Using biomass to provide energy services is promoted by many governments globally as a means to combat rising energy prices, support rural development and help mitigate climate change. Biomass is also expected to play an increasing role as a feedstock for the chemicals industry, reducing this sector’s reliance on coal and oil. Yet the increased use of biomass is not without controversy. While advocates argue that developing biomass resources represents an opportunity that society cannot afford to miss. Opponents point to the potential for conflicts with food supply, water availability, biodiversity and land use – arguing that rapid and injudicious expansion risks environmental ruin.
To examine the facts of the debate, scientists at Imperial College London undertook a systematic review of the evidence base, re-examining all the global biomass potential estimates published over the last 20 years. Publishing their results in a landmark report, Energy from biomass: the size global resource, the authors describe how societal preferences around food, energy and environmental protection will ultimately determine the extent to which biomass is used, and whether production happens in a sustainable or unsustainable way.
Sources of biomass
The most important sources of biomass are energy crops, agricultural and forestry residues, wastes, and existing forestry. By far the widest range of potentials, however, relates to energy crops: with estimates ranging from negligible to a contribution that exceeds current global primary energy supply. Because these crops require land and water, they also stimulate the most discussion about whether deployment at scale could be beneficial – e.g. mitigating some of the environmental damage caused by conventional agriculture; or detrimental – e.g. increasing competition for land, contributing to food price increases and damaging ecosystems. The other categories of biomass – agricultural and forestry residues, wastes and existing forestry – are comparatively neglected in global studies but could make a contribution comparable in size to the existing use of biomass for energy (around 10% of global primary energy supply). Practical and environmental constraints will also limit the use of agricultural and forestry residues.
Biomass estimates are most often discussed in terms of a hierarchy of potentials: theoretical; technical; economic; and realistic. Different studies interpret these terms in different ways making comparison difficult and increasing the risk of misunderstanding. Yet while differences in definitions can be detrimental to effective communication they do not by themselves account for why the range of estimates is so large.
The methods used to estimate global biomass potentials, and energy crop potentials in particular, have advanced greatly over the last 20 years. Whereas the earliest studies used simple assumptions about the area of land that could be dedicated to energy crops and the quantity of residues that could be extracted from agriculture and forestry. Recent innovations include using spatially explicit modelling techniques and scenario based assessments. These models can provide a greater level of insight into the trade-offs and impacts associated with biomass development.
Assumptions underpinning estimates of biomass potential
Biomass potential studies can be broadly divided into two categories, those that test the boundaries of what might be physically possible and those that explore the boundaries of what might be socially acceptable or environmentally responsible. Because many of the most important factors affecting biomass potentials cannot be predicted with any certainty, all resource estimates must be viewed as what if scenarios rather than predictions.
The assumptions leading to the full range of global biomass potentials found in the literature reviewed are described in Figure 1. Estimates up to ~100EJ (~1/5th of current global primary energy supply) assume that there is very limited land available for energy crops. This assumption is driven by scenarios in which there is a high demand for food, limited intensification of food production, little expansion of agriculture into forested areas, grasslands and marginal land, and that diets evolve based on existing trends. The contribution from energy crops is correspondingly low (8-71EJ). The contribution from wastes and residues is considered in only a few studies, but where included the net contribution is in the range 17-30EJ.
Estimates falling within the range 100-300EJ (roughly half current global primary energy supply), all assume that food crop yields keep pace with population growth and increased meat consumption. Little or no agricultural land is made available for energy crop production, but these studies identify areas of marginal, degraded and deforested land ranging from twice to ten times the size of France (<500Mha). In scenarios where demand for food and materials is high, a decrease in the global forested area (up to 25%), or replacing mature forest with young growing forest is also assumed. Estimates in this band include a more generous contribution from residues and wastes (60-120EJ) but this is partly because a greater number of waste and residue categories are included.
Estimates in excess of 300EJ and up to 600EJ (600EJ is slightly more than current global primary energy supply) all assume that increases in food-crop yields will outpace demand for food, with the result that an area of high yielding agricultural land the size of China (>1000Mha) becomes available for energy crops. In addition these estimates assume that an area of grassland and marginal land larger than India (>500Mha) is converted to energy crops. The area of land allocated to energy crops could occupy over 10% of the world’s land mass, equivalent to the existing global area used to grow arable crops. For most of the estimates in this band a high meat diet could only be accommodated with extensive deforestation. It is also implicit that to achieve the level of agricultural intensification and residue recovery required, most animal production would have to be landless.
Only extreme scenarios envisage biomass potential in excess of current global primary energy supply. The primary purpose of such scenarios is to illustrate the sensitivity of biomass estimates to key variables such as population and diet, and to provide a theoretical maximum upper bound.
Figure 1. Common assumptions for high, medium and low biomass potential estimates
Issues and implications for policy
Seeking to predict future biomass supply remains a highly speculative endeavour. There are uncertainties that cannot be resolved, and trade-offs that will always be contested, such as land-use choices and both positive and negative environmental impacts. Nevertheless, the literature indicates that there is considerable potential to expand biomass use before these more contested elements begin to dominate. Doing so could assist our understanding of impacts and implications.
Policy-making in an area beset by data gaps, scientific uncertainties and ethical debates is necessarily difficult. Moreover, policies related to diet, agriculture and land use are at least as important as those focused on biomass and bioenergy. Nevertheless it is possible to identify the following broad areas for policy action which could help address the opportunities and risks associated with biomass production for energy and chemicals:
- A near term focus on tangible opportunities could expand biomass deployment while allowing sustainability concerns to be evaluated. At a global level concentrating on how the first 100EJ could be made available sustainably would improve understanding of what is possible and the level of effort involved in going to higher levels of biomass use.
- There is a need to address key uncertainties through research and experimentation. For example: evaluating the sustainability of biomass production on marginal and degraded lands, the integration of food and biomass for energy systems, implications of energy crops on water use at regional level, and the environmental implications of land use change and related carbon flows.
- Developing and testing different approaches to environmental and land use governance that set biomass for energy, and agricultural systems, on a sustainable path.
This article is adapted from an Imperial College and UK Energy Research Centre report: Energy from biomass: the size global resource. The full report may be downloaded free of charge from: http://tinyurl.com/globalbiomass