact on allocating indirectresource requirements, Wiedmann et al. (2006) propose theapplicationof input–output (IO) analysis toallocate the footprintinto detailed consumption categories. In simple terms, this isbasedon theeconomic linkages between sectors anddistributesthe ecological footprint based on the amount spent by othersectors in order generate output in a given sector. It seems likelythat IO will display a greater prominence in future ecologicalfootprint studies (Lenzen and Murray, 2001).

Each step in methodological evolution reinforces the pointthat footprinting is an iterative process, with each successivemodification providing additional tools for environmentalmanagement. Ireland's ecological footprint over the last decadehas been the subject of a recent study (Lammers et al., 2008),which demonstrates the ascendancy of the ‘Celtic Tiger’ and itseffect in increasing resource consumption.

3. The carbon footprint

In most footprint studies the footprint of CO2 is calculated byestimating the area of annual forestry required to sequester itwhile allowing for ocean absorption. This is not to proposethat forestry is the solution to climatic change but ratherdemonstrates howmuch larger the world would have to be inorder to negate the effects of carbon emissions (Monfredaet al., 2004). As such an increase in land area is impossible, sothat the only appropriate action is to reduce emissions. Thismethod has been criticized as young forests experiencethe highest rates of sequestration. Also, as wood will inevi-tably decay, forests only act as a sink in the medium term(Herendeen, 2000). This method is further complicated by thediscovery that forests emit methane through oxidation(Keppler et al., 2006) and that in the future the oceans maybecome a source of CO2 emissions. In light of these and otherfacts, Kitzes et al. (2007b) present possible alternatives to thetraditional footprint.

One of the most common debates is whether other bio-types (such as natural grass land) should be included in thecalculation. This of course will face the same obstacles asmentioned above and in some cases (such as intensively usedpasture) may be more pronounced. Another potential alter-native is to calculate the area of bio-fuels necessary to provideequivalent energy (within a thermodynamically equivalentframework). This may be seen as a measure of the substitut-ability of bio-fuel as opposed to accounting for the impactsand demands of energy use (Kitzes et al., 2007b). Also thismethod would have to estimate the additional energy neededto successfully cultivate these plants.

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Another potential alternative as proposed by Lenzen et al.(2007), and discussed in a report commissioned by the UKDepartment for Environment, Food and Rural Affairs (Risk &Policy Analysts Ltd., 2007) is a dynamic approach, which allo-cates the impactofCO2emissions to changes inbio-productivitydue to future climatic instability. Instead of calculating the areaneeded tosequesterCO2 emittedcurrently (representinganareagreater than can be providedwithin the biosphere), thismethodwould estimate the actual impact, in terms of the future re-lationship between footprint and bio-capacity. The impact inthis sense will be the future degradation of bio-capacity due tolikely climatic instability. In actuality we cannot use more landarea than currently exists; therefore overshootwill be evidencedby the convergence of the actual ecological footprint withavailable bio-capacity.

Given the fact that the carbon footprint currently accountsfor 50% of the global footprint, any departure from traditionalfootprinting is likely to have a pronounced effect on the overallvalue. That, however, is insufficient reason not to seriouslyquestion the methods that are currently applied.

4. The problem of incorporating methane inecological footprinting

Kitzes et al. (2007b) recognise the importance of incorporatingother GHG into footprinting methods but accept that thequestion of which method should be accepted needs to becomprehensively debated. Themost apparent problemwithinmethane accounting is the large degree of uncertainty asso-ciated in estimating methane consumption, as demonstratedin Penman et al. (2000). This is due to the fact that methaneemissions are often dependent on regional factors; this isespecially relevant for ‘top down’ studies as presented here, asthey draw heavily from national averages. (This is a problemfaced by many sustainability indicators but may be moresignificant for methane). This is further complicated by thefact that anthropogenic methane is emitted by numeroussources, both organic and inorganic. The overall mechanismfor many of these sources is not thoroughly understood, par-ticularly within the global context (Penman et al., 2003). Therecent findings by Keppler et al. (2006) that growing plantmatter is a source of methane in aerobic conditions, was metwith considerable surprise and exemplifies the fact that theglobal methane cycle is not fully understood.

Possibly the most important impediment to the incorpora-tion of methane into ecological footprint method is the factthat methane has a longer atmospheric lifetime and is not aseasily assimilated into the biosphere as CO2. This is compli-cated by the fact that while methane emissions are quantita-tively less than CO2, it has a greater GWP. The problemsassociated with including GWP within footprinting are basedaround the ramification of including weighting factors, whichmay not be directly applicable to bio-productivity. It has beenargued that net radiative forcing (the net increase in irra-diance) is a more meaningful metric of climatic change thanGWP. Another issue is the multiple paths taken by methane,which has both an atmospheric and terrestrial sink. Thisraises the possibility of double counting in relation to trans-lating methane emissions into a bio-productive area. Instru-

mental to the ecological footprint's success is its use as acommunication tool; however, the inclusion of methane mayreduce its effectiveness. For example, methane emissions aregenerally lower for activities which people can directlyinfluence, such as transport or home heating. The most me-thane intensive processes such as cattle farming (Steinfeld etal., 2006) or fuel production generally take place before theconsumer plays a role. Despite these issues, the inclusion ofmethane is an important consideration for the continuingdevelopment in the ecological footprint methodology.

