09/24/2013 - 05:21

The devil’s in the details

Bioenergy, because of its potential to mitigate climate change and contribute to energy security and rural development, has attracted increased attention in recent years. It is a highly versatile energy source whose most common applications are heat energy for cooking and biofuels for transportation, but it can also encompass electricity. Biofuels such as ethanol and biodiesel can be produced from crops like sugarcane or corn; biomass energy can be derived from (usually woody) feedstock through processes running the gamut from simple combustion in a cookstove to biochemical conversion.

Another advantage of bioenergy is that, compared to fossil fuels, it is distributed equitably across the world and is accessible to communities everywhere—including poor people in rural areas, who tend to be very dependent on traditional biomass-based energy for cooking and heating and even for mechanical applications such as lift irrigation. Traditional biomass is often inefficient, harmful to the environment, and associated with low quality of life, but several modern bioenergy technologies have emerged that can, in an environmentally sustainable way, meet rural people's energy needs. These technologies include efficient biomass cookstoves, biogas systems for cooking and for decentralized power generation, gasification of woody biomass, and biofuels for transportation.

Meanwhile, bioenergy technologies are increasingly being recognized for their potential to mitigate climate change. According to the 2012 Global Energy Assessment, bioenergy is essential if global temperature increases are to be limited to between 1.5 and 2 degrees Celsius. In many applications, bioenergy may be able to displace fossil fuels, as with biofuels for transportation. But another critical approach is to combine biomass energy with carbon capture and storage. This technology involves growing crops that absorb carbon dioxide, burning them to produce energy, and capturing and storing the carbon that results from the combustion. Capture and storage of carbon dioxide emissions from bioenergy conversion has the potential to generate negative emissions—to remove carbon from the atmosphere.

But the technology's potential as a mitigation option is still uncertain due to constraints on carbon capture and storage itself, and due to the difficulties associated with producing supplies of biomass. Biomass energy combined with carbon capture and storage must be deployed on a large scale if it is to have a significant impact on global emissions of greenhouse gases; this is true of biofuels as well. Large-scale deployment of these two technologies implies sustained, large-scale production of bioenergy feedstock, and this carries potential implications for food security.

Potential and contention. The food-security implications differ for the two technologies, not least in terms of their feedstock. For biomass energy combined with capture and storage, woody (or ligneous) feedstock is generally required. This can be sourced from tree plantations, but if either croplands or forests were used on a commercial scale for woody biomass production, food security and biodiversity could be adversely affected. However, if sustainable tree plantations were established on degraded land or fallow crop lands, implications for food security would be minimal. And using residues from forests or croplands would have no implications for food security.

Feedstock for biofuels, meanwhile, can be thought of as belonging to two generations: first-generation crops like palm oil (which can be used to produce biodiesel) and sugarcane (which can be used to make ethanol); and next-generation sources such as microalgae (used to produce biodiesel) and woody biomass, tall grasses, and agricultural residues (used for ethanol). First-generation biofuels pose the greater risks to food security, and imply negative environmental effects like reduced biodiversity and increased water usage. These negative effects are likely to be particularly acute in the developing world, where, due to low production costs, the bulk of future biofuel production is likely to occur. So even though biofuels can promote rural development, create rural jobs, and reclaim degraded land, they have become highly contentious.

Nonetheless, technology for converting second-generation feedstock into biofuels holds out substantial promise for avoiding many of the challenges associated with first-generation feedstock. Agricultural or forest residues and short-rotation woody crops could be sourced from marginal or degraded lands. This would be unlikely to have significant implications for food security, livestock feed, and fiber production. Moreover, new biofuel technologies can be expected to provide net benefits in emissions of greenhouse gases.

Taking all this into account, it is difficult to generalize about bioenergy's ability to meet energy needs and mitigate climate change while avoiding adverse effects on food production, biodiversity, and so forth. The impacts of bioenergy use depend on the technology used (biofuel production versus other forms of biomass energy), the feedstock used (forest or crop residues versus food grains, for example), and the scale of production.