D.O. Hall & J.I. House
Division of Life Sciences, King's College London,
Campden Hill Rd, London W8 7AH, UK
Studies on climate change and energy production increasingly recognise the crucial role of biological systems. Carbon sinks in forests (above and below ground), CO2 emissions from deforestation, planting trees for carbon storage, and biomass as a substitute for fossil fuels are some of the key issues which arise. Halting deforestation is of paramount importance, but there is also great potential for reforestation of degraded lands, agroforestry and improved forest management. We conclude that biomass energy plantations and other types of energy cropping could be a more effective strategy for carbon mitigation than simply growing trees as a carbon store. Using the biomass for production of modern energy carriers such as electricity, and liquid and gaseous fuels also has a wide range of other environmental, social and economic benefits. In order for biomass projects to succeed, it is necessary to ensure that these benefits are felt locally as well as nationally, furthermore, environmental sustainability of bioenergy projects is an essential requirement. The constraints to achieving environmentally-acceptable biomass production are not insurmountable. Rather they should be seen as scientific and entrepreneurial opportunities which will yield numerous advantages at local, national and international levels in the long term.
The International Climate Convention has been signed by over 150 nations, however, it does not as yet make any binding commitments or provide any realistic solution to reducing carbon dioxide. In 1990, CO2 was responsible for an estimated 60% of the enhanced global warming effect, and this percentage is likely to increase. Emissions of CO2 in 1989/90 were 6.0 GtC from fossil fuels and 1.6 (±1.0) GtC from deforestation1. There is currently a net increase in atmospheric CO2 equivalent to 3.8 GtC/yr. The United Nations Intergovernmental Panel on Climate Change (IPCC) has concluded that 60 to 80 % cuts in CO2 and other greenhouse gas emissions are needed in order to stabilise the world's climate. Since the demand for energy will continue to rise, and there is no constraint in the near-term of fossil fuel supplies, it is necessary to find an increased sink for the increasing carbon emissions2,3.
Plants, from minute algae through to large forests, absorb carbon and thereby act as a carbon sink for varying periods of time. The biomass they produce can also be used as a renewable energy source, burning it directly, or converting it into liquid or gaseous fuels. Therefore protecting forests, promoting tree planting and substitution of fossil fuels with renewable forms of energy, and energy efficiency could have a large impact on atmospheric CO2 levels.
This mini-review examines the options for mitigating atmospheric CO2 increases using biomass in an environmentally acceptable manner, either as a carbon store, or as a fossil fuel replacement. It discusses the perceived problems of land availability and conflicts with food production, environmental impacts, social considerations and economic and political barriers, none of which are found to be insurmountable. It finds that there is a large potential for reducing atmospheric CO2 using biomass, particularly as an energy source, and that this compares well with many other options in terms of economics and social benefits.
Trees can store large amounts of carbon in their standing biomass. Most assessments of forestry potential for carbon storage have only considered plantations. However, other options for maintaining and increasing biomass may be more practical and cost less, often have a wider range of benefits, and could be more socially and economically acceptable. Preventing deforestation is the most important forest strategy, yet this cannot happen without substantial changes in policy, reduction in population growth, and improvements in agricultural productivity and sustainability. There is also considerable potential for improving the health and productivity of existing forests by managing them better, coppicing, for example, can help maintain a higher growth rate, and if the wood is used for long-lived building products this constitutes an additional carbon sink. Regeneration and rehabilitation of degraded lands by planting trees (or other biomass crops) is low cost, relatively easy to implement and can protect environmentally sensitive areas unsuitable for intensive plantations.
Agroforestry can increase and stabilise agricultural yields and reduce soil erosion4. The biomass can provide fuelwood, foods, fodder, basic construction materials, shade, medicines, etc, and thereby decreases pressure on natural forests. Furthermore, it may allow land to be taken out of fallow rotation in shifting cultivation systems; for example, one hectare of land sustainably managed with agroforestry could replace 5-10 ha of land under shifting rotation slash and burn5. Urban and community forestry can provide local biomass and help the public to recognise the usefulness of tree planting. In the U.S. and Europe alone, 50 MtC/yr could be absorbed by urban tree planting6. Indirect effects, such as substitution for fossil fuels, could prevent the release of 17 MtC/yr worldwide7.
Plantations can produce large numbers of desired tree species at rapid growth rates under uniform management practices. Unfortunately, many reforestation programmes in the past have been unsuccessful for a variety of reasons7,8. Some successful plantations, including those in Brazil and Ethiopia, incorporated extensive prior research, site-specific projects, thorough site preparation, regular maintenance and management, environmental sustainability, local involvement and benefits, and government commitment7,8,9. The World Bank has estimated that the world's plantations cover 90 Mha, less than 5% of total forest area, and the annual rate of planting is about 7 Mha10.
It has been proposed that industrialised countries could offset some of their carbon emissions to meet their national targets by financing forestry schemes in developing countries, where higher biomass productivities can be achieved at lower cost. Several US power companies have already begun such projects11, and the FACE (Forests Absorbing Carbondioxide Emission) Foundation in the Netherlands is looking at the cost effectiveness of fixing CO2 by planting new forests of various types12. While promoting the planting of new forests in tropical countries may be a good idea in principal, there are many practical and equity reasons why it will be difficult to implement long-term carbon sequestration programmes in developing countries. As Grace Akumu of the Climate Network Africa so clearly points out, this enables industrialised countries to "exploit low-cost emission reduction opportunities in the South" while "evading the necessary changes in production systems and lifestyles at home"13. The question arises as to whether carbon reduction targets both nationally and globally can be equitably achieved by trading internationally in CO2 sources and sinks14.
