Pergamon

Renewable Energy, Vol.5. Part I. pp. 58-66. 1994 Elsevier Science Lid

Printed in Great Britain 0960-1481/94 $7.00+0.00

B I O M A S S E N E R G Y A N D T H E G L O B A L C A R B O N B A L A N C E

D.O. Hall & J.I. House

Division of Life Sciences, King's College London, Campden Hill Rd, London W8 7AH, UK

Paper prepared for: The World Renewable Energy Congress

University of Reading, UK, 1994

ABSTRACT

Studies on climate change and energy production increasingly recognisc the crucial role of biological systems. Carbon sinks in forests (above and below ground), CO 2 emissions from deforestation, planting trees for carbon storage, and biomass as a substitute for fossil fucis 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 biomass energy plantations and other types of energy cropping could he a more effective strategy for carbon mitigation than simply growing trees as a carbon store, particularly on higher productivity lands. Use of the biomass produced as an energy source has the added advantage of a wi~le range of other environmental, social and economic benefits. The constraints to achieving environmentally- acceptable biomass production are not insurmountable. Rather they should he seen as scientific and entrepreneurial opportunities which will yield numerous advantages in the long term.

KEYWORDS

Forest strategies, CO2, land availability, environmental snstainability, biomass energy, fossil fuel substitution.

INTRODUCTION

The prospect of global warming is now heing taken seriously by sciantists and politicians alike, The International Climate Convention has been signed by over 150 nations, however, it does not make any real commitments or provide any realistic solution to reducing carbon dioxide. In 1990, CO 2 was responsible for an estimated 60% of the enhanced global warming effect, and this percentage is likely to increase. There is currently a net increase in atmospheric CO z equivalent to 3.8 GtC/yr. Emissions of CO 2 in 1989/90 were 6.0 GtC from fossil fuels and 1.6 (__.1.0) GtC from deforestation 1. The United Nations Intergovemmantal Panel on Climate Change (IPCC) have concluded that 60 to 80 % cuts in CO 2 and other greenhouse gas emissions arc 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 emissions. Plants, from minute algae through to large forests, absorb carbon acting as a sink. The biomass they produce can also he used as a renewable energy source. 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 CO 2 levels. This mini-review examines the options for mitigating atmospheric CO 2 increase using biomass in an environmentally acceptable manner.

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. Sedjo 2 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 helieve that the future role of forests in mitigating global warming could he much greater if they are used to provide an energy crop which can substitute for fossil fuels.

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59

FOREST OPTIONS FOR MITIGATING THE GREENHOUSE EFFECT

Most assessments of forestry potential only consider 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 imprnvements in agricultural productivity and sustainability. There is also considerable potential for improving the health and productivity of emlsfing foresls 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 coustitutes an additional carbon sink. Regeneration and rehabilitation of degrmled l u d s by planting trees (or other hiornass crops) is low cost, relatively easy to implement and can protect environmentally sensitive areas unsuitable for intensive plantations.

Agroforestr y can increase and stabilise agricultural yields and reduce soil erosion 3. The biomass can provide fuelwood, foods, fodder, basic construction materials, shade, medicines, etc, and thereby decreases pressure on natural forests. Furthermore, land may 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 burn 4. 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 planting s. Indirect effects, such as substitution for fossil fuels, could prevent the release of 17 MtC/yr worldwide 4.

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 reasons 6'7. 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, and government commitment) 6'zs. In 1990 plantations were estimated to cover 130 Mha (3.7% of the world's natural forested area), 9 and average plantation establishment in the 1980s was 2.2 Mha/yr in the tropics and 0.5 Mha in temperate regions 1°.

BIOMASS ENERGY AS A 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 E,l/yr; 25 million barrels of oil equivalent) of 1990 primary energy. 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, xl estimated that by 2050 biomass could provide nearly 38% of the world's direct fuel use and 17% of the world's electricity. Currently developing countries as a whole derive 38% of their energy from biomass, which therefore constitutes their number one energy source. In some countries it provides over 90% of the energy used in the form of traditional fuels eg. fuelwood, residues and dung, and demand is likely to rise with increasing population, although per capita biomass consumption may decline. A number of developed countries also use biomass quite substantially; for example the USA derives 4% of its total energy from biomass (equivalent to 1.5 million barrels of oil a day, nearly as much as from nuclear power), Finland derives 18%, Sweden 16%, and Austria 10%. These countries plan to significantly increase bioenergy production.

