Field biomass as global energy source

Current (1997–2006) and future (2050) global field biomass bioenergy potential was estimated based on FAO (2009) production statistics and estimations of climate change impacts on agriculture according to emission scenario B1 of IPCC. The annual energy potential of raw biomass obtained from crop residues and bioenergy crops cultivated in fields set aside from food production is at present 122–133 EJ, 86–93 EJ or 47–50 EJ, when a vegetarian, moderate or affluent diet is followed, respectively. In 2050, with changes in climate and increases in population, field bioenergy production potential could be 101–110 EJ, 57–61 EJ and 44–47 EJ, following equivalent diets. Of the potential field bioenergy production, 39–42 EJ now and 38–41 EJ in 2050 would derive from crop residues. The residue potential depends, however, on local climate, and may be considerably lower than the technically harvestable potential, when soil quality and sustainable development are considered. Arable land could be used for bioenergy crops, particularly in Australia, South and Central America and the USA. If crop production technology was improved in areas where environmental conditions allow more efficient food production, such as the former Soviet Union, large areas in Europe could also produce bioenergy in set aside fields. The realistic potential and sustainability of field bioenergy production are discussed.


Introduction
The global surface temperature has increased during the last century (1850-1899 to 2001-2005) by an average of 0.76 ºC.The warming has been especially rapid during the last decade, the period 1995-2006 being the warmest ever recorded (IPCC 2007a).The observed increase in average global temperature is mostly due to increases in anthropogenic greenhouse gas concentrations, the most important of which is CO 2 (IPCC 2007a).With increases in global temperatures, the entire global climate system has changed: precipitation has increased in northern Europe, eastern parts of North and South America and northern and central Asia, while it has decreased across the Sahel, the Mediterranean, southern Africa and parts of southern Asia (IPCC 2007a).Because of the differences in regional changes, the effects on agricultural production differ in different parts of the world.In general, small increases in temperature (under 3 ºC) will improve agricultural production at high latitudes (e.g.northern Europe, North America), but increases in temperatures as small as 1-2 ºC would worsen conditions at low latitudes (India, China, dry areas in Africa) (IPCC 2007b).On general, increases of temperatures higher than 3 ºC are projected to decrease global food production and food production at high latitudes will also be threatened, depending on the region (IPCC 2007b).At the same time, populations in areas with the highest vulnerability to climate change are projected to increase most (IPCC 2007b, United Nations 2007).
Because of the obvious severity of the impacts of climate change, governments around the globe have agreed on measures to reduce greenhouse gas emissions.The best known agreement, the Kyoto protocol, was first adopted in 1997, and by the end of 2008 had been ratified by 177 countries and the European Community.It entered into force on 16 February 2005.Industrialized countries agreed on reducing (relative to year 1990) their greenhouse gas emissions by an average of 5% from the 1990 emission levels, during the period 2008-2012 (United Nations 1998), with 8% reduction assigned for EU (UNFCCC).
Greenhouse gas emissions decreased by 7.7% in the EU-27 countries between 1990 and 2006.However, in the EU-15 group originally committed to the Kyoto protocol, the decrease was only 2.7%.The projections for 2010 suggest, however, that the 8% target reductions will be met during the period 2008-2012, partly through use of the Kyoto mechanisms such as joint implementation or adoption of clean technology (EEA 2008).An important way to reduce greenhouse gas emis-sions is to use renewable energy sources.Sunlight, water flows, wind and biomass from forests and fields have always been used for different energy needs.Currently renewable energy sources make up only about 18% of all consumed energy, and traditional biomass energy 13% (REN21 2008).Thus, in 2004, when the global primary energy demand was calculated to be 464 EJ (Sims et al. 2007) the share of biomass energy in this figure was 44.6 EJ (altogether 9.6%), of which wood fuel comprised 39 EJ, agro fuels 4.2 EJ and municipal waste 1.1 EJ.However, the energy demand in 2050 will be about double compared to 2004 (baseline about 850 EJ and policy scenario of 2 ºC temperature increase about 810 EJ), and the assumed bioenergy potentials would be 270 EJ (wood fuel 57 EJ and agro fuels 213 EJ) in 2050 (evaluated with the VTT version of the ETSAP TIAM energy system model described in Koljonen et al. 2009).To efficiently contribute to mitigation of climate change, EU has taken a further decision in December 2008, where the 27 EU countries are committed to further cutting their greenhouse gas emissions by 20% (compared with the 1990 level), increasing the share of renewable energy sources to 20% of all energy needed, and cutting energy use by 20% by 2020.In addition, 10% of transport fuel should originate from renewable sources by 2020.
When biomass production potential for bioenergy has been considered on basis of soil and climatic suitability, the possible energy crop production values have ranged from <100 EJ to >400 EJ (Berndes et al. 2003, Hoogwijk et al. 2005), even reaching 648 EJ when all land suitable for biomass production is used efficiently (Wolf et al. 2003).With technological development, and development of infrastructure, the bioenergy production figures presented e.g. for Africa (Hoogwijk et al. 2005) could be reached.However, much less is actually being produced at the moment, not even enough food, with the percentage of undernourished people remaining high in Africa (FAO 2009).Thus, looking at the present field crop production values gives a more realistic picture of the crop production situation.Therefore, in the present simple survey based on FAO pro-duction statistics (FAO 2009) we estimated the sufficiency of crop production at the moment and in the future (2050) and how much raw material for bioenergy, either as crop residues or specific bioenergy plants, could realistically be harvested from the field, taking into account the field area demand for food production.For the future we estimated how increases in population (United Nations 2007) and climate change would affect the production of field biomass energy.The world in this study is divided into 15 areas (Annex 1) according to the targets set by the umbrella project SEKKI, "The competitiveness of Finnish energy industry under developing climate policy" (Syri et al. 2008a).This project monitored the worldwide availability of energy now and in the future (2050), employing the global TIMES model (Syri et al. 2008b).The studied areas would normally be trading food among each other, but here they are for simplicity considered as independent units.For the future, the assumptions were that development will proceed according to the emission scenario B1 of IPCC (Nakicenovic et al. 2000, IPCC 2007a), that all arable land of the present day is used for field biomass production, and that field area does not increase.Emission scenario B1 was chosen, as efficient employment of renewable energy sources, including field bioenergy, aims at radical reductions of greenhouse gas emissions, as is also assumed in the B1 scenario of IPCC.