While deforestation is still outstripping reforestation in the tropics by a large amount, several studies show forest cover is on the increase in temperate zones, and that this is creating a major carbon sink. Sedjo15 and Dixon et al.16 estimated all mid- and high-latitude forests were sequestering 0.7 GtC/yr. Meanwhile the Tropical Forest Action Plan (TFAP) and other programmes aim to increase forest cover in the tropics. We believe that the future role of forests in mitigating global warming could be much greater if they are used to provide an energy crop which can substitute for fossil fuels.
Biomass can be burnt directly or it can be converted into solid, gaseous and liquid fuels using conversion technologies such as fermentation to produce alcohols, bacterial digestion to produce biogas, and gasification to produce a natural gas substitute. Burning plant biomass as a fuel source does not result in net carbon emissions since the biofuels will only release the amount of carbon they have absorbed during growth (providing production and harvesting is sustainable). If these biofuels are used instead of fossil fuels, carbon emissions from the displaced fossil fuels are avoided as well as other associated pollutants such as sulphur. The development of large-scale energy production from biomass will rely on specifically-grown energy crops. Nevertheless residues (from forestry, crops and dung) are invaluable as an immediate and relatively cheap energy resource. Wood can also be removed sustainably from existing secondary forests and plantations.
Biomass is already the fourth largest source of energy in the world supplying about 13% (55 EJ/yr; 25 million barrels of oil equivalent) of 1990 primary energy (Figure 1). It is also considered one of the main renewable energy resources of the future due to its large potential, economic viability and various social and environmental benefits. Johansson et al.17 estimated that by 2050 biomass could provide nearly 38% of the world's direct fuel use and 17% of the world's electricity.
Figure 1.World primary energy consumption 1990 (Source: Hall and House, in preparation, based on Scurlock and Hall, 1989).
Developing countries are the biggest users of biomass for energy where it is the number one energy source providing 33% of all energy. In some countries it provides over 90% of the energy used in the form of traditional fuels eg. fuelwood, residues and dung. As populations continue to rise, so will demand for biomass fuels. Since 90% of the population will probably reside in developing countries in 2050, it is important to address their energy needs.
The traditional uses of biomass in developing countries - burning wood, agricultural residues and dung - is usually very inefficient (typically only 5-15% of the energy is actually utilised) and can give rise to harmful indoor air pollutants, while the energy delivered is often less convenient to use compared to electricity, gas and kerosene. However, biomass is a virtually "free" resource costing only labour to collect it. In all countries there is a type of "energy ladder" connecting energy sources to income. The poorest people use "free" but less convenient biomass sources, then as incomes rise (or urban migration occurs) they purchase charcoal, kerosene, LPG (liquefied petroleum gas), and eventually electricity. Nevertheless, poverty will always exist and hence there will be a continuing reliance on traditional biomass; poor people tend to use biomass resources sustainably for their own use, so it is important to ensure that people have this chance by not removing their land rights and other resources. Nonetheless, it is inevitable and desirable that the incomes of many will rise, and this will lead to a shift up the energy ladder. If developing countries move towards using more coal and other fossil fuels, this will have serious effects on the global atmosphere. Similarly, charcoal is generally a very inefficient means of using biomass for households, increasing per capita wood use and contributing to deforestation. This is the real energy crisis of the future, how to encourage development while protecting the environment18,19.
If biomass is produced more efficiently and used with modern conversion technologies, it can supply a considerable range and diversity of fuels at small and large scales. Much more useful energy could be extracted from biomass than at present and this could allow a break in the "energy ladder" and some relief to the energy problem. Modernised bioenergy systems can then form part of a matrix of fuel sources offering increased flexibility of fuel supply and energy security. Local and efficient production of modern biomass fuels will enable communities to remain self-sufficient in energy while promoting development. National projects could reduce dependency on oil imports, already badly affecting the economies of many countries, and will keep expenditure within the local economy. This could enable the release of land previously needed to grow crops to earn foreign currency, and the land could then be used for plantations, agriculture or returned to secondary forest.
Since bioenergy can be used at small and large scales in a decentralised manner this can bring substantial benefits to rural (and even urban) areas which don't usually have access to modern energy carriers. Furthermore, growing biomass is a labour intensive activity which can create jobs in rural areas whilst providing convenient energy carriers to promote other rural industries, and thereby help to stem urban migration. Job creation will also be of importance in industrialised countries, and growing biomass could provide an economically viable use for the agricultural land being taken out of production in "set-aside" schemes in Europe and North America.
In Brazil, almost 5 million vehicles run on pure bioethanol produced from sugarcane and many more run on a gasoline/ethanol blend. In 1989 over 12 billion litres of ethanol replaced about 250,000 barrels of imported oil a day, and between 1976 to 1987 the total savings equivalent in gasoline that did not have to be imported was $ 12.48, while total investment in the programme was $ 6.97. Costs were falling at 4% per annum between 1979 and 1988, and could fall much further if sugar cane residues were fully utilised. 700,000 direct jobs have been created along with perhaps 3 to 4 times this number of indirect jobs20,21. In Zimbabwe production of 40 million litres of ethanol has been possible since 1983 (apart from during the 1991/2 drought) enabling the country to begin to tackle it's energy problem (it is landlocked making oil imports difficult) while fostering the agro-industrial base22.
