Article: Cherubini, F. et al. (2011). CO2 emissions from biomass combustion for bioenergy: atmospheric decay and contribution to global warming. GCB Bioenergy.
Background: Bioenergy from biomass combustion is often assumed to have a net zero impact on GHG emissions in LCA and national GHG inventories. It is presumed that keeping the harvested area forested offsets the carbon released into the atmosphere. However, these assumptions ignore the climate impact of CO2 from temporary change; CO2 is emitted rapidly when biomass is burned at one point in time, but then is sequestered over a longer period of time that depends on the growth rate of the biofuel crop.
Summary: Global warming potential (GWP) of bioenergy can be represented on a scale of 0 to 1; it is a metric that allows scientists to aggregate emissions of different gases to a common unit (kg CO2 equivalent). The assumption that the net flux balances to zero leads some to assign a GWP of zero to direct CO2 emissions. Others assign the GWP equal to 1 (equivalent to impact of fossil fuels), believing that the emissions from combustion must be offset by an equal amount of sequestration (credit) somewhere else. Cherubini et al. challenges both of these assumptions, in order to improve the methodology for quantifying CO2 emissions from bioenergy combustion with a unit-based indicator that can be used in future carbon accounting studies. The group ultimately concluded that the GWP should be some fraction between 0 and 1 and should be a function of the biomass rotation period (although it should be noted that a minority of model scenarios imply a GWP above 1, higher than that for fossil fuels).
Cherubini et al. modeled the atmospheric decay of CO2 emissions to determine the fraction of emissions that still remain in the atmosphere, as limited by the equilibrium response of the ocean-atmosphere system. Their model was simplified with the following assumptions: biomass combustion creates a pulse of emissions, the biomass is clear-cut, the land is immediately re-vegetated with the same species, and the whole process is carbon flux neutral. They ran their model of CO2 over time with three variations of impulse response functions or IRF (mathematical ways of representing CO2 atmospheric decay), which differ from each other in the assumed extent of ocean vs. terrestrial removal. The IRF that considered the full carbon cycle with ocean and terrestrial sinks was found to be most reliable and accurate. The three different IRF were compared for different rotation period lengths (between 1 – 100 years) and for different time horizons (20, 100, 500 years); for all of the IRF, the mean residence time of CO2 in the atmosphere was longer for longer biomass rotation periods. This means that even though both are “flux neutral,” short rotation biomass has less climate impact than long rotation biomass per unit of CO2 emitted from the combustion of the biomass. However, this does not mean that one rotation length is better than the other, since other properties of the biomass growth/management and all climate forcing agents need to be considered as well. Cherubini et al. also concludes, “bioenergy is a climate change mitigation strategy particularly effective for long-term targets,” or longer time horizons.
CATF take-away message: The assumptions made in the models allow the results to be applied to CO2 emissions from combustion of a variety of biomass species ranging from annual crops to slower growing trees. The study does not necessarily endorse fast-growing over slow-growing biocrop species, but does highlight the relationship between GWP, rotation period, and time horizon. Studies like this are instrumental for understanding that carbon flux neutral systems can still contribute to climate change.