Biofuels or Forests?
23 Aug, 2007 11:37 am
Carbon-free transport fuels present some of the most difficult problems in adapting to a low carbon economy and, while there are solutions like hydrogen in the offing, it will probably be 30 years or more, before the bulk of transport fuel could be replaced. Liquid biofuels offer a superficially attractive option because they can substitute fossil fuels more or less directly in internal combustion engines and use the existing fuel distribution system.
Because agriculture and production of biofuels itself uses fossil carbon (for fertilisers, fuels, buildings etc), the use of these fuels spares only a proportion of the emissions of the fossil fuel equivalent. After taking into account the fossil inputs, bioethanol and biodiesel can still give useful net emissions reductions compared with the use of fossil fuels of between one third and two thirds 1, 2, 3. Depending on the crop, 1000 – 3000 litres of fossil fuel equivalent can be produced per hectare, though because of the fossil carbon costs of producing and converting the crop, the emissions avoided are lower. A range of studies analysed by the International Energy Authority 4 show the emissions avoided by bioethanol compared with gasoline to be around 30% (21-38%) in the case of corn and 33% (19 – 47%) for wheat. These are equivalent to avoided emissions of about 1.5 tonnes CO2 /hectare.
Because of their higher carbohydrate yields, the sugar crops are more effective. Sugar beet yielding 5-6,000 litres of ethanol per hectare, avoiding 45% (35-56%) of the energy equivalent emissions of gasoline, can provide avoided emission of around 4 tonnes CO2/hectare. Sugar cane has the added advantage of the by-product, bagasse, the fibrous waste from the sugar cane plant that is burnt to fuel the conversion process, and is more effective at around 5.5 – 7.5 tonnes CO2/hectare 5.
Vegetable oils can be efficiently converted to a diesel substitute (biodiesel). In the case of oilseed rape, producing around 1,000 litres of diesel equivalent per hectare and providing avoided emissions of around 2 tonnes CO2/hectare 1.
So, it appears that biofuels can make some contribution to reducing overall carbon emissions. For it to be significant in the context of global carbon emissions, huge areas of land would be required. Even a 10% substitution of petrol and diesel - a limited target proposed by the EU for 2015 - is estimated to require 43% and 38% of current cropland area in the US and Europe respectively 4. As even this low substitution level cannot be met from existing arable land: forests and grasslands would need to be cleared to produce the fuel crop itself or other crops displaced by it. Before embarking on such large scale production, we should also look at the alternative of maintaining or restoring forests and grasslands and compare that with arable production of fuel crops.
When arable land is restored to forest instead of using it for biofuel production, carbon stores build up in the soil and vegetation and outweigh the emissions avoided by the production of biofuel. Converting cropland to tropical forest can sequester 20-30 tonnes CO2/hectare per year-6, 3-4 fold more than the emissions avoided by sugar cane-derived bioethanol. In temperate regions, forest regrowth is slower but the rates of carbon sequestration are still several fold higher than the avoided emissions from biofuels produced from temperate crops. The sequestration rates fall as forests mature, but only after 50 – 100 years might cumulative avoided emissions exceed the carbon sequestered by forest restoration.
Where natural forests or grasslands are converted to arable land to permit the production of the fuel crop, the loss of carbon stored in the biosphere has to be factored in. In the tropics, the amount of carbon released into the atmosphere in conversion of secondary forest to burnt cropland is approximately 600 tonnes carbon dioxide per hectare 7 6. Most of this loss occurs through burning and biodegradation in the months following the initial clearance and its impact on global CO2 and warming is immediate. Balancing this amount of carbon in the atmosphere with the emissions avoided through the use of biofuels would take around 100 years. Furthermore, removal of forest cover may reduce downwind rainfall, causing a cascade of further forest loss, further reducing the biosphere’s capacity to sequester carbon and accelerating warming.
- EUCAR/JRC/CONCAWE Study, 2006. “Well to wheels analysis of future automotive fuels and powertrains in the European context.” http://ies.jrc.ec.europa.eu/wtw.html 28/07/06
- Larson, E, 2005. A review of LCA studies on liquid biofuels for the transport sector. http://stapgef.unep.org/docs/folder.2005-12-07.8158774253/folder.2005-12-08.9446059805/folder.2005-12-08.0238464777/file.2006-06-21.8962502645
- Elsayed, M.A., Mathews, R., and Mortimer, N.D., 2003. “Carbon and energy balances for a range of biofuel options”. Resources Research Institute, Sheffield Hallam University, UK, March, 71 pages.
- International Energy Authority, 2004. “Biofuels for transport: an international perspective”, chapter 6. http://www.iea.org/textbase/nppdf/free/2004/biofuels2004.pdf
- Macedo, I., Leal M.R.L.V., da Silva, J.E.A.R. 2003. “Greenhouse gas emissions in the production and use of ethanol in Brazil: the present situation (2002)”. http://www.senternovem.nl/mmfiles/135550_tcm24-124345.pdf.
- Watson, R.T., Noble, I.R., Bolin, B., Ravindranath, N.H., Verardo, D.J., Dokken, D.J. 2001. “Land use, land-use change and forestry” p 184. Intergovernmental Panel for Climate Change, Geneva.
- Palm C. A., Woomer P. L., Alegre J., Arevalo L., Castilla C., Cordeiro D. G., Feigl B., Hairiah K., Kotto-Same J., Lasco R., Mendes A., Moukam A., Murdiyarso D., Njomgang R., Parton W. J., Ricse A., Rodrigues V., Sitompul S. M., van Noordwijk M., 1999. “Strategic information on changes in carbon stocks and land use” . CGIAR. http://www.asb.cgiar.org/data/dataset/IDAOJYZB.htm.