Biogas production (from the anaerobic digestion of organic wastes in special digesters) is relatively simple and can operate at small and large scales practically anywhere, with the producer gas as versatile as natural gas. In India over a million biogas plants of various capacities had reportedly been installed by August 198823. China's rural population of 900 million relies on biomass for about 54% of energy requirements (about 282 Mt of coal equivalent)24, improved efficiency of biomass production and use thus has become imperative25. China has about five million household digesters in working order, used by about 25 million people mainly for cooking and lighting; 10,000 large and medium size biogas digesters are operating in food factories, wineries, livestock farms, etc; and large enterprises transfer biogas to centralised supply stations, biogas motive power stations, or biogas electric power stations (there are 822 such stations with a total of 7,836 kW)26.
It is not only developing countries that can benefit from modern bioenergy use, a number of industrialised countries already use biomass quite substantially. For example the USA derives 4% of its total energy from biomass (7.5 Mtoe, 3.2 EJ, nearly as much as from nuclear power) with a biomass electric generating capacity of about 9,000 MWe17. Finland obtains 18%, of its primary energy from waste liquors from pulping, peat, firewood, wood waste and municipal refuse. Government support is strong with 10% of the total financing of energy research (US $ 2.5 million) going to bioenergy27. Sweden obtains 16% of its energy from biomass, and actually imported some biofuels, indicating the potential for development of international trade28. Austria obtains 10% of its energy from wood, mostly for space heating from 11,000 biomass heating systems with a combined capacity in excess of 1200 MW. Since they are dependant on import for over 80% of their fossil fuels, development of a bioenergy industry will be of great importance29. In Denmark, more than 50 straw-fired district heating plants totalling 170 MW are competitive with coal- and oil-fired heating plants30. All these countries plan to significantly increase bioenergy production.
The most likely technology to be used to convert biomass to electricity in the near- to medium-term is biomass integrated gasifier/gas turbine cycles (BIG/GT), which will be more efficient than conventional coal steam-electric power generation and coal gasification, and will have lower capital costs17. Elliott31 concludes that the emerging technology of gasifier/turbines can give electricity at about 5c/kWh and that the investment costs could be as low as $ 1300/kw after the 10th plant was installed. Presently two such plants are coming on stream in Sweden and Finland (one of 6 MWe + 9MWth & the other 15 MW, at a cost of $ 45 million each) (FT, 1993) and a 30MWe plant is planned for Brazil32. The EU plans to shortly construct an 8MWe plant using short rotation forestry as the feedstock.
Over the last 2-3 years a number of global energy scenarios have been published which include substantial roles for energy efficiency and renewable energies, while some have incorporated large roles for bioenergy.
The renewables - intensive global energy scenario (RIGES) prepared by Johansson et al.17 as part of the UNCED Rio de Jeneiro Conference in 1992 proposes a significant role for biomass in the next century. They propose that by 2050 that "renewable sources of energy could account for three-fifths of the world's electricity market and two-fifths of the market for fuels used directly" and that "global CO2 scenarios would be reduced to 75% of their 1985 levels....and such benefits could be achieved at no additional cost". Within this scenario biomass should provide 38% of the direct fuel and 17% of the electricity use in the world. Detailed regional analyses shows how later America and Africa might become large exporters of biofuels.
The Environmentally Compatible Energy Scenario developed by IIASA for 202033 "assumes that past trends of technological and economic structural change will continue to prevail in the future (and thereby) serve to some extent economic and environmental objectives at the same time". Primary energy supply is predicted to be 12.7 Gtoe (533 EJ) of which biomass energy would contribute 11.6% (62 EJ) derived from wastes and residues, energy plantations and crops, and forests - this excludes traditional uses of non-commercial biomass energy for fuelwood in developing countries.
A Fossil-Free Energy Scenario (FFES) was developed by Lazarus et al.34 of the Stockholm Environment Institutes as part of Greenpeace International's study of global energy warming. They forecast that in 2030 biomass could supply 24% (91 EJ) of primary energy supply (total 384 EJ) compared to today's low of only 7% (22 EJ) out of a total of 338 EJ. The biomass supply could be derived equally from developing and industrialized countries. The study indicates that most regions of the world except for Eastern Europe, Middle East, Japan/Australia/New Zealand could be major suppliers of biofuels - these 3 regions could still however each supply 2.6 - 4.3 EJ annually from biomass.
The World Energy Council examined four "Cases" for global energy supply to 2020 spanning energy demand from a "low" (ecologically driven) of 475 EJ to a "very high" of 722 EJ with a "reference" case total of 563 EJ3,35. In the ecologically driven case traditional biomass could contribute about 9% of total supply while modern biomass would supply 5% of the total equal to 24 EJ or 561 Mtoe. New renewables (modern biomass, solar and wind etc) could supply 12% of the total. In the high growth case these contributions could be 8% and 5%, respectively, of a higher total supply.
