CO2 emissions from biomass combustion for bioenergy: atmospheric decay and contribution to global warming

Carbon dioxide (CO₂) emissions from biomass combustion are traditionally assumed climate neutral if the bioenergy system is carbon (C) flux neutral, i.e. the CO₂ released from biofuel combustion approximately equals the amount of CO₂ sequestered in biomass. This convention, widely adopted in life cycle assessment (LCA) studies of bioenergy systems, underestimates the climate impact of bioenergy. Besides CO₂ emissions from permanent C losses, CO₂ emissions from C flux neutral systems (that is from temporary C losses) also contribute to climate change: before being captured by biomass regrowth, CO₂ molecules spend time in the atmosphere and contribute to global warming. In this paper, a method to estimate the climate impact of CO₂ emissions from biomass combustion is proposed. Our method uses CO₂ impulse response functions (IRF) from C cycle models in the elaboration of atmospheric decay functions for biomass‐derived CO₂ emissions. Their contributions to global warming are then quantified with a unit‐based index, the GWP_bio. Since this index is expressed as a function of the rotation period of the biomass, our results can be applied to CO₂ emissions from combustion of all the different biomass species, from annual row crops to slower growing boreal forest.     

Water use and yield of bioenergy poplar in future climates: modelling the interactive effects of elevated atmospheric CO2 and climate on productivity and water use

Vegetation exerts large control on global biogeochemical cycles through the processes of photosynthesis and transpiration that exchange CO₂ and water between the land and the atmosphere. Increasing atmospheric CO₂ concentrations exert direct effects on vegetation through enhanced photosynthesis and reduced stomatal conductance, and indirect effects through changes in climatic variables that drive these processes. How these direct and indirect CO₂ impacts interact with each other to affect plant productivity and water use has not been explicitly analysed and remains unclear, yet is important to fully understand the response of the global carbon cycle to future climate change. Here, we use a set of factorial modelling experiments to quantify the direct and indirect impacts of atmospheric CO₂ and their interaction on yield and water use in bioenergy short rotation coppice poplar, in addition to quantifying the impact of other environmental drivers such as soil type. We use the JULES land‐surface model forced with a ten‐member ensemble of projected climate change for 2100 with atmospheric CO₂ concentrations representative of the A1B emissions scenario. We show that the simulated response of plant productivity to future climate change was nonadditive in JULES, however this nonadditivity was not apparent for plant transpiration. The responses of both growth and transpiration under all experimental scenarios were highly variable between sites, highlighting the complexity of interactions between direct physiological CO₂ effects and indirect climate effects. As a result, no general pattern explaining the response of bioenergy poplar water use and yield to future climate change could be discerned across sites. This study suggests attempts to infer future climate change impacts on the land biosphere from studies that force with either the direct or indirect CO₂ effects in isolation from each other may lead to incorrect conclusions in terms of both the direction and magnitude of plant response to future climate change.     

Contrasting effects of Miocene and Anthropocene levels of atmospheric CO2 on silicon accumulation in a model grass

Grasses are hyper-accumulators of silicon (Si), which they acquire from the soil and deposit in tissues to resist environmental stresses. Given the high metabolic costs of herbivore defensive chemicals and structural constituents (e.g. cellulose), grasses may substitute Si for these components when carbon is limited. Indeed, high Si uptake grasses evolved in the Miocene when atmospheric CO₂ concentration was much lower than present levels. It is, however, unknown how pre-industrial CO₂ concentrations affect Si accumulation in grasses. Using Brachypodium distachyon, we hydroponically manipulated Si-supply (0.0, 0.5, 1, 1.5, 2 mM) and grew plants under Miocene (200 ppm) and Anthropocene levels of CO₂ comprising ambient (410 ppm) and elevated (640 ppm) CO₂ concentrations. We showed that regardless of Si treatments, the Miocene CO₂ levels increased foliar Si concentrations by 47% and 56% relative to plants grown under ambient and elevated CO₂, respectively. This is owing to higher accumulation overall, but also the reallocation of Si from the roots into the shoots. Our results suggest that grasses may accumulate high Si concentrations in foliage when carbon is less available (i.e. pre-industrial CO₂ levels) but this is likely to decline under future climate change scenarios, potentially leaving grasses more susceptible to environmental stresses.     