The traditional uses of biomass in developing countries - burning wood, agricultural residues and dung -can be associated with the increasing scarcity of hand gathered wood, nutrient depletion and the problems of deforestation and desertification. Burning is usually very inefficient (typically only 5-15% of the energy is actually utilised) and can give rise to harmful air pollutants, while the energy delivered is often less convenient to use compared to electricity, gas, kerosene, etc. For these reasons, biomass is generally and wrongly regarded as a low status fuel, and rarely finds its way into the energy statistics when, in fact, it should be considered as a renewable equivalent to fossil fuels.

If biomass is produced more efficiently and used with modern conversion technologies, it can offer considerable flexibility of fuel supply due to the range and diversity of fuels which can be produced. Furthermore, much more useful energy could be extracted from biomass than at present. Modernised bioencrgy systems can then form part of a matrix of fuel sources offering increased

60

flexibility of fuel supply and energy security. This in turn 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 cash crops to earn foreign currency for oil purchases. The land could then be used for plantations, agriculture or returned to secondary forest.

Since binenergy can be used at small and large scales in a deeentralised 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. 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 costs lz.

Despite the numerous benefits, there are various concerns regarding the use of bioenergy that must be addressed. In common with the establishment of any plantations are the problems of environmental impacts of any large-scale crop production, the issues of land availability and the possible conflict with food production. As discussed later, these problems can be avoided with careful planning and management. Other major worries include the economic viability and the energy ratio (how much energy must be put in to obtain a given amount of energy out). Many bioenergy projects are already economically viable and have a favourable energy ratio; 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 this situation. Currently, energy ratios for solid biofuels are better than those for liquid biofuels produced as gasoline or diesel substitutes lz. The biggest problem facing bioenergy is market penetration which will require overcoming peoples' preconceived ideas of bioenergy, and changing the current system of taxes and subsidies that are biased against renewable energies in general.

Carbon Storaoe vs. Fossil Fuel Substitution

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 and which is the best option to be pursued where?

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 tl~refore fossil fuels to be replaced) indefinitely ~3'14. 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 impact 15'5. Energy crops also have relatively short rotations enabling them to adapt more rapidly to environmental changes. Vitonsek 16 compared the impact of plantations for timber and biomaas energy; based on a planting rate of 10 Mha/yr for 10 years, he found that timber plantations could reduce atmospheric CO 2 levels by about 10%, while biomass energy plantations reduced levels by 15%.

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 CO 2 emissions by substitution could be near zero or negative. In addition to this economic advantage, there are also the numerous environmental and social benefits associated with bioenergy provision that were mentioned earlier ~3.

Marland & Marland 17 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 expectec~ 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 CO 2 levels using available land, as long as environmental sustainability is ensured a. 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.

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LAND AVAILABILITY

Many studies have been carried out on land availability and they give very wide ranging results depending on sources of data and assumptions used. A summary of several estimates is presented in Table 1. Most estimates for the tropics are based on degraded land areas, but also include surplus land after accounting for food production, and fallow land. Tl~y are generally in the range of 500 Mha availability. 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 schemes TM. 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 under the reforms of the Common Agricultural Policy, and this could reach over 50 Mha in the next century a. According to our "back-of-the- envelope" calculations ~8 - 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 16.6% 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 1~.

Therefore 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. In any case, food shortages are caused more by distribution problems, a lack of purchasing power, bias towards the production of export crops and livestock, other political issues and, in Africa particularly, droughts and war. It should be remembered that both food and fuel are important requirements that need not compete, particularly if bioenergy crops are grown on degraded lands or in place of cash crops grown to pay for oil imports, and if agroforestry practices are adopted. Forestry policies and programmes can in fact improve food security by providing food (from the tree directly and from animals in the habitat provided), fodder, and income for food purchase and by reducing erosion. It is now being recogrtised that intensive agriculture cannot be maintained on fragile lands, but an alternative must be found for farmers to maintain their livelihoods.

PLANTATION MANAGEMENT AND ENVIRONMENTAL IMPACTS

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 with good 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 100t/ha/yr are common in a number of areas such as Hawaii, South Africa and Queensland s.

