researchers, rather than hidden away. comparisons of ghg emissions at the point of combustion ghg emissions per produced energy unit at the point of combustion depend on moisture content in the fuel, chemical differences between fuels and conversion efficiencies. for example, raw biomass can have higher moisture content compared to some common coal types. when this is the case, more of the wood's inherent energy must be spent solely on evaporating moisture, compared to the drier coal, which means that the amount of co2 emitted per unit of produced heat will be higher. many biomass-only combustion facilities are relatively small and inefficient, compared to the typically much larger coal plants. further, raw biomass (for instance wood chips) can have higher moisture content than coal (especially if the coal has been dried). when this is the case, more of the wood's inherent energy must be spent solely on evaporating moisture, compared to the drier coal, which means that the amount of co2 emitted per unit produced heat will be higher. this moisture problem can be mitigated by modern combustion facilities. forest biomass on average produces 10-16% more co2 than coal.: 3 however, focusing on gross emissions misses the point, what counts is the net climate effect from emissions and absorption, taken together.: 386 : 3–4 iea bioenergy concludes that the additional co2 from biomass " is irrelevant if the biomass is derived from sustainably managed forests.": 3 climate impacts expressed as varying with time time-dependent net emission estimates for forest bioenergy pathways, compared against coal and natural gas alternative scenarios. plus signs represents positive climate effects, minus signs negative climate effects. the use of boreal stemwood harvested exclusively for bioenergy have a positive climate impact only in the long term, while the use of wood residues have a positive climate impact also in the short to medium term. short carbon payback/parity times are produced when the most realistic no-bioenergy scenario is a traditional forestry scenario where "good" wood stems are harvested for lumber production, and residues are burned or left behind in the forest or in landfills. the collection of such residues provides material which " would have released its carbon (via decay or burning) back to the atmosphere anyway (over time spans defined by the biome's decay rate) ." in other words, payback and parity times depend on the decay speed. the decay speed depends on a.) location (because decay speed is " roughly proportional to temperature and rainfall "), and b.) the thickness of the residues. residues decay faster in warm and wet areas, and thin residues decay faster than thick residues. thin residues in warm and wet temperate forests therefore have the fastest decay, while thick residues in cold and dry boreal forests have the slowest decay. if the residues instead are burned in the no-bioenergy scenario, e.g. outside the factories or at roadside in the forests, emissions are instant. in this case, parity times approach zero. like other scientists, the jrc staff note the high variability in carbon accounting results, and attribute this to different methodologies. in the studies examined, the jrc found carbon parity times of 0 to 400 years for stemwood harvested exclusively for bioenergy, depending on different characteristics and assumptions for both the forest/bioenergy system and the alternative fossil system, with the emission intensity of the displaced fossil fuels seen as the most important factor, followed by conversion efficiency and biomass growth rate/rotation time. other factors relevant for the carbon parity time are the initial carbon stock and the existing harvest level; both higher initial carbon stock and higher harvest level means longer parity times. liquid biofuels have high parity times because about half of the energy content of the biomass is lost in the processing. climate impacts expressed as static numbers greenhouse gas emissions from wood pellet production and transport from the us to the eu.eu's joint research centre has examined a number of bioenergy emission estimates found in literature, and calculated greenhouse gas savings percentages for bioenergy pathways in heat production, transportation fuel production and electricity production, based on those studies. the calculations are based on the attributional lca accounting principle. it includes all supply chain emissions, from raw material extraction, through energy and material production and manufacturing, to end-of-life treatment and final disposal. it also includes emissions related to the production of the fossil fuels used in the supply chain. it excludes emission/absorption effects that takes place outside its system boundaries, for instance market related, biogeophysical (e.g. albedo), and time-dependent effects. the authors conclude that "ost bio-based commodities release less ghg than fossil products along their supply chain; but the magnitude of ghg emissions vary greatly with logistics, type of feedstocks, land and ecosystem management, resource efficiency, and technology." because of the varied climate mitigation potential for different biofuel pathways, governments and organizations set up different certification schemes to ensure that biomass use is sustainable, for instance the red (renewable energy directive) in the eu and the iso standard 13065 by the international organization for standardization. in the us, the rfs (renewables fuel standard) limit the use of traditional biofuels and defines the minimum life-cycle ghg emissions that are acceptable. biofuels are considered traditional if they achieve up to 20% ghg emission reduction compared to the petrochemical equivalent, advanced if they save at least 50%, and cellulosic if the save more than 60%. the eu's renewable energy directive (red) states that the typical greenhouse gas emissions savings when replacing fossil fuels with wood pellets from forest residues for heat production varies between 69% and 77%, depending on transport distance: when the distance is between 0 and 2500 km, emission savings is 77%. emission savings drop to 75% when the distance is between 2500 and 10 000 km, and to 69% when the distance is above 10 000 km. when stemwood is used, emission savings varies between 70% and 77%, depending on transport distance. when wood industry residues are used, savings varies between 79% and 87%. since the long payback and parity times calculated for some forestry projects is seen as a non-issue for energy crops (except in the cases mentioned above), researchers instead calculate static climate mitigation potentials for these crops, using lca-based carbon accounting