CO2 fluxes of transitional bioenergy crops: effect of land conversion during the first year of cultivation

The present study examined the effect of land conversion on carbon (C) fluxes using the eddy covariance technique at seven sites in southwestern Michigan (USA). Four sites had been managed as grasslands under the Conservation Reserve Program of the USDA. Three fields had previously been cultivated in a corn/soybean rotation with corn until 2008. The effects of land use change were studied during 2009 when six of the sites were converted to soybean cultivation, with the seventh site kept as a grassland. In winter, the corn fields were C neutral while the CRP lands were C sources, with average emissions of 15 g C m^?2 month^?1. In April 2009, while the corn fields continued to be a C source to the atmosphere, the CRPs switched to C sinks. In May, herbicide (Glyphosate) was applied to the vegetation before the planting of soybean. After tilling the killed‐grass and planting soybean in mid June, all sites continued to be C sources until the end of June. In July, fields previously planted with corn became C sinks, accumulating 15–50 g C m^?2 month^?1. In contrast, converted CRP sites continued to be net sources of C despite strong growth of soybean. The conversion of CRP to soybean induced net C emissions with net ecosystem exchange (NEE) ranging from 155.7 (±25) to 128.1 (±27) g C m^?2 yr^?1. The annual NEE at the reference site was ?81.6 (±26.5) g C m^?2 yr^?1 while at the sites converted from corn/soybean rotation was remarkably different with two sites being sinks of ?91 (±26) and ?56.0 (±20.7) g C m^?2 yr^?1 whereas one site was a source of 31.0 (±10.2) g C m^?2 yr^?1. This study shows how large C imbalances can be invoked in the first year by conversion of grasslands to biofuel crops.     

Second generation bioenergy crops and climate change: a review of the effects of elevated atmospheric CO2 and drought on water use and the implications for yield

Second-generation, dedicated lignocellulosic crops for bioenergy are being hailed as the sustainable alternative to food crops for the generation of liquid transport fuels, contributing to climate change mitigation and increased energy security. Across temperate regions they include tree species grown as short rotation coppice and intensive forestry (e.g. Populus and Salix species) and C₄ grasses such as miscanthus and switchgrass. For bioenergy crops it is paramount that high energy yields are maintained in order to drive the industry to an economic threshold where it has competitive advantage over conventional fossil fuel alternatives. Therefore, in the face of increased planting of these species, globally, there is a pressing need for insight into their responses to predicted changes in climate to ensure these crops are 'climate proofed' in breeding and improvement programmes. In this review, we investigate the physiological responses of bioenergy crops to rising atmospheric CO₂ ([Ca]) and drought, with particular emphasis on the C₃ Salicaceae trees and C₄ grasses. We show that while crop yield is predicted to rise by up to 40% in elevated [Ca], this is tempered by the effects of water deficit. In response to elevated [Ca] stomatal conductance and evapotranspiration decline and higher leaf–water potentials are observed. However, whole-plant responses to [Ca] are often of lower magnitude and may even be positive (increased water use in elevated [Ca]). We conclude that rising [Ca] is likely to improve drought tolerance of bioenergy crop species due to improved plant water use, consequently yields in temperate environments may remain high in future climate scenarios.     

