Published August 2021.
9.1% of world total primary energy in 2017 was supplied by biofuels and waste, and almost all (92.4%) was produced using solid biofuels. 1 (the remaining 8% was supplied by liquid biofuels, biogases and waste). Solid biofuel is also known as biomass, which is simply vegetation, or biological matter that was created by photosynthesis.
Of that 92%, about half (or 4.6% of world total primary energy supply) was supplied as dung and wood for residential cooking and heating,2 by about 2.5 billion people.3 4 This causes millions of deaths annually, damages health, and inhibits education and development.5
The other half of solid biofuel supply was almost all wood pellets and wood chips from forests. Note that the term ‘solid biofuel’ does not distinguish between slow growing biofuels such as trees, and fast growing biofuels such as grass.
Unfortunately the assessment of carbon emissions from energy produced by solid biofuels is a mire, distorted by perverse carbon-accountancy, a lack of regulation and deceptive marketing by trade associations and biofuel companies. A report by Chatham House, a policy institute based in London,6 explains that to classify energy from solid biofuel as carbon-neutral is a misconception, due to two assumptions –
The use of wood for electricity generation and heat in modern 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.
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.Woody Biomass for Power and Heat, Impacts on the Global Climate.7
Chatham House’s report explains this first assumption in greater detail below, and is also explained in the article ‘Is Biomass Really Renewable?‘ by Columbia University.8
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. 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.Woody Biomass for Power and Heat, Impacts on the Global Climate.7
Note the above states that the time taken for carbon emissions from combustion of woody-biomass to be equal to that from a coal or gas power station can range from less than 20 years to decades or centuries. For such energy to be carbon-neutral would take even longer.
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.Woody Biomass for Power and Heat, Impacts on the Global Climate.7
The second assumption that creates the misconception that energy from solid biofuels is carbon-neutral is the potentially mistaken understanding that carbon emissions from solid biofuel energy are actually tallied for a given country, as explained by Chatham House –
In order to avoid double-counting emissions from biomass energy within the energy sector (when the biomass is burned) and the land-use sector (when the biomass is harvested), the rules provide that emissions should be reported within the land-use sector only.
While this approach makes sense for reporting, it has resulted in significant gaps in the context of accounting – measuring emissions levels against countries’ targets under the Kyoto Protocol (or, potentially, the Paris Agreement), largely deriving from the different forest-management reference levels that parties have been permitted to adopt.
The problem of ‘missing’, or unaccounted-for, emissions arises when a country using biomass for energy:
Imports it from a country outside the accounting framework – such as the US, Canada or Russia, all significant exporters of woody biomass that do not account for greenhouse gas emissions under the second commitment period of the Kyoto Protocol;
Accounts for its biomass emissions using a historical forest-management reference level that includes higher levels of biomass-related emissions than in the present; or
Accounts for its biomass-related emissions using a business-as-usual forest-management reference level that includes, explicitly or implicitly, anticipated emissions from biomass energy (since the associated emissions built in to the projection will not count against its national target).Woody Biomass for Power and Heat, Impacts on the Global Climate.7
Neither the US nor Japan account for emissions from their land-use sectors under the Kyoto Protocol, while Germany accounts against a business-as-usual projection that does not explicitly include bioenergy policies, and France uses a business-as-usual projection that includes bioenergy demand from policies up to, but not including, the EU Renewable Energy Directive. Woody biomass emissions from all these countries, therefore, have the potential to go unaccounted for.Woody Biomass for Power and Heat, Impacts on the Global Climate.7
The impact on the forests of the southern US for the EU’s wood pellet supply is detailed by Dogwood Alliance.9
Is burning wood really carbon neutral?The UK’s move away from coal means they’re burning wood from the US, Public Radio International (PRI)10
Southern forest ecosystems do a lot for both people and wildlife. But perhaps most valuable today is their role in storing carbon. And on that key point, critics take strong exception to the industry’s claim that wood pellets are a carbon neutral fuel.
“That’s just not correct,” says John Sterman, a professor at MIT’s Sloan School of Management who recently published a lifecycle analysis of wood bioenergy.
“What we found is that contrary to your intuition, burning wood to make electricity in places like the Drax power plant actually makes climate change worse for the rest of the century” Sterman says.
- Using IEA’s 2017 biofuels and waste figures from the Renewables and Waste balances table at https://www.iea.org/statistics/, Domestic Supply row for year 2017: % = Primary_Solid_Biofuels / (Municipal_Waste + Industrial_Waste + Primary_Solid_Biofuels + Biogases + Liquid_Biofuels) (↩)
- p. 14, https://www.iea.org/reports/technology-roadmap-delivering-sustainable-bioenergy(↩)
- p. 18 https://www.iea.org/reports/technology-roadmap-delivering-sustainable-bioenergy(↩)
- Is Biomass Really Renewable? by Renee Cho, Earth Institute, Columbia University, blogs.ei.columbia.edu/2011/08/18/is-biomass-really-renewable/(↩)