carbon sequestration and biomass energy offset: theoretical, potential and achievable capacities...

Biomass and Bioenergy 24 (2003) 97–116

Carbon sequestration and biomass energy o�set:theoretical, potential and achievable capacities globally,

in Europe and the UKMelvin G.R. Cannell

Centre for Ecology and Hydrology, Bush Estate, Midlothian, EH26 0QB, UK

Received 21 March 2002; received in revised form 5 July 2002; accepted 8 August 2002

Abstract

The extensive literature on the capacity to o�set fossil fuel carbon emissions by enhancing terrestrial carbon sinks orbiomass energy substitution is confused by di�erent interpretations of the word ‘potential’. This paper presents an overview ofthese capacities for the world, the EU15 countries and the UK over the next 50–100 years, divided into what are considered:(i) theoretical potential capacities, (ii) realistic potential capacities, and (iii) conservatively achievable capacities. The range ofcapacities is determined principally by judgements of the areas of land that are likely to be devoted to sequestration or energycrops. Theoretically, enhanced carbon sequestration and energy cropping could o�set 2000–5000 Mt C=yr globally, but a morerealistic potential o�set is 1000–2000 Mt C=yr and there are good reasons to suppose that only 200–1000 Mt C=yr is actuallyachievable. Similarly, ‘conservative achievable’ estimates for the EU15 and the UK are about 10 times less than theoreticalpotentials. In the EU15, ‘realistic potential’ and ‘conservative achievable’ estimates for energy crop substitution were 21–32% and 11–21% of current annual emissions, respectively, compared with 5–11% and 2–5% for carbon sequestration. Inthe UK, the ‘realistic potential’ and ‘conservative achievable’ estimates for energy crop substitution were 3.4–13.6% and0.7–4.1% of current annual emissions, respectively, compared with 2.0–3.4% and 0.7–1.3% for carbon sequestration.? 2002 Published by Elsevier Science Ltd.

Keywords: Carbon sequestration; Energy crops; Substitution; Biomass; Biofuels

1. Introduction

This paper summarizes and reviews global, Euro-pean and UK estimates of the amount of carbon thatcould be sequestered in biomass (plants and soils) dur-ing the coming 50–100 yr, and the amount of carbonemitted by burning fossil fuels that could be avoidedby growing biomass for energy.

The literature on these two aspects is extensive, butit is scattered and bedevilled with di�erent interpreta-tions of what is ‘potential’ and what is actually achiev-

able. In this review, estimates are given of the rangeof values for:

• Theoretical potential capacity, meaning some orall practical constraints have been ignored.

• Realistic potential capacity, meaning account hasbeen taken of most constraints, but some optimisticassumptions are made about land availability,socio-economic and policy drivers.

• Conservative, achievable capacity, meaning a cau-tious prognosis, based on current trends, with fewoptimistic assumptions.

0961-9534/03/$ - see front matter ? 2002 Published by Elsevier Science Ltd.PII: S0961 -9534(02)00103 -4

98 M.G.R. Cannell / Biomass and Bioenergy 24 (2003) 97–116

Each section is prefaced with a likely range of val-ues, in italics, followed by a review of the literatureand calculations upon which the range of values isbased.

2. Carbon sequestration

The estimates given here are the amounts of carbonthat can be stored in vegetation and soils as a directresult of man’s activities over the next 50–100 yr—additional to the carbon sink that exists ‘naturally’,most of which is essentially free. This net ‘natural’ ter-restrial carbon sink was 1:5+0:7 Gt C=yr in the 1990s,the net result of a sink of 3:2 Gt C=yr and an emissiondue mainly to deforestation of 1:5 Gt C=yr. Currentglobal fossil fuel emissions were 6:4+0:4 Gt C=yr inthe 1990s [1]. Thus, about 23% of the carbon emittedfrom fossil fuels is being taken up by the land withoutany change in current practices [2].

This ‘natural’ sink of 3:2 Gt C=yr can be attributedto two factors. First, forest and soil carbon stores arenaturally recovering from past depletion. Thus, thepresent distributions of age classes in the forests ofUS, Canada, Europe and parts of Russia are not in aquasi-equilibrium state—they are aggrading. The liv-ing biomass in forests is increasing by 0:17 Gt C=yrin US and 0:11 Gt C=yr in W Europe [3]. Some ofthis sink is due to man’s activities or decisions toconserve forests, prevent Hres and not to harvest asmuch timber each year as grows, but much of it canbe regarded as ‘natural’. Second, the sink is due toa global acceleration of photosynthesis by increasingatmospheric CO2 levels and, in some areas, nitrogendeposition.

Recent estimates would place 0:88 Gt C=yr of this3:2 Gt C=yr global sink in temperate and boreal re-gions [4] and 0.3–0:6 Gt C=yr in the US [5].

2.1. World

To provide a perspective on the amount of car-bon that can be sequestered in vegetation and soils,it should be recognized that the total amount of car-bon emitted from fossil fuels plus land use changeup to 2000 was about 420 Gt C. Projections of theamount of carbon emitted from 2000 to 2100 rangefrom 690 following IPCC scenario IS92c, 1430 Gt C

following IS92a (approximately business-as-usual) to2090 Gt C following IS92c [6]. As mentioned, currentfossil carbon emissions are about 6:4 Gt C=yr.

2.1.1. Theoretical potential capacityLikely range 100–200 Gt C; 2–4 Gt C=yr for 50

yearsIt could be argued that an upper boundary on the

amount of carbon that could be added to the store ofcarbon on land is roughly equal to the amount that hasbeen lost historically as a result of man’s activities.That is, all vegetation and soil carbon pools could bebrought back to quasi-equilibrium with the climate (asit was)—all naturally forested land restored to forest,and so on. This boundary condition would be achievedby a mixture of current ‘natural’ processes and new ac-tivities. The amount of carbon that has been lost fromglobal soils and vegetation (principally forests) in theperiod 1700–1985 is estimated to be about 170 Gt C(41 Gt C from soils) [7,8]. Assuming some loss priorto 1700, the total loss is about 200 Gt C. Some wouldargue that the upper boundary is higher than this be-cause of elevated CO2 and climate change, combinedwith the use of fertilizers and irrigation—but thosefactors are uncertain and/or carry fossil fuel costs.

Optimistic estimates of the land area available tostore carbon in trees (a�orestation, regeneration, agro-forestry) and agricultural soils (restoration, set-aside,minimal tillage) and of the amount of carbon storedover about 50 years, give totals of 76–147 Gt C.This is made up from 52 to 104 Gt C in trees and 24–43 Gt C in agricultural soils [9,10]. These estimatesassume that 300–600 Mha of land in the tropics isconverted to forests or agroforests and that soils arerestored in 10–50% of the 1200 Mha of tropicaldegraded lands. Also, over half of the carbon losthistorically from agricultural soils would be restored.

All of these estimates of potential carbon storageviolate or ignore the ideals of sustainable develop-ment, encompassing economic, social and environ-mental goals.

2.1.2. Realistic potential capacityLikely range 50–100 Gt C; 1–2 Gt C=yr for 50

yearsEstimates have been made of the amount of

carbon that could be stored by: (i) planting newforest and agroforest plantations, (ii) managing

M.G.R. Cannell / Biomass and Bioenergy 24 (2003) 97–116 99

existing forests, and (iii) managing croplands—takinginto account competing demands on land and thetimescale of tree planting or cropland management.These three estimates are expanded below. However,all assume no adverse climate change and an aggres-sive policy to support carbon sequestration, requiringthe commitment of considerable Hnancial resources.

(i) The best estimates of sequestration in newforests/agroforests are still those of Nilsson andSchopfhauser [11] and Trexler and Haugen [12], usedin the IPCC Second Assessment [13]. The existingglobal plantation forest area of 80–120 Mha (about40 Mha in the tropics) could be expanded by an ad-ditional 275 Mha, plus 70 Mha of agroforests, from1995 to 2050, by planting at rates consistent withnational forestry objectives (8:5 Mha=yr globally forplantations, which is reasonable when compared with3:9 Mha=yr in China alone in the 1980s). It is as-sumed that all technically suitable lands at mid- andhigh-latitudes are used (215 Mha) but only 6% oftechnically suitable lands at low latitudes (130 Mha)are used because of demands for food and otherconstraints. By 2050, a total of 38 Gt C would besequestered in planted forests and agroforests, plus aless certain 12–29 Gt C as a result of regeneration ofdegraded tropical lands, giving a total of 50–67 Gt C.Estimates of carbon emissions prevented by slowingdeforestation are ignored in this analysis.

(ii) Existing forests can be managed to raise thecarbon store in trees and forest soils, by lengthen-ing rotations, preventing Hres and insect pest out-breaks, applying fertilizers, and so on. These actionscould potentially increase the sink in existing foreststo 0:7 Gt C=yr by 2040 [14]. However, this estimateassumes that half of the forest area is managed withcarbon storage as an objective, it includes activitiesthat will occur anyway and ignores adverse e�ectson, for instance, biodiversity, Hre hazard and waterresources.

(iii) Lal and Bruce [15] estimated the potential car-bon storage in croplands (essentially in soils) over 20–50 years to be somewhere in the range 10–30 Gt C.This is equivalent to 0.4–0:6 Gt C=yr for 20–50years, achieved by accumulating 0.08–0:12 Gt C=yrby erosion control, 0.02–0:03 Gt C=yr by restorationof degraded soils, 0.02–0:04 Gt C=yr by reclamationof salt-a�ected soils, 0.15–0:175 Gt C=yr by con-servation tillage and crop residue management, and

0.18–0:24 Gt C=yr by improved cropping systems(see also, [4]). In a separate estimate, Keller andGoldstein [16] considered that 0:8 Gt C=yr could besequestered by restoring drylands (woodland, grassand semi-desert), which if continued over 20 yearswould accumulate 16 Gt C (partly in vegetation,partly in soils).

Overall, the IPCC Special Report on land use, landuse change and forestry concluded that, by 2040, thepotential sequestration rate by managing lands (crop-land, grazing, forest management, urban, etc.) couldbe 0:8 Gt C=yr in Annex I countries and 0:7 Gt C=yrin non-Annex I countries, totalling 1:5 Gt C=yr. In ad-dition a further 0:6 Gt C=yr could be sequestered bythe adoption of agroforestry in unproductive croplandsand grasslands [17].

