Woody Biomass for Power and Heat

Impacts on the Global Climate

Current biomass policy frameworks are not fit for purpose and require substantial changes to ensure they contribute to mitigating climate change rather than exacerbating it.

Research paper Updated 18 May 2023 ISBN: 978 1 78413 190 6

— Biomass power station.

Duncan Brack

Associate Fellow, Environment and Society Programme

1. Is Biomass Carbon-neutral?

This chapter reviews the argument that biomass emissions are part of a natural cycle in which forest growth balances the carbon emitted by burning wood for energy. The following issues are discussed:

The level of greenhouse gases emitted by woody biomass when burnt, compared to those of the fossil fuels it potentially replaces.
The types of woody biomass used for energy and their potential impact on carbon emissions.
The relationship between the emissions from burning woody biomass and forest growth or regrowth, and the time forest growth may take to absorb the emissions from burning woody biomass (the ‘carbon payback period’).
The debate around bioenergy with carbon capture and storage.

Most of the studies carried out on these topics relate to the sourcing of woody biomass from the US, generally for export and use in the EU. This is a relatively small proportion of total global use of woody biomass for energy, even in modern technologies. Across the UN Economic Commission for Europe region (Europe, North America, and north, west and central Asia), forest-based industries form the largest end-use sector, consuming over 40 per cent of wood energy.8 However, the use of woody biomass for heat and power is growing more quickly, particularly in the EU, and imports from outside the EU, chiefly from the US and Canada, have risen sharply in recent years. This is likely to continue. It is estimated that, if it is to achieve its aim of providing 27 per cent of its energy consumption from renewable sources by 2030, the amount of biomass the EU will need is the equivalent to the total EU wood harvest for all purposes in 2015.9 While studies based on the US may not always be applicable to the sourcing of woody biomass in other regions, they focus attention on the country that has experienced most rapid recent changes in this respect and many of the conclusions they reach are applicable more broadly.

Greenhouse gas emissions from burning woody biomass

Since in general woody biomass is less energy dense than fossil fuels, and contains higher quantities of moisture and less hydrogen, at the point of combustion burning wood for energy usually emits more greenhouse gases per unit of energy produced than is the case with fossil fuels.10 Table 1 presents the emission factors agreed by the IPCC in 2006 and widely used, for example, in emissions calculations under the EU Emissions Trading Scheme and for some national inventory reports under the UN Framework Convention on Climate Change.

Table 1: Greenhouse gas emissions of wood, coal and natural gas, net calorific basis

	Emissions (kg CO2/TJ) (1 TJ = 278 MWh)
Source	Wood	Anthracite	Bituminous	Lignite	Natural gas
Carbon dioxide	112,000 (95,000–132,000)	98,300 (94,600–101,000)	94,600 (89,500–99,700)	101,000 (90,900–115,000)	56,100 (54,300–58,300)
Methane	30 (10–100)	1 (0.3–3)	1 (0.3–3)	1 (0.3–3)	1 (0.3–3)
Nitrous oxide	4 (1.5–15)	1.5 (0.5–5)	1.5 (0.5–5)	1.5 (0.5–5)	0.1 (0.03–0.3)

Emissions (kg CO2/TJ) (1 TJ = 278 MWh)

Source

Wood

Anthracite

Bituminous

Lignite

Natural gas

Carbon dioxide

112,000

(95,000–132,000)

98,300

(94,600–101,000)

94,600

(89,500–99,700)

101,000

(90,900–115,000)

56,100

(54,300–58,300)

Methane

(10–100)

(0.3–3)

Nitrous oxide

4 (1.5–15)

1.5

(0.5–5)

1.5

(0.5–5)

1.5

(0.5–5)

0.1

(0.03–0.3)

Source: Intergovernmental Panel on Climate Change (2006), Guidelines for National Greenhouse Gas Inventories, Vol. 2 (Energy), Table 2.2, pp. 2.16–2.17.

The emission levels from wood are compared with emissions from natural gas and three different types of coal (anthracite, bituminous coal and lignite). The table includes ranges of factors together with the central default values agreed by the IPCC. As can be seen, wood has a wider range of carbon dioxide emissions than all of the fossil fuels. Nevertheless, while some types of wood may have lower levels of carbon emissions than some types of coal, in general wood is more carbon intensive than coal and significantly more so than natural gas, as well as having higher levels of emissions of methane and nitrous oxide.

