Forest Bioenergy Research Articles

Policy coherence in climate change mitigation: An ecosystem service approach to forests as carbon sinks and bioenergy sources

Policies governing forest ecosystems can mitigate climate change in many ways, making use of various ecosystem services. Although identification of ecosystem service trade-offs has received increasing analytical attention, the policies and mixes of policies generating the trade-offs have remained outside the focus. To advance the policy relevance of ecosystem service trade-off analysis, we analyse the coherence of Finnish policies affecting forest bioenergy and carbon sequestration, two contrasting means to use forests for climate change mitigation. In particular, we focus on the interactions that different policy outputs have with respect to these two ecosystem services. The analysed policy outputs represent different foci and levels and rely on different mechanisms. We identify the direct and indirect impacts that the policy outputs have on the supply and demand of the services by utilizing natural science and policy assessment approaches. We find forest bioenergy, representing a tangible ecosystem service exchanged in the market, to be governed more positively and with more explicit instruments compared to carbon sequestration. Carbon sequestration policies remain at a higher level of abstraction, stating merely strategic objectives, possibly because these markets are only emerging and remain political and highly uncertain. Our analysis shows that trade-offs between the two ecosystem services are generated by policies supporting bioenergy, whilst general policies advance both services. The entire mix of policy outputs and its differentiated impacts on ecosystem services should be thoroughly considered when assessing the strategies for mitigating climate change and designing new policy instruments.

Forest carbon accounting methods and the consequences of forest bioenergy for national greenhouse gas emissions inventories

While bioenergy plays a key role in strategies for increasing renewable energy deployment, studies assessing greenhouse gas (GHG) emissions from forest bioenergy systems have identified a potential trade-off of the system with forest carbon stocks. Of particular importance to national GHG inventories is how trade-offs between forest carbon stocks and bioenergy production are accounted for within the Agriculture, Forestry and Other Land Use (AFOLU) sector under current and future international climate change mitigation agreements. Through a case study of electricity produced using wood pellets from harvested forest stands in Ontario, Canada, this study assesses the implications of forest carbon accounting approaches on net emissions attributable to pellets produced for domestic use or export. Particular emphasis is placed on the Forest Management Reference Level (FMRL) method, as it will be employed by most Annex I nations in the next Kyoto Protocol Commitment Period. While bioenergy production is found to reduce forest carbon sequestration, under the FMRL approach this trade-off may not be accounted for and thus not incur an accountable AFOLU-related emission, provided that total forest harvest remains at or below that defined under the FMRL baseline. In contrast, accounting for forest carbon trade-offs associated with harvest for bioenergy results in an increase in net GHG emissions (AFOLU and life cycle emissions) lasting 37 or 90 years (if displacing coal or natural gas combined cycle generation, respectively). AFOLU emissions calculated using the Gross-Net approach are dominated by legacy effects of past management and natural disturbance, indicating near-term net forest carbon increase but longer-term reduction in forest carbon stocks. Export of wood pellets to EU markets does not greatly affect the total life cycle GHG emissions of wood pellets. However, pellet exporting countries risk creating a considerable GHG emissions burden, as they are responsible for AFOLU and bioenergy production emissions but do not receive credit for pellets displacing fossil fuel-related GHG emissions. Countries producing bioenergy from forest biomass, whether for domestic use or for export, should carefully consider potential implications of alternate forest carbon accounting methods to ensure that potential bioenergy pathways can contribute to GHG emissions reduction targets.

Forest biomass for bioenergy: opportunities and constraints for a good governance. A case study from Italy

Interest in the use of biomass for energy has increased significantly in the last few years. The latest report by the Intergovernmental Panel on Climate Change highlights the influence mankind has had on the climate: an unprecedented increase in GHG levels in the last 800,000 years and a rise of 40% in CO 2 concentrations since pre-industrial times. The challenge now is to find energy alternatives, and in this context, one important option is bioenergy, one of the most important energy sources of the future. In light of this, the goal of this paper was to assess the sustainable potential of woodfuel resources in Italy using WISDOM methodology. WISDOM, developed by the FAO, has been applied in many countries around the world. From this study, at national level, household consumption was at 19.3 Mt in 2003 (average value), while the potential supply of woody biomass (productivity) was 24.9 Mt (average value), with a surplus of almost 6 million tons between household consumption and productivity. This study represents an advance in knowledge of the biomass potential for energy use in Italy, and, as such, is subject to possible future improvement. Forest bioenergy development creates good opportunities to mobilize the production potential of European forests, and to contribute to a more climate-friendly, bio-based economy.