Mention may also be made of the pool of carbonin forests products. Globally, this pool is estimatedto be 10–20 Gt C, increasing by 0:139 Gt C=yr—thusrepresenting a small sink [18,19].

2.1.3. Conservative, achievable capacityLikely range 10–50 Gt C; 0.2–1.0 Gt C=yr for 50

yearsCurrent policies and trends make it very likely that

many direct human actions and decisions will, indeed,be made that will enhance the ‘natural’ terrestrial car-bon sink in the coming decades. These actions aredriven by goals other than carbon sequestration.

(i) Global timber models predict that, as a result ofincreased demand, global forests will become a sinkof 0.11–0:25 Gt C=yr between 1995 and 2045 (at leasthalf in tropical/subtropical countries). This is a resultof increased demand for timber and normal marketactivity [20].

(ii) Conservation tillage (minimum and no-till),which saves 40% of tractor fuel and lessens soil or-ganic matter decomposition, is increasing in the USand Canada [21,22]—although the primary motivesare to lessen production, reduce costs, conserve water,reduce erosion and enhance biodiversity.

(iii) Some countries are vigorously pursuing poli-cies to increase their forest cover, notably China andIndia, in order to lessen erosion and provide fuelwood,while others are promoting agroforestry. Many devel-oped countries place increasing emphasis on forestconservation. Also, there is growth in the area of forest


plantations worldwide, led by Asia; half of all planta-tions in the world are less than 15 years old [23].

The IPCC Special Report on land use, landuse change and forestry, estimated that, globally,with an ambitious policy agenda, new a�oresta-tion/reforestation could sequester 0.20–0:58 Gt C=yrby 2008–2012, and ‘additional activities’ (forest,cropland and grassland management, agroforestry andcropland to grass conversion) may lead to seques-tration of about 1:0 Gt C=yr (0:4 Gt C=yr from agro-forestry alone). These estimates total 1.2–1:6 Gt C=yrby 2008–2012 [17]. By contrast, new a�orestationand reforestation since 1990 in Annex I countries islikely to sequester only 0.007–0:046 Gt C=yr in 2012.

There are, however, good reasons to be cautious. Inthe absence of Hnancial incentives, carbon sequestra-tion is likely to remain incidental to the main gaols ofland managers. All 10 of the existing major forestryprojects in tropical countries designed speciHcally tosequester carbon or avoid emissions (by lesseningdeforestation) will save only about 0:05 Gt C overtheir lifetime (covering 2:9 Mha, [17]). This esti-mate assumes no negative leakage—the displacementof activities which release carbon elsewhere—anddoes not take account of the fossil fuel cost of im-plementation. Any large-scale global a�orestationprograme to sequester carbon would result in an over-supply of wood, undermining incentives for currenta�orestation—causing negative leakage [24]. Accountmust also be taken of the e�ect of land use changeon the sources and sinks of N2O and CH4, and ofthe risk of climate change itself adversely a�ectingforests, notably by increasing the risk of Hre [25].

Even with Hnancial incentives, there are formidableconstraints to increasing forest cover in many parts ofthe tropics. These are the same forces that are drivingdeforestation, namely, the demand for cropland, fuel-wood and, in Latin America, grazing land—fuelled byinsecure tenure, landless people, land speculation, ex-ternal debt and migration. Sathaye and Ravindranath[26] estimated that, in tropical and temperate Asia, lessthan half of the potential sequestration of 26 Gt C islikely to be feasible, and that is probably optimistic.

2.2. Europe

In 2000, anthropogenic emissions from the EU15totalled about 0:94 Gt C=yr (Eurostat website). Be-

cause of underharvesting, increased growth rates,conservation and new planting, forests in EU15 coun-tries are estimated to be net carbon sinks, totalling63 Mt C=yr in 1995 (about 14 Gt C=yr in Germany,11 Gt C=yr in Sweden, 10 Gt C=yr in France and7 Gt C=yr in Finland) [27]. Thus, the current forestsink takes up about 7% of the annual emission.

The EU15 countries have about 116 Mha of forestand 142 Mha of agricultural land, of which 72 Mhais arable and 35 Mha is cropped with cereals. Theestimates of potential sequestration depend primarilyon judgements of the areas that can be converted fromagriculture to forest and from arable to pasture as wellas changes in cropland management.

2.2.1. Theoretical potential capacityLikely range 20–30 Gt C; 200–500 Mt C=yr for

50–100 yearsIf all of the 142 Mha of agricultural land were

a�orested, and accumulated 100–150 tC=ha over 50–100 years (200–300 t=ha dry biomass, [28]), thiswould store 14–21 Gt C in biomass, plus possibly anadditional 5 Gt C in the soils on previously arableland, totalling 19–26 Gt C. This would give an aver-age sequestration rate of 0.38–0:52 Gt C=yr over 50years or 0.19–0:26 Gt C=yr over 100 years, represent-ing 20–55% of current EU15 emissions. Scurlock etal. [29] made similar back-of-envelope calculations,showing that 40–120% of the total land area of Europewould need to be devoted to new forests to absorb allEurope’s emissions.

2.2.2. Realistic potential capacityLikely range 5–10 Gt C; 50–100 Mt C=yr for 100

yearsMore credible estimates of potential carbon stor-

age in the EU15 countries were made by Smithet al. [30]. They estimated that a�oresting 20% ofcurrent arable land (i.e. 14 Mha) would sequesterabout 5 Gt C over 100 years in biomass and soils.Incorporating straw, manures and sewage sludge intoagricultural soils would sequester another 2–3 Gt Cover 100 years.

Alternatively, if all EU15 arable land were con-verted to no-till farming, this would sequester about23 Mt C=yr (Smith et al. [32]). In addition, we shouldattribute some fraction of the current European forest


sink to management practices that will continue withinthe existing forest area.

In a further study, Smith et al. [31] considered thecarbon sequestration and biofuel o�set consequencesof a number of policy options for the use of 10% sur-plus arable land in the EU15 (ca. 7 Mha) and man-agement of the remaining arable land. In their optimalscenario, 50% of the surplus land is used for bioen-ergy, 50% for woodland and the remaining arable landis managed with a high rate of organic matter incor-poration into soils. This optimal scenario produced acarbon mitigation potential of 105 Mt C=yr.

These estimates suggest that a realistic potentialsequestration capacity resulting from all types ofland use change and management might lie in therange 5–10 Gt C, giving a sequestration rate of 50–100 Mt C=yr for 100 years. However, Smith et al.[30–32] stressed that these are potentials and do notconsider all the constraints.

2.2.3. Conservative, achievable capacityLikely range 2–5 Gt C; 20–50 Mt C=yr for 100

yearsActual land use change in Europe in the past 50

years has been slow relative to the potential changesconsidered above. If historic constraints on land usechange persist, future change might be slow and poten-tial rates of carbon sequestration will not be achieved.At present, it seems unlikely that carbon sequestra-tion will feature strongly in changes in the CommonAgricultural Policy or incentives for forestry, althoughSolberg [33] has argued that the net value of carbonsequestered in forests is greater than the net value ofthe timber. History suggests that any large expansionin plantation forestry is likely to meet local objections,especially in southern Europe where water suppliesmay become increasingly critical.

The European Forest Institute scenario modelsuggests that, in a business-as-usual scenario, thegrowing stock of carbon in 27 European countrieswill increase by 74 Mt C=yr between 1990 and 2050,mainly due to continued increase in the biomass ofexisting forests. In an alternative scenario, whichincludes forest expansion of 4 Mha and conserva-tion forestry on 4 Mha, the increase in carbon was94 Mt C=yr from 1990 to 2050 [34]. Much of thiscarbon storage is the legacy of past actions.

There is no objective way of estimating the level ofcarbon sequestration that might actually be achievedby new forest planting in the EU15, but a conserva-tive range would be 2–5 Gt C, or 20–50 Mt C=yr for100 years, o�setting the equivalent of 2–5% of currentEU15 emissions.

2.3. United Kingdom

In 2000, the UK emitted 147 Mt C=yr by burningfossil fuels. A further 4 Mt C=yr were emitted as aresult of past land use change (primarily from soils),partially o�set by about 3 Mt C=yr absorbed by forests[35]. The forest sink is the result of expansion in thearea of forest since the 1950s. A detailed breakdown ofUK carbon sources and sinks in 1990—due to changesin the stores of carbon in soils and vegetation—isgiven by Cannell et al. [36].

The UK has about 11:4 Mha of agricultural land(about 5:3 Mha of which is arable cropland and6:1 Mha grassland), 6:6 Mha of semi-natural veg-etation (mostly in the uplands) and 2:6 Mha offorest.

2.3.1. Theoretical potential capacityLikely range 2.5–3.5 Gt C; 30–70 Mt C=yr for 50

–100 yearsIf all the non-forest land in the UK (18 Mha) were

a�orested instantly and accumulated 150–200 tC=haover the next 50–100 years, it would store 2.7–3:6 Gt C, equivalent to 54–72 Mt C=yr over 50 yearsor 27–36 Mt C=yr over 100 years, equal to 18–49%of current emissions.

Alternatively, if current agricultural land weremanaged to maximize carbon sequestration (mostlyin soils) about 7 Mt C=yr could be sequestered for,perhaps, 50–100 years, storing an additional 0.35–0:70 Gt C ([37], revision of [38]). This estimateassumes that 20 t=ha=yr of manure is applied to 45%of arable land (1:75 Mt C=yr), 10 t=ha=yr of strawis incorporated into 40% more arable land than in1990 (0:93 Mt C=yr), no-till is applied to 37% ofarable land (1:32 Mt C=yr) extensiHcation is appliedto 29% of arable land (1:66 Mt C=yr), sewage sludgeis applied at 1 t=ha=yr to an extra 5% of arable land(0:14 Mt C=yr) and 10% of arable land is convertedto woodland (2:52 Mt C=yr) [37,39]. Clearly, this


potential storage in agricultural soils is less, the-oretically, than could be stored by converting allagricultural land to forest.