Projection of the future EU forest CO2 sink as affected by recent bioenergy policies using two advanced forest management models

Forests of the European Union (EU) have been intensively managed for decades, and they have formed a significant sink for carbon dioxide (CO₂) from the atmosphere over the past 50 years. The reasons for this behavior are multiple, among them are: forest aging, area expansion, increasing plant productivity due to environmental changes of many kinds, and, most importantly, the growth rates of European forest having been higher than harvest rates. EU countries have agreed to reduce total emissions of GHG by 20% in 2020 compared to 1990, excluding the forest sink. A relevant question for climate policy is: how long will the current sink of EU forests be maintained in the near future? And could it be affected by other mitigation measures such as bioenergy? In this article we assess tradeoffs of bioenergy use and carbon sequestration at large scale and describe results of the comparison of two advanced forest management models that are used to project CO₂ emissions and removals from EU forests until 2030. EFISCEN, a detailed statistical matrix model and G4M, a geographically explicit economic forestry model, use scenarios of future harvest rates and forest growth information to estimate the future carbon balance of forest biomass. Two scenarios were assessed: the EU baseline scenario and the EU reference scenario (including additional bioenergy and climate policies). Our projections suggest a significant decline of the sink until 2030 in the baseline scenario of about 25–40% (or 65–125 Mt CO₂) compared to the models’ 2010 estimate. Including additional bioenergy targets of EU member states has an effect on the development of this sink, which is not accounted in the EU emission reduction target. A sensitivity analysis was performed on the role of future wood demand and proved the importance of this driver for the future sink development.     

Subtle direct effects of rising atmospheric CO2 on insect eggs

Ocean acidification is an important consequence of rising levels of atmospheric CO₂. The chemistry of acidification is, however, general and may disturb pH in terrestrial systems. The present study examines the effects of rising CO₂ on insect eggs, which may be vulnerable to acidification because they are small, have (at least initially) poorly developed physiological systems, and support important developmental events. Newly‐laid eggs of the moth Manduca sexta are exposed to levels of CO₂ between 0 and 2200 p.p.m., in air, and effects on yolk pH, total developmental time, and survival are measured. Altered CO₂ has no effect, over several hours, on the pH of egg yolk, suggesting that yolk fluids are well buffered. By contrast, there is a large developmental change in yolk pH. Eggs exposed to eight different levels of CO₂ for the duration of development show a small but significant parabolic response in development time. Eggs develop fastest at intermediate levels of CO₂, between 400 and 1200 p.p.m., and slower at 0, 1600 and 2000 p.p.m. These results suggest that future rises in CO₂ may not have strong direct effects on insect development.     

The likely impact of elevated [CO2], nitrogen deposition, increased temperature and management on carbon sequestration in temperate and boreal forest ecosystems: a literature review

Temperate and boreal forest ecosystems contain a large part of the carbon stored on land, in the form of both biomass and soil organic matter. Increasing atmospheric [CO2], increasing temperature, elevated nitrogen deposition and intensified management will change this C store. Well documented single-factor responses of net primary production are: higher photosynthetic rate (the main [CO2] response); increasing length of growing season (the main temperature response); and higher leaf-area index (the main N deposition and partly [CO2] response). Soil organic matter will increase with increasing litter input, although priming may decrease the soil C stock initially, but litter quality effects should be minimal (response to [CO2], N deposition, and temperature); will decrease because of increasing temperature; and will increase because of retardation of decomposition with N deposition, although the rate of decomposition of high-quality litter can be increased and that of low-quality litter decreased. Single-factor responses can be misleading because of interactions between factors, in particular those between N and other factors, and indirect effects such as increased N availability from temperature-induced decomposition. In the long term the strength of feedbacks, for example the increasing demand for N from increased growth, will dominate over short-term responses to single factors. However, management has considerable potential for controlling the C store.     