Applying scenarios of climate change effects on crop production
Crop production data were derived from FAO (2009).The production data were from 1997 to 2006 and averages from that period were used in calculations of food production, availability of arable area for bioenergy crops production and production potential of crop residues for bioenergy.

Calculation of crop residue potential
The theoretical crop residue potential was estimated using yield, yield dry matter (DM) content and harvest index (HI) of each individual crop species (Annex 2).HI describes the share of harvested yield of the total biomass of a crop on a DM basis.Based on published literature and our own results, a single harvest index was chosen per crop and the theoretical residue potential was calculated as: (1-HI) × yield DM)/HI.For calculation of the harvestable residue, or technical residue potential, the estimated biomass of the crop stubble left on the field as well as the residue lost through shedding of the straw material at harvest was reduced from the theoretical residue potential.For cereals, oil crops and pulses the stubble is normally 15-30 cm high, depending on crop and the harvesting conditions.According to Finnish research results, 15 cm barley (Hordeum vulgare L.) stubble represents about 27% of all straw biomass (Pahkala et al. 2007, Pahkala andKontturi 2008).Studies of other cereals reached similar conclusions (Staniforth 1979).The technical residue potential in this study is thus a product of the theoretical potential reduced by 30% for cereals, oil crops and pulses, 25% for grain maize (Zea mays L.) (Graham et al. 2007), and 50% for root crops including sugar beet (Beta vulgaris L. subsp.vulgaris).After determination of the technical crop residue potential of each individual species for the year 2006 (Table 1, Pahkala et al. 2009), the values were corrected according to the 10 year production averages (1997)(1998)(1999)(2000)(2001)(2002)(2003)(2004)(2005)(2006) of the same crop groups (cereals, pulses, oil crops, root crops and sugar crops), and variation in residue potential was estimated according to the variation in production (Table 2).When crop residue production was estimated for the future (2050), the average technical residue potential values for 1997-2006 were corrected at group level for climate change effects (Annex 1).The resulting residue production figures for the future thus assume similar division of crop groups into individual crop species as at present.The energy value of each residue type was assumed to be 18 MJ kg -1 DM.