2.3.2. Realistic potential capacityLikely range 0.3–0.5 Gt C; 3–5 Mt C=yr over 50

–100 yearsThe maximum rate at which the forest area ex-

panded in UK during the 20th century was about40 kha=yr in the early 1970s, mainly in the uplands[40]. From 1950 to 1990 the average rate of forestexpansion was about 25 kha=yr. The eventual estab-lishment of about 1:3 Mha of mainly conifer forestsduring the 20th century met with considerable objec-tion, because of its impacts on the landscape, acidiH-cation, water resources, wildlife and recreation.

Even with an aggressive carbon sequestration pol-icy, it seems unlikely that the rate of forest expansioncould exceed 50 kha=yr. Furthermore, it would be un-realistic to imagine that this rate of expansion couldbe maintained for much more than 50 years, whichwould give an additional 2:5 Mha of forest, approx-imately doubling the existing forest area in the UK.If that aggressive planting programme were imple-mented, another 0.38–0:50 Gt C would eventually bestored in forests (assuming all felled forests were re-planted) with a sink averaging 3–5 Mt C=yr over 50–100 years [40] equivalent to 2–4% of current emis-sions.

The potential sequestration rate on agricultural landof 7 Mt C=yr given above [37] takes little account ofthe suitability of soils or climate for optimal practices,nor of the availability of manure, sewage sludge orstraw relative to other demands, much less of the prac-ticalities of transport, the fuel cost and adverse envi-ronmental e�ects (e.g. nitrate leaching, heavy metaland persistent organic pollutants). Thus, the realisticpotential carbon sink from changing agricultural prac-tices is probably no more than 1–2 Mt C=yr. The re-alistic potential is therefore taken as the aggressivea�orestation option.

2.3.3. Conservative, achievable capacityLikely range 0.05–0.10 Gt C; initially 1–2

Mt C=yr, then falling to zero over 50–100 yearsThe actual forest sink in the UK was estimated to be

2:9 Mt C=yr in the year 2000 [35]. If the area of new

forest continues to expand at the 1997 rate of 7 kha=yrof conifers and 10 kha=yr of broadleaved woodland(assuming, additionally, that all existing clearfelledforests are replanted) the UK forest sink will be about2:7 Mt C in 2020. This is the business-as-usual sce-nario.

Alternatively, if forest area expansion occurred ata higher rate of 10 kha=yr of conifers and 20 kha=yrof broadleaved woodland, the forest sink would be3:1 Mt C=yr in 2020, storing an extra 50 Mt C by 2020[35]. By 2020, approximately a further 0:2 Mha of up-land conifer forest would be created and 0:4 Mha ofwoodland on land mainly taken out of farming. This isrealistically achievable, but possibly not on a 20-yeartimescale. Also, the planting rate of 30 kha=yr is un-likely to be sustained much beyond 2020 as forestrywould then encroach on land that is in demand forother purposes. When the forest expansion rate de-clines, as it inevitably must, the forest sink will di-minish towards zero about 40 years later [40].

A conservative, achievable storage capacity, tak-ing into account the constraints on land use changein the UK, might be somewhere in the range 50–100 Mt C over the next 100 years, sustaining a sinkof 1–2 Mt C=yr for a few decades but then falling tozero. Sequestration of 1–2 Mt C=yr is equivalent to0.7–1.3% of current UK carbon emissions.

Meanwhile, estimates of emissions of carbon fromagricultural soils as a result of past land use change(excluding the forest sink) will change from 4.0 to4:7 Gt C=yr in 2000 to anywhere in the range 0.0–6:3 Gt C=yr in 2020 [35] but would be expected todiminish towards zero if some agricultural land wasa�orested and other land was managed less intensively(e.g. with minimal tillage).

3. Conclusions

Table 1 summarizes the ranges of carbon sequestra-tion capacities and rates concluded from this review.In all cases, a realistic assessment of the potential isabout an order of magnitude less than the theoreticalpotential, and a conservative estimate of what maybe achievable is about half of the realistic potential.Clearly, what is achievable is based largely on sub-jective judgement, but it nevertheless signals reasonsfor caution in the use of potential estimates.


Table 1Biological carbon sequestration estimates for the world, the EU15countries and the UK

Theoretical Realistic Conservativepotential potential achievable

Carbon storage capacity (Gt C)World 100–200 50–100 10–50Europe 20–30 5–10 2–5UK 2.5–3.5 0.3–0.5 0.05–0.10

Carbon sequestration rates for 50–100 yr (Mt C=yr)World 2000–4000 1000–2000 200–1000Europe 200–500 50–100 20–50UK 30–70 3–5 1–2

The estimates given of sequestration rates are ap-proximate time-averages. In reality, sequestrationrates following a�orestation and land use changechange over time, reaching a peak and eventuallyfalling to a time-average of zero as the vegetation–soilsystem reaches a new equilibrium.

Globally, land management to enhance carbon se-questration could, conservatively, add another 200–1000 Mt C=yr to the ‘natural’ terrestrial carbonsinks. Most of this could be achieved by enhancingcurrent good land management practices, such asa�orestation for erosion control and wood supply,agroforestry and minimal tillage. This is a substantialcontribution, representing 3–15% of current emissionsof 6400 Mt C=yr.

Within the EU15, readily achievable carbon se-questration may be only 20–50 Mt C=yr (additionalto the natural sinks), 2–5% of current EU emissions of940 Mt C=yr. In the UK, only 1–2% of current emis-sions of 147 Mt C=yr can readily be o�set by carbonsequestration activities (1–2 Mt C=yr). 1 More thanthis is clearly possible, but would require a major shiftin land use policies and public perception of land usepriorities.

1 The Kyoto target for the EU is a reduction of 132 Mt C=yr(8% below 1990 emissions of 1655 Mt C=yr), so 20–50 Mt C=yrrepresents 15–38% of that target. The Kyoto target for the UKis a reduction of 26 Mt C=yr (12.5% of 1990 emissions of209 Mt C=yr), so 1–2 Mt C=yr represents 4–8% of that target.

4. Biomass energy substitution

4.1. De8nitions

In any discussion of biomass energy potential, andthe extent to which it o�sets carbon emissions fromfossil fuels, it is Hrst necessary to deHne the terms andconversion factors. There are many options, which arenot always clear in the literature.

4.1.1. Energy unitsThe energy units used here are Joules, the quantity

of energy, rather than Watts, the rate of energy sup-ply or consumption. Conversion factors are given inAppendix.

4.1.2. Biomass energyThe energy considered here is that coming from

‘modern’ biomass, which is either produced from en-ergy crops and plantations or recovered from industrialforest residues. These may be used to produce elec-tricity, combined heat and power or liquid fuels. Weare not concerned with ‘traditional’ biomass energy,from dung, charcoal and fuelwood collected by house-holds. Furthermore, we should be clear that biomassenergy is only one component of all biofuels, whichincludes energy from waste and landHll gas.

4.1.3. Carbon content of fuelsVarious conversion factors have been used in the

literature for the amount of carbon emitted per unit ofenergy for di�erent fuels. The conversion factors usedhere are 27 kg C=GJ for biomass and coal, 21 kg C=GJfor oil and 15 kg C=GJ for natural gas, which equateto 37 GJ=tC for biomass and coal, 48 GJ=tC for oiland 66 GJ=tC for gas [41,42]. 2

4.2. Biomass fuel carbon displacement of coal, oiland gas

Calculation of the amount of carbon not emit-ted to the atmosphere as a result of using energyfrom biomass rather than fossil fuels is potentially

2 Values used by the Oak Ridge National Laboratory, USA are25.4, 19.9 and 14:4 kg C=GJ for coal, oil and gas, respectively.


complex, primarily because, (i) fossil fuel carbon isemitted during the cultivation and transport of biomass(and in alternative land management systems) and isalso emitted during the mining and transport of fossilfuels, and (ii) substitution (displacement) values canobtained based on the ‘primary energy’ content of fu-els as supplied to the primary user (power station orconversion plant) or the electrical or liquid fuel energysupplied to the end user. Complete life cycle analy-ses of biomass and fossil fuel emissions have led to arange of substitution values, depending, for instance,on what operations/processes are included, the choiceof fuel mix, the reference land use system, the energycosts and valuation of by-products, and the e�ects onnon-CO2 gases emitted. It is beyond the scope of thisstudy to cover all of these aspects.

In this review, the substitution values used are that

• one tonne of dry biomass used to generate electricityprevents 0:50 tC being emitted to the atmospherefrom coal, 0:44 tC from oil or 0:28 tC from naturalgas, using the primary energy contents of biomass,coal, oil and gas before electricity generation; and

• in the UK and the rest of Europe, biomass usedto produce liquid fuels prevents 0.2–2:0 tC beingemitted to the atmosphere for every hectare plantedwith energy crops, based on complete fuel cycleanalyses of Kaltschmitt et al. [43] and Bauen [44].

The justiHcation for using these conversion factors isgiven below.

4.2.1. Electricity generationConsider the simpliHed diagrams in Fig. 1 of carbon

and energy Mow from the production of biomass, coaland gas to electricity generation.

One tonne of dry biomass from energy crops con-tains about 500 kg of carbon, which is Hxed by photo-synthesis from the atmosphere. One tonne of biomass,delivered to produce electricity (by combustion, pyrol-ysis or gasiHcation), contains about 18:5 GJ of energy.The production of biomass involves the use of fossilenergy (for cultivation, fertilizers, transport, and soon), typically with an emission of 0.5–2:0 kg C=GJ ofprimary biomass energy delivered to a power plant—the ‘emission factor’ [45,46]. This means that the cul-tivation and production of 500 kg C of biomass emitsabout 10–40 kg C (approximately 18:5× 1:5–18:5×

2:0) and so consumes about 0.5–1:5 GJ, assuming thatthe fossil fuels used in cultivation and transport emitan average of about 25 kg C=GJ. This 10–40 kg C isthe net cost to the atmosphere of producing biomassfuel, because the 500 kg C Hxed by photosynthesis isassumed to be returned to the atmosphere during elec-tricity generation. 3 If we take the scheme to electric-ity generation, then only about 40% of the 18:5 GJ isrecovered as electrical energy. 4

Note that, in this simpliHed scheme, the ratio ofenergy input (0.5–1:5 GJ) to primary energy output(18:5 GJ) is in the range 12–37, in accord with pub-lished values for wood production systems of about 25[46] while the ratio of electrical energy output (7:4 GJ)to energy input is 5–15, or 9–26 if the biomass isused to produce combined heat and power (CHP). Inthis scheme (Fig. 1) the ‘emission factor’ for electric-ity generation (10–40 kg C divided by 7:4 GJ) is 1.4–5.4, at the lower end of the range in the literature(1.6–14:7 kg C=GJ, equivalent to 22–193 gCO2=kWhof electricity [46]).