5. Calculating methane emissions due to Irishconsumption

The domestic emissions due to Irish economic activity in 2003have been comprehensively calculated and presented in thelatest GHG inventory report (McGettigan et al., 2007a). How-ever, in order to satisfy the responsibility principle the me-thane embodied in traded materials needs to be incorporatedinto the domestic methane emissions. All material importswill have inorganic methane emissions associated with em-bodied energy. Certain organic products will have methaneemissions embodied in its production. These include emis-sions due to enteric fermentation, manure management, ricecultivation and wood production (Hongmin et al., 2007).

The direct combustion of fossil fuel is a source of methaneemissions. These however are quantitatively less than thefugitive emissions released in the production and consump-tion of fuel (Muller and Bartsch, 1999). In order to calculateinorganic methane emissions embodied in trade, themethane intensity of energy has to be calculated. Forexporting countries that have published GHG inventories,this calculation can be readily made. Where countries do notpublish GHG inventories, emissions were estimated hereusing energy balances published by the International EnergyAgency (IEA) and default fugitive emissions published by theIPCC. Since these balances are expressed in terms of energyunits, emission factors from the revised 1996 IPCC Guidelines(Houghton et al., 1997)were adopted, as the2006 IPCCGuidelines(Eggleston et al., 2006) provide units that are not as readilycompatible, suchasGg/m3produced, orGg/m3 rawgas feed. Therole of fugitive emissions has implications for the application ofthe responsibility principle, which is discussed later.

It should be noted that all fugitive emission factors sufferfrom high degrees of uncertainty. This is to be expected giventhe considerable variety in fuel production technologies and isinevitable when attempting to aggregate data on a nationalscale. For combustion estimates, emission factors for eachcategory from the IPCC default emission factor database wereaveraged. The totalmethane emissions due to combustion andfugitive emissions (due to domestic production and fuelconsumption) were divided by the total energy consumption.These intensities, (specific to each country and expressed inkg/MJ) were used in conjunction with embodied energy esti-mates, (expressed inMJ/kg), to translate import tonnages (CSO,2004) into methane emissions estimates.

The organicmethane embodied in imported foodstuffswascalculated, following Subak (1995). Live animal imports andexports were available as animal numbers andwere converted

Table 1 – Estimation of embodied methane emissions intonnes.

Imports Produced Exports Consumption

Inorganic 86,880 35,899 34,639 88,140Livestock 10,825 396,450 166,935 240,340Dairy 35,386 149,454 126,616 58,223Rice 611 N/A 0 611Wood 8,670 3,290 7,230 4,730Waste N/A 79,209 N/A 79,209Sum 142,372 664,301 335,420 471,253


into methane emission equivalents. Material imports weretranslated into units that can be readily translated intomethane emissions using default emission factors. For meatand dairy products this involved converting imports into livehead equivalents. This involved a number of steps. Firstlymeat product tonnages were translated into carcass weightequivalents (CWE) using conversion factors provided by theEconomic Research Service of the US Department of Agriculture(ERSUDA, 2006). An average “Dressing Percentage” of 62% wasused to translate carcass weight, based on recommendationsfrom the University of Dakota beef-grading scheme, into liveweight. Based on the same system, an averageweight of 500 kgwas assumed to translate live weight into cattle numbers andcattle were assumed to live for 2.5 years. This is an attempt tocompensate for the methane emitted throughout the life ofthe animal. Animal age is significant in relation to cattle, aslivestock are generally slaughtered at amuchearlier age. Regionspecific emission factors from Houghton et al. (1997) were usedto estimate total methane emissions. A similar method wasadopted for other meat types, applying different CWE factors,dressing percentages, live weights and ages.

An attempt was made to account for prevailing farmingpractices using the IPCC “tier 2” methodology. This methodestimates the total energy required to maintain the herd(including animals up to point of slaughter) and translates thisinto methane emissions. The overall estimates calculatedherewere similar to emissions reported in Ireland's latest GHGinventory report (McGettigan et al., 2007a), which are basedon farm surveys. This included an alternative allocation ofmanure management practices as reported in Menzi (2002).The estimates calculated are mostly comparable with pub-lished emission factors: discrepancies may be explained bythe fact that the data presented in the GHG inventory reportwere based on farm surveys, and assumed specific slaughtertimes for individual categories. The estimates calculated hereand those published in McGettigan et al. (2007a), were ave-raged and applied to meat exports. These combined emissionfactors, including both enteric fermentation and manuremanagement, were estimated to be 140 kg CH4 per head fornon-dairy cattle (assuming 2.5 years old) and 130 kg CH4 perhead per year for dairy animals (as milk is produced peren-nially, an annual emission factor is used). These emissionfactors are also comparable to the combined emission factorsof 130 kg CH4 per head and 121 kg CH4 per head per year (fornon-dairy and dairy animals respectively) adopted in Ireland'sGHG report (McGettigan et al., 2007a).

