Over the last 10,000 years large areas of Earth’s ter-restrial surface have undergone radical changes as the growing world population has used land for its various needs. The most important human land-use activities include the clearance or commercial use of forests, agriculture and the expansion of human set-tlements (Foley et al., 2005). Farmland alone, com-prising both cropland and pastures, now covers around 40 per cent of the land surface (Foley et al., 2005). Almost a quarter of Earth’s potential net pri-mary production is already subject to human influ-ence through harvesting, productivity changes result-ing from land use, and fires (Haberl et al., 2007).
The human use of land thus competes directly with natural land cover, which plays an important part in the conservation of biological diversity and also func-tions as a carbon reservoir in the climate system. The increasing deployment of biomass for energy pro-duction increases the pressure on previously unused land; on existing farmland, it competes with the need to produce food for the growing world population (Chapter 5).
It is against this backdrop that WBGU sets out to identify the size of the sustainable global poten-tial for energy crops until the middle of the century, using the modelling system that it has commissioned and which is described in this chapter (Beringer and Lucht, 2008). The plant primary production avail-able for bioenergy will be determined on a region-by-region basis, taking account of the guard rails for food and environmental, climate and soil protection (Chapter 3). Using simple scenarios, the guard rails described in Chapter 3 will be used to identify exclu-sion areas within which the cultivation of energy crops would not be defined by WBGU as sustain-able.
Whether this global sustainable potential for the deployment of bioenergy from the cultivation of energy crops can be realized depends mainly on eco-nomic and social conditions in those regions in which farmland that meets the WBGU criteria is available.
WBGU’s assessment of global potential at the end of the chapter is therefore preceded by a detailed socio-economic analysis of the countries in question.
96 6 Modelling global energy crop potential
Bioenergy production on degraded land lies in the range of 8–110 EJ per year, while production from biogenic wastes and residues (agricultural and for-estry residues, dung, organic waste) amounts to 62–108 EJ per year. Figures for the feedstock use of biomass range from 83 EJ to 116 EJ per year (Hoog-wijk et al., 2003). These figures highlight the impor-tance of assumptions about the area of land that will be needed in future to secure the world food supply.
Very high potentials (in the range of around 1000 EJ per year) for the contribution of bioenergy to the world energy supply are only possible if it is assumed that land previously used for food production can be released, as a result either of efficiency improve-ments or less land-intensive dietary habits.
This is also illustrated by a study of Wolf et al.
(2003) which considers the land available for food, feed and biomass and investigates the influence of agricultural production systems and dietary habits.
However, the study does not distinguish between different energy crops and their yields on different soils. Restricting consideration to land currently in agricultural use and assuming medium population growth and a moderate nutrition style, global techni-cal bioenergy potential is estimated at between 59 EJ (extensive cultivation for food, feed and biomass) and 417 EJ per year (intensive cultivation for food, feed and biomass). If not only existing farmland but also all potentially available agricultural land is used, consideration given to the competing claims of other
forms of land use and from the fact that some other estimates have assumed unrealistically high yields (WBGU, 2004a). Some more recent studies of glo-bal bioenergy potential are discussed below. All fig-ures for potentials represent the gross energy contri-bution – i.e. any losses that occur during conversion to final energy are not included.
Hoogwijk et al. (2003) evaluate existing studies and investigate the influence of various factors on the proportion of bioenergy from different sources in global energy production in 2050. The studies they review vary in their estimate of the world’s future food requirement (influenced by population devel-opment and dietary habits), in the food and feed culti-vation systems that they consider, and in the assump-tions they make about productivity, land availabil-ity and requirements for biomass feedstock cultiva-tion. Only existing conservation areas are excluded from bioenergy production. The resulting estimates for the year 2050 span a wide range of possible val-ues from 33 EJ to 1135 EJ per year. The way in which these bioenergy potentials are distributed between sources is interesting. Estimates for bioenergy pro-duction on existing agricultural land (after the food requirements of the growing world population have been met) range from 0 to 988 EJ per year (the fig-ure of 0 arises from the assumption that all exist-ing agricultural land is needed for food production).
Box 6.1-1
Types of potential
Discussion of the potentials of various energy carriers usu-ally distinguishes between theoretical potential, technical potential, economic potential and sustainable potential (WBGU, 2004a). In the context of this report, these terms are defined as follows:
Theoretical potential
Theoretical potential describes the physical upper limit of the energy available from a particular source. In the case of solar energy this is the total solar radiation incident on the area in question. This potential therefore takes no account of land-use restrictions or of the efficiency of the conver-sion technologies used.
