Comprehensive carbon accounting for identification of sustainable biomass feedstocks
The study estimates carbon impacts of bioenergy from ten biomass feedstock harvesting pathways feeding into three different production pathways. The harvesting pathways include forestry, agricultural residue, and dedicated energy crops. The production pathways are electricity generation, biochemical ethanol production, and thermochemical ethanol production. This is the first study to analyze climate implications of bioenergy from so-called reduced-impact logging.
The analysis takes account of all major carbon sources and sinks, including soil carbon and nutrient losses from residue harvesting, to calculate carbon payback times, carbon intensities, and carbon savings, with the goal of identifying those pathways that have systematically smaller or larger climate impacts.
Harvesting pathways are classified into three groups, according to their potential to deliver greenhouse gas (GHG) reductions: one that can be expected to deliver at least 50% carbon savings with a maximum 10-year payback period, which includes bioenergy from agricultural residues and energy crops; a second offering some GHG savings depending on the choice of production pathway, with payback periods up to 25 years, which consists of bioenergy from forest residues; and another, including bioenergy from whole trees via forest thinning, reduced-impact logging, and short-rotation temperate forestry, that offers no GHG savings over 30 years. Biomass pathways resulting in the lowest land-use change emissions, causing limited forgone carbon sequestration and with low processing emissions, are by far the best candidates for contributing to climate-change mitigation goals.
The authors take an innovative approach to accounting for the temporal character of bioenergy emissions. If biogenic carbon emissions are not resequestered for many years, the carbon dioxide released by harvest and combustion will temporarily contribute to radiative forcing (and hence global warming), just as fossil carbon would. This temporary radiative forcing from biogenic carbon can be modeled by calculating a global warming potential (GWPbio) for a given cycle of carbon emission and sequestration. This compares the warming impact from the temporary emission of a quantity of biogenic carbon dioxide to the warming impact over 100 years of emitting the same quantity of fossil carbon dioxide. The longer the biogenic carbon remains in the atmosphere, the higher its GWPbio factor will be.
The impact on net effective emissions and on carbon payback times of including GWPbio factors is potentially substantial for harvesting cycles longer than 10 years. In some cases, the use of GWPbio factors could change the eligibility and classification of particular biofuels with respect to GHG mitigation in biofuel regulations.
The study has several implications for policymaking. Consistent with earlier studies, it shows that pathways based on whole-tree logging in forests offer little or no climate mitigation over 50 years, but also indicates that reduced impact logging does not deliver GHG savings over the same time period. It delivers a clear signal that agricultural residues and dedicated energy crops should be given priority in research and development.
Although national and regional bioenergy policies do in some cases include life-cycle accounting of bioenergy, there is room for improvement in accounting for key carbon emissions sources, particularly soil carbon losses and the GHG costs of nutrient replacement after residue removal, which could be major emissions sources for cellulosic biofuel feedstocks. For longer harvesting cycles, introducing consideration of global warming potential for a given cycle of biogenic carbon emission and sequestration into the life-cycle analysis could reduce the risk of mischaracterizing climate-change mitigation potential.