Earlier in this blog an article from Helsingin Sanomat was referred. In the article particularly use of stumps for energy was estimated to be as bad as use of fossil coal from the global warming point of view.
The study that Helsingin Sanomat refers, compares the alternatives to leave the biomass at the site or to convert it into energy. Immediate CO2 emissions of wood are comparable with oil, because 50% of wood is coal and 10% hydrogen. CO2 emissions per energy unit are directly dependant on carbon hydrogen ratio of the fuel. On very long observation periods (e.g. 120 years) biomass will decompose totally in the forest and its carbon is released to the atmosphere. Thus, study concludes that only then wood becomes CO2 neutral fuel (because the emissions from stumps burned and left on the ground is converge) and at shorter observation periods it is not CO2 neutral (because stumps left in the forest retain part of carbon away from the atmosphere).
Energy is necessity in the Nordic countries. If stumps or other woody biomass is not available, we replace it with the fossil coal or peat. Therefore, the alternative is not to keep wood away from energy generation but burn solid fossil fuels instead. It would not be very clever to let the woody biomass compost in the forest releasing CO2 and import and combust fossil fuels on top of that.
The decomposition functions used in the study that Helsingin Sanomat refers are based on samples of small wood pieces. In the two independent empirical studies conducted by the Finnish Forest Research Institute and the Swedish University of Agricultural sciences found that stumps decompose more than two times faster than the above mentioned article and its model (Yasso 07) assumes. Only 10-20% of carbon is left in stumps after 40 years of clear cut. As a result, even in the 10-15 year observation time, stump wood would be equal to natural gas in terms of CO2 emissions and from 15 year span onwards it is clearly the best (among other wood fuels) alternative from the climate point of view.
In addition, we must keep in mind that the carbon stock of e.g. Finland’s forests has been increasing by about 30 million tonnes of CO2 annually since 1990. The total growing stock has increased by more than 1 billion m3 since 1950's and continues to increase. This trend can be found in the most countries in the boreal and temperate regions of the northern hemisphere.
As a carbon stock, the forest and forest soil are very unstable compared with the reservoirs of fossil carbon (coal). Storm damages, large insect outbreaks and forest fires have turned hundreds of millions of tonnes of wood into CO2 sources. These risks increase as the forests become overstocked and especially, when extreme weather conditions become more common due to climate change.
I feel that the interpretations made in the Helsingin Sanomat article are somewhat misleading and, partially, based on biased decomposition models. Therefore, I would still think that sustainable use of wood for energy is the second best means to mitigate the climate change after energy saving. It is not necessarily carbon neutral, but as carbon neutral as any fuel can be. When wood production, extraction and energy generation are observed at the system level, forest based energy is clearly carbon negative.
Antti Asikainen, professor, Finnish Forest Research Institute
Bioenergy from forests research programme