1 Introduction
2 Materials and methods
2.1 Description of the system under study
2.2 Selection and description of assessment methods
2.3 Analysis of selected assessment methods
Carbon cycle methods | Land use methods | ||||
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GWPbio | GWPnetbio | WF | GWPsoil | CRP | |
Does the method use a reference situation? | Yes, but reference situation not explicitly stated | Yes, uses several reference situations that are not all explicitly stated | Yes, but reference situation not explicitly stated | Yes, explicitly stated | Yes, explicitly stated |
Does the method account for potential timing differences between emission release and uptake? | Yes, the timing of carbon release and uptake is at the core of the method. | Yes, the timing of carbon release and uptake is at the core of the method. | Yes, but it is assumed that all carbon is emitted at the time of harvest. | No, but time is used to distribute land transformation impacts. | No, but time is used to distribute landtransformation impacts. |
Does the method consider all carbon pools (above and below ground) related to the biomass system? | No clear description, seems to depend on available data | No clear description, seems to depend on available data | No clear description, seems to depend on available data | Below ground only (soil organic carbon) | Above and below ground (vegetation and soil organic carbon) |
Does the method account for temporary carbon storage in biomass-based products? | Yes, the timing of product oxidation is considered. | Yes, the timing of product oxidation is considered. | No, it is assumed that all carbon is emitted at the time of harvest. | No, only the impact on land quality due to land use is included. | No, only the impact on land quality due to land use is included. |
Does the method consider product substitution effects? | No, only carbon uptake and release due to biomass use is considered. | Yes, the method considers displaced emissions | No, only carbon uptake and release due to biomass use is considered. | No, only changes in land quality due to land use are considered. | No, only changes in land quality due to land use are considered. |
2.4 Life cycle inventory
Unit | Sugarcane, fermentation | Wood, fermentation | Wood, gasification | Fossil based | Name in Fig. 1 | |
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Feedstock use | [t dry matter/t PE] | 12.4a | 3.8 | 5.0 | n/a | n/a |
CO2 uptake during growth | [t CO2/t PE] | 18.2a | 7.0 | 9.2 | n/a | CO2,regrowth |
Biogenic CO2 emissions from production | [t CO2/t PE] | 15.1 | 3.9 | 6.1 | n/a | CO2,prod,bio |
Biogenic CO2 emissions from final oxidation | [t CO2/t PE] | 3.1 | 3.1 | 3.1 | n/a | CO2,burn,bio |
Fossil CO2 emissions over life cycle | [t CO2/ t PE] | 1.1 | 1.4 b | 0.4 b | 4.4 | CO2,fossil |
Land occupation flow, CRP | [ha.year/t PE] | 0.085 | 1.7 c | 2.3 c | n/a | OccCRP |
Land transformation flow, CRP | [ha/t PE] | 1.3 × 10−4 | 0 d | 0 d | n/a | Trans |
Carbon flow during land occupation, GWPsoil | [t C/t PE] | 0.049 | 0 e | 0 e | n/a | n/a |
Land transformation flow, GWPsoil | [ha/t PE] | 2.6 × 10−4 | 0 d | 0 d | n/a | Trans |
Wood | Sugarcane | Name in Fig. 1 | |
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C100 in vegetation [t C/ha] | 40a | 20c | n/a |
C200 in vegetation [t C/ha] | 50a | 29b | n/a |
C in soil [t C/ha] | 344b | 117b | Csoil,before harvest (wood) Csoil, before transformation (sugar cane) |
Annual carbon backflow [t C/ha.year] | 1.1d | 0.6b | CO2,regrowth |
2.5 Data and modelling choices and assumptions
3 Results
3.1 Results for the carbon cycle methods
3.2 Results for the land use methods
4 Discussion
4.1 Reference states and reference points
4.2 Spatial system boundaries—stand versus landscape approach
4.3 Approaches towards time in land use and carbon cycle methods
4.4 Carbon pools considered
4.5 Temporary carbon storage in products
4.6 Application challenges
5 Conclusions
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Consideration of time. Time differences between uptake and release of carbon need to be considered in LCIA methods for biogenic carbon flows. Currently, such time differences are considered only in what we here have called the carbon cycle methods, but not in the methods here called land use methods. Furthermore, there are still unresolved issues regarding amortization periods (although recommendations exist, see Sect. 4.3) and regarding what time horizons are used to assess the global warming potential of biogenic carbon emissions.
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Applicability to short and long rotation crops. The LCIA methods for biogenic carbon flows need to be applicable equally well to short rotation and long rotation crops, which is related to their consideration of time. If for no other reason, this is because LCAs of biomass-based products often compare products from different types of feedstock. Our case study showed that the application of carbon cycle methods to short rotation crops was open to interpretation, and hence, the results were not robust, which is why developers of carbon cycle methods in particular need to consider the methods’ applicability to short rotation crops.
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Coherent terminology and methodology aligned with general LCIA frameworks. More work is needed to arrive at a coherent terminology and set of concepts in the field, although it is recognized that the work leading up to the UNEP-SETAC guideline (Koellner et al. 2013) has included such efforts. For instance, there is a need to incorporate the issue of time lags between uptake and release of carbon into a more general framework for LCIA of land use activities. In addition, there are many terms used in the field that (presumably) have the same meaning, or roughly the same meaning, such as foregone sequestration, delayed relaxation and re-evolution. Work on definitions and translations between concepts would help to further the development of the field. Moreover, a stricter adherence to general LCIA frameworks would be valuable and make the methods easier to interpret by LCA analysts. For example, elements normally belonging to the inventory phase of LCA should preferably not be built into LCIA methods (as the GWPnetbio method does). It would also be helpful if the characterization factors that are calculated using the methods are more clearly defined and explicitly spelled out as such.
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Value laden methodological choices. As any LCIA method, the tested LCIA methods include value-laden choices. For LCIA of carbon flows related to biomass production and use, the choice of reference state(s) is one such choice of particular importance. For example, while being very clear about the need for a reference state, the UNEP-SETAC guideline (Koellner et al. 2013) is less clear about what state to use exactly. A similar issue also applies to the GWPnetbio and the reference state for lost uptake, in particular for short rotation crops. Other points of reference used by the carbon cycle methods are less explicit. This applies in particular to the fact that all the carbon cycle methods tested here start their assessment with the harvesting of a mature stand and model the re-growth to this state. We argue that this means that the fully grown forest stand is used as point of reference and that this needs to be clearly stated. We recognize that LCIA methods cannot be constructed without making value laden methodological choices, and recommend method developers to be explicit about them. We also recommend methods to be tested using different modelling choices regarding starting points, e.g. a fully grown forest vs. a forest that starts to grow.
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Stakeholder involvement. Finally, the choice of reference states and other value laden methodological choices need to be discussed with stakeholders in the field, including land owners, policy makers, industrial stakeholders, NGOs, as well as experts from the fields the LCIA methods draw on. Otherwise, there is a risk that stakeholders feel excluded and might simply ignore and hence not act upon assessments. Even though such discussions might not lead to consensus, they will pave the way for informed choices of reference states and other value laden elements of the methods, and possibly with time and experience, the development of conventions.