For imported dairy products, tonnages of products wereconverted into raw milk equivalents using conversion factorstaken from FAO (1978). Total raw milk imports were translatedinto cattle numbers using regional milk yields taken fromHoughton et al. (1997). As with meat products, region specificemission factors from this sourcewere adopted to estimate totalmethane emissions. For exported dairy produce, the NationalDairy Council (NDC, 2004) provided the amounts of rawmilk usedin production of dairy goods. By comparing dairy productionwith export estimates, the quantity of fresh milk exported wasestimated. This was then used, with dairy yield estimates pub-lished in DAF (2004), to estimate animal population numbers.

The methane emissions embodied in rice imports incur thehighest degree of uncertainty for the organic methane calcu-

lation. As the trade data do not identify the origin of importedrice, an average emission factor for Asian rice fields wascalculated. Harvested paddy areas in each Asian country wereestimated in conjunction with the distribution of water man-agement schemes and nutrient management, upon which theemission factors are based. These were taken from Houghtonet al. (1997) andwereaugmentedwithmore recentdata (Li, 2006).The main reason for this uncertainty lies in the scale of riceplantations, particularly in the case of India and China, wherethe large areas used for rice production reduce the accuracy ofanynational estimateof irrigationpractices (Frolkingetal., 2006).The overall emissions fromAsian rice paddies are comparable tothe aggregated value calculated by Yan et al. (2003), but differ incertain countries. A complication to the calculation is con-tinuous uptake of new water management schemes in anattempt to increase resource efficiency (Wassmann et al., 2000).

Keppleretal. (2006)demonstrate that forestproducts arealsosources ofmethane emissions. Kirschbaum et al. (2006) suggestthat leaf matter is themain determinant of methane emissionsand that theconversion factors publishedbyKeppler et al. (2006)should be applied to it rather than the entire plant. In order toestimate importedmethane, the tonnage of importedwood andpaper products were translated into Wood Raw Material Equi-valents or standing tree volume equivalent, based on datapublished by the UK Forestry Commission (UKFC, 2004). A furtherconversion factor of 1.5 (supplied by the sameorganisation)wasapplied to translate standing volume into total abovegroundbiomass. It was assumed that foliage accounted for 10% of totalaboveground biomass. A standard fresh density of 0.95 g/cm3

and dry matter content estimate of 41% were taken from Vileet al. (2005). Conversion factors inKeppler et al. (2006)were usedto convert dry matter into methane. This is dependent on thegrowing period and sunlight hours experienced in the regionwhere the trees grew.

Once the overallmethane embodied in traded commoditieshas been calculated it can be applied to thedomestic emissionsto estimate nationalmethane consumption. Domestic produc-tion and traded emissions are summarised in Table 1.

6. Methodological choices for the inclusion ofmethane in ecological footprinting

Perhaps the most obvious method for calculating a footprint ofmethane is to emulate standardCO2 footprinting as it is theonlyGHG currently included in the national footprint accounts.This requiresmethane to be translated into carbon equivalents,(based on differences in molecular weight, and accounting for


oceanic absorption) which allows an estimation of the area ofnew forest growth required to sequester it and subsequentlyinto a land area of global average bio-productivity (GFN, 2005).The argument for translating methane directly into carbonequivalents is based on the fact that the prevailing sink formethane is oxidationwith the hydroxyl (OH) radical to form themethyl radical. This removal of methane takes place largely inthe troposphere, stratospheric oxidation playing a less signifi-cant role. These two OH reactions account for approximately90% of methane removals. Baird (1999) describes a processwhereby methane is ultimately converted into CO2. The overallsequence of reactions is summarised below.

UV� ACH4 þ 5O2 þ NOþ 2OHYCO2 þ H2Oþ NO2 þ 4HOO

However methane has a GWP of 25 times that of CO2 (IPCC,2007). This effectively means that a tonne of methane has thesame integrated capacity to increasenet irradiance as 25 tonnesof CO2 over a given time horizon (in this case 100 years). Thusmultiplying by the GWP translates methane into CO2 equiva-lents. Because GWP does not directly affect bio-productivity, itcan arguably be applied once a land area estimate is calculated(although this was not done in this paper). The results basedcarbon sequestration (both with and without GWP) are sum-marised in Table 2. Alternatively methane emissions can betranslated into CO2 equivalents before they are translated intobio-productive land area. The effect each method has on theoverall footprint value is discussed later. These atmosphericprocesses together account for 93% of the global methane sink.Oxidation of methane by microbial activity accounts for theremaining 7%. A methane footprint based on terrestrial pro-cesses may demonstrate greater resonance with establishedfootprinting methods.