Technical potential
Technical potential is defined specifically for each tech-nology; it is derived from the theoretical potential and the annual efficiency of the respective conversion technology.
Restrictions relating to the land realistically available for energy production are also taken into account. The criteria used in selecting land are not applied uniformly in the lit-erature. Technical, structural and ecological restrictions and statutory specifications are sometimes included. The level of the technical potential of different energy sources is thus not a clearly defined value but dependent on a wide range of conditions and assumptions.
Economic potential
Economic potential describes how much of the technical potential is economically usable under the given economic conditions (at a particular point in time). For biomass, for example, the economic potential is the quantity of biomass that it is economical to extract in the face of competition with other products and land uses. It may be possible to exert significant influence on economic conditions through policy measures.
Sustainable potential
The sustainable potential of an energy source takes account of all the dimensions of sustainability. A range of ecologi-cal and socio-economic factors must usually be considered.
Sustainable potential is not clearly delineated, since some authors already include ecological factors in their consid-eration of technical or economic potential.
It should be borne in mind that these terms are used in very different ways by different authors; in consequence, the sequence described above does not necessarily represent an increasingly tight progression. Hence the modelling com-missioned by WBGU and described in this chapter refers to a ‘technical sustainable potential’, because the absence of integrated models has made it impossible to also assess economic viability.
Previous appraisals of bioenergy potential 6.1 97
year, while that from additional forest growth was estimated at 74 EJ per year (Smeets et al., 2007).
Hoogwijk et al. (2005) analyse the energy poten-tial of short-rotation plantations of woody biomass for the years 2050–2100 for the four IPCC scenar-ios A1, A2, B1 and B2. The area of land set aside for nature conservation that cannot be used for biomass cultivation is assumed under the A scenarios to be 10 per cent and under the B scenarios to be 20 per cent of the global land area. Assumptions about world population, dietary habits and technological devel-opment are based on the storylines of the IPCC sce-narios. The technical potential for bioenergy pro-duction arising from the use of abandoned agricul-tural land is put at 130–410 EJ per year in 2050 and at 240–850 EJ per year in 2100. The potential of land not previously used for agriculture, after deduction of grasslands, forests, urban areas and existing pro-tected areas, is estimated at 35–245 EJ per year for 2050 and 35–265 EJ per year for 2100 (Hoogwijk et al., 2005; Smeets et al., 2007).
In its World Energy Outlook 2007 the Interna-tional Energy Agency (IEA) puts the annual global primary energy use from biomass and residues in 2030 for its four scenarios at 68 EJ (Reference Scenario), 73 EJ (Alternative Policy Scenario), 69 EJ (High Growth Scenario) and 82 EJ (450 ppm Stabilisation Case; IEA, 2007a). This economic potential was cal-culated using an economic energy system model, tak-ing account of various policy scenarios.
A study by the Institute for Energy and Environ-mental Research, Heidelberg (Institut für Energie und Umweltforschung, IFEU) commissioned by the German chemical industry association (Verband der Chemischen Industrie, VCI) estimates global bioen-ergy potential in 2050 to be 240–620 EJ per year. Of this, 215–420 EJ per year arises from the cultivation of energy crops on surplus agricultural land. The study takes account of the future need for feedstock use of biomass, and extreme scenarios for yield increases in agriculture have been excluded. In addition, tim-ber growth contributes 0–45 EJ per year to the glo-bal potential, while all types of biogenic wastes and residues contribute 25–155 EJ (IFEU, 2007).
The OECD Round Table on Sustainable Develop-ment estimates that the sustainable global potential of bioenergy in 2050 totals 245 EJ per year (Doorn-bosch and Steenblik, 2007). Of this potential, 109 EJ per year arise from the cultivation of energy crops and 136 EJ per year from the use of agricultural and forestry residues, dung and organic waste for energy.
The authors estimate the area of land available for the cultivation of energy crops at 440 million hec-tares. They exclude land currently used for food pro-duction, an additional 200 million hectares for secur-these figures rise to 257 EJ for extensive cultivation
and 790 EJ per year for intensive cultivation (Wolf et al., 2003). The influence of dietary habits is interest-ing. For extensive farming for food, feed and biomass on existing land the potential decreases from 59 EJ per year to 0 EJ per year if nutrition styles involve large quantities of meat and milk products and are therefore very land-intensive; if nutrition styles are less land-intensive the figure rises to 194 EJ per year.