If we now compare the biomass fuel chain with thatfor 500 kg of coal, we see a similar picture (Fig. 1).Coal is carbonised biomass, so it is not surprisingthat 500 kg coal also contains about 18:5 GJ. Further-more, the energy required to mine coal and deliver itto a power station is similar to that required to pro-duce biomass, per unit of carbon or energy content(in Fig. 1, given as 0.5–2:0 GJ, emitting 10–50 kg C,to produce 500 kg coal). Moreover, the conversioneNciency of coal to electricity is also around 40%. 5

3 A small additional amount of the carbon in biomass may bereturned to the atmosphere by respiration and decay before it isused for electricity generation [46].

4 The eNciency of converting energy in biomass to electricalenergy by pyrolysis and gasiHcation (integrated gasiHcation com-bined cycle) is about 44% for energy crops and 31% for agri-cultural and forestry waste (or municipal solid waste) [41]. TheARBRE biomass plant in the UK has achieved eNciencies of 43–53%, depending on the gasiHer operating pressure and plant ca-pacity. By contrast, the energy conversion eNciency of combinedheat and power plants is commonly 70% (heat eNciency 52%,electrical eNciency 19% with a heat:power ratio of 2.6) and canbe up to 80%.

5 Conventional coal Hred power stations have a conversion ef-Hciency of about 38%, whereas coal ‘integrated gasiHcation com-bined cycle’ plants and those with pulverized Muid bed combustionhave eNciencies of about 46%.


500 kgC from atmosphere 10 - 40 kgC 500 kgC

7.4 GJ electricity

ENERGY CROPS Harvestable biomass Production and 18.5 GJ Combustion 40%1 dry tonne transport primary Pyrolysis

energy Gasification 70% 13.0 GJ CHP

0.5 - 1.0 GJ from fossil fuels

- 50 kgC 500 kgC

COAL 500 kgC Mining and 18.5 GJ Combustion 40% 7.4 GJ electricity in coal transport primary

energy


10 – 50 kgC 500 kgC

NATURAL GAS 500 kgC Extraction 33.0 GJ Combustion 50% 16.5 GJ electricityin gas and transport primary energy


10

Fig. 1. SimpliHed schemes of the Mows of energy and carbon when generating electricity from biomass, coal or natural gas. CHP is‘combined heat and power’.

Consequently, for electricity generation, biomass andcoal are approximate energy equivalents. The onlysubstantial di�erence in Fig. 1 is that coal emits500 kg C to the atmosphere whereas energy cropsrecycle it. Consequently, to a reasonable approxima-tion, 1.0 dry tonne of biomass displaces 0:5 tC fromcoal both as a primary fuel and when used to generateelectricity. This analysis is consistent with Bauen [44]who estimated that short rotation coppice yieldingabout 10 t=ha=yr in the UK displaces 5:4 tC=ha fromcoal used to produce electricity. In the terms used bySchlamadinger and Marland [47], it is assumed thatthe ‘displacement factor’ of biomass relative to coalused to generate electricity is one (i.e. the eNciencyof the energy systems and C emissions per Joule arethe same).

For natural gas, the situation is di�erent, becausegas contains more energy per unit of carbon emitted

(15 kg C=GJ) and is converted to electricity more ef-Hciently (Fig. 1). 6 In terms of primary energy, 1 t ofbiomass (500 kg C) displaces 280 kg C of gas (500×18:5=33). In terms of electrical energy, assuming 50%conversion eNciency for gas, 500 kg biomass dis-places 224 kg C from gas (500× 7:4=16:5). 7 It maybe noted that the ‘emissions factors’ for coal and gasin Fig. 1 for electricity generation are 69–74 and31–33 kg C=GJ, respectively, at the lower end of therange in the literature (73–98 kg C=GJ for coal and31–39 kg C=GJ for gas [46]).

6 Combined cycle gas turbines have a conversion eNciency of53%, with new designs reaching 60% [54].

7 Stated di�erently, 18:5 GJ (500 kg C) of biomass displaces500 kg C of coal or 280 kg C gas, so that 1 GJ of biomass dis-places 27 kg C coal or 15 kg C gas, or 1 EJ displaces 0:027 Gt Ccoal or 0:015 Gt C gas.


Oil contains about 21 kg C=GJ, so the equivalentcalculation for oil is that 1 t of biomass (500 kg C)displaces 440 kg C of oil (500× 18:5=21).

4.2.2. Equivalence between sequestration andelectrical energy substitution

Assuming an average yield of oven dry biomassfrom temperate short rotation woody plantations of10 t=ha=yr [48], every 100 Mha of land yields ap-proximately 18:5 EJ of primary energy and o�sets0:50 Gt C from coal or 0:28 Gt C from gas. Bycomparison, 100 Mha might have the capacity tosequester 100–200 tC=ha (200–400 t dry matter=ha)totalling 10–20 Gt C, o�setting 0.2–0:4 Gt C=yr over50 years or 0.1–0:2 Gt C=yr over 100 years. Thus,over a 50–100 year timescale, the carbon o�set bene-Ht derived from biomass grown for energy is roughlycomparable to that derived from growing it to se-quester carbon. This equivalence is the direct result ofthe carbon-energy equivalence between biomass andcoal. Within the growing lifetime of forests (whenthey are sequestering carbon) it makes little di�er-ence whether their biomass is left as a carbon storeor used as an energy substitute for coal [29].

4.2.3. Liquid fuel productionLiquid biofuels derived from biomass (ethanol,

methanol, biodeisel, etc.) have approximately thesame energy content as diesel and petroleum (19–22 kg C=GJ, 45–53 GJ=tC). However, the energycosts of production are high compared with the en-ergy costs of producing bio-electricity and a limitednumber of studies have been made of the completeenergy or fuel chain [49].

Brazilian ethanol from sugar cane (with bagasse)probably has the highest eNciency, with only 30–40%of the fuel energy being used in production—i.e. withenergy output/input ratios of 2.5–3.5 [46]. Estimatesof this ratio for ethanol from maize, taking accountof total energy costs of production are 1.01–1.13 inthe USA [50,51], 1.02 in France [52] and only 0.6 inItaly, or 1.36 when residues are used to provide en-ergy for the process and some energy credit is givenfor byproducts [53]. Matthews and Robertson [46] re-viewed the literature and concluded that the likelyrange was 2–15, but noted large variation in reportedenergy ratios due in part to variation in accountingprocedures.

Overall, it is clear that, in non-tropical countries,biomass used to produce liquid fuel o�sets much lessfossil carbon than biomass used to generate electricity,and in the worst case it may be carbon neutral [53].Only in some tropical situations will the carbon o�setbe appreciable. Ethanol production from sugar cane inBrazil has been estimated to replace about 2 tC=ha=yrrelative to petrol, but excluding the emissions involvedin the production of petrol [55].

Given the uncertainty, this review comments onlyon the potential for carbon o�sets using liquid biofuelsin the EU15 andUK, based on themost comprehensivefuel chain analyses of Kaltschmitt et al. [43], Ulgiati[53] and Bauen [44]. Their analyses of a range ofcrops growing on agricultural land (cereals, oilseedrape, short rotation coppice and Miscanthus) and arange of liquid fuels and processes, suggest that thepotential for fossil diesel substitution lies in the range0.2–2:0 tC=ha of land.

4.3. World

Current global emissions of 6:4 Gt C=yr are associ-ated with the generation of about 400 EJ of primaryenergy per year. By 2100, global primary energy usemay be 1400 EJ per year in a business-as-usual sce-nario, or 700 EJ in a low demand scenario [56].

Whereas forests planted for carbon storage can beplanted on poor land, biofuel plantations of trees orannual crops require productive land to sustain highyields [57]. The current global cropland area is about1450 Mha.

4.3.1. Theoretical potential capacityLikely range of fossil fuel o9set: 2–5 Gt C=yr by

2050–2100The physical upper boundary for biomass energy

production is set by global annual net primary pro-duction of dry biomass. Global NPP is about 100, or50 Gt C=yr [58], which contains about 1850 EJ, equalto the energy in 50 Gt C=yr of coal or 28 Gt C=yr ofgas. But, clearly, this is not a helpful potential Hgure,as it assumes no other uses for land. It is better to con-sider the theoretical potential in terms of maximumpotential land use for energy crops.

The most extreme estimates of the area of landthat could be used worldwide for biomass energyare 800 Mha by 2100 [56] and 600 Mha by 2050


[59–62]. The former is equivalent to 55% of the cur-rent cropland area (although made up in large partfrom degraded lands) while the latter assumes that40% of the current cropland area is used, plus 15%of the current forest area [60].

Assuming an average yield of 10 dry t=ha, 800 Mhawould give 148 EJ=yr of biomass energy and dis-place 4:0 Gt C=yr coal or 2:2 Gt C=yr gas, while600 Mha would give 111 EJ=yr of energy and dis-place 3:0 Gt C=yr coal or 1:7 Gt C=yr gas. A further0:9 Gt C=yr coal o�set might potentially be obtainedby 2050 by using biomass residues (wood and straw)and 0:9 Gt C=yr by generating electricity from sugarcane and kraft pulp [59].

With assumptions of high yields (¿ 10 t=ha=yr ofbiomass) and high conversion eNciencies, estimateshave been made of over 300 EJ=yr being derivedfrom biomass by 2100, equivalent to a coal o�setof 8:1 Gt C=yr or gas o�set of 4:5 Gt C=yr, but notall of this is ‘modern’ biomass [63,64]. In the IPCCSpecial Report on Land Use, Land Use Change andForestry ([17], Fact Sheet 4.21) an extreme scenariois given of 250 Mha devoted to energy crops by 2020and 500 Mha in 2100, which, with yields rising from10 to 25 t=ha=yr, could produce about 300 EJ=yr by2050.

