Conclusion
The use of woody biomass for energy cannot be considered to be automatically carbon-neutral under all circumstances, though most policy frameworks treat it as though it is. In reality, carbon dioxide and methane will be emitted from the combustion of woody biomass (generally at higher levels than from the fossil fuels it replaces) and from its supply chain of harvesting, collecting, processing and transport. In addition, where the feedstock derives from harvesting whole trees, net carbon emissions will increase from the foregone carbon sequestration that would have occurred had the trees been left growing.
Some types of biomass feedstock can be carbon-neutral, at least over a period of a few years, including in particular sawmill residues. These are wastes from other forest operations that imply no additional harvesting, and if otherwise burnt as waste or left to rot would release carbon to the atmosphere in any case. Black liquor is a waste from the pulp and paper industry that would otherwise have to be disposed of. It can make sense to burn these types of woody biomass for energy (particularly on-site, with no need for processing or transport), and in any case in many instances this will be economic without the need for subsidy. Fast-decaying (small-diameter) forest residues are unlikely to be usable by biomass plants, and burning slowly decaying forest residues for energy may mean that carbon emissions stay higher than if fossil fuels had been used for decades, which is a matter of considerable concern given the current rate of global warming. If mill residues are diverted from use as wood products to use as energy, net carbon emissions will be higher as a result.
Policies providing financial and regulatory support to woody biomass should discriminate between the different feedstocks on this basis. It cannot make sense to support practices that raise greenhouse gas concentrations over the short, medium and sometimes long term. Yet this is precisely what most existing policy frameworks do, ignoring changes in forest carbon stock and providing support to all biomass feedstocks irrespective of their impact on the climate. The international rules designed to account for changes in forest carbon levels in the land-use sector do not do this comprehensively, and some of the emissions from woody biomass may go unaccounted for.
Although comparisons are generally made between the use of woody biomass and the use of fossil fuels, particularly coal, in practice biomass energy may be more likely to displace other sources of renewable energy rather than fossil fuels. This is particularly the case where governments have adopted national targets for the growth of renewables (as in the EU) and where they have limited budgets for providing subsidies (as in, for example, the UK). In these cases, if biomass is not available, is constrained by sustainability criteria or is not subsidized, other forms of renewable energy may grow faster. (This raises questions of the costs of competing renewables – which for many, particularly wind and solar PV, are falling much faster than those of biomass – and the role of biomass as a system balancer, being a dispatchable rather than a variable source – which will be considered at more length in the companion paper, Woody Biomass for Power and Heat: Global Patterns of Demand and Supply.)
For all these reasons, current biomass policy frameworks are not fit for purpose. Sustainability criteria should be used to restrict support to mill residues that are produced from legal and sustainable sources (as defined in many timber procurement policies and forest certification schemes) and do not divert raw material away from wood products. This requires substantial changes in current policies in the EU and elsewhere to ensure that biomass policies contribute to mitigating climate change rather than exacerbating it.