- Burning biomass for energy emits carbon that is part of the continuous exchange of carbon between the biosphere and the atmosphere (biogenic carbon flows). In contrast, fossil fuel emissions represent a linear flow of carbon from geological stores to the atmosphere. Therefore, the effect on the atmospheric GHG concentrations of switching from fossil fuels to biomass cannot be determined by comparing CO2 emissions at the point of combustion.
- To determine the full effect of bioenergy on atmospheric GHG concentrations, assessments need to consider biogenic carbon flows together with other GHG emissions associated with the life cycle of the bioenergy system, and compare with GHG emissions in a realistic reference situation (counterfactual scenario) where energy sources other than bioenergy are used.
- Forest stands are typically not cut for bioenergy alone, but to produce a range of forest products (e.g., construction wood, biomaterials, fuels and chemicals) that can contribute to climate change mitigation by replacing greenhouse-gas intensive products such as cement, steel, and petroleum-based plastics and chemicals, as well as fossil fuels.
- Managed forests usually consist of a mosaic of stands of different ages, which are harvested at different times to obtain a continuous flow of wood for the forest industry. The harvest/replanting cycle maintains the forest in an active stage of growth, thus maintaining the forest carbon sink3. Due to the staggered harvest, carbon losses in harvested stands are balanced by carbon gains (growth) in other stands, so across the whole forest the carbon stock in managed forests is roughly stable.
- Effects on the climate from increased production and use of biomass for energy should therefore be assessed at the forest landscape level (i.e. at the scale that forest estates are managed), not the stand level. Determination of the counterfactual is a critical step in assessments. Some studies make the unrealistic assumption that forests planted for commercial use are left unharvested when there is no demand for bioenergy, ignoring that most forest biomass used for bioenergy is a by-product of higher value timber production.
- The longer-term development of the forest carbon stock depends on biophysical context such as soil and climate conditions, historic and current management regimes, and events such as storms, fires, and insect outbreaks. If harvest volumes (for wood products and energy) and losses related to mortality and disturbances (e.g. storms, insects, fire) do not exceed the growth across the whole forest, there is no net reduction in forest carbon stock.
- Increased biomass use for energy could lead to lower carbon stock and lower sequestration rate in the forest compared to a scenario with less biomass use. However, an increase in demand for bioenergy and other forest products can also incentivise reforestation and improved forest management to increase growth, potentially increasing forest carbon stock compared to the without-bioenergy situation. Forest management generally also reduces the risk of carbon stock losses due to wildfire and diseases/insect outbreaks, issues that are increasingly prevalent in warming climates.
- Concerning GHG emissions, in addition to impacts of bioenergy systems on biogenic carbon flows, full supply chain emissions must be considered. Fuel use for collection, chipping/pelletising and truck transport typically corresponds to less than 10-15% of the energy content in the supplied biomass. Moreover, studies have found that long-distance transport does not negate the climate benefits of biomass as a renewable energy source. For example, GHG emissions associated with transporting pellets between North America and Europe represent less than 5% of the life cycle GHG emissions of hard coal.
- Sustainability governance is required to avoid or mitigate adverse outcomes for the climate and to manage trade-offs with other societal goals. A key requirement is that forests are regenerated and that carbon uptake capacity in the forest is maintained (such as specified in the Recast of the EU Renewable Energy Directive).
Concluding, the most important climate change mitigation measure is the transformation of energy, industry and transport systems so that fossil carbon remains in the ground. Bioenergy plays a strategic role in supporting this transformation. Switching from fossil fuels to biomass from sustainably-managed forests can reduce atmospheric CO2 over time scales relevant to climate stabilisation.
This text has been developed by members of IEA Bioenergy Task 45, see http://task45.ieabioenergy.com/
1 This is an update of a document titled “The use of forest biomass for climate change mitigation: response to statements of EASAC”, from November 2019
2 For example, press releases by the European Academies Science Advisory Council, and an associated publication Norton et al. 2019 “Serious mismatches continue between science and policy in forest bioenergy” GCB Bioenergy, 11(11), pp.1256-1263
3 In the absence of management, forest growth rates decline and disturbance risks increase as trees become mature. Therefore, while old and unharvested forests can hold large amounts of carbon per hectare, they have a lower sink strength and may become carbon sources instead of sinks. READ MORE
The use of forest biomass for climate change mitigation: dispelling some misconceptions (IEA Bioenergy)
USIPA responds to CNN's biomass reporting (U.S. Industrial Pellet Association/Biomass Magazine)
.jpg)
Comments are closed.