Houghton et al. (2001) suggest that the overall global me-thane soil sink accounts for an annual removal of 30 Tg. Twoformsofmethaneoxidation are recognised in soils. High affinityoxidation takes place atmethane concentrations similar to thatof the atmosphere. This would seem to take place inmost soilsthat have not been exposed to high NH4 concentrations and isestimated to account for 10% of soil removals (Le Mer andRobért, 2001). The second form of oxidation, low affinityoxidation, occurs at concentrations greater than 40 ppm dueto theactionofmethanotrophbacteria. CurryandVanderKamp(2005) list the physical determinants on a soil's ability to fostermethanotrophic activity. Themain factor determiningmethaneoxidation is the rate of diffusion of CH4 and O2 through theuppermost soil layer, as described by a diffusion coefficient.Soilswith a poor diffusion coefficient, suchaswaterlogged soils,can become sources of methane. This coefficient is dependenton soil physical properties such as bulk density and texture andalso on hydrology, both of which determine the extent of

Table 2 –Methane footprint based on carbon equivalentsand GWP.

Total consumption Using massequivalents

Using GWP

Tonnes gha gha/cap gha gha/cap

471,253 356,055 0.089 3,242,760 0.81

available pore space for gas exchange (LeMer and Robert, 2001).The biological oxidation of methane also depends on theavailability of soil water, the nitrogen content (with fertilizedsoils having a reduced oxidation rate) and, to a lesser extent,ambient soil temperature (Brumme and Borken, 1999).

Research by Smyth et al. (2000) established methane oxi-dation rates for a number of European sites. They also reviewedprevious work on oxidation rates for soil types associated withtemperate forests and grassland, boreal and tropical forests,wetlands and deserts. For aerobic soils, most of the observedoxidation rates fell between 0.8 kg CH4 ha−1 yr−1 and 3.2 kgCH4 ha−1 yr−1. In order to calculate a global oxidation rate, thegeometric mean (selected because of the large range inpublished values) of the values for each soil typewas calculatedandmultiplied by its global area, as published in the IPCC SpecialReport On Land-use Land Change and Forestry (Watson et al., 2002).This resulted in a global annual oxidation rate of 32 Tg, whichwhen divided by the overall terrestrial land area, yielded anaverage oxidation rate of 2.2 kg CH4 ha−1 yr−1. However, not allland is oxidative, and the proportion of land which is notoxidative is not known, so that any global average estimate ofsoil oxidation rates will be approximate. However the globaloxidativeestimate adoptedheremaybe justifiedon thegroundsthat it accords with current practice in calculating EcologicalFootprints inwhich bio-productive yields are assumed to reflectthe optimal function in satisfying demand for each land type(e.g. cropyieldestimatesadoptedrepresentonlycultivated land,and exclude non-cultivated food production).

It should be mentioned that the standard footprint methodof determining hectares of global average productivity usingequivalence factors does not apply here as these factors arebased on the Global Agro-Economical Zones (GAEZ) index, andmany factorswhich increasebio-productivitywill reducea soil'scapacity to oxidise methane. It is unclear as to whether such afootprint can be reported in units of global hectares, as the termis associated with a very specific methodology. Within this me-thodology, as oxidative areas also provide other services, theyare as amatter of course includedwithin the global bio-capacityestimate. For that reason, it is argued here that oxidative areasshould not be included within the calculation of national(or global) bio-capacity. (This is comparable to how forestlandrepresents an individual footprint for both wood products andcarbon sequestration but is treated as a single category withinnational bio-capacity estimates). The extent to which theinclusion of an oxidative area within an Ecological Footprintestimate may represent double counting is considered later inthis paper.

However, while some of the methane-derived carbon isutilised by methantrophic bacteria, the majority is returned tothe atmosphere as CO2. Roslev et al. (1997) suggest an upperlimit for the mean carbon conversion efficiency of 40%. The re-emitted CO2 may be converted into global hectares in themanner described above. Given the quantity of methane oxi-dised, the carbon content of methane, the microbial assimila-tionof carbonand theeffect of oceanabsorption, the footprintofcarbon ultimately released through soil oxidation will be com-paratively insignificant. When reviewing this methodology it isimportant to decide whether a footprint based on terrestrialoxidation should attempt to account for all methane consump-tionorwhether it should incorporate the roleof theatmospheric

Table 3 – Footprint based on oxidation area and related atmospheric attenuation.

Consumption Oxidation area Remitted from soil Atmosphere(no GWP)

Atmosphere (GWP)

Tonnes CH4 ha ha/cap gha gha/cap gha gha/cap gha gha/cap

471,253 14,994,408 3.77 14,954 0.0038 331,131 0.083 3,015,767 0.758

Table 4 – Additional area necessary to reduce NRF.