Bearing in mind that biomass primary energy mayo�set a mix of fossil fuels, most of these optimisticestimates suggest that modern biomass could poten-tially produce anything in the range 150–300 EJ by2050–2100, displacing perhaps 2–5 Gt C=yr, similarto the theoretical global potential for carbon seques-tration. However, these estimates ignore and violatethe constraints of sustainable development outside theenergy sector.

4.3.2. Realistic potential capacityLikely range of fossil fuel o9set: 1–2 Gt C=yr by

2050–2100The area of high-quality land available for energy

crops will be constrained by the need for increasedfood production as world population increases fromabout 6 to 10 billion. The relationships betweenland availability, land quality and development needsshould not be oversimpliHed. Locally, the tradeo�sbetween energy, food and water security may be re-solved in di�erent ways. The notion that half of the

current cropland could be devoted to energy crops ishighly questionable.

Even in the USA, there is a two-fold di�erence inthe more optimistic estimates of biomass energy po-tential. Thus, optimistic projections by Wright andHughes [65] suggest that 20% of current US emissionscould be met by 2030, requiring 28 Mha of land, tobe planted at a rate of 1 Mha=yr, with yield increasesof 1.5%=yr (to over 22 t=ha=yr) and the installationof 500 MW new capacity each year. A more realistic,yet still optimistic, assumption is that 14 Mha couldbe used for energy crops, displacing 3–6% of currentUS emissions [66,67].

In developing countries, the di�erence between op-timistic and realistic potential land availability is likelyto be greater than in the USA, because of faster pop-ulation growth and demand for food and water. Also,degraded lands, which are included in many theo-retical calculations, may require too large an invest-ment of energy and capital to be viable for energycropping.

Rather than 500–800 Mha being devoted to en-ergy crops worldwide by 2050–2100, a more reason-able potential land area, suggested by some authors,is 200–400 Mha (14–28% of current cropland area)[67]. Assuming, again, an average biomass yield of10 dry t=ha=yr, this area could produce 37–74 EJ=yr,o�setting 1–2 Gt C=yr from coal or 0.6–1:1 Gt C=yrfrom gas.

4.3.3. Conservative, achievable capacityLikely range of fossil fuel o9set: 0.2–1.0 Gt C=yrThe main reasons for caution in projecting a large

role for biomass energy crops worldwide are as fol-lows:

(i) Most developing countries are facing continu-ous or periodic food and water shortages, as well asshortages of traditional biofuels. Increasing popula-tions, combined with shifts in the patterns or reliabil-ity of rainfall as a result of climate change, will makematters worse. Global food production kept pace withglobal population doubling during the 20th centurylargely by increasing yields per hectare. This increasehas been possible by increased use of fertilizers andbreeding to increase the ‘harvest index’. In future, alarger fraction of increased food demand may have tobe met by increasing the cropland area. Competition


for land is bound to intensify in order to simultane-ously achieve food, energy and water security. It isworth recalling the concern that water use by eucalyp-tus plantations has caused in India and SE Asia andthat food shortages in Brazil have been linked to theProAlcohol Program.

(ii) Substantial economic and policy obstacles mustbe overcome to allow biomass energy to penetratethe energy market. Fossil fuel energy costs must ap-proximately double, or biomass energy costs halve.Changes would be required not only in energy policies,but also in policies on agriculture, forestry, conserva-tion and transport. Biomass is used most eNciently insmall local generator plants, with combined heat andpower, requiring a restructuring of energy consump-tion patterns. Such radical change is not likely to occurquickly.

(iii) The environmental costs of woody biomassenergy crops may be greater than those of annualcrops [68]. Water use will be greater (owing largely togreater rainfall interception), landscapes will change,the harvesting and transport of bulky material will cre-ate nuisance, and there may be adverse impacts onbiodiversity and freshwater quality.

(iv) As mentioned, total life cycle energy balancesof crops grown for liquid fuel sometimes reveal onlya modest energy/carbon beneHt in non-tropical coun-tries, unless residues are used as a fuel in the processand co-products are credited as outputs. Ulgiati [53]calculated that, in Italy, it would take 37% of all cropand pasture land, and 22% of available water, to pro-duce 5% of the current total energy used in transport.The Brazilian ProAlcohol Program succeeds because4 Mha are available in an area with a small popu-lation (20 persons=km2) where sugar cane yield 50–60 t=ha=yr. However, it supplies only about 4% ofBrazil’s total energy in the transport sector [53]. Thefossil energy used in production means that, evenhere, only 29% of the energy in the cane is captured—likely to be the maximum for any crop in theworld.

(v) The total greenhouse gas beneHt of biomass en-ergy plantations way be less if N fertilizers lead to in-creased N2O emissions and if plantations are plantedon organically rich soils (e.g. former forest or natu-ral land) which may then lose carbon to the atmo-sphere. Also, non-ideal combustion produces methaneand N2O.

In a recent study, Yamamoto et al. [69] tookinto account the carbon emitted when convertingforests to short rotation plantations, and derived anachievable global fuelwood programme that wouldo�set only 2–7% of the accumulated global fossilemission of 1260 Gt C from 1990 to 2100 (slightlybelow IS92a). Bauen and Kaltschmitt [70] gave aconservative estimate of 150 Mha of land potentiallyavailable for energy crops—based on current FAOagricultural and forestry statistics and regional assess-ments of land suitability and future cropland demand.Finally, the World Energy Council’s estimate ofthe carbon o�set that could realistically be achievedin 2020 by global deployment of modern biomass isin the range 0.12–0:28 Gt C=year with current policiesand 0.30–0:65 Gt C=year with ‘ecologically driven’policies [71].

Overall, a conservative global estimate of energycropland by 2050–2100 might lie anywhere in therange 50–200 Mha, producing 9–37 EJ=yr, displacing0.24–1:00 Gt C=yr from coal or 0.14–0:56 Gt C=yrfrom gas.

4.4. Europe

In 2000, the EU15 had a primary energy useof 59 EJ (1442 Mt oe; 41% from oil, 23% gas,15% nuclear, 15% coal, 6% renewables) of which3.7% was from biomass (mostly in France andScandinavia) (Eurostat website). This energy useequates to the emission of about 0:51 Gt C from oil,0:20 Gt C from gas and 0:23 Gt C from coal, totalling0:94 Gt C=yr.

As mentioned above, there are 142 Mha of agricul-tural and 72 Mha of arable land in the EU. In 2000,about 3:9 Mha of agricultural land was set aside.

4.4.1. Theoretical potential capacityLikely range of fossil fuel o9set: 0.6–0.9 Gt C=yrIf all of the 142 Mha of agricultural land in

the EU15 were devoted to energy crops yielding10 t=ha=yr dry biomass with 18:5 GJ=t, this wouldcapture about 26:3 EJ=yr, 44% of the total primaryenergy consumption in 2000 (or 52% excluding nu-clear).

If this 26:3 EJ=yr of biomass energy were usedto o�set all of the coal energy (8:6 EJ) and the re-maining 17:7 EJ=yr (26.3–8.6) were used to o�set oil


consumption (it could o�set 73% of the 24:4 EJ=yr ofoil consumption in the EU15), it would displace about0:65 Gt C=yr (0:23 Gt C from coal and 0:42 Gt Cfrom oil), 69% of the EU15 total emission in 2000.A further 0.2–0:3 Gt C=yr may be displaced by mak-ing full use of forest residues. There would be noagricultural residues in this scenario.

Thus, if all agricultural land were converted to en-ergy crops and forest residues were fully exploited—andwith the most carbon-saving fuel substitution—thetotal carbon displacement would be 0.8–0:9 Gt C=yr,similar to the EU15 emission of 0:94 Gt C=yr in 2000.With less optimum energy substitution (substitutingsome natural gas use) the theoretical potential is likelyto lie in the range 0.6–0:8 Gt C=yr.

This estimate of theoretical carbon displacementby energy substitution is greater than the theoret-ical calculation made above of 0.19–0:52 Gt C=yrsequestered in biomass and soils for 50–100years if all 142 Mha of agricultural land werea�orested.

4.4.2. Realistic potential capacityLikely range of fossil fuel o9set: 200–300Mt C=yrA more realistic potential substitution scenario

might involve the use of, say, 30–40 Mha for energycrops, substituting 150–200 Mt C from coal plus thefuller use of forest and agricultural residues, to makea total o�set in the range 200–300 Mt C=yr, 21–32%of EU15 carbon emissions in 2000.

In a realistic analysis of the potential use ofshort-rotation woody crops and forest residues forfuel, Schwaiger and Schlamadinger [72] concludedthat up to 30% of the current EU15 carbon emissions(about 300 Mt C=yr) could be displaced—mainlyby the proliferation of small generating plants, com-bined heat and power, and individual or district househeating. Small-scale cogeneration for district heat-ing is currently an attractive alternative to gas-basedsystems, based on costs and eNciency as well asemissions [73].

4.4.3. Conservative, achievable capacityLikely range of fossil fuel o9set: 100–200Mt C=yr

by 2050The reality is that, in the EU15, biomass energy

crops are being promoted as part of the policies con-tained in the Renewables Directive (12% of primary

energy consumption by 2010), Biofuels Directive (5%by 2010) and Combined Heat and Power Directive(18% by 2010) as well as the Kyoto agreement to re-duce emissions by 8% below 1990 levels by 2008–2012. These pressures accompany changes to improveair quality (Large Combustion Plant Directive) and re-form the CommonAgricultural Policy (Agenda 2000).

Sweden is leading the way in the use of biofu-els in Europe. Currently, 15% of Swedish energy issupplied from biomass and the potential is over 20%[74,75]. However, Sweden has a low population den-sity, a large forest area, consumes only 3.5% of theEU15 total energy demand and, in fact, produces only14% of the EU15 biomass fuel.