The effects of elevated CO2 on symbiotic N2 fixation: a link between the carbon and nitrogen cycles in grassland ecosystems

The response of plants to elevated CO₂ is dependent on the availability of nutrients, especially nitrogen. It is generally accepted that an increase in the atmospheric CO₂ concentration increases the C:N ratio of plant residues and exudates. This promotes temporary N-immobilization which might, in turn, reduce the availability of soil nitrogen. In addition, both a CO₂ stimulated increase in plant growth (thus requiring more nitrogen) and an increased N demand for the decomposition of soil residues with a large C:N will result under elevated CO₂ in a larger N-sink of the whole grassland ecosystem. One way to maintain the balance between the C and N cycles in elevated CO₂ would be to increase N-import to the grassland ecosystem through symbiotic N₂ fixation. Whether this might happen in the context of temperate ecosystems is discussed, by assessing the following hypothesis: i) symbiotic N₂ fixation in legumes will be enhanced under elevated CO₂, ii) this enhancement of N₂ fixation will result in a larger N-input to the grassland ecosystem, and iii) a larger N-input will allow the sequestration of additional carbon, either above or below-ground, into the ecosystem. Data from long-term experiments with model grassland ecosystems, consisting of monocultures or mixtures of perennial ryegrass and white clover, grown under elevated CO₂ under free-air or field-like conditions, supports the first two hypothesis, since: i) both the percentage and the amount of fixed N increases in white clover grown under elevated CO₂, ii) the contribution of fixed N to the nitrogen nutrition of the mixed grass also increases in elevated CO₂. Concerning the third hypothesis, an increased nitrogen input to the grassland ecosystem from N₂ fixation usually promotes shoot growth (above-ground C storage) in elevated CO₂. However, the consequences of this larger N input under elevated CO₂ on the below-ground carbon fluxes are not fully understood. On one hand, the positive effect of elevated CO₂ on the quantity of plant residues might be overwhelming and lead to an increased long-term below-ground C storage; on the other hand, the enhancement of the decomposition process by the N-rich legume material might favour carbon turn-over and, hence, limit the storage of below-ground carbon.     

Interaction effects of zooplankton and CO2 on phytoplankton communities and the deep chlorophyll maximum

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Effects of elevated CO2 and temperature on leaf characteristics,photosynthesis and carbon storage in aboveground biomass of a boreal bioenergy crop (Phalaris arundinacea L.) under varying water regimes

This work examined the effects of elevated CO₂ and temperature and water regimes, alone and in interaction, on the leaf characteristics [leaf area (LA), specific leaf weight (SLW), leaf nitrogen content (N_L) based on LA], photosynthesis (light‐saturated net carbon fixation rate, P_sat) and carbon storage in aboveground biomass of leaves (C_l) and stem (C_s) for a perennial reed canary grass (Phalaris arundinacea L., Finnish local cultivar). For this purpose, plants were grown under different water regimes (ranging from high to low soil moisture) in climate‐controlled growth chambers under the elevated CO₂ and/or temperature (following a factorial design) over a whole growing season (May–September in 2009). The results showed that the elevated temperature increased the leaf growth, photosynthesis and carbon storage of aboveground biomass the most in the early growing periods, compared with ambient temperature. However, the plant growth declined rapidly thereafter with a lower carbon storage at the end of growing season. This was related to the accelerated phenology regulation and consequent earlier growth senescence. Consequently, the elevation of CO₂ increased the P_sat, LA and SLW during the growing season, with a significant concurrent increase in the carbon storage in aboveground biomass. Low soil moisture decreased the P_sat, leaf stomatal conductance, LA and carbon storage in above ground biomass compared with high and normal soil moisture. This water stress effect was the largest under the elevated temperature. The elevated CO₂ partially mitigated the adverse effects of high temperature and low soil moisture. However, the combination of elevated temperature and CO₂ did not significantly increase the carbon storage in aboveground biomass of the plants.     