The International Energy Agency examined world total primary energy demand over the next 15 years and estimated a need for 486 EJ by 2010 compared to 330 EJ today35. Most of the increased demand is predicted to arise in non-OECD countries. Within the IEA analyses biomass is part of "coal and other solid fuels" where worldwide demand is predicted to rise by 2.1% per annum. What fraction of solid fuels is comprised of biomass is not easy to ascertain but "renewable energy such as waste" in N. America is "thought to have a capacity for such fuels for up to 9.7 GW which is likely to grow over the outlook period".
What is evident from examining these five scenario studies is that biomass could be a major contributor to future supplies especially as a modern fuel - while still playing an important role as a traditional fuel mainly in developing countries. How much bioenergy will contribute in the next century will depend on many factors which are implicit in the various scenarios proposed in each study.
The possibility of using biomass as an energy crop raises the issue of which is the best option to pursue: growing trees as a carbon store or using the biomass produced to offset fossil fuel emissions. There are many factors to consider, and ultimately the best method will depend on local circumstances. For example, where natural forest already exists, the most appropriate method is protection and/or improved management. In agricultural and urban areas, agroforestry and urban tree planting are likely to be the preferred options. But what of the areas available for plantation establishment?
Firstly, it is sometimes argued that more land would be needed to grow sufficient biomass to displace fossil fuels than would be needed to absorb and store (sequester) all the carbon produced. However, carbon sequestration by trees usually ceases at forest maturity whereas periodic harvesting of biomass for energy allows land to be used (and therefore fossil fuels to be replaced) indefinitely36,37. Furthermore, since short rotation energy crops have relatively high growth rates and higher yields compared to conventional forestry, they could be replacing fossil fuels before conventional tree plantations would be large enough to make a significant impact6,38. Energy crops also have relatively short rotations enabling them to adapt more rapidly to environmental changes.
Costs are an important consideration and while the production of bioenergy is more costly than simply growing trees as standing stock, there are revenues from the sale of the energy produced which can offset the costs of plantation establishment. Since biomass-derived electricity and liquid fuels can be produced competitively in certain circumstances, the net cost of offsetting CO2 emissions by substitution could be near zero or negative36. In addition to this economic advantage, there are also the numerous environmental and social benefits associated with bioenergy provision that are mentioned elsewhere in this paper.
Marland & Marland39 compared the two strategies of carbon storage or fossil fuel substitution by modelling carbon flows. They found that, depending on the assumptions made, growing trees for carbon storage may be more appropriate on low productivity land (or indeed where the biomass cannot be practically harvested). However, they conclude that "where high productivity can be expected, the most effective strategy is to manage the forest for a harvestable crop and to use the harvest for maximum efficiency either for long-lived products or to substitute for fossil fuels".
Carbon-sequestration strategies will be important where the creation of new forest reserves is deemed desirable for environmental or economic reasons. Using biomass to substitute for fossil fuels is likely to be a more advantageous and appropriate method for reducing atmospheric CO2 levels using available land, as long as environmental sustainability is ensured9. Nevertheless, a combination of carbon storage and fossil fuel substitution may be realistic in circumstances where harvesting the biomass at frequent intervals is difficult or financially unrewarding.
Despite the numerous benefits, there are various concerns regarding the production of biomass, and particularly its use as a fuel source. In common with the establishment of any large-scale crop production are the problems of environmental impact, the issues of land availability and the possible conflicts with food production. There are also worries about costs and social impacts. These problems are discussed in more detail under separate sub-headings. One point often raised is that of the energy ratio (how much energy must be put in to obtain a given amount of energy out) but these are more favourable than most critics imagine. Table 140 shows energy balances as well as CO2 balances and costs of different biofuel options. This study by the OECD and others show that energy aand carbon ratios for solid biofuels are better than those for liquid biofuels produced as gasoline or diesel substitutes; production of electricity from wood produces 23 times the amount of energy that is used for planting, harvesting, transport, conversion, etc40. Improvements in selection and management of plants for higher yields, the development of modern energy conversion technologies, and the use of biomass residues for energy requirements during production will continue to improve ratios. The biggest problem facing bioenergy is market penetration which will require overcoming peoples' preconceived ideas of bioenergy in government, the energy market and the general public.
Table 1.Biofuels for transport: summary of energy, CO2 balances and costs of best case scenarios (ie. use of byproducts, efficient technology and high yields).
Ethanol Ethanol Rape Methanol Electricity from from methyl from wood wheat beet ester from wood Net energy yield (GJ/ha) 58.4 200.6 48.9 158.1 209.9 Net energy ratio-energy input 4.74 12.21 4.98 13.33 22.85 output % gasoline or diesel 52% 38% 43% 22% 8% life-cycle energy use Gasoline or diesel 1,695 3,961 885 4,909 - substitution (l/ha) Net CO2 abatement (kg/ha) 4,072 13,628 3,722 10,661 9,480 % of gasoline/diesel 40% 28% 31% 18% 9% life-cycle CO2 emissions Cost of gasoline or diesel replaced 89-103 92-100 98-136 33-39 6.4-9,5 - EU feedstock prices 54-62 64-72 50-59 33-39 c/kWh - World feedstock prices (USA) Cost ratio to gasoline or 5.1-5.9 5.3-5.7 5.6-7.8 3.4-4.2 1.3-1.9 diesel (EU) Net cost (subsidy) required 1207-144 2960-328 710-1052 1068-1436 142-447 (annual $/t) 4 4 Cost of CO2 abated ($/t) 424-507 374-415 304-450 186-250 27-86 Net energy yield if 20% of OECD Europe cropland set 1.4 4.8 1.2 3.8 5.0 aside and used in 2050 (EJ)
Source: OECD, Biofuels, OECD/IEA, Paris, 1994.