The European Commission’s White Paper on Re-newables proposes a target of doubling the contri-bution of renewables from 6% to 12% of the EU’stotal primary energy needs by 2010 involving acapital investment of 165 billion Euros. The sub-sidiary target for all biofuels by 2010 is 5:6 EJ=yr(135 Mt oe) [76]. It is assumed that energy cropswill contribute 1:85 EJ=yr, requiring 10 Mha, andthat agricultural and forest residues will contribute1:23 EJ=yr. The 1:85 EJ=yr of energy crops will o�-set about 50 Mt C=yr of coal (28 Mt C=yr of gas)and the residues will o�set about 33 Mt C=yr of coal(or 18 Mt C=yr of gas), making a total target o�setof 83 Mt C=yr of coal (or 46 Mt C=yr of gas) by2010.

In addition, liquid biofuels will make a small con-tribution. In 1998, about 0:4 Mha of set-aside landin the EU15 were dedicated to liquid fuels, supply-ing about 0.3% of liquid fuel demand. This produc-tion gives an o�set of fossil liquid fuels in the range0.09–0:88 Mt C=yr (0.2–2:0 tC=ha, see above). If itwere assumed that this area increased by a factor of2 to 5 by 2010, the o�set would be in the range 0.2–4:4 Mt C=yr—still a small contribution.

Thus, the EU15 target carbon o�set in 2010 isaround 83–87 Mt C=yr of coal or 46–50 Mt C=yrof gas, representing about 9% of current EU15 car-bon emissions for coal substitution and 5% for gassubstitution.

Looking ahead to 2050, a reasonable guess might bethat an o�set in the range 100–200 Mt C=yr is achiev-able with continued expansion in land areas, improve-ments in yields, use of heat and power and improvedconversion technologies.


4.5. United Kingdom

In 2000, UK primary energy consumption was9:5 EJ=yr (232 Mt oe; 36% oil, 35% gas, 16% coal,11% nuclear). All renewables contributed only0:12 EJ=yr, 1.3% of primary energy consumption,which can be divided into hydro (14%), all formsof biofuels (82%) and ‘other’ (4%, including windand geothermal). The biggest contributors to biofuelswere landHll gas (0:030 EJ=yr), refuse combustion(0:026 EJ=yr) and wood for industrial and domes-tic use (0:021 EJ=yr). Straw combustion for heatcontributed 0:003 EJ=yr and similarly a small contri-bution was made from a total of about 2000 ha ofenergy crops (mostly for the ARBRE power plant).

The consumption of 9:5 EJ=yr was accompanied bythe emission of 147 Gt C=yr, mostly from liquid fuelsand gas.

There are 11:4 Mha of farmland in the UK. Thecurrent set-aside area is about 0:5 Mha. The UK hasabout 2:7 Mha of forest, with a harvest of about 7 Mgreen tonnes/yr.

4.5.1. Theoretical potential capacityLikely range of fossil fuel o9set: 40–60 Mt C=yrIf all of the 11:4 Mha of farmland in the UK were

devoted to energy crops yielding 10 t=ha=yr, thiswould capture 2:1 EJ=yr, 22% of the total primaryenergy consumption of the UK in 2000 (or 25% ex-cluding nuclear). If this biomass energy were usedto o�set all of the coal energy (1:5 EJ=yr) and theremainder used to o�set oil energy it would displace57 Mt C=yr (41 Mt C=yr from coal and 16 Mt C=yrfrom oil), 39% of current emissions.

The forest residues available for energy productionare about 15% of the timber removals, approximately0:5 M dry tonnes of wood. Not all of this could be re-moved without depleting soil organic carbon. If 70%were removed (0:35 Mt dry wood) this would o�-set only 0:18 Mt C=yr from coal. In this scenario, noresidues would be available from farmland.

Thus the theoretical o�set in the UK is about 50–60 Mt C=yr, but involving the disappearance of vir-tually all farming activities. This estimate may becompared with the calculation made above of 30–70 Mt C=yr sequestered in biomass and soils for 50–100 years if all 11:4 Mha of farmland were a�orested.

4.5.2. Realistic potential capacityLikely range of fossil fuel o9set: 5–20 Mt C=yrClearly, a more realistic potential o�set capacity

would involve the transfer of large areas of farmlandto energy cropping, but not all. There will be the fol-lowing constraints:

• Clearly, it is unlikely that the UK will abandonhome production of agricultural produce. Even ifagricultural subsidies were reduced, the economicbeneHts of non-agricultural alternative land uses, in-cluding wind farms, may be greater than those ofenergy crops in many areas.

• Given the strength of public resistance to a�oresta-tion of about 1:3 Mha of UK uplands from 1920to 2000, it is unlikely that millions of hectares ofagricultural land, closer to human habitation, willbe acceptable without large beneHts or incentives.Large-scale energy cropping could be resisted be-cause of its impacts on water quality, wildlife, recre-ation, landscape, transport use and noise. 8

• Short rotation coppice is likely to be economiconly in areas where yields of about 10 t=ha yr (drywood) can be sustained. Poplar and willow cop-pice transpire about one kg water for every 3:5 gof stemwood produced, so 10 t=ha=yr is equivalentto 286 mm of rainfall [77]. Transpiration consti-tutes only about 65% of total evaporation, evenassuming no groundwater recharge. Thus, wateruse may be a constraint on extensive cropping withshort rotation coppice in much of southern andeastern England, especially in areas dependent ongroundwater resources [78].

• The greatest energy and economic beneHts are ob-tained from small-scale combined heat and powerunits, located close to energy users. Only certainlocalities may be geographically suitable.

There is no objective basis upon which to set a re-alistic ‘potential’ land area for energy crops, exceptthat it will less than 11:4 Mha and probably greaterthan the existing set-aside area of 0:5 Mha. Smith etal. [37] chose an arbitrary Hgure of 10% of arable

8 Note that 1 Mha yielding 10 Mt dry wood per year with a5 yr rotation means that 200; 000 ha are clearfelled each yearin di�erent regions and that about 20 Mt of green material istransported each year.


land, equal to 0:7 Mha in their estimation of arable ar-eas, assuming that a further 0:7 Mha would be plantedwith woodland for carbon sequestration. A reason-able guess for a realistic potential energy crop areamight be in the range 1–4 Mha, giving an o�set of 5–20 Mt C=yr from coal.

4.5.3. Conservative, achievable capacityLikely range of fossil fuel o9set: 1–6 Mt C=yr by

2050The current policy drivers for biomass energy in the

UK are (i) the Renewables Obligation—to generate10% of electricity from renewable sources by 2010,(ii) the post-Kyoto target to obtain a 12.5% reduc-tion in greenhouse gas emissions compared to 1990by 2010, and (iii) the Green Fuels Challenge, whichprovides a 20% tax rebate for biodiesel. Achieving the10% Renewables Obligation is thought to be feasibleprovided Government subsidies enable the electricityto be sold at 3:5 p=kWh (cf. 2:6 p=kWh in 1998). How-ever 3:5 p=kWh is currently too low to promote sig-niHcant investment in biomass energy schemes, otherthan those using low cost wastes as fuels—but energyanalysts agree that, to meet the Renewables Obliga-tion, energy crops will be required as well as greateruse of waste and wind.

Bauen [44] suggested that up to 125; 000 ha ofenergy crops will be required by 2010 to meet the10% Renewables Obligation. At 10 t=ha=yr, this areawould produce 0:23 EJ=yr and o�set 0:625 Mt C=yrfrom coal. If we project forward to 2050, this areacould conceivably be multiplied by 2–10, with ano�set of 1–6 Mt C=yr from coal.

The current reality is that energy crops are onlyjust beginning to be introduced. The UKs Hrst sig-niHcant power plant to be fuelled by energy crops isthe ARBRE project at Eggborough in North York-shire. It will generate 8 MW of electricity from anannual input of 43,500 dry tonnes of wood, initiallyfrom forest residues and in time from willow cop-pice. It is supported by a UK Renewables Obligationgrant, Forestry planting grants and the EU-THERMIEprograme. Meanwhile, in Northern Ireland, B9 En-ergy Biomass Limited has set up a combined heat andpower plant (200 kW) at Blackwater Valley Museumin County Armargh.

These pioneering projects suggest that, in the longerterm, the costs could become competitive. However,

Table 2Estimates of carbon substitution using energy crops for the world,the EU15 and the UK. (Mt C=yr)

Theoretical Realistic Conservativepotential potential achievable

World 2000–5000 1000–2000 200–1000Europe 600–900 200–300 100–200UK 40–60 5–20 1–6

the UK Department of Trade and Industry [76] states:‘at present, competing agricultural activities receiveHnancial support and it is unrealistic to expect thefarming industry to switch to an untested crop forwhich there is no equivalent Hnancial support. It willtherefore be necessary to extend equivalent Hnancialsupport at least temporarily to energy crops to pumpprime the industry if it is to progress’. To this end,in 2001 the UK Government allocated $29m to en-courage planting of up to 6000 ha of energy crops and$33m for power generation technologies.

The Green Fuels Challenge tax rebate seems to beinsuNcient to make biodeisel competitive with diesel,except when produced from recycled vegetable oil[44]. There is, nevertheless, a vigorous lobby forbiodiesel, although the carbon o�set is likely to besmall (0.2–2:0 t C=ha).

5. Conclusions

Table 2 summarizes the carbon substitution valuesthat are possible using modern biomass and industrialresidues, derived in this review. As for carbon se-questration, achievable levels of substitution are wellbelow potential levels.

6. Discussion

The ranges of sequestration and substitution valuesderived here are shown in relation to each other andto current fossil carbon emissions in Figs. 2a (world),b (EU15) and c (UK).

