Bioenergy: how much?

The perspective by Haberl et al (2013 Environ. Res. Lett. 8 031004) entitled ‘Bioenergy: how much can we expect for 2050?’ is timely and valuable. It deals with an important subject since contrasting views on the subject make it very difficult for policy makers to adopt policies that would allow ‘production and consumption of energy at sustainable levels’, in the words of the authors. It is therefore very important to sort out from the abundant literature on the issue which are the facts and which are the biases and preferences.

Haberl et al [1] basically argue the following.
(I) 'It seems impossible that bioenergy could physically provide more than ≈250 EJ yr −1 in 2050, substantially below many published bioenergy projections.' (II) 'Meeting global food demand might be achieved without reducing the amount of annual plant production remaining in ecosystems, but only in the absence of large-scale additional bioenergy production.' (III) 'Plant growth is an inefficient way of converting sunlight into useable energy. The energy efficiency of photosynthesis is usually <1% under field conditions-far below the efficiency of commercial solar photovoltaic cells of 12-20%.' Regarding statement (I), it seems adequate to quote a paper by Dornburg et al [2], which analysed a number of projections and pointed out that studies on the potential of biomass as an energy source are in the range of 0-1500 EJ. Such a wide range is due to differences in methodology as well as assumptions on crop yields and available land. The higher value resulting from an optimistic approach assumes a highly developed agricultural system, the lower is the result of a pessimistic approach with high population growth and extreme measures to avoid biodiversity loss [3]. A sensitivity analysis conducted by Dornburg et al narrows that range to approximately 200-500 EJ yr −1 in 2050 when taking into consideration water limitations, biodiversity protection and food demand. Also important are the results reached in the Global Energy Assessment [4], which concludes on a potential equal to 160-270 EJ yr −1 in 2050. Another recent paper [5] presents a very comprehensive overview of bioenergy potentials, also discussing the different types of potential which are often misunderstood. It is stated that 'the broad variety of approaches, methodologies, assumptions and datasets is due to a lack of a commonly accepted approach to determine biomass energy potentials' and it is concluded that 'these differences in bioenergy resource assessment estimates make it difficult to clearly inform decision-making by policy-makers and investors'. Regarding statement (II), it was pointed out by Nogueira et al [6] that the 'perception that expansion of bioenergy use will set serious competition with food is not accepted by many experts'. According to FAO [7], more than 80% of the food/feed global future demand will be fulfilled by increment in productivity. In fact, between 1961 and 2009, global cropland grew by about 12% and agricultural production expanded by 150%, due to productivity gains. As a relevant outcome, the world food security situation is steadily improving, as indicated by a consistent rise of average food consumption 'per capita' and the progressive reduction of undernourishment in the developing world [8].   Regarding statement (III), the argument that solar photovoltaic cells (PV) are a better way to use solar energy than photosynthesis is very questionable, as pointed out also by Nogueira et al [6]. The use of PV generated electricity requires solving the problem of storing energy which is still uncertain. In addition to that, solar power systems present strong seasonal and daily variability. As a result the capacity factor of solar systems is around 25% at best. In bioenergy systems, the solar radiation is naturally stored as chemical energy in the biomass and further in the biofuel, allowing full dispatchability. As a consequence, the current and prospective prices of bioelectricity and sustainably produced biofuels are competitive with regards to the photovoltaic alternative in many cases [9].
Finally, to understand better the problem of bioenergy one should put it in the wider context of agricultural production and use of land: a total of 1553 Mha of land was in use for agricultural production in 2011, it was 1371 Mha in 1961, an increase of 182 Mha in 50 years over pastures, deforested (in some cases) and degraded land [10].
Modern forms of bioenergy in use in 2011 amount to 23.6 EJ as heat, biofuel and electricity. An additional 31.4 EJ of traditional biomass is used very inefficiently for cooking/heating in poor rural areas, mainly in Africa (figure 1).
Traditional biomass use is decreasing and being replaced by more efficient methods of cooking/heating such as modern cooking stoves and biomass residues for biogas production and electricity and other modern fuels [12].
While biofuels are frequently blamed for a number of ills such as dramatic increases in food prices and biodiversity loss [13] they represent only 2.6 EJ or approximately 0.5% of the present world's energy consumption. The area used for the production of biofuels is shown in table 1 and amounts to 36.1 Mha, approximately 2.3% of the world's agricultural land in use.
There is thus a lot of room to improve technologies for bioenergy use and promote development that is sustainable.
Efforts in this direction should be encouraged and negative statements such as 'large-scale promotion of bioenergy could result in economic incentives to divert land from food production to bioenergy which puts the world's poor at risk, driving up hunger and inequality' in Haberl's perspective [1] do not seem to be good advice to policy makers.