Carbon debt repayment or carbon sequestration parity? Lessons from a forest bioenergy case study in Ontario, Canada

AbstractForest bioenergy can contribute to climate change mitigation by reducing greenhouse gas (GHG) emissions associated with energy production. We assessed changes in GHG emissions resulting from displacement of coal with wood pellets for the Atikokan Generating Station located in Northwestern Ontario, Canada. Two contrasting biomass sources were considered for continuous wood pellet production: harvest residue from current harvest operations (residue scenario) and fibre from expanded harvest of standing live trees (stemwood scenario). For the stemwood scenario, two metrics were used to assess the effects of displacing coal with forest biomass on GHG emissions: (i) time to carbon sequestration parity, defined as the time from the beginning of harvest to when the combined GHG benefit of displacing coal with biomass and the amount of carbon in regenerating forest equalled the amount of forest carbon without harvest for energy production; and (ii) time to carbon debt repayment, defined as the time from the beginning of harvest to when the combined GHG benefit of displacing coal with biomass and the amount of carbon in the regenerating forest equalled forest carbon at the time of harvest. Only time to carbon sequestration parity was used for the residue scenario. In the residue scenario, carbon sequestration parity was achieved within 1 year. In the stemwood scenario, times to carbon sequestration parity and carbon debt repayment were 91 and 112 years, respectively. Sensitivity analysis showed that estimates were robust when parameter values were varied. Modelling experiments showed that increasing growth rates for regenerating stands in the stemwood scenario could substantially reduce time to carbon sequestration parity. We discuss the use of the two metrics (time to carbon sequestration parity and time to carbon debt repayment) for assessing the effects of forest bioenergy projects on GHG emissions and make recommendations on terminology and methodologies for forest bioenergy studies.

The Ability of Cellulosic Ethanol to Compete for Feedstock and Investment with Other Forest Bioenergy Options

Abstract The economic performance of cellulosic ethanol production from forest resources was contrasted with that of other bioenergy options: biopower, combined heat and power, wood pellets, and Fischer-Tropsch liquids (biomass-to-liquids). Techno-economic models were designed for each conversion facility and a trucking model was used to determine delivered feedstock cost in central British Columbia, Canada. Facilities and processes were analyzed for their sensitivity to feedstock type (sawdust/shavings, whole logs, pulp chips, harvest residues, and hog fuel), scale (0.05–4 million bone dry tonnes per year), and product revenue volatility (based on historical volatility of proxy fossil fuels). Internal rate of return (IRR) was used as the primary metric for economic performance comparison. Under the base case scenario, with 0.2 million bone dry tonnes per year in the form of whole logs (roundwood), all facilities had a negative IRR, indicating that dedicated bioenergy harvest may be economically challengi...

Can we produce carbon and climate neutral forest bioenergy?

AbstractHarvesting branches, stumps and unmercantable tops, in addition to stem wood, decreases the carbon input to the soil and consequently reduces the forest carbon stock. We examine the changes in the forest carbon cycle that would compensate for this carbon loss over a rotation period and lead to carbon neutral forest residue bioenergy systems. In addition, we analyse the potential climate impact of these carbon neutral systems. In a boreal forest, the carbon loss was compensated for with a 10% increase in tree growth or a postponing of final felling for 20 years from 90 to 110 years in one forest rotation period. However, these changes in carbon sequestration did not prevent soil carbon loss. To recover soil carbon stock, a 38% increase in tree growth or a 21% decrease in the decomposition rate of the remaining organic matter was needed. All the forest residue bioenergy scenarios studied had a warming impact on climate for at least 62 years. Nevertheless, the increases in the carbon sequestration from forest growth or reduction in the decomposition rate of the remaining organic matter resulted in a 50% smaller warming impact of forest bioenergy use or even a cooling climate impact in the long term. The study shows that carbon neutral forest residue bioenergy systems have warming climate impacts. Minimization of the forest carbon loss improves the climate impact of forest bioenergy.

