Is bioenergy really sustainable?

Words by Patricia Thornley, SUPERGEN Hub Director

The use of wood for electricity generation has grown rapidly in recent years and this has led to concerns being raised about the sustainability and carbon savings associated with bioenergy, particularly imported pellets manufactured from U.S. forest residues. Firstly, this needs to be understood in context: only 2.4% of harvested wood in the south east U.S. was used for pellet production in 2014. That is largely because forestry is a commercial enterprise and the highest value product is the saw-log. Where possible foresters will sell this to high value markets to maximize profit and only the lower grade (cheaper) material will be used for bioenergy. It is possible that the bioenergy market could drive higher harvesting rates, but historically even when prices increased by 100% only a 3% harvest increase was observed. It is possible that there could be further increases when new power plants are built, but the UK governments’ renewables obligation incentive has already implemented precautions to curb expansion.

The international accounting frameworks for greenhouse gases were developed in order to deliver national autonomy and responsibility: countries are assessed on the emission levels that are within their control/jurisdiction. This makes a lot of sense in terms of placing responsibility with the parties most able to deal with it, but is effectively usurped by the role of global trade. This isn’t just an issue with biomass: every time the UK imports goods from overseas (food, electronics, plastics etc.) we are incentivizing carbon emissions in the producer country.

Bioenergy is not “carbon-neutral”. No currently available energy source is! However, sustainably sourced biomass can provide low carbon energy, which is valuable in reducing UK carbon emissions. Bioenergy is actually the only UK energy source subject to protocols to calculate the actual carbon footprint of the energy being delivered. Many of us carry out these calculations regularly and there is wide diversity, with some systems giving huge reductions compared to fossil fuels and others performing at a level that could be worse than fossil fuels. The important thing is not to throw the baby out with the bath water and to continue to support sustainable bioenergy systems that are delivering real and much-needed GHG reductions today.

Sustainability criteria can ensure that only biomass with the lowest impact on the climate are used. It is important that such calculations take a comprehensive view of the supply chain and changes in forest carbon stock should be taken into account. It is relatively straightforward to carry out carbon balance assessments of supply chains and account for transport, processing and other emissions, but accounting for changes in forest carbon stock is complex. It requires modelling of forest management and growth, apportionment of the carbon stock change to different forest products and an understanding of the drivers of forest management. It is also the case that the fastest carbon sequestration periods are actually in the earlier growth stages of a forest. So once maturity is reached we have a large standing, sequestered carbon stock; but the amount of additional carbon being sequestered annually will have reduced significantly. So it may make sense to fell the forest, use the prime products for applications that lock up the carbon sequestered and that frees up the land for re-afforestation, increasing the annual rate of CO sequestration again. It must also be remembered that forests are not generally established with the prime objective of sequestering carbon: they do this alongside delivering other important ecosystem and socio-economic objectives. So it may not always make sense to manage for maximize carbon stock, since this might reduce biodiversity, recreational value or economic return and the reality is that forest carbon is not valued in any way that rewards management to maximize carbon.

It is right that we should focus on bioenergy systems with legislation and incentives to maximize greenhouse gas reductions, but we must also remember that over 90% of global biomass use is inefficient use of fuel wood in developing countries. As populations and industrialisation in those countries increase there is a risk that deforestation may be driven by unsustainable fuel wood use. Investing UK expertise to switch away from those inefficient systems would have much greater global impact than focusing on the relatively small proportion of global biomass that supports first world energy systems.

The Supergen bioenergy comic which you can download on this website, provides a general introduction to biomass sustainability and carbon balances, but for those who would like more detail, check out the other blogs from 3 leading UK researchers who explain the issues associated with: