and consume water resources and synthetic fertilisers. approximately one-third of all wood used for fuel is harvested unsustainably. bioenergy feedstocks typically require significant amounts of energy to harvest, dry, and transport; the energy usage for these processes may emit greenhouse gases. in some cases, the impacts of land-use change, cultivation, and processing can result in higher overall carbon emissions for bioenergy compared to using fossil fuels. use of farmland for growing biomass can result in less land being available for growing food. in the united states, around 10% of motor gasoline has been replaced by corn-based ethanol, which requires a significant proportion of the harvest. in malaysia and indonesia, clearing forests to produce palm oil for biodiesel has led to serious social and environmental effects, as these forests are critical carbon sinks and habitats for diverse species. since photosynthesis captures only a small fraction of the energy in sunlight, producing a given amount of bioenergy requires a large amount of land compared to other renewable energy sources. second-generation biofuels which are produced from non-food plants or waste reduce competition with food production, but may have other negative effects including trade-offs with conservation areas and local air pollution. relatively sustainable sources of biomass include algae, waste, and crops grown on soil unsuitable for food production. if the biomass source is agricultural or municipal waste, burning it or converting it into biogas provides a way to dispose of this waste. short-term vs long-term climate benefits regarding the issue of climate consequences for modern bioenergy, ipcc states: "life-cycle ghg emissions of modern bioenergy alternatives are usually lower than those for fossil fuels." consequently, most of ipcc's ghg mitigation pathways include substantial deployment of bioenergy technologies. some research groups state that even if the european and north american forest carbon stock is increasing, it simply takes too long for harvested trees to grow back. bioenergy from sources with high payback and parity times take a long time to have an impact on climate change mitigation. they therefore suggest that the eu should adjust its sustainability criteria so that only renewable energy with carbon payback times of less than 10 years is defined as sustainable, for instance wind, solar, biomass from wood residues and tree thinnings that would otherwise be burnt or decompose relatively fast, and biomass from short rotation coppicing (src). the ipcc states: "while individual stands in a forest may be either sources or sinks, the forest carbon balance is determined by the sum of the net balance of all stands." ipcc also state that the only universally applicable approach to carbon accounting is the one that accounts for both carbon emissions and carbon removals (absorption) for managed lands (e.g. forest landscapes.): 2.67 when the total is calculated, natural disturbances like fires and insect infestations are subtracted, and what remains is the human influence. iea bioenergy state that an exclusive focus on the short-term make it harder to achieve efficient carbon mitigation in the long term, and compare investments in new bioenergy technologies with investments in other renewable energy technologies that only provide emission reductions after 2030, for instance the scaling-up of battery manufacturing or the development of rail infrastructure. forest carbon emission avoidance strategies give a short-term mitigation benefit, but the long-term benefits from sustainable forestry activities provide ongoing forest product and energy resources. most of ipcc's ghg mitigation pathways include substantial deployment of bioenergy technologies. limited or no bioenergy pathways leads to increased climate change or shifting bioenergy's mitigation load to other sectors. in addition, mitigation cost increases. carbon accounting system boundaries carbon positive scenarios are likely to be net emitters of co2, carbon negative projects are net absorbers of co2, while carbon neutral projects balance emissions and absorption equally. it is common to include alternative scenarios (also called "reference scenarios" or "counterfactuals") for comparison.: 83 the alternative scenarios range from scenarios with only modest changes compared to the existing project, all the way to radically different ones (i.e. forest protection or "no-bioenergy" counterfactuals.) generally, the difference between scenarios is seen as the actual carbon mitigation potential of the scenarios.: 100 alternative system boundaries for assessing climate effects of forest-based bioenergy. option 1 (black) considers only the stack emissions; option 2 (green) considers only the forest carbon stock; option 3 (blue) considers the bioenergy supply chain; option 4 (red) covers the whole bioeconomy, including wood products in addition to biomass. in addition to the choice of alternative scenario, other choices has to be made as well. the so-called "system boundaries" determine which carbon emissions/absorptions that will be included in the actual calculation, and which that will be excluded. system boundaries include temporal, spatial, efficiency-related and economic boundaries. for example, the actual carbon intensity of bioenergy varies with biomass production techniques and transportation lengths. temporal system boundaries the temporal boundaries define when to start and end carbon counting. sometimes "early" events are included in the calculation, for instance carbon absorption going on in the forest before the initial harvest. sometimes "late" events are included as well, for instance emissions caused by end-of-life activities for the infrastructure involved, e.g. demolition of factories. since the emission and absorption of carbon related to a project or scenario changes with time, the net carbon emission can either be presented as time-dependent (for instance a curve which moves along a time axis), or as a static value; this shows average emissions calculated over a defined time period. the time-dependent net emission curve will typically show high emissions at the beginning (if the counting starts when the biomass is harvested.) alternatively, the starting point can be moved back to the planting event; in this case the curve can potentially move below zero (into carbon negative territory) if there is no carbon debt from land use change to pay back, and in addition more and more carbon is absorbed by the planted trees. the emission curve then spikes upward at harvest. the harvested carbon is then being distributed into other carbon pools, and the curve moves in tandem with the amount of carbon that is moved into these new pools (y axis), and