Sustainability of forest bioenergy in Europe: land‐use‐related carbon dioxide emissions of forest harvest residues

AbstractIncreasing bioenergy production from forest harvest residues decreases litter input to the soil and can thus reduce the carbon stock and sink of forests. This effect may negate greenhouse gas savings obtained by using bioenergy. We used a spatially explicit modelling framework to assess the reduction in the forest litter and soil carbon stocks across Europe, assuming that a sustainable potential of bioenergy from forest harvest residues is taken into use. The forest harvest residue removal reduced the carbon stocks of litter and soil on average by 3% over the period from 2016 to 2100. The reduction was small compared to the size of the carbon stocks but significant in comparison to the amount of energy produced from the residues. As a result of these land‐use‐related emissions, bioenergy production from forest harvest residues would need to be continued for 60–80 years to achieve a 60% carbon dioxide (CO2) emission reduction in heat and power generation compared to the fossil fuels it replaces in most European countries. The emission reductions achieved and their timings varied among countries because of differences in the litter and soil carbon loss. Our results show that extending the current sustainability requirements for bioliquids and biofuels to solid bioenergy does not guarantee efficient reductions in greenhouse gas emissions in the short‐term. In the longer‐term, bioenergy from forest harvest residues may pave the way to low‐emission energy systems.

Stakeholder frames in the making of forest bioenergy legislation in Finland

The Finnish forest bioenergy production has mainly evolved in close connection to the industrial forestry system producing pulp, paper and building materials, with tensions also related to these competing forms of forest resource utilization. However, in recent years, the tensions have been growing in terms of climate change, sustainability and innovations, and forest bioenergy production is facing important pressures pushing simultaneously for both increasing and restricting the production. Focusing on stakeholder perspectives in the preparation processes of two recent bioenergy related pieces of legislation, and drawing from framing theory, this study demonstrates the existence of the tensions. Only parts of the tensions are acknowledged in the making of bioenergy policies. The most significant tension evolves between actors related to forest industry and bioenergy production. Environmental tensions and wider sustainability aspects including local actors are largely neglected. The results show sustenance of old energy and forest policy paradigms with resistance related to new emerging interests. The resistance is explained by the ability of established actors to frame themselves as important in the policy arena, which hinders the entrance of new actors. The resistance presents a risk for aggravated tensions in the future and may constrain innovations related to the development of different types of bioenergy production.

On the Economics of Forest Carbon: Renewable and Carbon Neutral But Not Emission Free

First-best optimal forest sector carbon policy is examined. Using a comprehensive forest sector model with a detailed carbon cycle section we show that the renewability and carbon neutrality arguments do not warrant emission free treatment of forest bioenergy. However, under the biomass stock change carbon accounting convention followed by UNFCCC and IPCC, the forest owners pay for the roundwood emissions and, to avoid double counting, the use of roundwood is treated as emission free. The bioenergy from harvest residues cannot be treated as emission free either. Their emission factors are determined through the decay time-scales specific to residue fractions and discount rate used in welfare assessment. In addition, we show that an optimal policy subsidizes the production of wood products sequestering carbon. The relative magnitude of the subsidy is based on the fraction of the carbon stored, the lifetime of the products and the discount rate. Correspondingly, the carbon removals by biomass growth are subsidized and the harvest residue generation taxed. Further, we show that the supply side policies are independent of final use of harvested timber. Numerical solution of the model shows that, although the use of wood is not emission free, it is optimal to increase the use of wood, possibly also in the energy sector. Before the wood use can be increased, the forest biomass has to be increased. This initial carbon sink speeds up the convergence to the lower steady-state atmospheric carbon stock.

Stakeholder engagement in scenario development process – Bioenergy production and biodiversity conservation in eastern Finland

In this study participatory approaches were used to develop alternative forest resource management scenarios with particular respect to the effects on increased use of forest bioenergy and its effect on biodiversity in Eastern Finland. As technical planning tools, we utilized a forest management planning system (MELA) and the Tool for Sustainability Impact Assessment (ToSIA) to visualize the impacts of the scenarios. We organized a stakeholder workshop where group discussions were used as a participatory method to get the stakeholder preferences and insights concerning forest resource use in the year 2030. Feedback from the workshop was then complemented with a questionnaire. Based on the results of the workshop and a questionnaire we developed three alternative forest resource scenarios: (1) bioenergy 2030 – in which energy production is more centralized and efficient; (2) biodiversity 2030 – in which harvesting methods are more nature friendly and protected forests make up 10% of the total forest area; and (3) mixed bioenergy + biodiversity 2030 scenario – in which wood production, recreation and nature protection are assigned to the most suitable areas. The study showed that stakeholder engagement combined with the MELA and ToSIA tools can be a useful approach in scenario development.