Lifetime and temporal occurrence of ectomycorrhizae on ponderosa pine (Pinus ponderosa Laws.) seedlings grown under varied atmospheric CO2 and nitrogen levels

Climate change (elevated atmospheric CO2, and altered air temperatures, precipitation amounts and seasonal patterns) may affect ecosystem processes by altering carbon allocation in plants, and carbon flux from plants to soil. Mycorrhizal fungi, as carbon sinks, are among the first soil biota to receive carbon from plants, and thereby influence carbon release from plants to soil. One step in this carbon release is via fine root and mycorrhizal turnover. It is necessary to know the lifetime and temporal occurrence of roots and mycorrhizae to determine the capacity of the soil ecosystem to sequester carbon assimilated aboveground. In this study, ponderosa pine (Pinus ponderosa Laws) seedlings were grown under three levels of atmospheric CO2 (ambient, 525 and 700 mol CO2 mol-1) and three levels of annual nitrogen additions (0,100 and 200 kg N ha-1) in open-top chambers. At a two-month frequency during 18 months, we observed ectomycorrhizal root tips observed using minirhizotron tubes and camera. The numbers of new mycorrhizal root tips, the numbers of tips that disappeared between two consecutive recording events, and the standing crop of tips at each event were determined. There were more mycorrhizal tips of all three types seen during the summer compared with other times of the year. When only the standing crop of mycorrhizal tips was considered, effects of the CO2 and N addition treatments on carbon allocation to mycorrhizal tips was weakly evident. However, when the three types of tips were considered collectively, tips numbers flux of carbon through mycorrhizae was greatest in the: (1) high CO2 treatment compared with the other CO2 treatments, and (2) intermediate N addition treatment compared with the other N addition treatments. A survival analysis on the entire 18 month cohort of tips was done to calculate the median lifetime of the mycorrhizal root tips. Average median lifetime of the mycorrhizal tips was 139 days and was not affected by nitrogen and CO2 treatments.     

The global pyrogenic carbon cycle and its impact on the level of atmospheric CO2 over past and future centuries

The incomplete combustion of vegetation and dead organic matter by landscape fires creates recalcitrant pyrogenic carbon (PyC), which could be consequential for the global carbon budget if changes in fire regime, climate, and atmospheric CO₂ were to substantially affect gains and losses of PyC on land and in oceans. Here, we included global PyC cycling in a coupled climate–carbon model to assess the role of PyC in historical and future simulations, accounting for uncertainties through five sets of parameter estimates. We obtained year‐2000 global stocks of (Central estimate, likely uncertainty range in parentheses) 86 (11–154), 47 (2–64), and 1129 (90–5892) Pg C for terrestrial residual PyC (RPyC), marine dissolved PyC, and marine particulate PyC, respectively. PyC cycling decreased atmospheric CO₂ only slightly between 1751 and 2000 (by 0.8 Pg C for the Central estimate) as PyC‐related fluxes changed little over the period. For 2000 to 2300, we combined Representative Concentration Pathways (RCPs) 4.5 and 8.5 with stable or continuously increasing future fire frequencies. For the increasing future fire regime, the production of new RPyC generally outpaced the warming‐induced accelerated loss of existing RPyC, so that PyC cycling decreased atmospheric CO₂ between 2000 and 2300 for most estimates (by 4–8 Pg C for Central). For the stable fire regime, however, PyC cycling usually increased atmospheric CO₂ (by 1–9 Pg C for Central), and only the most extreme choice of parameters maximizing PyC production and minimizing PyC decomposition led to atmospheric CO₂ decreases under RCPs 4.5 and 8.5 (by 5–8 Pg C). Our results suggest that PyC cycling will likely reduce the future increase in atmospheric CO₂ if landscape fires become much more frequent; however, in the absence of a substantial increase in fire frequency, PyC cycling might contribute to, rather than mitigate, the future increase in atmospheric CO₂.     