Net radiativeforcing

Additional area required

% W/m2 MW ha ha/cap

100 0.0001045 53,306 0 080 0.0000836 42,644 12,751,640,000 3,20560 0.0000627 31,983 34,004,373,333 8,54640 0.0000418 21,322 76,509,840,000 19,22920 0.0000209 10,661 204,026,240,000 51,277


sink. As themethaneoxidation rate is quantitatively low, itmaybe unfeasible to apply it to overall national consumption as itmay overwhelm other land types. The method adopted here isto allocatemethane to both an atmospheric and terrestrial sinkin the same proportion as is observed in the biosphere. In thiscase the footprint of atmospheric methane can be calculatedwith or without incorporating GWP. The results of this method,applying both oxidation rates and carbon sequestration can beseen in Table 3.

Some commentators suggest that net radiative forcing(NRF) is a more significant indicator that GWP. NRF estimatesthe overall increase in irradiance reaching the tropopause dueto the action of a climate change driver such as GHG emissionsor increased solar insolation. In terms of anthropogenic emis-sions, net radiative forcing is related to GWP andmeasures theincrease innet energy gain due to emissions realised since pre-industrial times. This quantifies incoming irradiance in W/m2

and a positive value indicates that human activity is con-tributing to a net increase in temperature. The main rationalefor its application is that it quantifies the actual effects ofclimate change as opposed to expressing methane emissionsin terms of a related (and somemight argue arbitrary) variable.Chapter 2 of the IPCC Fourth Assessment Report: Climate Change—The Physical Science Basis, entitled Changes in Atmospheric Con-stituents and in Radiative Forcing (Foster et al., 2007) states thatanthropogenic methane emissions since 1750 have contribu-ted to a radiative forcing of 0.48 W/m2. In order for radiativeforcing to be incorporated into a national footprint account,annual methane consumption for the study year needs to betranslated into a unit comparable with pre-industrial condi-tions, in this case concentration in parts per billion (ppb). Thiseffectively treats the relative emissions as being the only an-thropogenic GHG present in the atmosphere. Ramaswamyet al. (2001) apply a simplified equation to calculate the ra-diative forcing of methane.

ΔF ¼ αð√M−√M0Þ−ð f ðM;N0Þ−f ðM0;N0ÞÞ ð1Þ

Where

• ΔF is radiative forcing in W/m2

• α=0.036• f(M,N)=0.47 ln[1+2.01×10−5×(MN)0.75+5.31×10−15×M(MN)1.52]• M is CH4 concentration in parts per billion (ppb)• N is N2O concentration in ppb• The subscript zero refers to pre-industrial concentrations.

The National Center for Atmospheric Research (NCAR)estimates that the overall mean mass of the atmosphere is5.1480×1018 kg. Assuming an averagemolarmass of 28.97 g/mol(basedoncurrentmixing ratios) allowsanapproximation for thetotal moles present in the atmosphere. By converting Ireland's

methane emissions due to consumption into nanomoles, theoverall concentration in ppb can be estimated. Once the ra-diative forcing of methane is calculated then the increasedirradiation the earth receives due to the methane embodied inIrish consumption canbe calculated.Thisallowsanestimate forthe quantity of additional land required to dilute its greenhouseeffect. This is a stark departure from traditional bio-capacity ledEcological Footprinting and its implications will be discussedlater.

7. Results

The method of calculation of Ireland's methane emissionsarising from consumption is summarised in Table 1. A sig-nificant proportion of methane emissions can be attributed tothe agricultural sector. The extent to which such commoditiesare exported means that despite being a net importer of ma-terials, Ireland is a net exporter of methane emissions.

Translatingmethane into CO2 equivalents using GWP (as op-posed to carbon equivalents) significantly increase themethanefootprint (Table 2).

Table 3 demonstrates the individual components of a mul-tifacetedmethane footprint. It is assumed that 7%of embodiedmethane is assimilated by oxidative soil and that 60% of itscarbon content is re-emitted as CO2. The remainingmethane isassumed to be converted into CO2 in the atmosphere andsubsequently sequestered by growing trees. At this stage GWPor carbon mass equivalents can be used. The majority of thefootprint is formedby the oxidative area. Therefore the issue ofincorporatingGWP,while still relevant, becomes less pivotal. IfGWP is not incorporated at this stage, this results in a final areavalue (including oxidative area and global hectares) of 3.85 ha/cap. Retaining the GWP gives a final value of 4.53 ha/cap.

Calculating the concentration ofmethane in the atmosphereresulted in a net radiative forcing estimate of 0.000104 W/m2.While this may seem insignificant, when applied to the globalsurface area of 510,065,600 km2 this becomes a global forcingestimate of over 53 GW. Table 4 illustrates the additional areaneeded to incrementally dilute the effect of this increase in


irradiation. When this is allocated to the Irish population, itresults in a value greatly in excess of any published EcologicalFootprint value based on bio-productivity. The issue of assign-ing responsibility is discussed later.