Estimation of food sufficiency and availability of field for bioenergy crops
Food sufficiency was estimated using the production statistics of FAO (2009).Grain equivalent (GE) values (on kg of wheat grain basis) were fitted for different crops, as described by Penning de Vries et al. (1997).In the calculation of GE, production quantities (averages of 1997-2006) of all cultivated crops listed in FAO statistics (FAO 2009) except temporary forage grasses were included in the total energy values for each of the 15 areas.Thus, in addition to cereals, pulses, oil crops, sugar crops and root crops, production of vegetables, fruits, nuts and fibre crops (hemp, flax, etc.) were also taken into account.Sufficiency of food production on arable land was then evaluated for each area for three different diets, vegetarian (GE usage 490 kg per capita per annum), moderate (860 kg) and affluent (1535 kg), using the United Nations population statistics.Estimation of food sufficiency in the future (2050) was based on United Nations estimations of population in the different areas (United Nations 2007) and estimations of changes in agricultural production (Parry et al. 2004) in the future (Annex 1).Before any of the areas were considered able to set aside field from food production, the GE required for each diet was doubled to cover yield fluctuations, storage losses (which can be substantial, particularly in developing countries) and other production uncertainties (Penning de Vries et al. 1997, Wolf et al. 2003).Food value of animal husbandry products relying solely on grazing was not taken into account, as data for calculations of productivity of permanent pastures was not available for all the studied areas.Also game and fish were excluded from the calculations.
Estimation of energy crop yields and energy values per hectare were done using average yields from 1997-2006 for each area, where enough land for energy crops was available.Energy crop species were chosen from typical crops grown or potentially grown in each area.The average yield levels (1997)(1998)(1999)(2000)(2001)(2002)(2003)(2004)(2005)(2006) of the conventional grain/ seed crops and sugar cane were derived from FAO (2009) statistics, and the yields of special energy crops were taken from literature (Mischantus: Woods et al. 2006; reed canary grass: Pahkala et al. 2008;switchgrass: Schmer et al. 2008).The hypothesised share of the crop was used for assessing the total bioenergy of the crops (Table 3).For estimation of values in 2050, the effect of climate change was taken into account, as stated in Annex 1.

Energy yield potential from harvestable crop residues
The total production of different food crops with harvestable residue (cereals, oil crops, pulses, sugar crops, root crops) varied in the studied period 1997-2006 from 4.8 to 5.1 billion tonnes (Table 2).The biggest group was cereals, the production of which was about 2.1 billion tonnes year -1 .The cur-rent technically harvestable residue energy potential of these crop groups is about 39-42 EJ at present, and 38-41 EJ in 2050 (Table 2).In practice, even the technical potential overestimates the real attainable crop residue yield as some of the crop residue, in addition to the stubble, has to be ploughed in or left on the ground for better organic matter content and functionality of the soil.The amount needed for satisfactory soil functioning varies according to area and yield of the crop (Graham et al. 2007), and is not defined reliably enough for all the studied areas to be taken into account in this study.