Carbon balances of bioenergy systems using biomass from forests managed with long rotations: bridging the gap between stand and landscape assessments

Studies report different findings concerning the climate benefits of bioenergy, in part due to varying scope and use of different approaches to define spatial and temporal system boundaries. We quantify carbon balances for bioenergy systems that use biomass from forests managed with long rotations, employing different approaches and boundary conditions. Two approaches to represent landscapes and quantify their carbon balances – expanding vs. constant spatial boundaries – are compared. We show that for a conceptual forest landscape, constructed by combining a series of time‐shifted forest stands, the two approaches sometimes yield different results. We argue that the approach that uses constant spatial boundaries is preferable because it captures all carbon flows in the landscape throughout the accounting period. The approach that uses expanding system boundaries fails to accurately describe the carbon fluxes in the landscape due to incomplete coverage of carbon flows and influence of the stand‐level dynamics, which in turn arise from the way temporal system boundaries are defined on the stand level. Modelling of profit‐driven forest management using location‐specific forest data shows that the implications for carbon balance of management changes across the landscape (which are partly neglected when expanding system boundaries are used) depend on many factors such as forest structure and forest owners’ expectations of market development for bioenergy and other wood products. Assessments should not consider forest‐based bioenergy in isolation but should ideally consider all forest products and how forest management planning as a whole is affected by bioenergy incentives – and how this in turn affects carbon balances in forest landscapes and forest product pools. Due to uncertainties, we modelled several alternative scenarios for forest products markets. We recommend that future work consider alternative scenarios for other critical factors, such as policy options and energy technology pathways.     

Simulating effects of changing climate and CO2 emissions on soil carbon pools at the Hubbard Brook experimental forest

Carbon (C) sequestration in forest biomass and soils may help decrease regional C footprints and mitigate future climate change. The efficacy of these practices must be verified by monitoring and by approved calculation methods (i.e., models) to be credible in C markets. Two widely used soil organic matter models – CENTURY and RothC – were used to project changes in SOC pools after clear‐cutting disturbance, as well as under a range of future climate and atmospheric carbon dioxide (CO₂) scenarios. Data from the temperate, predominantly deciduous Hubbard Brook Experimental Forest (HBEF) in New Hampshire, USA, were used to parameterize and validate the models. Clear‐cutting simulations demonstrated that both models can effectively simulate soil C dynamics in the northern hardwood forest when adequately parameterized. The minimum postharvest SOC predicted by RothC occurred in postharvest year 14 and was within 1.5% of the observed minimum, which occurred in year 8. CENTURY predicted the postharvest minimum SOC to occur in year 45, at a value 6.9% greater than the observed minimum; the slow response of both models to disturbance suggests that they may overestimate the time required to reach new steady‐state conditions. Four climate change scenarios were used to simulate future changes in SOC pools. Climate‐change simulations predicted increases in SOC by as much as 7% at the end of this century, partially offsetting future CO₂ emissions. This sequestration was the product of enhanced forest productivity, and associated litter input to the soil, due to increased temperature, precipitation and CO₂. The simulations also suggested that considerable losses of SOC (8–30%) could occur if forest vegetation at HBEF does not respond to changes in climate and CO₂ levels. Therefore, the source/sink behavior of temperate forest soils likely depends on the degree to which forest growth is stimulated by new climate and CO₂ conditions.     

Climatic effect of atmospheric CO2 doubling on radial tree growth in south eastern France

The climatic effect of a doubling of atmospheric CO₂ on radial growth of trees was studied in ten populations of three species in south eastern France using an Atmospheric General Circulation Model (AGCM) predicting a 3°C increase of mean temperature and a light rise of precipitation. Results are based on empirical growth climate models, involving an Artificial Neural Network (ANN) technique. Only two of the studied populations, on the boundaries of their ecological area, are sensitive to the climatic variations. One is the larch ( Larix decidua Mill.) population located at 2300 m on elevation (near the timberline) which shows a radial growth increase. The other is the most southern French Scots pine ( Pinus sylvestris L.) population which reacts with a severe growth rate reduction.     