8. Discussion

The quantity of imported inorganic methane exceeds that oforganic imports. This is due to the fact that Ireland is largelydependent on material imports. It should be noted that theestimates calculated do not include the emissions from im-ported fuel, as these are already included within the GHGinventory (McGettigan et al., 2007b). If the methane embodiedin tradedmaterials were incorporated into the GHG inventory,methane emissions would be reduced by approximately 25%.

8.1. Evaluation of footprint methods

The advantage of simply converting methane into carbonequivalents lies in its comparability with established Ecologi-cal Footprinting methods. The main rationale for this methodis the assumption that methane is converted into CO2 in theatmosphere. While this represents a simplified approxima-tion, it should be readily comprehensible to those familiarwith Ecological Footprinting methodology, and easily under-stood by those who are not. While this may be seen as themost logical choice, it does present several disadvantages.Firstly, asmethane emissions are quantitatively less than CO2,this results in a significantly smaller carbon footprint. As canbe seen in Table 1, the methane footprint based solely onthe carbon content of methane is considerably less than Ire-land's reported CO2 footprint (WWF, 2006). This may reducethe communicative impact of such a calculation. As withtraditional Ecological Footprinting, the use of carbon seques-tration is perhaps better understood as a measure of sub-stitutability as opposed to ameasure of impacts. Perhapsmoresignificant is the fact that methane does not behave like CO2,as it has a greater capacity to increase net radiative forcing aswell as a longer functional life in the atmosphere. In order toaccount for these differences, multiplying by the GWP willconvert methane emissions into the amount of CO2 with anequivalent radiative forcing over a given period. The questionof whether the methane footprint should incorporate GWP isin essence the question of whether a weighting factor shouldbe applied.

Adopting GWP significantly increases the effect of methaneon the overall footprint. The advantage of GWP is that it reflectsthe greater climatic impact of methane compared to CO2,whereas simply translating to mass of carbon does not, aswhile methane has a GWP factor of 25 times that of CO2, it doesnot require 25 times the land area to sequester its carbon con-tent. Thishighlights that a land-based indicator isnot capableofrepresenting the full range of anthropogenic impacts. In otherwords as GWP is not directly relatable to bio-productivity, anincreased ‘CO2 equivalent’ footprint is seen as being a suitablesubstitute for a different environmental impact. At with manymethodological debates, the need to inclusively account for thewide-ranging effects of human activities is judged against theneed to accurately represent biological processes. For that

reason it is preferable if the GWP is applied to methane emis-sions, as opposed, for example, to a resultant carbon footprintvalue.

One of the concerns implicit in the use of such weightingfactors is how they are reconciled with the ‘global hectare’concept. By applying a weighting factor in a calculation of agiven land type (in this case new forest growth), it presupposesthat this relationship remains meaningful once the area inquestion is translated into global hectares. Different landareas may react inconsistently to different stimuli and mayreduce the validity of equivalence factors as they are currentlycalculated. Also, if GWP is adopted this suggests that othergases with higher GWP estimates should be incorporated.Unfortunately many of these gases are not readily assimilatedinto the biosphere, complicating their inclusion into EcologicalFootprinting methods. From Table 2, when the GWP ofmethane is used to generate CO2 equivalents, this results ina value comparable to Ireland's overall carbon energy foot-print. Many commentators would likely agree that it is pre-ferable to include all relevant impacts (even inaccurately),rather than to omit them from analysis.

The application of methane oxidation rates in estimatingland area requirements is a departure from standard Ecologi-cal Footprint calculation. Themain rationale for this departureis the need to approximate the actual processes by which car-bon cycles through the biosphere. There is an additionalreason to apply such a method. While Baird (1999) describes ameans by whichmethanemay return to the biological system,it is dependent on the chemistry of free radicals, which maybe unpredictable. While CO2 remains the most likely resultof methane's reactions within the atmosphere, it may notapply to all anthropogenic methane emissions. As mentionedpreviously, calculating a methane footprint entirely based onoxidation rates is unwieldy, resulting in an estimate of56.4 gha/cap. As suggested, a preferable method may be toallocate the soil and atmospheric sinks in proportion to theiractual uptake within the methane cycle (7% and 93% re-spectively). This method results in a smaller footprint thanone based solely on terrestrial oxidation and perhaps repre-sents a more realistic approximation (see Table 3). It allowsthe methodological choice as to whether the methane at-tenuated in the atmosphere should be translated into globalhectares based simply on its carbon content or its relativeglobal warming potential.

