The use of wood for electricity generation and heat in modern (non-traditional) technologies has grown rapidly in recent years. For its supporters, it represents a relatively cheap and flexible way of supplying renewable energy, with benefits to the global climate and to forest industries. To its critics, it can release more greenhouse gas emissions into the atmosphere than the fossil fuels it replaces, and threatens the maintenance of natural forests and the biodiversity that depends on them. Like the debate around transport biofuels a few years ago, this has become a highly contested subject with very few areas of consensus. This paper provides an overview of the debate around the impact of wood energy on the global climate, and aims to reach conclusions for policymakers on the appropriate way forward.
Although there are alternatives to the use of wood for biomass power and heat, including organic waste, agricultural residues and energy crops, they tend to be less energy-dense, more expensive and more difficult to collect and transport. Wood – and particularly wood pellets, now the dominant solid biomass commodity on world markets – is therefore likely to remain the biomass fuel of choice for some time.
Biomass is classified as a source of renewable energy in national policy frameworks, benefiting from financial and regulatory support on the grounds that, like other renewables, it is a carbon-neutral energy source. It is not carbon-neutral at the point of combustion, however; if biomass is burnt in the presence of oxygen, it produces carbon dioxide. The argument is increasingly made that its use can have negative impacts on the global climate. This classification as carbon-neutral derives from either or both of two assumptions. First, that biomass emissions are part of a natural cycle in which forest growth absorbs the carbon emitted by burning wood for energy. Second, that biomass emissions are accounted for in the land-use sector, and not in the energy sector, under international rules for greenhouse gas emissions.
Is biomass carbon-neutral?
The first assumption is that woody biomass emissions are part of a natural cycle in which, over time, forest growth balances the carbon emitted by burning wood for energy. In fact, since in general woody biomass is less energy dense than fossil fuels, and contains higher quantities of moisture and less hydrogen, at the point of combustion burning wood for energy usually emits more greenhouse gases per unit of energy produced than fossil fuels. The volume of emissions per unit of energy actually delivered in real-world situations will also depend on the efficiency of the technology in which the fuel is burnt; dedicated biomass plants tend to have lower efficiencies than fossil fuel plants depending on the age and size of the unit. The impact on the climate will also depend on the supply-chain emissions from harvesting, collecting, processing and transport. Estimates of these factors vary widely but they can be very significant, particularly where methane emissions from wood storage are taken into account. Overall, while some instances of biomass energy use may result in lower life-cycle emissions than fossil fuels, in most circumstances, comparing technologies of similar ages, the use of woody biomass for energy will release higher levels of emissions than coal and considerably higher levels than gas.
The impacts on the climate will also vary, however, with the type of woody biomass used, with what would have happened to it if it had not been burnt for energy and with what happens to the forest from which it was sourced.
Biomass energy feedstocks
The harvesting of whole trees for energy will in almost all circumstances increase net carbon emissions very substantially compared to using fossil fuels. This is because of the loss of future carbon sequestration from the growing trees – particularly from mature trees in old-growth forests, whose rate of carbon absorption can be very high – and of the loss of soil carbon consequent upon the disturbance.
The use of sawmill residues for energy has lower impacts because it involves no additional harvesting; it is waste from other operations of the wood industry. The impact will be most positive for the climate if they are burnt on-site for energy without any associated transport or processing emissions. However, mill residues can also be used for wood products such as particleboard; if diverted instead to energy, this will raise carbon concentrations in the atmosphere. The current high levels of use of mill residues mean that this source is unlikely to provide much additional feedstock for the biomass energy industry in the future (or, if it does, it will be at the expense of other wood-based industries). Black liquor, a waste from the pulp and paper industry, can also be burnt on-site for energy and has no other use; it is in many ways the ideal feedstock for biomass energy.
The use of forest residues for energy should also imply no additional harvesting, so its impacts on net carbon emissions can be low (though whole trees can sometimes be misclassified as residues). This depends mainly on the rate at which the residues would decay and release carbon if left in the forest, which can vary substantially. If slow-decaying residues are burnt, the impact would be an increase in net carbon emissions potentially for decades. In addition, removing residues from the forest can adversely affect soil carbon and nutrient levels as well as tree growth rates.
Many of the models used to predict the impacts of biomass use assume that mill and forest residues are the main feedstock used for energy, and biomass pellet and energy companies tend to claim the same, though they often group ‘low-grade wood’ with ‘forest residues’, although their impact on the climate is not the same. Evidence suggests, however, that various types of roundwood are generally the main source of feedstock for large industrial pellet facilities. Forest residues are often unsuitable for use because of their high ash, dirt and alkali salt content.