Transport fuel use in 1990 = 11.7 EJ & predicted in 2050 = 16.6 EJ (see Table 2 also for electricity.
Many studies have been carried out on land availability and they give very wide ranging results depending on sources of data and assumptions used. There are large areas of degraded and abandoned lands in the tropics which could benefit greatly from the establishment of biomass plantations. A summary of several estimates for land availability in developing countries is presented in Table 2. Most of the estimates for the tropics are based on degraded land areas, but also include surplus land after accounting for food production, and fallow land. They are generally in the range of 500 Mha availability.
Table 2.Estimates of potential land available for reforestation.
Reference Total available Comments area (Mha) Grainger (1988), 758 2007 Mha of degraded land in the 54 tropics of which areas with high priority for forest replenishment are: 137 Mha of logged forest that could be managed for natural vegetation, 203 Mha of fallow forests, 87 Mha of deforested watersheds and 331 Mha of desertified drylands that could be managed for plantations. These estimates are from crude satellite images, and the author acknowledges that some of this land is probable in use. Myers (1989), 69 300 200 Mha "needs reforestation for reasons other than the greenhouse effect. 160 Mha of the above 200 Mha is upland watersheds which urgently needs reforestation, the rest is required as woodlots. Houghton (1990), 865 Land deforested and now unused in Asia 70 (100 Mha), Latin America (100 Mha), and Africa (300 Mha). The other 365 Mha is equivalent to 95% of the land in shifting cultivation which "could be returned to forest if permanent agriculture were to replace shifting cultivation". Dixon et.al. Tropical - This is just what is "technically (1991), 71 620-2000 available" ie. it does not take account Temperate - 605 of the constraints. Boreal - 426 Alpert et.al. 952 Estimated as the total area available (1992), 64 for halophyte culture. 125 Mha of this is assumed to be feasible due to restrictions for saline irrigation. Bekkering 553 Theoretical land available for (1992), 66 reforestation in 11 out of 117 tropical countries analysed which had over 10 Mha of surplus land once future crop requirements are fulfilled. 385 Mha of land is available after accounting for climate, and a further 168 Mha could be available from fallow land. Some of this land may be in other uses eg. permanent pasture. Nakicenovic 265 A further 84.5 Mha is considered et.al. (1993), available for agroforestry. Takes 33 account of actual availability. Trexler (1993), 67 This what the author feels could be 5 realistically converted over the next 60 years out of 70 Mha for which it would make "economic sense" to convert to plantations. More than 200 Mha (out of 300 Mha possible) could realistically be regenerated and 63 Mha are available for agroforestry. This covers 50 tropical countries in detail and takes into account future trends, policy, infrastructure, and other constraints.
Large areas of surplus agricultural land in North America and Europe could become significant biomass producing areas. In the USA, farmers are paid not to farm about 10% of their land, and in the EC, 15% of arable farmland will be "set-aside" under present schemes41. Apart from over 30 Mha of cropland already set aside in the USA to reduce production or conserve land, another 43 Mha of croplands have erosion rates exceeding the maximum rate consistent with sustainable production and a further 43 Mha have "wetness" problems. In the EC 15-20 Mha could be set aside by 2000 (up to a third of present agricultural land) under the reforms of the Common Agricultural Policy9, and this could reach over 50 Mha in the next century42. According to our calculations43 - assuming that 10% of "usable land" (cropland, permanent pasture, and forestry and woodland) in OECD Europe could be used to produce biomass for energy at productivities of 10 ODt/ha in 2020, and that 25% of potentially harvestable residues are also used, this could provide 17% of present primary energy consumption. By 2050, yields might be expected to have reached 15 ODt/ha, in which case 10% of this land plus residues could provide 30% of the primary energy requirement predicted by Johansson et al.17.
The "food versus fuel" issue is controversial and complex. Food and biomass require the same resources for production ie. land, water and agrochemicals. As we have just shown, on a global scale, there is enough land available to allow biomass to make a significant impact on atmospheric carbon levels without impinging on food production, by using degraded land and surplus agricultural land. Biomass energy crops can also be managed for minimum water and nutrient inputs to a greater extent than food crops, although much more can and should be done along these lines in agricultural production. In any case, food shortages are caused more by distribution problems, a lack of purchasing power, bias towards the production of export crops, use of food and land for livestock, underutilised agricultural production potential, political issues and, in Africa particularly, droughts and war44,45.
It should be remembered that both food and fuel are important requirements that need not compete, particularly when there is careful planning to ensure ecological conservation and sustainability of production methods. Forestry policies and programmes such as agroforestry and integrated farming systems can in fact improve food security by providing food (from the tree directly and from animals in the habitat provided), fodder, energy, and income for food purchase. In Brazil, the area used for ethanol from sugarcane represents less than 0.2% of total land area, crop rotation in sugarcane areas has led to an increase in certain food crops, while some byproducts are used as animal feed21. Planting trees can reduce run-off and erosion, conserve water and rehabilitate land that can then be used once more for food crops. It is now being recognised that intensive agriculture cannot be maintained on fragile lands, but an alternative must be found for farmers to maintain their livelihoods. Nationally, bioenergy crops can be produced in place of cash crops grown to pay for oil or other energy imports (as has happened in Brazil and Zimbabwe). Additionally, improving agricultural practices and yields can help stabilise carbon emissions by protecting forest areas that would otherwise have to be cut down to increase food production.
