The Implications of Bioenergy Derived From Forests for Net Greenhouse Gas Emissions

Greenhouse gas (GHG) accounting systems consider bioenergy from forests as carbon neutral because, under international greenhouse gas accounting rules, the carbon contained in wood is deemed emitted to the atmosphere at the time of transfer out of the forest ecosystem. This accounting simplification eliminates the need to track the fate of harvested carbon and to quantify the emissions from harvested wood when and where they occur. Life Cycle Analyses typically assume carbon neutrality of bioenergy with the justification that the regrowing forest will remove emitted carbon from the atmosphere. These simplifying assumptions fail to account for the timing of emissions and removals and the net impact on atmospheric GHG concentrations. To contribute to climate change mitigation bioenergy does not have to be carbon neutral: it merely has to produce fewer net GHG emissions than the fossil fuel alternative. Quantification of the timing of net GHG reductions from forest-derived bioenergy use is thus a prerequisite to the development of effective climate change mitigation portfolios. Net GHG emission estimates are obtained by accounting for carbon stock changes in forest ecosystems, the emissions from biomass use, and the avoided emissions from the displaced fossil fuels. Forest biomass typically causes higher emissions per unit of energy than fossil fuels. The excess emissions are reduced over time as regrowing forests remove C from the atmosphere. The type of forest biomass used, the conversion efficiency, the fossil fuel type that is displaced, and the rate of forest regrowth all affect the substitution benefits of bioenergy. Results indicate that the break-even point at which net emissions from bioenergy are equal to those from the displaced fossil fuel can be years or decades into the future. After that break even point, the net benefits of bioenergy use increase over time. Copyright: Her Majesty the Queen in right of Canada, Natural Resources Canada, Canadian Forest Service.