米亚罗林区土地利用变化对土壤有机碳和微生物量碳的影响

为了解土地利用变化对土壤有机碳和微生物量碳的影响,分析了川西米亚罗林区原始冷杉林、20世纪60年代云杉人工林、20世纪80年代云杉人工林和农地的土壤有机碳和微生物量碳状况.结果表明,土地利用变化明显地影响了土壤有机碳和微生物量碳含量.土壤有机碳和微生物量碳含量原始林最高,其次为60年代人工林和80年代人工林,农地最低.农地土壤有机碳含量分别比原始林、60年代人工林和80年代人工林低83%、53%和52%,微生物量碳含量分别低23%、25%和21%.土壤有机碳和微生物量碳含量均随土壤深度的增加而降低,并且两者在不同土地利用类型的变化趋势基本一致.相关分析表明,土壤有机碳和土壤微生物量碳与全氮、水解氮、速效磷呈极显著相关(P＜0.01),说明土壤微生物量碳可作为衡量土壤有机碳变化的敏感指标,而土壤有机碳和微生物量碳含量可作为衡量土壤肥力和土壤质量变化的重要指标.     

Range and uncertainties in estimating delays in greenhouse gas mitigation potential of forest bioenergy sourced from Canadian forests

Accurately assessing the delay before the substitution of fossil fuel by forest bioenergy starts having a net beneficial impact on atmospheric CO₂ is becoming important as the cost of delaying GHG emission reductions is increasingly being recognized. We documented the time to carbon (C) parity of forest bioenergy sourced from different feedstocks (harvest residues, salvaged trees, and green trees), typical of forest biomass production in Canada, used to replace three fossil fuel types (coal, oil, and natural gas) in heating or power generation. The time to C parity is defined as the time needed for the newly established bioenergy system to reach the cumulative C emissions of a fossil fuel, counterfactual system. Furthermore, we estimated an uncertainty period derived from the difference in C parity time between predefined best‐ and worst‐case scenarios, in which parameter values related to the supply chain and forest dynamics varied. The results indicate short‐to‐long ranking of C parity times for residues < salvaged trees < green trees and for substituting the less energy‐dense fossil fuels (coal < oil < natural gas). A sensitivity analysis indicated that silviculture and enhanced conversion efficiency, when occurring only in the bioenergy system, help reduce time to C parity. The uncertainty around the estimate of C parity time is generally small and inconsequential in the case of harvest residues but is generally large for the other feedstocks, indicating that meeting specific C parity time using feedstock other than residues is possible, but would require very specific conditions. Overall, the use of single parity time values to evaluate the performance of a particular feedstock in mitigating GHG emissions should be questioned given the importance of uncertainty as an inherent component of any bioenergy project.     

Diverting residual biomass to energy use: Quantifying the global warming potential of biogenic CO2 (GWPbCO2)

To calculate the global warming potential of biogenic carbon dioxide emissions (GWP_bCO2) associated with diverting residual biomass to bioenergy use, the decay of annual biogenic carbon pulses into the atmosphere over 100 years was compared between biomass use for energy and its business-as-usual decomposition in agricultural, forestry, or landfill sites. Bioenergy use increased atmospheric CO₂ load in all cases, resulting in a ¹⁰⁰GWP_bCO2 (units of g CO₂e/g biomass CO₂ released) of 0.003 for the fast-decomposing agricultural residues to 0.029 for the slow, 0.084–0.625 for forest residues, and 0.368–0.975 for landfill lignocellulosic biomass. In comparison, carbon emissions from fossil fuels have a ¹⁰⁰GWP of 1.0 g (CO₂e/g fossil CO₂). The fast decomposition rate and the corresponding low ¹⁰⁰GWP_bCO2 values of agricultural residues make them a more climate-friendly feedstock for bioenergy production relative to forest residues and landfill lignocellulosic biomass. This study shows that CO₂ released from the combustion of bioenergy or biofuels made from residual biomass has a greenhouse gas footprint that should be considered in assessing climate impacts.