One of the main issues facing the establishment of forest plantations is that of environmental impact. Large monocultural plantations such as those established in the past have often had negative environmental impacts and this must be avoided at all costs. Yet it is necessary to increase the productivity from around 5 t/ha/yr which is common now without good management. It is now possible in favourable climates, with good soils and management, and planting of appropriate species and clones to obtain 10 to 15 t(dry weight)/ha/yr in temperate areas and 15 to 25 t/ha/yr in tropical countries. Record yields of 40 t/ha/yr have been obtained with Eucalyptus in Brazil. High yields are also feasible with herbaceous (non-woody) crops which can also be used to substitute for fossil fuels. For example, in Brazil, the average yield of sugar cane has risen from 47 to 65 t/ha (harvested weight) over the last 15 years while over 100 t/ha/yr are common in a number of areas such as Hawaii, South Africa and Queensland9.
High yields can be achieved with monocultures of selected clones, but these are often associated with high inputs of fertilisers and water, and may be prone to disease. Moreover, exotic species are often used as they may be more productive than indigenous species, but they are generally less well adapted to local environmental conditions, pests and diseases. The use of clonal strategies could facilitate the incorporation of desirable characteristics, but in general the development of mixed blocks and stands with a variety of indigenous and other species and clones will produce plantations which are more likely to survive46.
In addition, it is beneficial to leave areas set aside for natural species to maintain biodiversity and also harbour natural predators for pests and diseases. In some areas of Brazil it is now common practice to leave 20-30% of the area in a natural or undisturbed state9. In order for these natural patches to be more effective, they should be connected with undisturbed corridors of natural vegetation to enable species migration. In any case, "horizon-to-horizon" monocultures are not necessary (or acceptable) since only a relatively small fraction of the landscape needs to be planted to biomass crops. Bioenergy systems favour medium-scale 20-50 MW power plants which can be modular, as opposed to 500-1000 MW fossil fuel plants. Figure 247 shows that a 20 MW plant (45% efficiency), using biomass produced at 10 ODt/ha, would require 7,000 ha which represents only 10% of the land within a 14 km radius of the generation plant. For a 50 MW plant with plantation yields of 15 ODt/ha & 11,000 ha requirements with a 15% land use, the radius of the planted area would be 16 km. These two examples correspond to the scenarios used in this paper.
Figure 2. Land requirements for biomass power plants (Booth, 1993). Example: 20 MW plant with 10 t (oven dry)/ha yields would require 7,000 ha which for 10% land use will require land within a 14 km radius of the power plant.
Plantation management can optimise nutrient and water conditions avoiding wastage and leaching. The nutrient status can be maintained by recycling wastes such as wood chippings and ash from biomass burning or gasification. Leaves and twigs should be left on the ground after harvesting as most of the nutrients are concentrated in those parts of the tree. Inter-cropping with nitrogen-fixing plants can increase yields while reducing the need for fertilisers.
There are potential negative impacts of plantations, many of which are similar to the problems of intensive agriculture, and foresters must be aware of these so that with careful management these problems can be avoided. Overall, the possible positive environmental effects far outweigh the negative effects, as long as plantations are established on degraded or set-aside land, and natural forests are not cut to make way for plantations. If the biomass is able to replace fossil fuels it will have many more environmental benefits such as reducing emissions of other pollutants and other environmental impacts associated with fossil fuel use. More research is needed to establish sustainable methodologies, appropriate species mixes, and good management practices which are economically viable and acceptable to the public. Incentives and regulations should be established to ensure sustainable practices are followed7,8,46 and guidelines should be encouraged such as those produced by Shell/WWF8.
Social factors can be major constraints. Many case studies show that local involvement, control and multiple benefits (both short and long term) are a prerequisite for success of almost any project, and that a great many projects have failed in the past through not considering this. Programmes which have won the widest acceptance have been those which are more readily integrated into the existing social and economic situations and which do not require radical change5,48. Flexibility and sustainability allows such schemes to catalyse development that is appropriate to the people it is meant to be benefiting. Reducing atmospheric carbon as an end in itself is not enough to ensure the implementation and success of most projects as the benefits are not felt directly enough for most people to feel any commitment, thus projects must also benefit people more directly and these benefits must be felt immediately and be sustainable in the long term. For example, growing multi-product plantations that include trees for energy, fruit, straight poles, etc. will create more local interest and help acceptability of plantations. Project managers must first find out what it is exactly that people want, then find the best way of providing it, consulting and involving local people at every stage in the process. Only then will we see successful projects that can sustainably remove carbon and provide the range of potential benefits we have mentioned.