Field et al. (2008) argue that sustainable cultiva-tion of energy crops is only possible on abandoned agricultural lands that have previously been used as cropland or pasture, provided that they have not been converted to urban or forest areas. The authors thus implicitly exclude land used for growing food and feed, existing protected areas and wilderness areas, which they consider essential for securing the world food supply and for nature conservation. On the basis of this land appraisal and taking account of the spatially differentiated and climatologically determined net primary production on this land, they arrive at a global potential for the additional cultiva-tion of energy crops that is sustainable according to their criteria of 27 EJ per year (Field et al., 2008).
Another study estimates that the sustainable pro-duction of bioenergy from the extensive use of high-diversity grassland on unused and degraded land could contribute around 45 EJ per year to global energy production. This type of use would in addi-tion entail low inputs of chemical fertilizers and pes-ticides, good carbon storage in the soil and relatively high biodiversity (Tilman et al., 2006). Without plac-ing restrictions on the type of energy crop grown, a new study arrives at a similar potential of 32–41 EJ per year on abandoned and degraded land (Campbell et al., 2008).
Smeets et al. (2007) explore global bioenergy potentials to 2050 for three types of biomass (bioen-ergy crops, agricultural and forestry residues and waste, and additional forestry yields) without tak-ing climate change into account. In view of the need to conserve biodiversity, this study excludes exist-ing protected areas, forests, barren land, scrubland and savannahs. Drawing on various assumptions for increasing yields in food production, the authors identify technical potentials of 215–1272 EJ per year for the cultivation of energy crops on surplus agricul-tural land, although the assumptions underlying even the lowest figure appear noticeably optimistic (Faaij, 2008). The higher figures assume major technological progress in food production as well as the use of irri-gated agriculture. The global potential of bioenergy production from agricultural and forestry residues and wastes in 2050 was projected to be 76–96 EJ per
98 6 Modelling global energy crop potential
additional forest growth is small, because of the rising demand for wood products (Section 5.3.2). The cas-cade use of these material products goes only some of the way towards alleviating the problem because of the inevitable losses that are entailed (Section 5.3.3). WBGU therefore puts a figure of 0 EJ per year on the sustainable potential of additional forest growth, but notes that further research in this area is needed.
There are very few studies of the sustainable potential arising from the use of biogenic wastes and residues. In its energy report (WBGU, 2004a), WBGU estimates that this potential amounts in total to 67 EJ per year. On the basis of more recent studies, WBGU regards a realistic figure for the global tech-nical potential from biological wastes and residues from agriculture and forestry and from dung to be 80 EJ per year. However, not all of this is usable, since these estimates do not necessarily take account of economic considerations and sustainability crite-ria. For example, for soil protection reasons residues from agricultural and forestry ecosystems cannot be removed completely, as this would result in too much organic material being removed from the soil (Münch, 2008). At a rough estimate it seems realistic to assume that the technical sustainable potential is around 50 EJ per year, of which approximately half is economically realizable. WBGU points out that this figure must be regarded as very uncertain, since there are issues relating to the sustainable and economic use of biogenic wastes and residues that still need to be clarified through research.
6.2
Global land-use models: The state of scientific knowledge
6.2.1
Effects and impacts of human land use
Human-induced changes in Earth’s land cover influ-ence the climate by changing the reflectivity (albedo) of Earth’s surface and affecting the carbon cycle (Lambin et al., 2003). It is estimated that around 35 per cent of anthropogenic carbon dioxide emissions since 1850 are the result of human land use (Foley et al., 2005). In addition, land use and land-use changes affect the water cycle, the nutrient cycle, biological diversity and soil quality (Lambin et al., 2003).
Conversely, biogeophysical variables such as cli-mate, water availability and soil quality, and changes in them, affect not only the natural vegetation;
together with political, economic and social factors ing the world food supply, and forests, but do not
reserve any land for nature conservation.
Under an ‘alternative scenario’ for climate-friendly future energy production, a study commis-sioned by Greenpeace and the European Renewable Energy Council (EREC) puts the sustainable con-tribution of bioenergy to global energy production in 2050 at around 105 EJ per year (Greenpeace and EREC, 2007).