High yields can be achieved with monocultures of selected dunes, 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 survive 19. Growing multi- product plantations that include trees for energy, fruit, straight poles, etc will also create more interest and help acceptability of plantations. In additions, 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 state a. In order for these natural patches to be more effective, they should be connected with undisturbed corridors of natural vegetation to enable species migration. Plantation management can optimise nutrient and water conditions avoiding wastage and leaching. Leaves and twigs should be left on the ground after harvesting as most of the nutrients are concentrated in those parts of the tree. The nutrient status can be maintained by recycling wastes such as wood chippings and ash from biomass burning or gasification. Intercropping with nitrogen-fixing plants can increase yields while reducing the need for fertilisers.

There are potential negative impacts of plantations, 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, particularly if the biomass is able to replace fossil fuels and offset emissions of other pollutants. But more research is needed and incentives and regulations should be established to ensure sustainable practices are followed 6'z~9.

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Tablel : Estimates of Potential Land Available for Reforestation.

:tefarence

rGraingar (1988), 25

Myers (1989), 39

Houghton (1990), 36

Dixon et.al. (1991), 40

eCai. (1992), 34

~ck~ng 0992), 35

Nilsson (1992) in Nakicenovic et.al. (1993~

,38 Trexler (1993), 4

Total Avm'lable

758

3O0

865

Temperate - 605 Boreel - 426

952

553

265

67

Comm~N~s

2007 Mha of degraded land in the tropk~ of which areas with a high priority for forest repkmishment are: 137 Mha of logged forest that could be mmlaged for natural vegetation; 203 Mha of fallow forests, 87 Mha of deforeeted watend~ds and 331 Mha o~ deeertiF~d drylands that could be managed for plantations. Theee mstimaee are from crude sa~el#e i ~ and the suthor

a c k n ~ thst some of thb |end is pr~oal~e'in use,

200 Mha "needs r e f ~ n for reesons othar tl'mn the greenhouse effect. 160 Mha of the above 200 Mha is upland w a t e n d ' ~ which urgenUy needs r~orutation, the re6t is required es woodlots.

Land deforested and now unused in Asia (100 Mha), Latin Amarica (100 Mha), and Africa (300 Mha). The oth~ 365 Mha is equivalent to 95% of the land in sh~ng cu#iv=ion ~ "could be MJrned to form= if pemtar.mt = g r ~ . r e ware to rapa~ shi~ng cultivation."

This is just wh~ ls "technically 8vailoble" ie. it does not take account of the ¢onstmira.

~ m the total area availabb for halophyte culture. 125 Mha of this is assumed to be feasible due to restrictions for saline i r d ~ .

Theorelic~ land available for reforestation in 11 out of 117 tropical countries analyeed which hed ovar 10 Mha of surplus land once future crop requirements are fulfilled. 385 Mha of land is available altar accounting for dime=e, and a further 168 Mha could be avail~ole from fa, ow land. Sorne of this land rnay be in other uses eg. permanent pasture.

A further 84.5 Mha is considered available for l agroforestry. Takes account of actual availability.

This is what the author feels could be 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 ~nd takes into account future trends, policy, infrastructure, and other constraints.

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SOCIAL CONSIDERATIONS

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, along with flexibility and sustainability so that these schemes can catalyse development. 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 change 4'~°.

ECONOMIC FACTORS

Economics is one of the main constraints, 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 sources u. 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 intemalised to allow biofuels and other renewables to compete in a "level playing field". Taxes, regulations and other policy instruments can be used ~n.

The US Environmental Protection Agency (EPA) z~ estimates that, in the tropics, regeneration would cost $0.5-~tC ($100-300/ha), agroforestry $2-1 l/tC ($300-2,500/ha) and reforestation $3-26/tC ($100-4,300/ha). Their estimates for temperate areas are $0.2-5/tC for afforestation and $3-29/tC for reforestation. Volz 22 estimates that plantations in temperate areas will cost around $100/tC/yr, and Moulton and Richards z3 put the cost of plantation in the US at $5-43/tC. Yet trees are a marketable resource and the biomass can be sold to pay for the costs of establishing and maintaining forest schemes. 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.