Forest Carbon Accounting Considerations in US Bioenergy Policy

Four research-based insights are essential to understanding forest bioenergy and “carbon debts.” (1) As long as wood-producing land remains in forest, long-lived wood products and forest bioenergy reduce fossil fuel use and long-term carbon emission impacts. (2) Increased demand for wood can trigger investments that increase forest area and forest productivity and reduce carbon impacts associated with increased harvesting. (3) The carbon debt concept emphasizes short-term concerns about biogenic CO2 emissions, although it is long-term cumulative CO2 emissions that are correlated with projected peak global temperature, and these cumulative emissions are reduced by substituting forest bioenergy for fossil fuels. (4) Considering forest growth, investment responses, and the radiative forcing of biogenic CO2 over a 100-year time horizon (as used for other greenhouse gases), the increased use of forest-derived materials most likely to be used for bioenergy in the United States results in low net greenhouse gas emissions, especially compared with those for fossil fuels.

Biomasfor: an open-source holistic model for the assessment of sustainable forest bioenergy

This paper presents an open-source spatial analysis model (Biomasfor) that can quantify the availability of forest wood-energy biomass in the light of ecological and economic sustainability. Several multi-functionality parameters were evaluated to highlight the potential impact of biomass extraction on different forest functions. The multistep approach used and the model’s internal structure permit the use of the model with highly differentiated input datasets. The introduction of biomass demand evaluation allows the quantification of the wood-energy supply/demand. The analysis is focused on the province of Trento (northeastern Italian Alps). The results are based on a scenario evaluation characterised by several degrees of biomass extraction and by a sensitivity analysis of biomass price, as well as on a typology of mechanisation. The model outputs define a reduction in biomass availability with the introduction of technical, economic and multi-functionality parameters. Furthermore, considerations on territorial characteristics outline the importance of woodchip production as a means of avoiding carbon dioxide emissions and achieving low-impact reductions of the risk of fires. The model appears to be an effective tool in bioenergy planning, particularly for the following purposes: (i) the estimation of the biomass supply/demand ratio under different scenarios; (ii) a preliminary analysis of biomass quality; and (iii) the influence of local environmental, economic and logistical characteristics on biomass production.

Greenhouse gas emissions of forest bioenergy supply and utilization in Finland

The paper assesses greenhouse gas (GHG) emissions of forest bioenergy supply and utilization in Finland. Each step in the supply chains of harvesting residues (HR), small-diameter energy wood (EW) and stumps (ST) is assessed separately, with geography-related differences between Northern and Southern Finland (NF and SF) taken into consideration. Furthermore, the GHG performance of five distinct bioenergy options—(1) combined heat and power production, (2) condensing power production, (3) torrefied pellets, (4) gasification, and (5) pyrolysis oil production—is assessed and compared with that of current reference systems in Finland and also the European Union (EU) sustainability criteria. If soil carbon stock (SCS) changes and possible storage emissions are omitted, the GHG emissions deriving from the supply chain of comminuted forest biomass to plants are 2.4, 3.0, and 2.6 gCO2eq MJ−1 for HR, EW, and ST in SF, respectively. In NF, the corresponding values are 2.9, 3.6, and 3.2 gCO2eq MJ−1, respectively. If SCS changes and possible emissions from storage are accounted for, the emissions for HR, EW, and ST are in the ranges 9.2–49.2, 24.4–64.4, and 33.1–73.1 gCO2eq MJ−1 in SF and 12.7–52.7, 29.4–69.4, and 39.5–79.5 gCO2eq MJ−1 in NF. Most supply-chain GHG emissions arise from SCS changes and possible emissions from storage of comminuted biomass, both of which may involve significant uncertainty factors. In comparison to local reference systems, significant GHG savings can be achieved through energy utilization of forest biomass, but if SCS changes and, in particular, storage emissions are taken into account, fulfillment of the EU sustainability criteria is not guaranteed.

