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To assess the net effect on greenhouse gas emissions of replacing fossil fuels by biofuels, we need to analyse emissions throughout the whole process of producing, transporting and using the fuel. Life-Cycle Analysis is the main tool used to do this. It compares a specific biofuel system with a reference system – in most cases petrol.
Greenhouse gas balances differ widely depending on the type of crop, on the location, and on how feedstock production and fuel processing are carried out. Biofuels from some sources can even generate more greenhouse gas emissions than fossil fuels.
A significant factor contributing to greenhouse gas emissions is the amount of fossil energy used for feedstock production and transport, including for fertilizer and pesticide manufacture, for cultivation and harvesting of the crops, and or in the biofuel production plant itself.
Emissions of nitrous oxide are another important factor. It is released when nitrogen fertilizers are used and its greenhouse gas effect is about 300 times stronger than that of carbon dioxide.
By-products from biofuel production such as proteins for animal feed make a positive contribution to climate change mitigation because they save energy and greenhouse gas emissions that would otherwise have been needed to produce the feed by other means.
Most studies have found that producing first generation biofuels usually yields reductions in greenhouse gas emissions of 20 to 60% when fossil fuels are replaced provided the most efficient systems are used and carbon dioxide emissions from changes in land-use are excluded.
Ethanol produced from sugar cane in Brazil and second-generation biofuels typically reduce emissions by 70 to 90%, again excluding carbon releases related to land-use change.
However, changes in land use can have dramatic effects on greenhouse gas emissions. When forest or grassland is converted to farmland to produce feedstocks, or to produce crops that have been displaced by feedstock production, carbon stored in the soil is released into the atmosphere. The effects can be so great that they negate the benefits of biofuels. Repaying this ‘carbon debt’ could take decades or even hundreds of years. In some cases it would be more cost-effective to strive for greater fuel efficiency and carbon sequestration through reforestation and forest conservation.
Since land-use changes have a significant impact on greenhouse gas emissions, it is important to know whether increased biofuels production will be met through improved land productivity or through expansion of cultivated area.
Expansion of cultivated land
Of the world’s 13.5 billion hectares of total land surface area, about 8.3 billion hectares are currently grassland or forest and 1.6 billion hectares cropland. After excluding forest land, protected areas and land needed for food, between 250 and 800 million hectares, are potentially available for the expansion of biofuel crop production.
In 2004, about 1% of global cropland was being used for biofuels, and the IEA expects this share to increase to 3 to 4 times this level by 2030. Some land that was not profitable in the past could be returned to production, for example in the Former Soviet Union. In practice, additional land is expected to come from non-cereal croplands and set-aside land in Australia, Canada, the USA and the EU, with some new, currently uncultivated land, especially in Latin America and Africa.
Intensive research has produced significant improvements in crop yields, but has focused on specific crops and regions. In many parts of the world actual yields are still below their potential. Africa, in particular, has not benefitted from modern high-yielding crop varieties and farming practices as much as other regions have.
Some potential biofuel crops such as jatropha, cassava, sweet sorghum, may be able grow on marginal land where food crops cannot strive. However, growing any crop, including those that are drought resistant, on land with low levels of water and nutrient inputs will result in lower yields. It is therefore likely that biofuels will intensify the pressure on the fertile lands where higher returns can be realised. More...
During biofuel production, water is used in large quantities for washing plants and seeds and for evaporative cooling. However, the biggest impact on local water availability stems from irrigation. Crops such as sugar cane, oil palm and maize have relatively high water requirements and are best suited to high-rainfall areas, unless they can be irrigated. Three quarters of the sugar-cane production in Brazil and slightly less of the maize production in the USA is rainfed.
The availability of water resources may constrain the production of biofuel crops in countries that would otherwise have a comparative advantage. The amount of irrigation water needed in lower rainfall areas can be significant. Many irrigated sugar-producing regions in southern and eastern Africa and north-eastern Brazil are already operating close to the limits of the available water. Even plants like jatropha that can be grown in semi-arid areas may require some irrigation during hot and dry summers.
