Wood energy, carbon sinks and global climate change1
Evidence for global climate change is accumulating, and today there is a growing consensus that the most important cause is humankind’s interference in the natural cycle of greenhouse gases, especially carbon dioxide (CO2). Since the beginning of the twentieth century the atmospheric concentration of greenhouse gases has increased from roughly 300 to 360 parts per million, and the two main causes have been identified as:
Using more bioenergy can help reduce dependence on fossil fuels and resulting emissions. In addition, plantation of trees and sustainably managed forests, including those managed for woodfuel, can help avoid or reverse deforestation and can offset carbon emissions by serving as caorbon “sinks”.
Plants capture CO2 from the atmosphere and release oxygen through photosynthesis. Some of the CO2 is lost through respiration, but a major part is sequestered in living and dead organic matter, for instance in wood, wood products and soils. While burning fossil fuels releases CO2 that has been locked up for millions of years, burning biomass simply returns to the atmosphere the CO2 that was absorbed as the plants grew. Under sustainable management, this CO2 is again recaptured by the growing forest, and there is no net release of CO2.
If an area of non-forest land is converted to forest, additional CO2 will be removed from the atmosphere and stored in the tree biomass. The carbon stock on that land increases. However, the newly created forest is a carbon sink only while the carbon stock continues to increase. Eventually an upper limit is reached where losses through respiration, death and disturbances from fire, storms, pests, diseases or harvesting approximately equal the carbon gain from photosynthesis.
Harvested wood from these forests is converted into wood products, which also act as a sink until the decay and destruction of old products matches the addition of new products. Since harvest cannot be increased beyond a sustainable limit, the forest and the products derived from it have a finite capacity to store CO2 from the atmosphere; they act as a perpetual carbon store only when managed sustainably, and otherwise release the carbon previously fixed.
If biomass, including wood, is substituted for fossil fuels, however, land used for sustainable biomass and bioenergy production can continue to provide emissions reductions indefinitely. Often there are opportunities for synergy between bioenergy and wood production and management of forests as carbon sinks, particularly on a regional scale. An example of synergy is that found in integrated management for wood, carbon sequestration and bioenergy, in which the stand is thinned to maximize the combined value of wood production and carbon sequestration, and where cleanings, precommercial fellings and logging residues are used for bioenergy.
Fossil energy is usually consumed in producing bioenergy, for instance during felling of trees in the forest or hauling of timber, but research shows that usually the energy used is a small fraction of the energy produced – roughly 25 to 50 units of bioenergy are produced for every 1 unit of fossil energy consumed in production. Net carbon emissions from generation of a unit of electricity from bioenergy are 10 to 20 times lower than emissions from fossil fuel-based electricity generation.
The approximate global potential for biological mitigation of climate change has been estimated as 100 gigatonnes of carbon by 2050, approximately 10 to 20 percent of total estimated fossil fuel emissions during that time. Roughly two-thirds of this carbon storage could occur in forests.
Ultimately, carbon stocks in vegetation will reach ecological or practical saturation. This potential might be achieved at the same time as greater bioenergy production is realized, with much of the future bioenergy supply probably coming from some of the newly created forests or adapted agricultural systems. It is estimated that bioenergy has the potential to reduce global CO2 emissions in the year 2050 by up to 25 percent of projected fossil fuel emissions, with a potential for further increases thereafter.
1 Adapted from: Matthews, R. & Robertson, K. 2001. Answers to ten frequently asked questions about bioenergy, carbon sinks and their role in global climate change, prepared by the International Energy Agency (IEA) Bioenergy Task 38, “Greenhouse Gas Balances of Biomass and Bioenergy Systems”. Graz, Austria, Joanneum Research. Available on the Internet: www.joanneum.at/iea-bioenergy-task38/ publication/task38faq.pdf. For further details please contact: email@example.com or firstname.lastname@example.org.