6.1.2
Summary and evaluation
Estimates of the potential contribution of bioen-ergy to global enbioen-ergy consumption are summarized in Table 6.1-1. Although the potentials quoted vary widely, ranging as they do from 30 EJ to 1,200 EJ per year, it is nevertheless possible to identify from this literature review some trends from which, despite some major uncertainties, a reasonably consistent picture emerges.
The greatest uncertainty results from the fact that the amount of land needed to meet the future food requirement of the world population is unknown.
This land requirement depends not only on popula-tion growth, but also on the development of dietary habits, technological progress and the level of intensi-fication of agricultural production (Section 5.2). Very high bioenergy potentials of the order of 1000 EJ per year are only technically realizable if land at present used for food production becomes available for the cultivation of energy crops as a result either of effi-ciency improvements or of less land-intensive die-tary habits. It is of course possible that securing the food supply of a growing world population might in fact require the use of even more land, as the FAO, among others, have forecast (FAO, 2003a).
If land that has been used until now for food pro-duction is excluded, the only land that remains avail-able for the cultivation of energy crops is marginal land (Box 4.2-1) with a very uncertain energy poten-tial of around 30–200 EJ per year for a non-irrigated and not highly intensified farming system.
According to the studies described here, this poten-tial from the cultivation of energy crops is supple-mented by additional forestry yields at around 80 EJ per year and by biogenic wastes and residues (includ-ing agricultural and forestry plant residues, dung and organic waste) at around 80 EJ per year.
It must, however, be borne in mind that the major-ity of these estimates relate to technical potential;
economic potential and in particular sustainable potential are likely to be less. Furthermore, compet-ing use has not always been considered. For example, WBGU estimates that the bioenergy potential from
Global land-use models: The state of scientific knowledge 6.2 99
Table 6.1-1
Technical (TP), economic (EP) and sustainable potential (SP) of bioenergy in EJ per year from various studies.
Compilation: WBGU Sources Potential,
year Forest
increment Energy crop cultivation Residues Total
Farmland Unused land
Degraded land
Agricul-ture
Forestry Other Studies that consider all contributions to the bioenergy potential
WBGU
(2004a) NP 0 37 17 42 8 104
Hoog-wijk et al.
(2003)
TP, 2050 0 0–988 8–110 10–32 42–482 10–28 33–1135
Smeets et al. (2007)
TP, 2050 74 215–1272 76–96 365–1442
IEA
(2007a) WP, 2030 68–824
IFEU (2007)
WP, 2050 0–45 200–390 15–30 15–70 5–30 5–55 240–620
Doorn-bosch und Steenblik (2007)
NP, 2050 109 35 91 10 245
Faiij (2008)
NP, 2050 60–100 1206 706 40–170 430–6006
Studies of the potential from the cultivation of energy crops Wolf et al.
(2003) TP, 2050 0–7901
Hoog-wijk et al.
(2005)
TP, 2050 130–410 35–245
TP, 2100 240–850 35–265
Tilman et al. (2006)
NP 453
Campbell et al.
(2008)
NP 32–41
Field et al.
(2008) NP 27
WBGU (2008)
NP, 2050 34–1207
1 depending on dietary habits and degree of intensification of agricultural production
2 including 32 EJ per year from cascade use of biomaterials
3 extensively used grassland of high biodiversity
4 for the four IEA scenarios (Reference Scenario, Alternative Policy Scenario, High Growth Scenario, 450 ppm Stabilisation Case)
5 Alternative Scenario
6 an additional 140 EJ per year in energy crop cultivation as a result of technological progress in agriculture is assumed
7 climate model HadCM3, emissions scenario A1B, depending on guard rail scenario and irrigation
100 6 Modelling global energy crop potential
• Integrated models seek to combine the strengths of both approaches; they attempt to provide a more realistic description of changes in the human use of land, since these changes are influenced by both biogeophysical and socio-economic factors.
6.3
Description of the model
The modelling carried out for this report uses the LPJmL (LPJ managed Land) model (Bondeau et al., 2007), which is based on the LPJ dynamic glo-bal vegetation model (Lund-Potsdam-Jena; Sitch et
The modelling carried out for this report uses the LPJmL (LPJ managed Land) model (Bondeau et al., 2007), which is based on the LPJ dynamic glo-bal vegetation model (Lund-Potsdam-Jena; Sitch et