Economics is always an important consideration, therefore, to encourage tree planting by individuals and private investors it will be necessary to provide grants and economic and social incentives. Care must be taken that this does not promote cutting of natural forests or loss of land rights, and measures should be built in to ensure environmental sustainability. In conjunction with this, there should be a review of current economic incentives that run contrary to sustainable forest strategies such as large financial incentives to landowners to cut forests for timber, grow cash crops, or plant pastures. A major criticism levied against renewable energy in general and biomass energy in particular, is the need for large subsidies. In fact energy from renewable resources generally receives far less subsidies than conventional energy sources17. Furthermore, energy prices mostly ignore the social and environmental costs and risks (including health expenditure and pollution) associated with fossil fuels, and the various benefits of renewable energies. These externalities should be internalised to allow biofuels and other renewables to compete in a "level playing field". Taxes, regulations and other policy instruments can be used9,17.
A recent study by the World Bank49 examined biomass electricity and ethanol costs worldwide on a comparable basis from a total of 114 reports. In the case of electricity it was concluded that "costs... compare well with the costs of fossil-fired generation and even hydro generation in favourable situations; some are as low as 2-4 c/kWh ... biomass gasification combined-cycle technologies show much promise for reducing costs for large-scale power generation...". Ethanol from biomass, with data mostly from the US and Brazil, show that the "decline in costs since the 1970's has been significant and is attributable to technology improvements and a shift toward cheaper crops". "The costs of ethanol were beginning to compare well with gasoline until the collapse of oil prices in the mid-1980's".
Several estimations of costs of various forestry projects are shown in Table 3. Volz50 estimates that plantations in temperate areas will cost around $ 100/tC, Moulton and Richards51 put plantation costs in the US at $ 5-43/tC, and Nacicenovic et al. put global costs at $ 4.4/tC. The US Environmental Protection Agency (EPA)52 estimates that, in the tropics, regeneration would cost $ 0.5-2/tC, agroforestry $ 2-11/tC and reforestation $ 326/tC; in temperate areas costs are $ 0.2-5/tC for afforestation and $ 3-29/tC for reforestation. Yet trees are a marketable resource and the biomass can be sold to pay for the costs of establishing and maintaining forest schemes. The OECD40 (Table 1) calculate the net cost of abatement via electricity production from wood at $ 27-86/tCO2 (to convert to $/tC multiply by 12/44 ie.$ 7-23/tC). The timber market can be stimulated by encouraging wood use instead of cement, concrete, steel, plastics, etc. Alternatively, biomass energy crops can offer greater flexibility of products and economic returns than timber.
Table 3.Estimates of carbon mitigation potential, land requirements and costs.
Source Offset Rate Comments Area Cost Goal carbon (t C/yr) uptake (Mha) (t C/ha/yr) ($/ha) ($/t C) Marland (1988), 72 5 7.5 Tropics 700 9.6 Tropics 500 Marland (1989), 73 1.2 7.5 USA 164 Myers (1989), 74 3 10 Tropics 300 400 Sedjo & Soloman (1990) 2.9 6.2 Tropical or 465 400-1400 53 temperate EPA (1989), 75 0.05 3.5-10 USA 4.5-13 432 Nacicenovic et.al. 1.6 * Global 265 4.4 (1993), 33 Trexler (1991), 76 50 Gt by 2050 ** Tropics 220 Volz (1991), 50 5.5 Temperate 4 Mha/yr 400 (US) 100 (average) 3000 (Europe) Moulton & Richards 20% US 60 $ 4,5 b/yr May-43 (1990), 51 emissions EPA (1991), 52 Tropics: Regeneratio 100-300 0.5-2 n 300-2,500 2-11 Agroforestr 100-4,300 3-26 y Reforestati on 0.2-5 3-29 Teperate: Afforestati on Reforestati on
** Rate depends on forestry option ie. regeneration, agroforestry or plantations.
Fossil fuel emissions are 6 GtC/yr (gigatons of carbon) globally.
Various estimates of carbon mitigation potentials are shown in Table 3. Sedjo & Soloman53 estimate 465 Mha of new forest is necessary at a stemwood growth rate of 15 m3/ha/yr to offset the annual net increase in CO2. Grainger54 considers this figure is more likely to be 600 Mha (a land area equivalent to 75% of Australia). He suggests that a reforestation rate of 3 Mha/yr is all that is feasible in the short term and this would cut the CO2 increment by 25% by 2020, however, if this was combined with a reduction in deforestation, then the increment could be close to zero by 2020. Cannell55 concludes that 500 Mha would need to be planted with a sequestration rate of 6 tC/ha/yr (12 oven dry t/ha/yr biomass) in order to absorb the 6 GtC emitted annually from fossil fuels and cement manufacture. However, he feels it is likely that only 50 Mha could be planted which would sequester 5 to 10% of the amount of carbon accumulating in the atmosphere. Trexler5 estimates that by 2050, forest measures could realistically be implemented on 220 Mha, only 42 Mha of which would be under large-scale plantations (a rate of establishment of just 0.7 Mha/yr), but that these efforts will help to slow deforestation, giving a net impact of 50 GtC by 2050.