Food usage and availability of field area for bioenergy crop production
The results show that enough food is produced at present in the world to satisfy the diet of every inhabitant, even without taking into account permanent grassland productivity ( Food production is, however, not evenly distributed.For example, South Korea (SKO) and Japan (JPN) are not self sufficient in food, but they are solvent enough to be able to import foodstuffs.The situation is more difficult in Africa (AFR), which is clearly deficient in food production, and will be more so in 2050 (Table 4).If considered only on the basis of the studied 15 districts, with no food trade assumed, fields could be set aside from food production for bioenergy crops both now and in the future in AUS, CAN, CSA, EEU, FSU, MEX and USA if a vegetarian diet were adopted (Table 4).With affluent diet, only AUS and CAN could still be producing bioenergy crops on fields.If an exercise is taken to look at technological development as above for Europe, filling the yield gap and positive effects of climate change in WEU would result in possibility of bioenergy production in this area as well (results not shown).If, however, food would be divided equally and food availability would be secured for everyone in a better world, no field area would be freed for bioenergy production, provided food is produced with the present technology and present crops.
For the calculation of potentially produced biomass energy on set-aside fields, the energy values of the energy crops and their yields were calculated per hectare (Table 3).The global gross yield of biomass energy from specifically cultivated energy crops would be (with vegetarian diet) 83-91 EJ now and 64-70 EJ in the future (Table 4).The biggest producers of field energy crops for both the present and for 2050 would be AUS, CSA and USA.Positive technological development, e.g.irrigation in areas where water resources could be taken into use, might change the figures for the future dramatically.E.g., if the production technology in Europe alone would proceed according to the scenarios of Ewert et al. (2005) and Olesen and Bindi (2002), the global biomass energy potential would increase to 132 EJ (results not shown).
The total field biomass energy potential is the sum of crop residue technical potential and bioenergy crop energy potential (Table 5).When this sum is used, all areas in the world are assigned a value.The biggest field energy producers would understandably be those that could produce most energy crop biomass (AUS, CSA and USA).The total energy yield from field biomass would be (if vegetarian diet would be assumed) 122-133 EJ now (1997)(1998)(1999)(2000)(2001)(2002)(2003)(2004)(2005)(2006) and 101-110 EJ in 2050 (Table 5).

Sustainability of residue collection for bioenergy
Agricultural residues are one of the most reliable bioenergy sources for the future because they are always produced when crops are grown.In this study,  (Wilhelm et al. 2007, Varvel andWilhelm 2008).According to these studies a safe amount of stover harvest depends on soil properties (sensitivity to erosion), climatic conditions and biomass yield of the corn crop.If the yield of corn is low and no-till is not used, then all crop residues must be left in the field (Wilhelm et al. 2007).If no-till cultivation (growing crops without tillage) is used, 30% of crop residues can be harvested without danger of increased soil erosion (Lindstrom 1986, Andrews 2006).On average, only about 30% of the corn crop residues can be sustainably collected in the USA for bioenergy or other uses without endangering soil fertility (Graham et al. 2007).In northern production areas, and if yields are high, up to 60% of corn stover can be safely harvested (Graham et al. 2007).In a Canadian study, 40% of wheat residue could be harvested in 2 years out of three without affecting soil productivity (Lafond et al. 2009).In cool climates, such as Finland, where the growing season is short and crop residues on the soil surface can reduce crop yields by slowing soil warming, their removal at least partly could even enhance yield formation.
In this study the technical biomass potential (harvestable biomass) was estimated by subtracting the portion of the crop left in the field at harvest (stubble and shed straw) from theoretical biomass potential.The calculation of sustainable biomass potential would require valid estimates of the amount of crop residue needed to retain soil fertility.As there is limited information concern-ing the amounts of crop residue needed to sustain soil fertility, numerical estimates of the sustainable biomass potential are not given in this report.Areas where crop residue removal is likely to impair soil fertility and cause erosion are those where water shortage currently limits crop production, and where the limitation will become more severe with climate change.These areas are IND, MEX, USA, AFR, AUS, MEA, CHI, and some countries in ODA (IPCC 2007b, Parry et al. 2004).In the northern hemisphere, where the climate is more humid, the extensive and sustainable use of crop residues for bioenergy is still possible.

