1. Carbon debt – Lost in the forest? Niclas Scott Bentsen Department of Geosciences and Natural Resource Management, Section for Forest, Nature and Biomass, University of Copenhagen, Rolighedsvej 23, DK-1958 Frederiksberg C Contact Information: nb@ign.ku.dk, tel.: +45 20206318nb@ign.ku.dk Year Million tons yr -1 Million tons BAU Biomass focused Environmental focused Combination scenario BAU Biomass focused Environmental focused Combination scenario References Bentsen NS & Stupak I (2013). Imported wood fuels: A regionalized review of potential sourcing and sustainability challenges. University of Copenhagen. Graudal L et al. (2013). Perspectives for the forests contributions to a transition towards a bio-based economy [in Danish]. Copenhagen, IGN/Skov & Landskab, IFRO/Skov & Landskab. Figure 2. Annual harvest of wood for energy and industry from Danish forests under different management scenarios (Left display), and carbon accumulation in living biomass (Right display). Graudal et al. (2013) Background The rationale for using biomass for energy is that it may displace fossil resources and potentially reduce the emissions of CO 2 to the atmosphere. Bioenergy is commonly, but incorrectly, considered to be carbon neutral. Increased use of biomass for bioenergy will to some extent influence the exchange of carbon between terrestrial and atmospheric carbon pools. Greenhouse gas emissions may, temporarily or permanently, increase or reduce depending on practices and circumstances surrounding the utilization of biomass. Carbon debt When forest biomass is harvested to generate an energy service and displace fossil energy resources, the carbon in the biomass is released to the atmosphere. As… 1) the carbon emitted from biomass is re sequestered by new biomass over a shorter or longer period, and 2) bioenergy technologies commonly are less efficient that their fossil counterparts i.e. emits more carbon per generated energy service, there is a temporal difference between the initial carbon emission from bioenergy and it’s fossil displacement benefit. Forests can displace and store more carbon Graudal et al. (2013) demonstrate that incorporating bioenergy production already in the forest planning phase could enable the Danish forests to produce significantly more biomass for energy purposes, while simultaneously build up more carbon in living biomass. As such increased bioenergy production does not by default conflict with the preservation of carbon in living biomass (figure 2). Implications There is an inherent conflict, when the time horizon of short-term policy goals is applied to systems that work over longer time periods. When focus is on achieving for example 2020 targets for reduction of greenhouse gas emissions, this may in some cases lead to conclusions that carbon emissions from forest energy are higher than emissions from e.g. oil. In the very long term perspective, forest energy that does not lead to a degradation of the forests productivity will have benefits over fossil fuels.. The crucial point is that carbon stored in biomass remains in the fast carbon cycle, while using fossils move carbon from the slow carbon cycle into the fast carbon cycle. Analysis A meta analysis of 11 published studies on carbon payback times including 271 different scenarios shows the considerable variation in the estimated time it takes forest bioenergy to generate real climatic benefits over fossil energy. Not only ecological and engineering parameters, but also the modelling approach itself influence the estimated payback time (Figure 1). Figure 1. Carbon payback time of forest bioenergy as influenced by modeling assumptions.