Another issue for discussion is whether this method re-presents double counting (accounting for the same area twice).This is complicated when bio-productive areas serve morethan one function. This discussion is related to the much-debated ‘single land-use’ used in Ecological Footprinting. Incurrent methods land area is assumed to provide one primary“mutually exclusive” function. While in reality an area mayprovide dual services, such as carbon sequestration and woodprovision, allowing them to functionwithin the same areawillprovide one area value for two distinct services. It can bedebated as to whether this applies to the oxidation area of amethane footprint. The aerobic land area, which oxidisesmethane, also produces commodities such as grass and wood.However a distinctionmay bemade. The oxidation ofmethaneis a biochemical service, undertaken by microbial activity,which is separate from appropriable bio-productivity. Indeed,


the factorswhich determine bio-productivity (whenmeasuredin terms of yield) may have little bearing on local aerobicoxidation and vice versa. In the National Footprint Accounts,the area required to supply forest goods and the forest arearequired to sequester carbon emissions are estimated sepa-rately, as commercial forests are not considered to providelong term storage of fossil carbon. In this instance it is recog-nised that the carbon footprint represents an additionalforested area that must be maintained specifically for thepurpose of carbon capture. Because factors such as the use offertilisers may reduce a soil's capacity for methane oxidation,this suggests that estimates for additional areas specificallyallocated for this purpose, should be included in the samemanner as forests in energy footprint calculations.

This point might be clarified by distinguishing between landtype and land function. The oxidation of methane provides adifferent function to the production of materials for humanconsumption, even if it takes place within the same area. Ifmethane oxidation and resource provision are seen to be tooclosely associated to exist simultaneously within the EcologicalFootprint framework, Kitzes et al. (2007b) provides a potentialsolution. It may be feasible to extract area estimates for in-terdependent services and highlight them in a non-additivecapacity or develop extended (or perhaps satellite) accounts. Inthis fashion such estimates may function as indicators withoutcontributing to the overall Ecological Footprint estimate.

This raises another important issue, the footprint of me-thane, being an indicator of human activity, does not includenatural methane emissions. Also it assumes that the soil sinkarea is entirely oxidative at all times. This may not be the case,as under certain conditions soils may become net methanesources: therefore this footprint represents a theoretical landarea. It could be argued that the oxidation rate should betemperedby includinganestimate fornaturalmethanesources.This would require an accurate estimate for the area distribu-tion (either nationally or globally) of soil sources and sinks.

The methane footprint based on NRF represents a starkdeparture from traditional Ecological Footprinting as it is notbased on bio-capacity but rather represents the additionalland area required to offset the increased irradiance experi-enced globally due to Irish methane consumption. While thismethod is presented primarily for illustrative purposes, it doesraise potentially useful questions. As NRF represents condi-tions at the top of the tropopause, it does not relate directly toterrestrial area. In this sense area is assumed to refer to anarea of tropopause surface above a corresponding terrestrialarea. The advantage of this method is that while calculationsutilise GWP and/or carbon sequestration in an to attemptcompensate for the effects of climate change, NRF demon-strates the actual result. However, the usefulness of such anindicator may rightly be questioned. The results illustrated inTable 4 indicate the additional land necessary to reduce theeffect of a relatively small impact. The main disadvantage ofthis method is that it cannot give a definitive area for thecomplete neutralisation of NRF. As NRF is measured in W/m2

there is no yield factor to apply. Even when the impact mayhave little or no measurable effect, increasing the area willreduce the anthropogenic irradiance per unit area but willnever remove it completely. The implied recommendations ofincreasing land area or reducing consumption provide no new

insight for informing behaviour or policy. Table 4 illustratesthe additional land as needed to reduce NRF and assigns it tothe population of Ireland. As the impacts of the emission ofmethane embodied in Irish consumption are global in nature,there is justification for allocating the additional area requiredon a global per capita basis. However, while the result of thismethod, as an indicator in itself, may be imperfect it may yetprove useful within Ecological Footprinting methodology. Byexamining the results of NRF, the hypothetical nature of amethane footprint may be reduced. The effect of an increasein irradiance and hence temperature on various land types(such as changes in forestry growth and the loss of arable landdue to soil erosion or coastal flooding) may be estimated andused in conjunction with other Ecological Footprint methodsas described earlier (Lenzen et al., 2007). Certainly this methodis complicated by the inherent uncertainty entailed in anyestimate of the likely effect of climate change. Also given themixing ratio of gases in the atmosphere, the footprint of GHGwill have to account for the global effect of future climatechange and not just the likely effects in the country respon-sible for emissions. While this highlights the global natureof such emissions, gauging such effects on a worldwide scalemay prove very difficult. However NRF should not be dis-missed but recognised as a potentially useful component offuture Ecological Footprint analysis.

The calculation of the methane footprint suggests thatthere is merit in considering all natural processes, as opposedto strictly biological ones, within Ecological Footprinting. Theatmospheric degradation of methane and the oceanic absorp-tion of CO2 are two examples of how such processes may beincluded. This raises the possibility of other wastes that arenot immediately assimilated into the biosphere being incor-porated into Ecological Footprinting. Non-biological cyclesmay be used to translate the results of consumption intometrics that are more compatible with current EcologicalFootprint methods: however, comparison against the provi-sion of natural capital must remain the primary focus ofecological footprinting. It is unclear if non-biological naturalprocesses will have the same identifiable limitations as thebiological ones currently applied in footprinting. Therefore theprovisions of the strong sustainability concept may be betterserved by an Ecological Footprint method that retains its coredependence on biological processes but may incorporate ap-propriate elements from any natural cycle. To summarise, theintegration of methane into Ecological Footprinting meritscareful consideration of a number of methodological issues.The issue ofmethane's environmental impact is predicated onits increased GWP. This is also complicated by the fact thatany substance that reacts with the OH radical (thus reducingits concentration in the atmosphere) may in turn cause anincrease in methane concentration. Regardless of which me-thod is adopted, the varied paths taken by methane throughthe biosphere means that any choice of method is likely tohave shortcomings. As methane is part of the carbon cycle, itcannot be said to have a definitive destination. The decision totreat carbon assimilated by soil microbial activity as beingeffectively sequestered may also be open to criticism. How-ever its inclusion does reflect the fact that soils provide animportant service in global carbon sequestration. Herendeen(2000) makes the point that as succession is completed, net