Although the theoretical potential o�sets are high,when critical consideration is given to the constraints,especially land use, the realistic and likely achievableo�sets are more modest. Expressed as a percentage of


0

1

3

5

6

Global fossil fuel emission

SEQUESTRATION

31-62%

16-31%

3-15%

EENERGYSSUBSITITUTION

31-78%

16-31%

3-15%

Theoretical potential

Realistic potential

Conservative achievable

GtC

/yr

2

4

7

0

200

400

600

800

1000

EU15 fossil fuel emission

SSEQUESTRATION

21-53%

5-11% 2-5%

EENERGYSSUBSITITUTION

64-96%

21-32%

11-21%


Realistic potential


MtC

/yr

0

50

100

150

UK fossil fuel emission

SSEQUESTRATION

20-48%

2.0-3.4%

0.7-1.3%

27-41%

3.4-13.6%


Realistic potential


MtC

/yr

200

0.7-4.1%

(a) (b)

(c)

ENERGYSUBSITITUTION

Fig. 2. (a) Global carbon sequestration and energy substitution capacity in the next 50–100 yr shown in relation to fossil fuel emissionsin the 1990s. (b) Carbon sequestration and energy substitution capacity in the next 50–100 yr in the EU15, shown in relation to fossilfuel emissions in 2000. (c) Carbon sequestration and energy substitution capacity in the next 50–100 yr in the UK, shown in relation tofossil fuel emissions in 2000.

current fossil carbon emissions, the ‘realistic poten-tial’ o�sets were estimated to be in the range 16–31%globally, 5–32% in the EU15 and 2–14% in the UK,while the ‘conservative achievable’ estimates were 3–15% globally, 2–21% in the EU15 and 1–4% in theUK. Thus, biological carbon sequestration and energysubstitution can play a signiHcant, but not a dominant,role in mitigating carbon emissions. Clearly, the per-centage contribution is smallest in countries and re-gions with large populations and high fossil carbonemissions per unit area of land available for carbonmanagement.

In the EU15 and the UK, the estimates of carbono�set by energy substitution are greater than those by

sequestration, when considering realistic and achiev-able o�sets (Fig. 2b and c). Thus, in the EU15, the‘realistic potential’ and ‘conservative achievable’ es-timates for energy crop substitution were 21–32% and11–21% of current emissions, respectively, comparedwith 5–11% and 2–5% for carbon sequestration. Thisdi�erence is due, in part, to the fact that sequestrationwas averaged over 100 years—in reality, sequestrationrates rise to a peak following a land use change andthen fall towards zero. A more rigorous comparisonbetween sequestration and energy substitution can bemade using an equivalence factor, based on radiativeforcing reduction over time ([79]; 1 tCO2 sequesteredfor one year is equivalent to about 0:018 tCO2


emitted). Inevitably, energy substitution becomesmore advantageous over long time periods, becauseit can be sustained indeHnitely [80].

Using land to sequester carbon in trees and soils,rather than to produce bioenergy, is the best optiononly when forest productivity is low (on poor qualityland) and/or the costs of harvesting and utilization forenergy are high. However, carbon storage in forestshas the problem of ‘permanence’, i.e. sequestrationis revered if the forest is destroyed. Energy cropsavoid this problem, while providing some carbon se-questration during the Hrst rotation (20–40 tC=ha inshort rotation coppice) and possibly in soils. How-ever, energy crops require good quality land, can haveenvironmental disbeneHts, involve the transport of abulky product, involve multiple stakeholders (com-pared with other renewables), and may be economi-cally advantageous only in certain localities.

The optimum o�set policies will be those which(i) provide win–win solutions, meeting other needson the same land area, such as soil conservation, bio-diversity enhancement and water conservation, and(ii) link short-term sequestration with long term sub-stitution. Read [81] showed that high, sustained car-bon substitution can be obtained by planting forestsfor sequestration which are subsequently harvested forfuel. As mentioned, the longer the time horizon, thegreater the likelihood that harvesting forests for en-ergy will give the greatest carbon o�set.

Acknowledgements

This review was funded by the Tyndall Centre forClimate Change Research, University of East Anglia,Norwich, UK.

Appendix.

Biofuels=landHll gas, energy from waste, biomassenergy.Biomass energy=energy crops, forest and agricul-

tural residues.Primary energy production = the energy con-

tained in fuels at the point at which they are pro-duced (coal, oil, gas, biomass, etc.). Primary energyconsumption = the energy contained in fuels at the

point at which they are consumed, in power stationsetc. Final energy consumption = the energy actuallyconsumed, taking into account the losses on conver-sion to electricity etc and distribution.

Mega (M) = 106, giga (G) = 109, tera (T) = 1012,peta (P) = 1015, exa (E) = 1018.

1 Tg = 1 Mt; 1 Pg = 1 Gt; 1 Gt = 1000 Mt.To convert TJ to Mt oe or GWhmultiply by 2:388×

10−5 or 0.2778, respectively.To convert Mt oe to TJ or GWh multiply by

4:1868× 104 or 11630, respectively.To convert GWh to TJ or Mt oe, multiply by 3.6 or

8:6× 10−5, respectively.

References

[1] Prentice IC and 9 others plus 50 contributors. The carboncycle and atmospheric carbon dioxide. In: Climate change2001. The scientiHc basis. Contribution of working group Ito the third assessment report of the Intergovernmental Panelon Climate Change. Cambridge, UK: Cambridge UniversityPress, 2001. p. 183–237 [chapter 3].

[2] Royal Society of London. The role of land carbon sinks inmitigating global climate change. The Royal Society, London,2000. 27p. ISBN 0 85403 561 3.

[3] Houghton RA. Counting terrestrial sources and sinks ofcarbon. Climatic Change 2001;48:525–34.

[4] Metz B, Davidson O, Swart R, Pan J. Climate change 2001:mitigation. Contribution of Working Group III to the ThirdAssessment Report of the Intergovernmental Panel on ClimateChange. Cambridge, UK: Cambridge University Press, 2001.

[5] Pacala SW, Hurtt GC, Baker D, Peylin P, Houghton RA,Birdsey RA, Heath L, Sunquist ET, Stallard RF, CiaisP, Moorcroft P, Caspersen JP, Shevliakova E, Moore B,Kohlmaier G, Holland E, Gloor M, Harmon ME, Fan SM,Sarmiento JL, Goodale CL, Shimel D, Field CB. Consistentland and atmosphere based US carbon sink estimates. Science2001;292:2316–20.

[6] Lenton T, Cannell MGR. Mitigating the rate and extentof global warming—a comment. Climatic Change 2002;52:255–62.

[7] Houghton RA, Skole DL. ‘Carbon’. In: Turner BL, ClarkWC, Kates RW, Richards JF, Matthews JT, Meyer WB,editors. The earth as transformed by human action. NewYork: Cambridge University Press, 1990. p. 393–408.

[8] Houghton RA. The annual net Mux of carbon to theatmosphere from changes in land use 1850–1990. Tellus1999;51B:298–313.

[9] Winjum JK, Dixon RK, Schroeder PE. Estimating the globalpotential of forest and agroforest management practicesto sequester carbon. Water, Air and Soil Pollution 1992;64:213–27.


[10] Paustain K, Cole CV, Sauerbeck D, Sampson N. CO2mitigation by agriculture: an overview. Climatic Change1998;40:135–62.

[11] Nilsson S, Schopfhauser W. The carbon-sequestrationpotential of a global a�orestation program. Climatic Change1995;30:267–93.

[12] Trexler MC, Haugen C. Keeping it green: evaluatingtropical forestry strategies to mitigate global warming. WorldResources Institute, Washington DC, USA, 1994.

[13] Brown S, Sathaye J, Cannell MGR, Kauppi P. Managementof forests for mitigation of greenhouse gas emissions. In:Watson RT, Zinyowera MC, Moss RH, Dokken DJ, editors.Climate change 1995. Impacts, adaptations and mitigationof climate change, scientiHc-technical analyses. Contributionof working group II to the second assessment report ofthe IPCC. Cambridge: Cambridge University Press, 1996.p. 773–97.

[14] Sampson RN and 2 others. Additional human inducedactivities—Article 3.4. In: Watson RT, Noble IR, Berlin B,Ravindranath NH, Verardo DJ, Dokken DJ, editors. Land use,land use change and forestry. Special Report of the IPCC.Cambridge: Cambridge University Press, 2000.

[15] Lal R, Bruce JP. The potential of world cropland soils tosequester C and mitigate the greenhouse e�ect. EnvironmentalScience and Policy 1999;2:177–85.

[16] Keller AA, Goldstein RA. Impact of carbon storagethrough restoration of drylands on the global carbon cycle.Environmental Management 1998;22:757–66.

[17] Watson RT, Noble IR, Bolin B, Ravindranath NH, VerardoDJ, Dokken DJ. Land use, land-use change and forestry. Aspecial report of the IPCC. Cambridge: Cambridge UniversityPress, 2001.

[18] Sampson RN, Apps M, Brown S, Cole CV, Downing J,Heath JS, Ojima DS, Smith TM, Solomon AM, Wisniewski J.Workshop summary statement—terrestrial biospheric carbonMuxes—quantiHcation of sinks and sources of CO2. WaterAir and Soil Pollution 1993;70:3–15.

[19] Winjum JK, Brown S, Schlamadinger B. Forest harvests andwood products: sources and sinks of atmospheric carbondioxide. Forest Science 1998;44:272–84.

[20] Sohngen B, Sedjo R. Potential carbon Mux from timberharvests and management in the context of a global timbermarket. Climatic Change 2000;44:151–72.

[21] Gerbhardt MR, Dabiel TC, Schweizer EE, Allmaras RR.Conservation tillage. Science 1985;20:625–30.

[22] Dumanski J, Desjardinds RL, Tarnocai C, Monreal C,Gregorich EG, Kirkwood V, Campbell CA. Possibilities forfuture carbon sequestration in Canadian agriculture in relationto land use changes. Climatic Change 1998;40:81–103.

[23] FAO. State of the world’s forests. Rome: FAO, 2001.[24] Sedjo RA, Solomon AM. Climate and forests. In: Rosenburg,

et al. editors. Greenhouse warming: abatement and adaptation.Resources for the Future, Washington DC, 1989.

[25] Stocks BJ, Fosberg MA, Lynham TJ, Mearns L, WottonBM, Yang Q, Jin J-Z, Lawrence K, Hartley GR, MasonJA, McKenney DW. Climate change and forest Hre potentialin Russian and Canadian boreal forests. Climatic Change1998;38:1–13.

[26] Sathaye J, Ravindranath NH. Climatic change mitigationin the energy and forestry sectors of developing countries.Annual Review of Energy and Environment 1998;23:387–437.

