Forests make an important contribution towards meeting net zero emissions targets by storing sequestered carbon. However, the growing use of forest biomass for energy has raised concerns over the immediate emissions from burning wood.
The growing number of countries adopting national targets for net zero greenhouse gas emissions is helping to focus attention on the policy mixes needed. The role of forests as stores of sequestered carbon is an important element of this debate, as is the use of forest biomass to generate electricity and heat. Policy frameworks in many countries, including the EU and UK, treat biomass energy as zero-carbon at the point of combustion and, accordingly, grant it access to financial and regulatory support available to other forms of renewable energy. These incentives have helped to drive a rapid increase in the consumption of biomass for energy and stimulated a debate about its impact on the climate.
When burnt in the presence of oxygen, wood and other forms of vegetation biomass emit carbon dioxide (CO₂), almost always at higher concentrations per unit of electricity or heat generated than fossil fuels. The classification of biomass as zero-carbon at the point of combustion derives from either or both of the following 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. The problems with this assumption are examined at length in the first paper published by Chatham House on biomass, Woody Biomass for Power and Heat: Impacts on the Global Climate (2017), and in many other publications. To summarize, harvesting and burning trees for energy results in a substantial initial increase in carbon emissions, creating a ‘carbon debt’ on land. While the regrowth of trees and the displacement of the fossil fuels that would have been used instead will eventually pay off this carbon debt, regrowth takes time and its effectiveness is uncertain because of the potential effects of climate and land-use changes. As many studies have shown, wood burning increases atmospheric CO₂ levels for decades or even centuries until the regrowing forest can accumulate enough carbon to replace that lost previously to harvesting – the ‘carbon payback period’ – with potentially significant impacts on global warming.
Wood burning increases atmospheric CO₂ levels for decades or even centuries until the regrowing forest can accumulate enough carbon to replace that lost previously to harvesting.
If natural forests are replaced by plantations, the stored carbon may never be replaced, even if the number or density of trees is maintained. Plantation forests have higher growth rates than natural forests and are typically harvested at a relatively young age, while naturally regenerated forests tend to be older and have larger trees when harvested. Therefore, more stored carbon is lost when natural forests are harvested, as it takes longer to replace the stored carbon that was emitted to the atmosphere. Where residues or wastes are used instead of whole trees, the impact on carbon levels in the atmosphere is lower – though potentially still significant – depending on what would have happened to these wastes and residues had they not been burnt for energy. However, current policy frameworks do not distinguish accurately between different categories of feedstock in allocating financial and regulatory support. Feedstock category definitions are too vaguely written to permit accurate distinctions between whole trees and residues or, sometimes, between wood from plantations and from natural forests.
Second, to avoid double-counting emissions from biomass energy within the energy sector (when the biomass is burnt) and the land-use sector (when the biomass is harvested), the international greenhouse gas reporting and accounting frameworks established under the UN Framework Convention on Climate Change (UNFCCC) and the 1997 Kyoto Protocol provide that emissions should be reported within the land-use sector only. As explored in detail in the 2017 Chatham House paper, this approach makes sense for the system of global reporting which allows overall carbon flows to be measured. However, it has resulted in significant gaps in the context of national accounting – measuring emissions levels against countries’ targets under the Kyoto Protocol, the 2015 Paris Agreement on climate change (where countries’ Nationally Determined Contributions – NDCs – contain such targets) or under national legislation, such as the UK’s Climate Change Act. This is the case particularly where the countries of production and consumption are different, as land-use sector reports do not distinguish between different uses for harvested biomass.
As a result, countries have used subsidies and regulatory incentives to encourage energy companies to replace fossil fuels with biomass, leading to a growth in CO₂ emissions, with at least a portion – specifically that associated with biomass combustion – not reflected anywhere in emissions accounting against national greenhouse gas reduction targets.
This paper examines this issue with regard to one particular source of woody biomass: wood pellets sourced from the US and burnt for electricity and combined heat and power (CHP) in the EU and UK. Although this represents less than 5 per cent of the total woody biomass consumed for energy in the EU, the market has grown rapidly in recent years. US-sourced pellets account for the majority of wood pellet imports to the UK, which alone consumed 21 per cent of all the wood pellets produced worldwide in 2018. Most UK imports have been burnt at one facility – the Drax power station – which since 2013 has converted four of its six units from coal to biomass. Although consumption of US wood pellets in the EU27 is lower, they remain an important source. Imports to Belgium, Denmark and the Netherlands are already significant, and are forecast to grow further to the latter two countries, as well as to Germany.
Chapter Two of this paper discusses in detail the issues summarized at the start of this chapter – the different feedstocks used for biomass energy and their impact on the climate, plus the challenges faced in accounting for these emissions against national greenhouse gas emission reduction targets.
Chapter Three reviews data on the use and sources of woody biomass for energy in the EU and UK, and on emissions from the consumption of biomass for energy, as reported to the UNFCCC. It then calculates the full emissions associated with the consumption of US-sourced biomass in the EU and UK, under three categories: 1) emissions from the combustion of wood pellets; 2) emissions from energy use in the supply chain, including harvesting, processing and transporting; and 3) the impact on forest carbon emissions and stocks in the forests of the southern US from which the pellets are sourced. The detailed methodology is set out in the Annex.
Chapter Four analyses likely future demand for US wood pellets in the EU27 and UK over the next 10 years, including estimates for imports in 2025 and 2030. It then uses the emissions figures calculated in Chapter Three to outline the associated impacts on the climate.
Chapter Five concludes by proposing a series of changes to the sustainability criteria currently used in the EU and UK, with the aim of restricting support for biomass energy. We recommend that the criteria should be changed to restrict eligible feedstocks to those with the lowest impacts on the climate, and also that much tighter definitions of feedstock categories be introduced to prevent whole trees being treated in the same way as genuine residues. Emissions from any non-residue feedstock used for energy should be counted in full against the consuming country’s climate targets.
