Replacing fossil fuels by logging-residue-based bioenergy has been proposed as a way to mitigate climate change. If residues are combusted for energy, their carbon content is released immediately. Residues, that are not combusted, decompose and emit carbon gradually. The relative harmfulness of bioenergy emissions therefore depends on how strongly we prefer the slow release of carbon to an immediate one. Two factors affect this judgment: (1) our time preference and (2) our expectations regarding the relative harmfulness of future carbon emissions. Neither aspect is included in established biomass emission factors. The Effective Emission Factor (EEF), outlined in this study, includes both aspects in a transparent and tractable way. We demonstrate the concept by deriving the EEFs for the logging residues of three Nordic tree species: Norway spruce (Picea abies), Scots pine (Pinus sylvestris) and silver birch (Betula pendula). We also examine the sensitivity of the results to variation in time preference and damage expectations. The derived factors can be used to compare the harmfulness of carbon emissions from residue-based bioenergy and fossil fuel combustion and to organize bioenergy carbon taxation consistently with the taxation of fossil fuels.