[27] Liski J, Karjalainen T, Pussinen A, Nabuurs GJ, Kauppi P.Trees as carbon sinks and sources in the European Union.Environmental Science and Policy 2000;3:91–7.

[28] Cannell MGR. World forest biomass and primary productiondata. London and New York: Academic Press, 1982.

[29] Scurlock JMO, Hall DO, House JI. Utilising biomass cropsas an energy source: a European perspective. Water, Air andSoil Pollution 1993;70:499–518.

[30] Smith P, Powlson DS, Glendining MJ, Smith JU. Potential forcarbon sequestration in European soils: preliminary estimatesfor Hve scenarios using results from long-term experiments.Global Change Biology 1997;3:67–79.

[31] Smith P, Powlson DS, Smith JU, Fallon P, Coleman K.Meeting Europe’s climate change commitments: quantitativeestimates of the potential for carbon mitigation by agriculture.Global Change Biology 2000;6:525–39.

[32] Smith P, Powlson DS, Glendining MJ, Smith JU. Preliminaryestimates of the potential for carbon mitigation in Europeansoil through no-till farming. Global Change Biology1998;4:679–85.

[33] Solberg B. Forest biomass as carbon sink—economic valueand forest management/policy implications. Critical Reviewsin Environmental Science and Technology 1997;27:S323–33.

[34] Nabuurs GJ, Pussinen A, Liski J, Karjalainen T. Upscalingbased on forest inventory data and EFISCEN. In: Long terme�ects of climate change on carbon budgets of forests inEurope (LTEEF II). Alterra Report 194. Alterra, Green WorldResearch, Wageningen, 2001.

[35] DEFRA (Department for Environment, Food and RuralA�airs). The UK’s third national communication under theUN Framework Convention on Climate Change, London,2001.

[36] Cannell MGR, Milne R, Hargreaves KJ, Brown TAW,Cruickshank MM, Bradley RI, Spencer T, Hope D, Billett MF,Adger WN, Subak S. National inventories of terrestrial carbonsources and sinks: the UK experience. Climatic Change1999;42:505–30.

[37] Smith P, Milne R, Powlson DS, Smith JU, Falloon P, ColemanK. Revised estimates of the carbon mitigation potential of UKagricultural land. Soil Use and Management 2000;16:293–5.

[38] Smith P, Powlson DS, Smith JU, Falloon P, Coleman K.Meeting the UK’s climate change commitments: optionsfor carbon mitigation on agricultural land. Soil Use andManagement 2000;16:1–11.

[39] Smith P, Goulding KWT, Smith KA, Powson DS, Smith JU,Falloon P, Coleman K. Including trace gas Muxes in estimatesof the carbon mitigation potential of UK agricultural land.Soil use and Management 2000;16:251–9.

[40] Cannell MGR, Dewar RC. The carbon sink provided byplantation forests and their products in the Britain. Forestry1995;68:35–48.

[41] Blundell T, editor. Royal Commission on EnvironmentalPollution. Energy—the Changing Climate. UK: HMSO, 2000.292pp.


[42] Marland G. Carbon dioxide emission rates for conventionaland hypothetical fuels. Energy 1983;8:981–92.

[43] Kaltschmitt K, Reinhardt GA, Stelzer T. Life cycle analysisof biofuels under di�erent environmental aspects. Biomassand Bioenergy 1997;12:121–34.

[44] Bauen A. 2002. Biomass energy, greenhouse gas abatementand policy integration. Aspects of Applied Biology, in press.

[45] Turhollow AF, Perlack RD. Emissions of CO2 fromenergy crop production. Biomass and Bioenergy 1991;1:129–35.

[46] Matthews R, Robertson K. Forest products and bioenergy. In:Karjalainen T, Apps M, Innes J, editors. Role of forestry incarbon sequestration. UK: CAB International, 2002, in press[chapter 3].

[47] Schlamadinger B, Marland G. The role of bioenergy andrelated land use in global net CO2 emissions. In: Pelkonen P,Hakkila P, Karjalainen T, Schlamadinger B, editors. Woodybiomass as an energy source—challenges in Europe. EuropeanForest Institute, Proceedings No. 39. Joensuu, Finland, 2001.p. 21–7.

[48] Cannell MGR, Smith RI. Yields of minirotation closelyspaced hardwoods in temperate regions: review and appraisal.Forest Science 1980;26:415–28.

[49] Giampietro M, Ulgiati S, Pimentel D. Feasibility oflarge-scale biofuel production. Does an enlargement of scalechange the picture? BioScience 1997;47:587–600.

[50] Marland G, Turhollow AF. CO2 emissions from theproduction and combustion of fuel ethanol from corn. Energy1991;16:1307–16.

[51] Shapouri H, DuNeld JA, Graboski MS. Estimating thenet energy balance of corn ethanol. US Departmentof Agriculture, Economic Research Service. AgriculturalEconomic Report No. 721, 1995.

[52] CCPCS. Commission consultative pour la production decarburant de substitution. Rapport des Travaux du Groupe,Numero 1, Paris, 1991.

[53] Ulgiati S. A comprehensive energy and economic assessmentof biofuels: when green is not enough. Critical Reviews inPlant Sciences 2001;20:71–106.

[54] Shao Y, Golomb D. Power plants with CO2 capture usingintegrated air separation and Mue gas recycling. EnergyConversion and Management 1996;37:903–8.

[55] Khesghi HS, Prince RC, Marland G. The potential ofbiomass fuels in the context of global climate change:focus on transportation fuels. Annual Review of Energy andEnvironment 2001;25:199–244.

[56] Leemans R, van Amstel A, Battjes C, Kreileman E, ToetS. The land cover and carbon cycle consequences oflarge-scale utilisations of biomass as an energy source. GlobalEnvironmental Change 1996;6:335–57.

[57] Marland G, Schlamadinger B. Biomass fuels and forestmanagement strategies: how do we calculate the greenhousegas emissions beneHts? Energy—The International Journal1997;20:1131–40.

[58] Cramer W, Kicklighter DW, Bondeau A, et al. Comparingglobal models of terrestrial net primary productivity(NPP): overview and key results. Global Change Biology1999;5(Suppl 1):1–15.

[59] Hall DO. Biomass energy. Energy Policy 1991;19:711–37.[60] Hall DO, Mynick HE, Williams RH. Cooling the greenhouse

with bioenergy. Nature 1991;353:11–2.[61] Rosillo-Calle F, Hall DO. Biomass energy, forests and global

warming. Energy Policy 1992;20:124–36.[62] Hall DO, House JI, Scrase JI. Introduction: overview of

biomass energy. In: Rosillo-Calle F, Bajay S, Rotman H,editors. Industrial uses of biomass energy. London: Taylorand Francis, 2000.

[63] Hall DO, Scrase I. Will biomass be the environmentallyfriendly fuel of the future? Biomass and Bioenergy1998;15:357–67.

[64] Nakivcenovic NA, Grublert A, McDonald A, editors. Globalenergy perspectives. WEC-IIASA. UK: Cambridge UniversityPress, 1998.

[65] Wright LL, Hughes EE. US carbon o�set potential usingbiomass energy systems. Water, Air and Soil Pollution1993;70:483–97.

[66] Graham RL, Wright LL, Turhollow AF. The potential ofshort-rotation woody crops to reduce US CO2 emissions.Climate Change 1992;22:223–38.

[67] Haroon SK, Prince R, Marland G. The potential of biomassfuels in the context of global climate change: focuson transportation fuels. Annual Review of Energy andEnvironment 2000;25:199–244.

[68] Cannell MGR. Environmental impacts of forest monocultures:water use, acidiHcation, wildlife conservation and carbonstorage. New Forests 1999;17:239–62.

[69] Yamamoto H, Yamaji K, Fujino J. Scenario analysis ofbioenergy resources and CO2 emissions with a global landuse and energy model. Applied Energy 2000;66:325–37.

[70] Bauen A, Kaltschmitt K. Current use and potential of solidbiomass in developing countries and their implications forCO2 emissions. Proceedings of First World Conference onBiomass for Energy and Industry. London: James and JamesScience Publishers, 2001.

[71] WEC. Renewable energy sources: opportunities andconstraints 1990–2020. London: World Energy Council,1993.

[72] Schwaiger H, Schlamadinger B. The potential of fuelwoodto reduce greenhouse gas emissions in Europe. Biomass andBioenergy 1998;15:369–77.

[73] Ossebaarde ME, Vanwijk AJM, Vanwees MT. Heat supplyin the Netherlands—a systems analysis of costs, energyeNciency, CO2 and NOx emissions. Energy 1997;22:1087–98.

[74] Gustavsson L. Biomass and district-heating systems.Renewable Energy 1994;5:838–40.

[75] Gustavsson L. Energy eNciency and competitiveness ofbiomass-based energy systems. Energy 1997;22:959–67.

[76] Department of Trade and Industry (DTI). New and RenewableEnergy. Prospects for the 21st century. London, 2001.

[77] Allen SJ, Hall RL, Rosier PTW. Transpiration by two poplarvarieties grown as coppice for biomass production. TreePhysiology 1997;19:493–501.

[78] Hall RL, Allen SJ. Water use of poplar clones grown asshort-rotation coppice at two sites in the United Kingdom.


In: Bullard MJ, Ellis RJ, Heath MC, Knight JD, LainsburyMA, Parker SR, editors. Biomass and bioenergy crops.Wellesbourne, Warwick: Association of Applied Biologists,1997. p. 163–72.

[79] Moura Costa P, Wilson C. An equivalence factor betweenCO2 avoided emissions and sequestration—description andapplications in forestry. Mitigation and Adaptation Strategiesfor Global Change 2000;5:51–60.

[80] Schlamadinger B, Marland G. The role of forest andbioenergy strategies in the global carbon cycle. Biomass andBioenergy 1996;10:275–300.

[81] Read P. Carbon sequestration in forests: supply curves forcarbon absorption. US Department of Energy/InternationalEnergy Agency workshop: Promising technologies formitigating greenhouse gases. Washington DC, May 1999.Available at: iea.org/workshop/engecon/.

carbon sequestration and biomass energy offset: theoretical, potential and achievable capacities...

Documents