Biomass and the forest carbon cycle
It is often argued that biomass emissions should be considered to be zero at the point of combustion because carbon has been absorbed during the growth of the trees, either because the timber is harvested from a sustainably managed forest, or because forest area as a whole is increasing (at least in Europe and North America). The methodology specified in the 2009 EU Renewable Energy Directive and many national policy frameworks for calculating emissions from biomass only considers supply-chain emissions, counting combustion emissions as zero.
These arguments are not credible. They ignore what happens to the wood after it is harvested (emissions will be different if the wood is burnt or made into products) and the carbon sequestration forgone from harvesting the trees that if left unharvested would have continued to grow and absorb carbon. The evidence suggests that this is true even for mature trees, which absorb carbon at a faster rate than young trees. Furthermore, even if the forest is replanted, soil carbon losses during harvesting may delay a forest’s return to its status as a carbon sink for 10–20 years.
Another argument for a positive impact of burning woody biomass is if the forest area expands as a direct result of harvesting wood for energy, and if the additional growth exceeds the emissions from combustion of biomass. Various models have predicted that this could be the case, but it is not yet clear that this phenomenon is actually being observed. For example, the timberland area in the southeast of the US (where most US wood pellet mills supplying the EU are found) does not appear to be increasing significantly. In any case, the models that predict this often assume that old-growth forests are replaced by fast-growing plantations, which in itself leads to higher carbon emissions and negative impacts on biodiversity.
The carbon payback approach argues that, while they are higher than when using fossil fuels, carbon emissions from burning woody biomass can be absorbed by forest regrowth. The time this takes – the carbon payback period before which carbon emissions return to the level they would have been at if fossil fuels had been used – is of crucial importance. There are problems with this approach, but it highlights the range of factors that affect the impact of biomass and focuses attention on the very long payback periods of some feedstocks, particularly whole trees.
The many attempts that have been made to estimate carbon payback periods suggest that these vary substantially, from less than 20 years to many decades and in some cases even centuries. As would be expected, the most positive outcomes for the climate, with very low payback periods, derive from the use of mill residues (unless they are diverted from use for wood products). If forest residues that would otherwise have been left to rot in the forest are used, the impact is complex, as their removal may cause significant negative impacts on levels of soil carbon and on rates of tree growth. The most negative impacts involve increasing harvest volumes or frequencies in already managed forests, converting natural forests into plantations or displacing wood from other uses.
Some have argued that the length of the carbon payback period does not matter as long as all emissions are eventually absorbed. This ignores the potential impact in the short term on climate tipping points (a concept for which there is some evidence) and on the world’s ability to meet the target set in the 2015 Paris Agreement to limit temperature increase to 1.5°C above pre-industrial levels, which requires greenhouse gas emissions to peak in the near term. This suggests that only biomass energy with the shortest carbon payback periods should be eligible for financial and regulatory support.
There is growing interest in the combination of bioenergy with carbon capture and storage (BECCS) with the aim of providing energy supply with net negative emissions. The latest assessment report of the Intergovernmental Panel on Climate Change (IPCC) relies heavily on bioenergy for heat and power, and specifically on BECCS, in most of its scenarios of future mitigation options. However, all of the studies that the IPCC surveyed assumed that the biomass was zero-carbon at the point of combustion, which, as discussed above, is not a valid assumption. In addition, the slow rate of deployment of carbon capture and storage technology, and the extremely large areas of land that would be required to supply the woody biomass feedstock needed in the BECCS scenarios render its future development at scale highly unlikely. The reliance on BECCS of so many of the climate mitigation scenarios reviewed by the IPCC is of major concern, potentially distracting attention from other mitigation options and encouraging decision makers to lock themselves into high-carbon options in the short term on the assumption that the emissions thus generated can be compensated for in the long term.
- In assessing the climate impact of the use of woody biomass for energy, changes in the forest carbon stock must be fully accounted for. It is not valid to claim that because trees absorb carbon as they grow, the emissions from burning them can be ignored.
- Along with changes in forest carbon stock, a full analysis of the impact on the climate of using woody biomass for energy needs to take into account the emissions from combustion (which are generally higher than those for fossil fuels) and the supply-chain emissions from harvesting, collection, processing and transport. There is still some uncertainty over some of these factors and further research would be helpful.
- The provision of financial or regulatory support to biomass energy on the grounds of its contribution to mitigating climate change should be limited only to those feedstocks that reduce carbon emissions over the short term.
- In practice, this means that support should be restricted to sawmill residues, together with post-consumer waste. Burning slower-decaying forest residues or whole trees means that carbon emissions stay higher for decades than if fossil fuels had been used.