Carbon accounting of forest bioenergy and stand thinning
March 16th, 2012 by Rachel Perlman,Article: Hudiburg, T.W. et al. (2011). Regional carbon dioxide implications of forest bioenergy production. Nature Climate Change: 1-5.
Background: Forests are valuable carbon sinks, since atmospheric carbon dioxide is taken up and stored as carbon in tree biomass (trunks, branches, foliage, and roots). However, tree thinning, which makes forests less crowded, is a strategy for preventing forest fires (which release carbon dioxide). Forests also can provide energy (as a biofuel) in the form of firewood, denser wood pellets, wood charcoal, wood-derived liquid fuel. Some strategies for reducing CO2 emissions include substituting forest biofuels for fossil fuels, which is assumed to have zero net emissions (i.e. carbon stored in growth of new tree biomass equals carbon emitted in burning).
Summary: Hudiburg et al. used LCA and inventory data from 80 forests on the US West Coast (California, Washington, and Oregon) to do carbon accounting analyses for current management practices, as well as for the following three forest management practices: (1) fire prevention by removing fuel ladders, (2) removing fuel ladders and enough marketable wood in fire-prone areas to be economically feasible (3) thinning all forestland to support energy production while contributing to fire prevention.
They found that in 90% of the diverse forest area studied, the carbon sink cannot be matched or exceeded by replacing fossil fuels with forest bioenergy. Thinning reduced net biome production in 90% of the region’s forest area. They show that the three scenarios lead to a 2-14% (46-405 Tg C) increase in carbon emissions over a 20-yr period. Forest bioenergy production could only reduce both fire risk and carbon emissions in the near future if forests in the region naturally became weaker sinks, trapping 30-60 g C/m2/yr less (due to insect infestations or higher rates of fires). At present, only three of the 19 ecoregions studied have high enough fire emissions that thinning for fire prevention and bioenergy would likely have a net emission savings.
CATF take-away message: Forests provide a wide variety of valuable services, and it is important to consider various management options in any given forested region by doing proper carbon accounting. Hudiburg et al. wrongly assume that biomass-based energy production is “carbon neutral” (an outdated assumption that neglects how the delay in re-sequestration affects climate, improperly credits bioenergy with sequstration that may have happened anyway, and fails to account for the possibility that regrowth (and thus re-sequestration) may not occur due to development decisions, etc.). Nevertheless, the article still suggests that it is typically not in our best interests from a carbon emissions standpoint to use temperate forests for bioenergy, unless net biome production is weakened there; generally, more carbon will be stored by minimal harvesting.
Identifying global warming potential of biofuels based on their rotation periods
March 16th, 2012 by Rachel Perlman,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.
RESEARCH: Expanding the scope of traditional life cycle analyses through the inclusion of nitrogen and land use intensity to assess the environmental impacts of biofuel generation
June 14th, 2010 by CATF,Article: Miller, S.A. (2010). Minimizing Land Use and Nitrogen Intensity of Bioenergy. Environmental Science and Technology. Vol.4(10) pp.3932-3939.
Synopsis: Shelie A. Miller of Clemson University developed a system of ranking biofuel feedstocks based on land use and nitrogen intensity in order to determine which feedstocks were the most environmentally favorable with respect to these factors. Miller’s results suggest that, “current energy policies either undervalue or do not consider nitrogen and land use impacts.”
This paper explains that widely practiced lifecycle analysis methods tend to focus almost entirely on greenhouse gas emissions, despite the evidence that many environmental systems are negatively impacted by bioenergy production. In particular, most biofuels are known to be water, nutrient and land intensive. This study ranked feedstocks according to nitrogen and land use intensity as these two factors are almost entirely limited to feedstock cultivation stages, whereas water and energy are factors throughout cultivation, manufacturing and “end-of-life” stages.
In terms of the ranking system, 14 biofuel feedstocks were evaluated to determine the “minimum nitrogen and land required to produce 1000GJ of unprocessed energy.” The calculations represent ideal values for maximum energy yield (Maximum Energy Yield = harvestable yield x high heating value) and maximum nitrogen requirements (Maximum nitrogen requirements = harvestable yield x percent nitrogen composition) of each system averaged for a range of conditions and regions for each feedstock but do not account for thermodynamic losses. Land use and nitrogen intensity were weighted equally and the feedstocks were then ranked relative to one another.
The results of the Miller article indicate that sugar crops and algae are the least land intensive when accounting for nitrogen and land intensity, with sugarcane ranking as the most favorable feedstock overall. Food crops, soy and rapeseed in particular, ranked consistently low.
Environmental damages are not limited to greenhouse gas emissions; water, land and nutrient cycling are all vital ecosystem services that may be impacted by biofuel production. Going forward, this research suggests that efforts taken to determine the ecological viability of a particular fuel should include a wider scope of environmental impacts and should not be limited to greenhouse gas emissions.
The full article can be downloaded here.
Study: Land Use-Related GHG Emissions from Biodiesel Production Overwhelm Climate Benefits
February 7th, 2008 by CATF,A Clean Air Task Force-supported study by Tim Searchinger and Ralph Heimlich looks at a critical but under-explored issue in biofuels and climate: the effect of increasing demand for arable land to grow biofuels crops. The study concludes that the carbon dioxide releases from land use change – that is, from the land clearing that is needed to support biofuel production – overwhelm the emissions reductions typically associated with biodiesel use. The biodiesel study is a companion piece to an article by Searchinger et al. that focuses on the land use-related climate impacts of ethanol production. The ethanol study was published in the journal Science on February 7, 2008.
Click here for the complete analysis.