methods. a particular energy crop-based bioenergy project is considered carbon positive, carbon neutral or carbon negative based on the total amount of co2 equivalent emissions and absorptions accumulated throughout its entire lifetime: if emissions during agriculture, processing, transport and combustion are higher than what is absorbed (and stored) by the plants, both above and below ground, during the project's lifetime, the project is carbon positive. likewise, if total absorption is higher than total emissions, the project is carbon negative. in other words, carbon negativity is possible when net carbon accumulation more than compensates for net lifecycle greenhouse gas emissions. typically, perennial crops sequester more carbon than annual crops because the root buildup is allowed to continue undisturbed over many years. also, perennial crops avoid the yearly tillage procedures (plowing, digging) associated with growing annual crops. tilling helps the soil microbe populations to decompose the available carbon, producing co2.: 393 there is now (2018) consensus in the scientific community that " the ghg balance of perennial bioenergy crop cultivation will often be favourable ", also when considering the implicit direct and indirect land use changes.: 150 albedo and evapotranspiration this section is an excerpt from albedo § trees. environmental impacts the environmental impacts of biomass production need to be taken into account. for instance in 2022, iea stated that "bioenergy is an important pillar of decarbonisation in the energy transition as a near zero-emission fuel", and that "more efforts are needed to accelerate modern bioenergy deployment to get on track with the net zero scenario while simultaneously ensuring that bioenergy production does not incur negative social and environmental consequences." sustainable forestry and forest protection old-growth spruce forest in france. plantation forest in hawaii. forest area increase in the eu 1990–2020. ipcc states that there is disagreement about whether the global forest is shrinking or not, and quote research indicating that tree cover has increased 7.1% between 1982 and 2016.: 367 the ipcc writes: "while above-ground biomass carbon stocks are estimated to be declining in the tropics, they are increasing globally due to increasing stocks in temperate and boreal forests .": 385 old trees have a very high carbon absorption rate, and felling old trees means that this large potential for future carbon absorption is lost.: 3 there is also a loss of soil carbon due to the harvest operations.: 3 old trees absorb more co2 than young trees, because of the larger leaf area in full grown trees. however, the old forest (as a whole) will eventually stop absorbing co2 because co2 emissions from dead trees cancel out the remaining living trees' co2 absorption. the old forest (or forest stands) are also vulnerable for natural disturbances that produces co2. the ipcc found that " landscapes with older forests have accumulated more carbon but their sink strength is diminishing, while landscapes with younger forests contain less carbon but they are removing co2 from the atmosphere at a much higher rate .": 386 the ipcc states that the net climate effect from conversion of unmanaged to managed forest can be positive or negative, depending on circumstances. the carbon stock is reduced, but since managed forests grow faster than unmanaged forests, more carbon is absorbed. positive climate effects are produced if the harvested biomass is used efficiently.: 351 there is a tradeoff between the benefits of having a maximized forest carbon stock, not absorbing any more carbon, and the benefits of having a portion of that carbon stock "unlocked", and instead working as a renewable fossil fuel replacement tool, for instance in sectors which are difficult or expensive to decarbonize. the "competition" between locked-away and unlocked forest carbon might be won by the unlocked carbon: "in the long term, using sustainably produced forest biomass as a substitute for carbon-intensive products and fossil fuels provides greater permanent reductions in atmospheric co2 than preservation does.": 39 iea bioenergy writes: "forests managed for producing sawn timber, bioenergy and other wood products can make a greater contribution to climate change mitigation than forests managed for conservation alone." three reasons are given: reducing ability to act as a carbon sink when the forest matures. wood products can replace other materials that emitted more ghgs during production. "carbon in forests is vulnerable to loss through natural events such as insect infestations or wildfires" data from fao show that most wood pellets are produced in regions dominated by sustainably managed forests, such as europe and north america. europe (including russia) produced 54% of the world's wood pellets in 2019, and the forest carbon stock in this area increased from 158.7 to 172.4 gt between 1990 and 2020. in the eu, above-ground forest biomass increases with 1.3% per year on average, however the increase is slowing down because the forests are maturing. sankey diagram that shows the flow of biomass from forest to wood products, paper and energy in sweden. biodiversity classification scheme for win-win (green), trade-off (orange), and lose-lose (red) scenarios caused by additional bioenergy pathways in the eu.: 107 short term climate and biodiversity impacts for 3 alternative bioenergy pathways in the eu (forest residues, afforestation and conversion to forest plantation.) short term is here defined as a period of 0–20 years, medium term 30-50 years, and long term over 50 years.: 146 biomass production for bioenergy can have negative impacts on biodiversity. oil palm and sugar cane are examples of crops that have been linked to reduced biodiversity. in addition, changes in biodiversity also impacts primary production which naturally effects decomposition and soil heterotrophic organisms. win-win scenarios (good for climate, good for biodiversity) include:: 8–149 increased use of whole trees from coppice forests, increased use of thin forest residues from boreal forests with slow decay rates, and increased use of all kinds of residues from temperate forests with faster decay rates; multi-functional bioenergy landscapes, instead of expansion of monoculture plantations; afforestation of former agricultural land with mixed or naturally regenerating forests. win-lose scenarios (good for the climate, bad for biodiversity) include afforestation on ancient, biodiversity-rich grassland ecosystems which were never forests, and afforestation of former agricultural land with monoculture plantations.: 125–147 lose-win scenarios