carbon sequestration approaches zero. Given this finding, amethane footprint may be considered as notional as aconventional CO2 footprint. The decision as to which methodshould be adopted depends on the value placed on accuratelysimulating natural processes as opposed to maintainingmaximum comparability with other Ecological Footprint stud-ies. Applying oxidation rates results in a per capita footprintthat is comparable to Ireland's overall Footprint (WWF, 2006).This may be a reason to consider publishing a national me-thane footprint in a satellite account supplementary to, butdistinct from, the mainstream National Footprint account.Regardless of which approach is taken, it should be noted thatthe inclusion of methane in Ecological Footprinting should beviewed as another step in the iterative processes of improvingthe standard Footprint methodology.

8.2. The issue of fugitive emissions

Here total methane emission intensities were estimated, in-cluding emissions due to national fuel production. It may beargued that such fugitive emissions should not be incorpo-rated into the energy intensity estimates of the producingstate, as they are not the result of direct energy consumed inthat country. However fuel production is geographically de-pendent and does require energy consumed within the pro-ducing country.Oneof thepotential disadvantagesof retainingfugitive production emissions within the calculation of themethane intensity of the producing country is that it mayreduce the apparent methane intensities of developed coun-tries, which often produce little fuel; that is countries that canimport the majority of their energy needs will have lowermethane intensities. These are generally countries that areeconomically stronger and consume large quantities of energyin order to satisfy resource intensive lifestyles. Ireland iscurrently dependent on imported energy. If the fugitive emis-sions of productionwere allocated to energy consumers ratherthan to geographic location, then Ireland's domestic methaneemissions would rise significantly. In addition, Ireland alsoimports many products from other European countries, someof which are also dependent on imported energy.

In order to satisfy the responsibility principle, only thefugitive production emissions of fuel consumed within thecountry should be included in fugitive production emissionestimates. This is problematic given the difficulties in distin-guishing domestically and foreign produced fuel, each withdiffering fugitive emission factors (Houghton et al., 1997). Analternative is to calculate a methane energy coefficient, whichretains all the fugitive emissions (based both on production andconsumption) contained within the study region and onlyincorporates direct emissions. The choice of method is depen-dent on the priority placed on allocating consumer responsi-bility as opposed to accurately representing resource demands.This issue is complicated by the fact that in many of thesedeveloping countries, the energy production sector is of majoreconomic importance. Altering energy supply may indirectlyaffect thequality of life ofwhole communities. Changing importpractices to favor wealthier countries (which generally have atleast some GHG abatement policies) may result in considerableloss of revenue for exporting developing countries. The abun-dance of re-exported goods, whereby materials are exported

from countries that did not produce them, further complicatesthe issue of impact allocation (Munksgaard and Pedersen, 2001).This may be of greater importance for developed countries thatare likely to benet importers ofmaterials. Bastianoni et al. (2004)reject the dichotomy of the geographic and responsibilityprinciple as application of the former favours more developedcountries andapplicationof the lattermaynotprovide incentivefor improving resource efficiency. A compromise is presentedbased on the “carbon emissions added approach”whereby eachstage in theproduction/consumptionprocess is considered tobeco-responsible for the emissions of each previous stage. In thatregard the emissions of every stagewill include its emissions aswell as theemissionsallotted to eachprevious stage.Theoverallemissions as allocated by co-responsibility are summedand thepercentage taken up by each stage is used to consign the actualoverall emissions as calculated using lifecycle analysis.

9. Conclusions

The inclusion of methane within the Ecological Footprintincreased Ireland's per capita footprint by 2% without ac-counting for GWP, but by 20% when GWP was accounted for.Applying a method based on soil oxidation rates results in amuch larger footprint, comparable to Ireland's overall foot-print for all other consumption categories. Altering the way inwhich fugitive emissions are allocated in calculations mayfurther increase this value. The task of reducing global me-thane emissions is a daunting one that has consequences forsocial, economic and environmental pillars of sustainabledevelopment. For Ireland, the biggest methane emission re-ductions are achievable by changes in agricultural practicesand reduced dependence on imported energy and materials.

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Incorporating methane into ecological footprint analysis: A case study of Ireland