Bioenergy from forestry and changes in atmospheric CO2: Reconciling single stand and landscape level approaches

Analyses of global warming impacts from forest bioenergy systems are usually conducted either at a single stand level or at a landscape level, yielding findings that are sometimes interpreted as contrasting. In this paper, we investigate and reconcile the scales at which environmental impact analyses of forest bioenergy systems are undertaken. Focusing on the changes caused in atmospheric CO2 concentration of forest bioenergy systems characterized by different initial states of the forest, we show the features of the analyses at different scales and depict the connections between them. Impacts on atmospheric CO2 concentration at a single stand level are computed through impulse response functions (IRF). Results at a landscape level are elaborated through direct application of IRFs to the emission profile, so to account for the fluxes from all the stands across time and space. Impacts from fossil CO2 emissions are used as a benchmark. At a landscape level, forest bioenergy causes an increase in atmospheric CO2 concentration for the first decades that is similar to the impact from fossil CO2, but then the dynamics clearly diverge because while the impact from fossil CO2 continues to rise that from bioenergy stabilizes at a certain level. These results perfectly align with those obtained at a single stand for which characterization factors have been developed. In the hypothetical case of a sudden cessation of emissions, the change caused in atmospheric CO2 concentration from biogenic CO2 emissions reverses within a couple of decades, while that caused by fossil CO2 emissions remains considerably higher for centuries. When counterfactual aspects like the additional sequestration that would have occurred in the forest if not harvested and the theoretical displacement of fossil CO2 are included in the analysis, results can widely differ, as the CO2 debt at a landscape level ranges from a few years to several centuries (depending on the underlying assumptions considered).

Development Trends and Future Prospects of Cut-to-Length Machinery

Over 30 years of cut-to-length harvesting machines experience have demonstrated their effectiveness to logging companies in the countries of the boreal forest zone; i.e. better labour conditions in terms of ergonomics and safety, and less environmental damage and reliability in combination with convenient operation and maintenance. The introduction of fully mechanized cut-to-length technology in Russia shows a number of challenges that reduces cut-to-length effectiveness. These include a lower productivity of harvester-forwarder chains and higher roundwood losses in comparison with the Nordic countries due to a number of objective and subjective reasons (service, training, forest management, motivation, etc). Wood harvesting machinery has developted in the Nordic countries in recent years. The major changes were in integrating the harvesting of logging residues and stumps into the traditional system. Future prospects of cut-to-length technology require the development of forest logistics and forest bioenergy, and better environmental compatibility of wheeled vehicles with forest loam and clay soils.

Potentials and costs of climate change mitigation in the Norwegian forest sector — Does choice of policy matter?

Forests are important contributors to the global carbon cycle and mitigate climate change through carbon sequestration and the supply of wood that substitutes for fossil fuels and greenhouse gas (GHG)-intensive building materials. However, current climate policies only partially credit forest carbon sequestration and bioenergy policies are handled independently of forestry. Using Norway as a case study, we analyze two sets of simulated carbon tax/subsidy policies, one crediting forest carbon sequestration while maintaining predetermined harvest levels and utilization of wood, and another targeting GHG fluxes in the entire forest industrial sector allowing harvest levels and wood markets to change in response to the policy. Results indicate that GHG emission reduction potentials differ substantially between the two policies, being several times higher for the latter than the former policy at a given carbon price. This suggests that (i) previous research efforts in Europe have not captured the full mitigation potential as they have not included adaptations in the harvest level and the wood market and (ii) climate policies should target GHG fluxes in the entire sector to utilize its potential contribution for mitigating climate change.

Best management practices for forest bioenergy programs

Forest ecosystems produce the best quality and most consistent supplies of water for human use. The increase in the use of woody biomass as a feedstock for bioenergy production has raised questions about potential impacts on water quality. Best management practices (BMPs) have been developed and implemented since the early 1970s to ensure that forest harvesting can be conducted with minimum impact on water quality. Although BMPs were originally designed to minimize water quality impacts, they can be used for a variety of environmental concerns. The use of BMPs is widespread in developed countries and it varies from mandatory to voluntary. In many countries BMPs are incorporated in ‘Codes of Forest Practice’ that guide forest managers through the complete bioenergy life cycle. The development and application of BMPs is not a static process, but one that relies on a continual cycle of application, assessment and monitoring, and refinement. Although some countries have “national standards,“ the complex matrix of forest ecosystems, climates, soils and topography, and harvesting systems requires ongoing assessment, monitoring, and refinement to craft BMPs to best suit local conditions. Research and development studies play a key part in the refinement and communication of improved BMPs. They are also crucial in validating the effectiveness of BMPs. This is especially important where local environmental conditions or operational standards are unique. BMPs ensure that forest bioenergy programs can be a sustainable part of forest management and renewable energy production.This article is categorized under: Bioenergy > Science and Materials Bioenergy > Climate and Environment

Feedstock specific environmental risk levels related to biomass extraction for energy from boreal and temperate forests