In temperate zones, Volz50 indicates that an annual afforestation rate of about 4 Mha might be feasible, taking up about 22 MtC/yr. Kulp6 estimates that the potential carbon sequestration in the USA is 0.05 GtC/yr (on 10% agricultural land), the same in Western Europe, 0.3 GtC/yr in China, and 0.02 GtC/yr in other temperate countries making a total of 0.42 GtC/yr. Moulton & Richards51 estimate that the USA could offset 20% of its emissions using 60 Mha at a cost of $ 4.5 billion/year. Studies at the US Oak Ridge National Laboratories show that Short Rotation Woody Crops (SRWCs) grown on 103 Mha could offset 80% of the total CO2 released from US electricity production56. Trexler estimates that within 20-30 years biofuels could be displacing 12-25% of current (1991) U.S. energy consumption57.
WRI58 suggest that a global strategy based on halving tropical deforestation and planting the equivalent of 130 Mha of trees in developing countries, and 40 Mha in industrial countries, could reduce worldwide carbon emissions by about one quarter of current levels. The IPCC59 Response Strategies Working Group estimate that if the most aggressive biological mitigation strategies are implemented, net emissions could be zero by 2000, and there could be a net sink of 0.5 GtC/per year.
Forestry options should not be considered as an alternative to energy efficiency, to other alternative energy sources or to emission-reducing technologies. There is no single option in order to reduce carbon by the amounts required on a long term, sustainable basis; such an aim will require a mixed strategy of measures. Spencer60 compared the costs and potential carbon mitigation of several options up to 2050. He found that efficiency improvements were the best option but that "a major international focus on increasing both terrestrial and marine phytomass is necessary to achieve global stability in CO2 emissions.....This is the only approach for achieving long-term stability in the atmospheric carbon burden".
Rubin et al.61 have compared a variety of options in the US. Efficiency could save 890 Mt CO2-equivalent annually at a net cost of minus $ 84/t C02-equivalent in the domestic sector, and 527 MtCO2 at a cost of minus $ 43/tCO2 in the industrial sector. Advanced coal technology could save 200 MtCO2 at a cost of $ 280/tCO2; natural gas substitution could save 850 MtCO2 at $ 32/tCO2; nuclear power could offset 1500 MtCO2 at $ 49/tCO2; and solar photovoltaic electricity could offset 400 MtCO2 at $ 87/tCO2. Generation of electricity from biomass is the cheapest alternative electric supply technology after natural gas and could save 130 MtCO2 at a cost of $ 36/tCO2. Reforestation could sequester 242 MtCO2 (about 5% of current US CO2 emissions) at a very low cost of $ 7/tCO2 (using 30 Mha of economically marginal cropland, pasture land and non-federal forest, about 3% of US land areas). The direct cost of providing economic incentives to practice sustainable forestry in developing countries would only be $0.4/tCO2 sequestered.
Future biological methods for atmospheric CO2 reduction include laboratory based research on photobiology and photobiochemistry. Photobiology (efficient microalgal growth in photobioreactors and tanks/ponds to remove CO2 from flue gases) can provide a source of energy, chemicals and food, while wastes can be converted or recycled into useful byproducts (eg. fertilizer). Photobiochemical systems use the CO2 fixing enzyme Rubisco to store energy via organic compounds62,63. These photobiological systems have a considerably higher photosynthetic efficiency than conventional biomass systems, and do not require high quality land or water so would not compete with agriculture and forestry. Assuming, for example, a 5% conversion efficiency, and average solar energy in the Mediterranean region to be about 200 W/m2 or 6.3 GJ/m2/yr, 18 Mha of land would theoretically be required to fulfil all Western Europe's energy requirements37.
Of the major alternatives to reduce atmospheric CO2 levels, biomass options are among the most promising and environmentally acceptable61,64. However, they are only part of an overall strategy that should include energy efficiency, alternative fuels, and emission control technologies. On available areas of land, it is generally more desirable to grow trees or other types of biomass for energy production rather than just planting trees as a carbon store, providing the biomass is produced in a sustainable manner. Such a strategy can make a large impact on atmospheric carbon levels and provide many ancillary benefits, such as restoration of degraded lands, energy security, foreign exchange savings, job creation and provision of electricity and other forms of energy to rural areas, thereby helping to promote development65. There is no evidence to suggest that biomass production will conflict with food production, and with agroforestry and integrated farming systems there is evidence that it could in fact enhance agricultural output.
Since the bioenergy can provide an income, it is a way of paying for CO2 mitigation and land restoration. To derive maximum benefits, bioenergy production and use must be modernised. Developing countries have been particularly instrumental in pushing modernisation of biomass for energy as they are so heavily dependant on it, and likely to remain so. Both developing and industrialised countries are now realising they can benefit environmentally and economically.
There are large areas of degraded land available for revegetation in the tropics as well as set-aside land in industrial countries. However, if large-scale programmes are to be successful there must be policy reforms such as to encourage environmental sustainability (including the establishment of environmental guidelines), increased productivities, improved infrastructure and planning, ongoing research and long term monitoring and a large financial commitment made at an international level. Almost all problems can be avoided with careful planning and management. A balanced system of taxes and subsidies needs to be put in place to allow biomass to compete fairly in the energy market, and perceptions of bioenergy must be improved. In particular, success will require that there must be local involvement, multiple benefits, sustainability and flexibility. As Bekkering66 concludes that "it is not realistic to boost tropical forestry for the sake of sequestering carbon dioxide alone. Rather, tropical forestry should focus on other more direct benefits whereas the fixation of carbon should be seen as a positive side effect". In this way, the various constraints may be overcome and entrepreneurial opportunities created9,36,67.
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