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

Abstract

Accurately assessing the delay before the substitution of fossil fuel by forest bioenergy starts having a net beneficial impact on atmospheric CO 2 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.

Bioenergy as climate change mitigation option within a 2 °C target—uncertainties and temporal challenges of bioenergy systems

“…In the case of perennial crops and forest systems, the main sources of uncertainty relate to carbon sequestration, soil carbon content, management practices and local climatic conditions. Additionally, timeframes when the carbon is sequestered and released play an important part in evaluating the real level of emissions [7,8]. In the biomass processing stage aspects such as drying, GHG emissions during storage and handling losses are major sources of emission uncertainties [8,25].…”

“…Various recent assessments, in particular of forest-based bioenergy systems, have shown that the emission profiles can range widely depending on the methodological choice of the assessment and included factors. These factors were mainly related to aspects such as type and location of the forests, albedo changes after harvesting, level of climate intensity, harvest frequency, rotation length, conversion technology or type of systemic feedback and direct and indirect impacts [5][6][7][34][35][36]. This and above named factors show the challenges related to natural variability, supply chain and activity characteristics, wider often indirect impacts and methodological choices.…”

“…System 3 is more challenging to assess, as the timeframe in which sequestration has occurred is longer and it does involve re-release of carbon that was sequestered a long time ago and would have been used for other things or could have stayed in place often for several years, with a much slower carbon release (e.g. through decomposition), if not used for bioenergy provision [7,39,43,44]. If the activity was solely releasing carbon, this would be unsustainable; but where there is simultaneous forest regeneration, it is possible to argue that there is resequestration within a relevant timeframe [7,38].…”

“…through decomposition), if not used for bioenergy provision [7,39,43,44]. If the activity was solely releasing carbon, this would be unsustainable; but where there is simultaneous forest regeneration, it is possible to argue that there is resequestration within a relevant timeframe [7,38]. The assessment then depends on what would have happened if the feedstocks were not used for energy-an assessment that cannot be based purely on the science of climate change or bioenergy systems but requires a holistic approach and considering wider impacts.…”

“…Bioenergy is linked to a number of challenges, such as real emission reduction, environmental impacts, sustainability, land use, food security, wider socio-economic impacts and financial incentives [1,4]. Past research showed large variation in the emission intensity of various bioenergy systems [5][6][7][8]. Considering the urgency of climate change mitigation, the net climate impacts of bioenergy systems must be within set emission thresholds, delivering the necessary emission reduction within the given timeframe.…”