Producing more biofuel crops will also affect water quality. For example, converting pastures or woodlands into maize fields may increase problems of soil erosion and runoff of excess nitrogen and phosphorous into surface and groundwaters. Pesticides and other chemicals can also wash into waterbodies. Of the principal feedstocks, maize is the one requiring the greatest amount of fertilizer and pesticides per hectare. More...
Changes in land-use and intensification of agricultural production both have the potential to harm soil condition, but these impacts depend on the way the land is farmed.
Various farming techniques can reduce adverse impacts or even improve environmental quality while still increasing biofuel crop production. These include conservation tillage and appropriate crop rotations.
Removing plant residues that would otherwise nourish the soil and permanent soil cover that prevents erosion can reduce the quality of soil. Only 25 to 33% of available crop residues from grasses or maize can be harvested without detrimental effects on soil quality, especially on soil organic content.
The use of perennial plants that can be harvested over several years such as palm, short-rotation coppice, sugar cane or switchgrass can also improve soil quality by increasing soil cover and organic carbon levels compared with annual crops like rapeseed, maize or other cereals. In the case of sugar cane, soil quality can be maintained by applying nutrients from sugar-mill and distillery wastes.
Crops such as eucalyptus, poplar, willow or grasses can be grown on poor-quality land, and soil carbon and quality will tend to improve over time. More...
Biofuel production can affect wild and agricultural biodiversity in some positive ways, for instance through the restoration of degraded lands, but many of its impacts will be negative, for example when natural landscapes are converted into energy-crop plantations or peatlands are drained.
Conversion of forest or grassland for crop production has a significant effect on wild biodiversity, because of the loss of habitat. Many current biofuel crops are well suited for tropical areas, and this creates an economic incentive to convert natural ecosystems into plantations causing a loss of wild biodiversity in these areas.
For existing arable land, positive impacts on farmland biodiversity can be obtained by using crops which increase soil cover, avoiding tillage and reducing fertilizer and pesticide inputs.
The genetic diversity of crops (agrobiodiversity) can be compromised where large-scale production is practiced. Most biofuel feedstock plantations are based on a single species, using a narrow pool of genetic material, with traditional varieties being used less and less. Such low levels of genetic diversity increase the susceptibility of crops to new pests and diseases.
Second-generation feedstocks raise their own concerns, since some of the proposed plant species can be invasive. Similarly, care will be required when dealing with genetically modified bacteria that produce enzymes used for cellulose conversion.
Crops which do well on fertile soils may not be as effective in poorer conditions. For example, switchgrass performs less well on poor soils than a diverse mixture of native grassland perennial plants. In addition, such diverse mixtures can provide better wildlife habitat, water filtration and carbon sequestration than maize or soybean alone. More...
The adoption of “good practices” in soil, water and crop protection, energy and water management, nutrient and agrochemical management, biodiversity and landscape conservation, harvesting, processing and distribution can contribute significantly to making bioenergy sustainable.
For instance, good agricultural practices, such as conservation agriculture, and good forestry practices, can reduce the adverse environmental impacts of biofuel production.
The environmental concerns about biofuel feedstock production are the same as for agricultural production in general, and existing techniques to assess the environmental impact offer a good starting point for analysing the biofuel systems. New complementary methodologies are being developed to assess bioenergy specific issues, for instance FAO’s analytical framework for bioenergy and food security.
The development of sustainability criteria or standards as already under way in a number of fora, such as the Global Bioenergy Partnership and the Roundtable on Sustainable Biofuels, should be established with the active collaboration of developing country partners and go hand in hand with training and support for implementation.
Payments for environmental services may also represent an instrument for encouraging compliance with sustainable production methods and standards.
Finally, for bioenergy to be developed sustainably, national policies will need to recognise the international consequences of biofuel development. More...
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