Food and bioenergy -prospects with and without fair share
Global food production is sufficient for every individual now and will be in the future if development occurs in a sustainable manner as suggested by emission scenario B1 (Nakicenovic et  and low technological development on a regional basis is taken into account in the actual production data (Table 2) and the resulting food sufficiency (Table 4).If permanent grassland production could be estimated, our study would probably have indicated higher GE and bioenergy production values.However, the present results seem realistic, at least for AFR, as the threat of increasing undernourished population in the area has been reported by the IPCC (IPCC 2007b).Fulfilling the need for food, and being able to produce bioenergy crops seems very unlikely for AFR without substantial technological progress occurring in the future.
In order to efficiently produce energy from field biomasses, the choice of the energy crop is crucial.E.g. maize and sugar cane are very efficient biomass and energy producers given the right conditions, whereas huge potential lies in the vast areas of permanent grasslands that form 70% of all agricultural area and are at the moment not efficiently used.For full exploitation of maize for bioenergy, taking into account that it also is used as food, its yield as well as the conversion of the yield and biomass to bioethanol has to be improved (Torney et al. 2007).The same demands apply to permanent grasslands, where improvement of productivity largely depends on adequacy of nutrients, water and transport logistics.
Sugar cane production for energy in suitable climates and areas could increase the energy yield from agricultural areas considerably.E.g. in Brazil, the total agricultural area is 264 million hectares, of which permanent pastures comprise almost 200 million hectares (FAO 2009).The increase in sugar cane production area from 4.8 to 6.4 million hectares in 1997-2006(FAO 2009) has according to Brazilian experts mainly taken place at the expense of the permanent pasture areas and small farms of varied crops with almost no impact on arable land (Goldemberg et al. 2008).Sugar cane production could still be increased on pasturelands, as the number of cattle km -2 is still very low and could be increased (Goldemberg et al. 2008).However, further increase in sugar cane production area in the coming decades may require deforestation and expansion to savannah (cerrado), which is an important natural habitat in Brazil.Luckily these kind of natural habitats are largely not suitable for intensive farming, because of soil quality, low precipitation and logistics, and also local laws tend to protect natural habitats (Goldemberg et al. 2008).
Usage of sugar cane and maize for bioenergy, while there still are areas in the world where population is undernourished has raised debate in public.Therefore, locally adapted natural plants such as Jathropa or castor bean could be taken into cultivation on large areas, provided their toxicity is reduced by breeding or genetic modification first (Gressel 2008).Genetic modification would also be required to improve cellulose biosynthesis and modify lignin content in lignocellulosic crops and straw to reduce the costs of lignin removal in this kind of biomass crops (Gressel 2008).
In this study we were not able to take into account international trade in foodstuffs.Thus, when JPN and SKO buy food, the GE overproduction will diminish in the areas providing that food.For example, Australia is a major wheat exporter and will most probably not start to produce bulk bioenergy crops on additional field area if it can export food profitably.Therefore, the bioenergy potential reported here has to be considered carefully.There is also danger of reduction in agricultural area.In Europe the arable land area is currently (average of years 2000-2005) 15% and the agricultural area, 30% lower than for the long-term average of 1977 to 1999 (FAO 2009).Some of this loss is attributable to urbanisation, but some results from yield improvement, technology development and reduced need for food production.Problems with land degradation can also occur.E.g. in Australia the agricultural area is decreasing because of drought and salinisation, but so far Australia has been able to keep the arable area constant (FAO 2009), probably with higher investments in technology.

Conclusions
According to our results total food production in the world should be just sufficient to provide a healthy diet for the entire population, both now and in the future, even considering only arable farming (not permanent grassland).If food were distributed evenly, however, no field area would be available for bioenergy crop production.Improvement of crop production technology and breeding for higher yields and better quality would increase the area freed from food production and improve the efficiency of energy production in these set aside fields substantially.Crop residues will always be a potential biomass energy source, but the extent of their sustainable use requires more information and studies that take local climate conditions into account.

Table 1 .
Production (Tg or million tonnes) and theoretical and technical crop residue energy yield potential (EJ year -1 ) for different crop types and different areas in 2006.The values for AUS are corrected to 10 year averages, as in year 2006 yields were exceptionally low in that area.

Table 2 .
Production (Tg or million tonnes) and technical crop residue energy yield potential (EJ year-1

Table 3 .
Energy crops and their energy values (GJ ha -1 ) for areas where arable land is likely to become available for biomass production for energy.At present: variation in average production in 1997-2006.In 2050, variation is assumed to be relatively the same as at present.

Table 4 .
Regional and global food production (excluding grassland and pasturing animals) in GE (grain equivalent, kg) year -1 A) at the moment (average of[1997][1998][1999][2000][2001][2002][2003][2004][2005][2006]and B) in 2050, the adequacy of the production for different diets, available field area for bioenergy production and the resulting bioenergy potential in EJ per unit area and year.Areas without a figure are not able to produce bioenergy crops.All available arable area is assumed to be used in both 1997-2006 and 2050.Bioenergy crops are assumed to be produced on the set aside arable land at the same intensity as crops.veget.=vegetarian diet, moder.=moderate diet, affl.= affluent diet.