Past research on identifying potentially negative impacts of forest management activities has primarily focused on traditional forest operations. The increased use of forest biomass for energy in recent years, spurred predominantly by policy incentives for the reduction of fossil fuel use and greenhouse gas emissions, and by efforts from the forestry sector to diversify products and increase value from the forests, has again brought much attention to this issue. The implications of such practices continue to be controversially debated; predominantly the adverse impacts on soil productivity and biodiversity, and the climate change mitigation potential of forest bioenergy. Current decision making processes require comprehensive, differentiated assessments of the known and unknown factors and risk levels of potentially adverse environmental effects. This paper provides such an analysis and differentiates between the feedstock of harvesting residues, roundwood, and salvage wood. It concludes that the risks related to biomass for energy outtake are feedstock specific and vary in terms of scientific certainty. Short-term soil productivity risks are higher for residue removal. There is however little field evidence of negative long-term impacts of biomass removal on productivity in the scale predicted by modeling. Risks regarding an alteration of biodiversity are relatively equally distributed across the feedstocks. The risk of limited or absent short-term carbon benefits is highest for roundwood, but negligible for residues and salvage wood. Salvage operation impacts on soil productivity and biodiversity are a key knowledge gap. Future research should also focus on deriving regionally specific, quantitative thresholds for sustainable biomass removal.

Damaged forests provide an opportunity to mitigate climate change

AbstractBritish Columbia (BC) forests are estimated to have become a net carbon source in recent years due to tree death and decay caused primarily by mountain pine beetle (MPB) and related post‐harvest slash burning practices. BC forest biomass has also become a major source of wood pellets, exported primarily for bioenergy to Europe, although the sustainability and net carbon emissions of forest bioenergy in general are the subject of current debate. We simulated the temporal carbon balance of BC wood pellets against different reference scenarios for forests affected by MPB in the interior BC timber harvesting area using the Carbon Budget Model of the Canadian Forest Sector (CBM‐CFS3). We evaluated the carbon dynamics for different insect‐mortality levels, at the stand‐ and landscape level, taking into account carbon storage in the ecosystem, wood products and fossil fuel displacement. Our results indicate that current harvesting practices, in which slash is burnt and only sawdust used for pellet production, require between 20–25 years for beetle‐impacted pine and 37–39 years for spruce‐dominated systems to reach pre‐harvest carbon levels (i.e. break‐even) at the stand‐level. Using pellets made from logging slash to replace coal creates immediate net carbon benefits to the atmosphere of 17–21 tonnes C ha−1, shortening these break‐even times by 9–20 years and resulting in an instant carbon break‐even level on stands most severely impacted by the beetle. Harvesting pine dominated sites for timber while using slash for bioenergy was also found to be more carbon beneficial than a protection reference scenario on both stand‐ and landscape level. However, harvesting stands exclusively for bioenergy resulted in a net carbon source unless the system contained a high proportion of dead trees (>85%). Systems with higher proportions of living trees provide a greater climate change mitigation if used for long lived wood products.

Global climate impacts of forest bioenergy: what, when and how to measure?

Environmental impact studies of forest bioenergy systems usually account for CO2 emissions and removals and identify the so-called carbon debt of bioenergy through comparison with a reference system. This approach is based on a simple sum of fluxes and does not consider any direct physical impact or climate system response. Other recent applications go one step further and elaborate impulse response functions (IRFs) and subsequent metrics for biogenic CO2 emissions that are compatible with the life-cycle assessment (LCA) methodology. However, a thorough discussion about the role of the different metrics in the interpretation of the climate impacts of forest bioenergy systems is still missing. In this work, we assess a single LCA dataset of selected bioenergy systems using different emission metrics based on cumulative CO2 emissions, radiative forcing and global surface temperature. We consider both absolute and normalized metrics for single pulses and sustained emissions. The key challenges are the choice of end point (emissions, concentration, radiative forcing, change in temperature, etc), the type of measure (instantaneous or time-integrated) and the treatment of time. Bioenergy systems usually perform better than fossil counterparts if assessed with instantaneous metrics, including global surface temperature change, and in some cases can give a net global cooling effect in the short term. The analysis of sustained, or continuous emissions, also shows that impacts from bioenergy systems are generally reversible, while those from fossil fuels are permanent. As shown in this study, the metric choice can have a large influence on the results. The dominant role traditionally assigned to cumulative metrics in LCA studies and climate impact accounting schemes should therefore be reconsidered, because such metrics can fail to capture important time dependences unique to the biomass system under analysis (to which instantaneous metrics are well suited).