Using logged trees as a bioenergy source – replacing fossil fuel – carries the risk of first increasing CO2 emissions before reducing them. It could in fact take up to 100 years before emissions begin to decrease. Using crop residues or wood waste does not have this downside, or to a lesser extent. If felling were to increase to provide more energy through biomass, this could hinder rather than help achieving CO2 targets. This is the main conclusion from the analysis ‘Climate effects of wood used for bioenergy’, released today by PBL Netherlands Environmental Assessment Agency in cooperation with Alterra Wageningen UR.
Currently, mainly wood waste and residues are used for bioenergy. Many woodworking companies, for instance, use sawdust for their energy supply, and wood waste is combusted to produce electricity and heat. Ambitious policy goals for renewable energy and greenhouse gas reduction may lead to increased demand for wood from other sources, such as forest residues and even whole trees.
The felling and thinning of trees often leaves wood residues behind in the forest, which slowly disintegrate. This is beneficial to biodiversity and soil quality, but some of these residues can also be harvested and used as an energy source. In the short term, combusting these residues will lead to more CO2 in the atmosphere than would otherwise be emitted through the slow process of disintegration in the forest. Furthermore, bioenergy is a less efficient energy source than fossil fuel. These two factors combined means that it may take several years – in some cases decades – before any real CO2 reductions are achieved.
The actual effect on the amount of CO2 in the air, in other words, strongly depends on what would have happened to the wood – and the carbon locked inside it – if it had not been used for bioenergy. This has also been investigated for additional tree felling. The vast majority of forests in Europe – as in many other regions – is relatively young. If trees are not felled, they are able to grow further and absorb large amounts of CO2. Newly planted trees do not grow as fast in the first years of their lives. It therefore not only takes time to regrow them, but also to compensate for the temporary reduction in CO2 absorption and the resulting increase in CO2 in the air. In many cases, it will take more than 100 years before an actual reduction in CO2 in the atmosphere is achieved, compared to a situation in which fossil fuels are used.
For the carbon balance it, therefore, remains preferable to use felled trees first as building material or in consumer goods, postponing their use as a bioenergy source until the very end of their life span. In this way the absorbed carbon remains stored within the wood for as long as possible. However, this does mean that wood is not immediately available as an energy source, making a rapid increase in the use of woody biomass not feasible.
Currently, the European Commission is preparing sustainability criteria for solid biomass. These criteria will be crucial to prevent a situation where climate policy that is aimed to reduce CO2 emissions by 2020, 2030 or 2050 actually leads to an increase in emissions.