CARBON MITIGATION POTENTIAL

Various estimates of carbon mitigation potentials are shown in Table 2. Sedjo & Soloman z4 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 CO z. Grainge~ 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 CO 2 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. Cannell ~ concludes that 500 Mha would need to be planted with a sequestration rate of 6 tC/ha/yr (12t/ha/yr dry biornass) in order to absorb the 6GtC emitted annually from fossil fuels and cement manufacture. However, he feels it is likely that only 50 Mha could be planted which would sequester 0.2 GtC/ha/yr for one rotation (or 5-10% of the amount of carbon accumulating in the atmosphere). Trexter 4 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, Volz 22 indicates that an annual afforestation rate of about 4Mha might be feasible, taking up about 22 MtC/yr at a cost of $400/ha in the USA and $3,000/ha in Europe with an average carbon sequestration cost of $100/tC. Kulp 5 estimates that the potential carbon sequestration in the USA is 0.05 GtC~r (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 & Richards ~ estimate that the USA could offset 20% of its emissions using 60 Mha at a cost of $4.5 billion/year. Based on this, Rubin et.al. 27 estimate that 30 Mha of economically marginal croplands, pasture lands and federal forests could sequester roughly 5% of current US CO 2 emissions at an average cost of $7/tCO z. 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 CO z released from US electricity production z~. Trexler estimates that within 20-30 years biofuels could be displacing 12-25% of current (1991) U.S. energy consumption ~.

WRI 3° 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 IPCC 3t 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.

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COMPARISON OF OTHER OPTIONS

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 for greenhouse gas mitigation 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. Rubin et.alY have compared a variety of options in the US. Efficiency could save 890 Mt COz-equlvalent annually at a net cost of minus $84/t C02-equivalent in the domestic sector, and 527 MtCO 2 at a cost of minus $43/tCO z in the industrial sector. Advanced coal technology could save 200 MtCO 2 at a cost of $280/tCO2; natural gas substitution could save 850 MtCO 2 at $32/tCO2; nuclear power could offset 1500 MtCO 2 at $49/tCO2; and solar photovoltaic electricity could offset 400 MtCO 2 at $87/tCO 2. Generation of electricity from biomass is the cheapest alternative electric supply technology after natural gas and could save 130 MtCO 2 at a cost of $36/tCO2. Reforestation could sequester 242 MtCO 2 (about 5% of current US CO 2 emissions) at a very low cost of $7/tCO 2 (using 30 Mha of economically marginal cropland, pasture land and non-federal forest, about 3% of US land areas). They further calculate that the direct cost of providing economic incentives to practice sustainable forestry in developing countries is only $0.4/tCO 2 sequestered.

PHOTOBIOLOGY AND PHOTOBIOCHEMISTRY FOR FIXING CO 2

Future biological methods for atmospheric C O z reduction include laboratory based research on photobiology and photobiochemistry. Photobiology (efficient mieroalgal growth in photohioreactors and tanks/ponds to remove CO 2 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 CO z fixing enzyme Rubisco to store energy via organic compounds 3~3. These photobiologieal 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 a 5% conversion efficiency, and average solar energy in the Mediterranean region to be about 200 W/m 2 or 6.3 GJ/m2/yr, 18 Mha of land would theoretically be required to fulfil all Western Europe's energy requirements 14.

CONCLUSIONS

Of the major alternatives to reduce atmospheric CO 2 levels, forestry options ave among the most promising and environmentally acceptable 27'34. 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 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, provision of electricity and other forms of energy to rural areas, etc, thereby helping to promote development. Since the biocnergy can provide an income, it is a way of paying for CO 2 mitigation and land restoration. To derive maximum benefits, bioenergy production and use should be modemisect, and a balanced system of taxes and subsidies needs to be put in place to allow biomass to compete fairly in the energy market.

There are large areas of degraded land available for reforestation 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 sus~nability, increased productivities, improved infrastructure and planning, long term monitoring and a large financial commitment made at an international level. In particular, success will require that there must be local involvement, multiple benefits, sustainability and flexibility. Bekkering 35 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 created.

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Table 2. Estimates of land reqttirements and costs of carbon mitigation.

Source

Marland (1988), 41

Marland (1989), 42

Myers (1989), 39

Sedio & Soloman (1990), 24

EPA (1989), 21

Trexler (1991), 37

Nacieenovic et.al. ~1993), 38

* Size of Auslxalia = 768 Mha. ** Size ofZaire = 235 Mha.

Offset Goal (tC/F)

1.2

3

2.9

0.05

0.2-0.5

1.6

Rate Carbon Uptake (tC/ha/~)

7.5 9.6

7.5

10

6.2

3.5-10

Location

Tropics Tropics

USA

Tropics

Tropical or temperate

USA

Tropics

Global

Area Cost tMha) tS~a) 700 * 5OO

164

300 ** 400

465 400- 1400

4.5 - 13 432

220

265 $4.4/tC

*** Rate depends on forestry option ie. regeneration, agroforestry of plantations (see text). **** Variable