Net land‐atmosphere flows of biogenic carbon related to bioenergy: towards an understanding of systemic feedbacks

Abstract

The notion that biomass combustion is carbon neutral vis-a-vis the atmosphere because carbon released during biomass combustion is absorbed during plant regrowth is inherent in the greenhouse gas accounting rules in many regulations and conventions. But this ‘carbon neutrality’ assumption of bioenergy is an oversimplification that can result in major flaws in emission accounting; it may even result in policies that increase, instead of reduce, overall greenhouse gas emissions. This commentary discusses the systemic feedbacks and ecosystem succession/land-use history issues ignored by the carbon neutrality assumption. Based on recent literature, three cases are elaborated which show that the C balance of bioenergy may range from highly beneficial to strongly detrimental, depending on the plants grown, the land used (including its land-use history) as well as the fossil energy replaced. The article concludes by proposing the concept of GHG cost curves of bioenergy as a means for optimizing the climate benefits of bioenergy policies.

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

“…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.…”

“…There is usually a significant point source release of GHGs at the point of biomass conversion to energy, predominantly in the form of CO 2 . This release has a global warming potential and affects the dynamics of the climate system in exactly the same way as a unit of CO 2 from fossil fuel combustion [34,36,37].…”

“…Recent research has shown that the assumption of carbon neutrality is not given in forest-based bioenergy systems due to the decay time of CO 2 in the atmosphere [5], direct and indirect impacts, alterations in plant growth for bioenergy and systemic feedbacks [34,36,38]. These different aspects play an important role in the evaluation of the real emission profile of bioenergy as alteration can lead to significant variations in the results [5,6,34,36,38,[40][41][42].…”

“…While some studies see a long-term reduction potential [5,34], others show that if wider impacts, e.g. continuously increasing biomass demand for energy, changes in forest and harvesting management and increasing climate change impacts lead to a significant increase in emissions even over a long-term time horizon [6,35,45].…”

Author’s response to commentary on “Carbon balance effects of U.S. biofuel production and use”

“…LCA's major limitation is that it is a static method of analysis that by construction treats biogenic carbon flows as if they are in balance over the defined Blifecycle.^However, that is an incorrect depiction of the situation because biofuel production engages the dynamic flows of the terrestrial carbon cycle (DeCicco 2013;Haberl 2013). Correct analysis must capture the system dynamics and reflect both the time-varying nature of key flows and the initial conditions of the system.…”