Why burning primary woody biomass is worse than fossil fuels for climate

In its current form, the EU’s renewable energy directives encourage the use of primary woody biomass from forests as an energy source. However, the directive gives a completely wrong picture of the associated greenhouse gas emissions, write a group of academics.

Klaus Josef Hennenberg and Hannes Böttcher are researchers at the Oeko-Institut in Berlin. Sampo Soimakallio is a researcher at the Finnish Environmental Institute, Helsinki. Edward Robinson is a consultant at Economy, Land and Climate Insight, London.

The debate around using wood from forests, so-called primary woody biomass, as a substitute for fossil fuels is a heated one but turns, essentially, on an obvious question: are the greenhouse gas emissions associated with either burning primary woody biomass as a source of fuel less than the emissions avoided by not using fossil fuels?

Remember, to qualify as a renewable energy source under the Renewable Energy Directive II (REDII), the emissions savings over fossil fuels must be at least 70% for new installations. The question is complicated because forests are sequesters of carbon with considerable roles to play in contributing to emissions reduction targets. Reducing their ability to sequester has a significant impact on carbon balances.

The problem is that the methodology currently used by REDII for judging greenhouse gas balances is far too narrow to provide an accurate answer and, as such, often gives the wrong answer, encouraging the harvesting of forest wood instead of the protection of forests (which would better serve the EU’s climate goals).

By contrast, the IPCC methodology for national greenhouse gas inventories assesses how much the carbon stock of the forest area changes as a result of the specific kind of forest management applied. This allows the amount of carbon removed from the harvested wood to be counted as a direct emission.

If forest wood is subsequently burned, e.g., in the energy sector, no further emissions need to be reported. If, on the other hand, wood is transformed into products, part of the harvested carbon is retained.

However, the GHG balancing method in REDII assesses neither the CO2 emissions from the combustion of harvested wood nor changes in carbon storage in the forest area. But when we apply a more complete greenhouse gas (GHG)-balancing methodology, also reflecting the carbon storage in the forest area in accordance with the IPCC principles, we find that the rationale for harvesting to provide wood-energy falls away.

To close this methodological gap, we have developed the idea of the ‘carbon storage balance in forests’. This describes how much carbon storage in forests changes with timber harvest and can be easily integrated into GHG balances.

To get a clearer picture of the interaction between harvesting and the carbon stored in different types of forests, we identified 45 suitable studies out of more than 450 forest modelling studies to capture the complex forest dynamics of diverse increments of different tree species and age classes up to the effects of forest management activities. These studies also captured the impact of forest management practices by examining scenarios with and without intensive forest management.

The carbon storage balance calculated in this manner reveals how strongly the sink capacity of forests changes in line with the amount of wood harvested. For boreal and temperate forests, a mean carbon storage balance of 1.2 tonnes CO2 cubic metre of wood harvested was found (Soimakallio et al. 2022). For German forests, it was in the range of 0.62 to 1.68 tonnes of CO2 m-³ (Fehrenbach et al. 2022). Interestingly, about 1.4 tonnes of CO2 m-³ is locked up in the wood of beech, oak and long-lived deciduous tree species in Germany (compared to about 1 tonne of CO2 m-³ in coniferous trees).

Thus, the carbon content in harvested wood can serve as an alternative estimate to assess the carbon stock changes in forests related to wood harvest.

So what happens when we include the loss of this amount of forest carbon in our calculations as to whether wood is a good substitute for fossil-fuel-derived fuels? The effect is to completely reverse the received wisdom.

The annexe to the Renewable Energy Directive II (page 174) states that if stem wood (i.e. whole trees) is used for energy, an emissions saving of over 80% over fossil equivalents can be assumed. If we assume a carbon storage loss of zero, these numbers will yield an emissions savings over fossil fuels.

But if we plug in a carbon storage loss of 0.62 tonnes of CO2 m-³, we find that wood harvesting for energy will actually serve to raise emissions 13% over a fossil fuel equivalent. If we plug in the mean level of carbon storage lost in Germany (of 1.15 tonnes of CO2 m-³) into the equations, we find that firewood and wood chips, when sourced from primary woody biomass, actually more than double the emissions associated with burning them as a substitute for fossil energy (see details in Hennenberg et al. 2022 and Fehrenbach et al. 2022).

This is a major finding. The debate over the carbon storage lost to wood harvesting is not purely academic. In its current form, REDII encourages using primary woody biomass from forests as an energy source. At the same time, the incomplete GHG balance of REDII gives a completely wrong picture of the associated GHG emissions.

Against this background, the European Parliament’s proposal to end support for harvesting primary woody biomass for energy and to count only the existing amount towards renewable energy targets appears to be the absolute minimum required.

According to our research, a much stricter approach to primary woody biomass for energy is needed, such as no longer classifying primary woody biomass as renewable energy and establishing a phase-out path for existing primary woody biomass used for energy.

This would also help the EU hit its targets for expanding the carbon sink of European forests.