Biomass energy: the scale of the potential resource.

Increased production of biomass for energy has the potential to offset substantial use of fossil fuels, but it also has the potential to threaten conservation areas, pollute water resources and decrease food security. The net effect of biomass energy agriculture on climate could be eithercoolingor warming, depending on the crop,the technology for converting biomass into useable energy, and the difference in carbon stocks and reflectance of solar radiation between the biomass crop and the preexisting vegetation. The area with the greatest potential for yielding biomass energy that reduces net warming andavoidscompetitionwithfoodproduction islandthat was previously used for agriculture or pasture but that has been abandoned and not converted to forest or urban areas. At the global scale, potential above-ground plant growth on these abandoned lands has an energy content representing � 5% of world primary energy consumption in 2006. The global potential for biomass energy production is large in absolute terms, but it is notenoughtoreplacemore thana fewpercentof current fossil fuel usage. Increasing biomass energy production beyond this level would probably reduce food security and exacerbate forcing of climate change. Biomass energy in context Biomass energy sources are among the most promising, most hyped and most heavily subsidized renewable energy sources. They have real potential to heighten energy security in regions without abundant fossil fuel reserves, to increase supplies of liquid transportation fuels and to decrease net emissions of carbon into the atmosphere per unit of energy delivered. However, increased exploitation of biomass energy also risks sacrificing natural areas to managed monocultures, contaminating waterways with agricultural pollutants, threatening food supplies or farm lifestyles through competition for land and increasing net emissions of carbon to the atmosphere, as a consequence of increased deforestation or energy-demanding manufacturing technologies. The opportunities are real, but the concerns are also justified. As investments in biomass energy increase, there needs to be an active, continuing discussion on strategies for balancing the pros and cons of biomass energy. The future of biomass energy in the global energy system is dependent on the complex interplay of four major factors. The first is conversion technology and the prospects for using new plant and microbe varieties as well as novel biomass-to-fuel conversion processes for increasing the yield of usable energy from each unit of available land or water. The second is the intrinsic productive capacity of the land and ocean ecosystems that can be used for biomass energy production. The third is alternative uses for the land and water resources that are candidate sites for biomass energy production. The fourth is offsite implications of biomass energy technologies for invasive species [1] and for levels of air and water pollution. These factors must be effectively integrated to maximize the benefits and minimize the ecosystem and societal costs of biomass energy production. In particular, constraints owing to ecosystem characteristics, competition from alternative land uses and offsite impacts can lead to practical or desirable levels of biomass energy production that are much smaller than theoretical potential levels. A clear picture of these constraints can be an important asset in encouraging rational development of the biomass energy industry. In this article we briefly review all four of these factors, with an emphasis on their integration. We first discuss the main types of biomass energy production systems, their relative efficiencies, and their environmental impacts. Next, we consider the role of existing vegetation in the distinction between energy and climate security, arguing that biomass energy production on current forest or crop lands is unlikely to result in significant climate benefits relative to fossil fuel use. Finally, we assess the potential total production of biomass on land other than forests or croplands.