the time it takes for the carbon to move out of the pools and return to the forest via the atmosphere (x axis). as described above, the carbon payback time is the time it takes for the harvested carbon to be returned to the forest, and the carbon parity time is the time it takes for the carbon stored in two competing scenarios to reach the same level. the static carbon emission value is produced by calculating the average annual net emission for a specific time period. the specific time period can be the expected lifetime of the infrastructure involved (typical for life cycle assessments; lca's), policy relevant time horizons inspired by the paris agreement (for instance remaining time until 2030, 2050 or 2100), time spans based on different global warming potentials (gwp; typically 20 or 100 years), or other time spans. in the eu, a time span of 20 years is used when quantifying the net carbon effects of a land use change. generally in legislation, the static number approach is preferred over the dynamic, time-dependent curve approach. the number is expressed as a so-called "emission factor" (net emission per produced energy unit, for instance kg co2e per gj), or even simpler as an average greenhouse gas savings percentage for specific bioenergy pathways. the eu's published greenhouse gas savings percentages for specific bioenergy pathways used in the renewable energy directive (red) and other legal documents are based on life cycle assessments (lca's). spatial system boundaries the spatial boundaries define "geographical" borders for carbon emission/absorption calculations. the two most common spatial boundaries for co2 absorption and emission in forests are 1.) along the edges of a particular forest stand and 2.) along the edges of a whole forest landscape, which include many forest stands of increasing age (the forest stands are harvested and replanted, one after the other, over as many years as there are stands.) a third option is the so-called increasing stand level carbon accounting method. the researcher has to decide whether to focus on the individual stand, an increasing number of stands, or the whole forest landscape. the ipcc recommends landscape-level carbon accounting. further, the researcher has to decide whether emissions from direct/indirect land use change should be included in the calculation. most researchers include emissions from direct land use change, for instance the emissions caused by cutting down a forest in order to start some agricultural project there instead. the inclusion of indirect land use change effects is more controversial, as they are difficult to quantify accurately. other choices involve defining the likely spatial boundaries of forests in the future. efficiency-related system boundaries the efficiency-related boundaries define a range of fuel substitution efficiencies for different biomass-combustion pathways. different supply chains emit different amounts of carbon per supplied energy unit, and different combustion facilities convert the chemical energy stored in different fuels to heat or electrical energy with different efficiencies. the researcher has to know about this and choose a realistic efficiency range for the different biomass-combustion paths under consideration. the chosen efficiencies are used to calculate so-called "displacement factors" – single numbers that shows how efficient fossil carbon is substituted by biogenic carbon. if for instance 10 tonnes of carbon are combusted with an efficiency half that of a modern coal plant, only 5 tonnes of coal would actually be counted as displaced (displacement factor 0.5). generally, fuel burned in inefficient (old or small) combustion facilities gets assigned lower displacement factors than fuel burned in efficient (new or large) facilities, since more fuel has to be burned (and therefore more co2 released) in order to produce the same amount of energy. the displacement factor varies with the carbon intensity of both the biomass fuel and the displaced fossil fuel. if or when bioenergy can achieve negative emissions (e.g. from afforestation, energy grass plantations and/or bioenergy with carbon capture and storage (beccs), or if fossil fuel energy sources with higher emissions in the supply chain start to come online (e.g. because of fracking, or increased use of shale gas), the displacement factor will start to rise. on the other hand, if or when new baseload energy sources with lower emissions than fossil fuels start to come online, the displacement factor will start to drop. whether a displacement factor change is included in the calculation or not, depends on whether or not it is expected to take place within the time period covered by the relevant scenario's temporal system boundaries. economic system boundaries the economic boundaries define which market effects to include in the calculation, if any. changed market conditions can lead to small or large changes in carbon emissions and absorptions from supply chains and forests, for instance changes in forest area as a response to changes in demand. macroeconomic events/policy changes can have impacts on forest carbon stock. like with indirect land use changes, economic changes can be difficult to quantify however, so some researchers prefer to leave them out of the calculation. system boundary impacts the chosen system boundaries are very important for the calculated results. shorter payback/parity times are calculated when fossil carbon intensity, forest growth rate and biomass conversion efficiency increases, or when the initial forest carbon stock and/or harvest level decreases. shorter payback/parity times are also calculated when the researcher choose landscape level over stand level carbon accounting (if carbon accounting starts at the harvest rather than at the planting event.) conversely, longer payback/parity times are calculated when carbon intensity, growth rate and conversion efficiency decreases, or when the initial carbon stock and/or harvest level increases, or the researcher choose stand level over landscape level carbon accounting. critics argue that unrealistic system boundary choices are made, or that narrow system boundaries lead to misleading conclusions. others argue that the wide range of results shows that there is too much leeway available and that the calculations therefore are useless for policy development. eu's join research center agrees that different methodologies produce different results, but also argue that this is to be expected, since different researchers consciously or unconsciously choose different alternative scenarios/methodologies as a result of their ethical ideals regarding man's optimal relationship with nature. the ethical core of the sustainability debate should be made explicit by