These figures are calorific values, i.e. the energy released from complete combustion of the fuel in the presence of oxygen. The energy actually delivered in real-world situations will differ from this depending primarily on the efficiency of conversion to ‘useful’ energy – i.e. thermal energy and electricity. Efficiency values vary substantially depending on the plant’s size, design, age and type of fuel used. The European Commission’s Joint Research Centre has reported average net thermal efficiencies of coal-burning plants of 40–45 per cent and average electric efficiencies of dedicated biomass plants of 20–30 per cent.11 More recent figures for biomass plants in the EU indicate electric efficiencies of 24–32 per cent.12 Very large modern plants such as the Drax power station in the UK, which has converted three of its six coal-fired units to biomass, may achieve electric efficiencies of around 38 per cent, though this depends on burning wood pellets rather than green chips.

Nevertheless, even in the case of Drax, carbon emissions per unit of energy are higher for woody biomass than for coal. Table 2 shows the figures for fuel use, electricity generation and carbon dioxide emissions reported by Drax for 2013. As can be seen, the carbon dioxide intensities of the fuels are 856 kg CO2/MWh (coal) and 965 kg CO2/MWh (biomass), i.e. a level of emissions from biomass about 13 per cent higher than from coal.

Table 2: Fuel used, electricity generated and carbon dioxide emissions, Drax, 2013

	Weight burnt (tonnes)	Electricity generated (TWh)	CO2 emissions (tonnes)	CO2 intensity (kg/MWh)
Coal and petcoke	9,301,000	23.4	20,089,607	856
Biomass	1,596,000	2.9	2,799,391	965

Source: Drax, Annual review of Environmental Performance 2013, pp. 3, 4, 8.

Similarly, data provided by the US Environmental Protection Agency show that power plants burning wood tend to have higher emissions per megawatt-hour than plants burning gas or coal. To take a particular example, the Schiller power station in New Hampshire has coal boilers and a wood boiler; emissions from the wood boiler are 1,444 kg CO2/MWh, compared to 1,243 kg CO2/MWh for the coal boilers.13 These solid fuel boilers are old and inefficient; new combined cycle gas boilers in the database have emission rates that are less than one third the emissions of the Schiller biomass boiler.

For biomass and fossil fuels, efficiency levels for combined heat and power (CHP), or cogeneration, plants, can be much higher – 80 per cent or more – as a much higher proportion of the heat produced during combustion is trapped and used. For example, DONG Energy’s Avedøre CHP plant near Copenhagen, which is converting from coal and gas to biomass (wood pellets and straw), is claimed to be one of the most efficient in the world, achieving fuel efficiencies up to 89 per cent.14

In addition to the emissions produced at the point of combustion, the production and processing of the biomass gives rise to additional greenhouse gas emissions, from the energy consumed in harvesting the forest or collecting the wood, to processing it (e.g. into pellets), and transporting it. Calculations of these supply-chain emissions vary substantially.15 A 2014 study estimated the emissions from supplying wood pellets from the southeastern US to power plants in the Netherlands, from truck, train and oceanic transport and from the process of pelletizing, as equivalent to 322 kg CO2 per tonne of pellets. Assuming 499 kg of pellets is burnt to generate 1 MWh of electricity, this gives additional emissions of 162 kg CO2/MWh – equivalent to about one-sixth of the emissions released during combustion (using the Drax figures above).16

In contrast, a 2016 study used the figure of 34.4 kg CO2 per tonne of pellets burnt, one-tenth of that of the 2014 study.17 The figures will vary with the particular scenario – e.g. with the distance between the forest and pellet plant, and between the plant and the power station, as well as with the amount and type of energy used in the plant – but this degree of variation seems excessive. A 2015 article calculated base-case figures of 132–140 kg carbon dioxide equivalent (CO2-eq)/MWh but then also considered the impact of methane emissions from wood chips and sawdust during storage, either at the pellet mill or the power station. It found this raised the associated emissions to 317 kg CO2- eq/MWh after storage for one month and 862 kg CO2-eq/MWh after four months – higher by itself (even ignoring emissions from combustion) than emissions from coal (estimated in this study as 752 kg CO2-eq/MWh).18

Given the considerable uncertainties associated with all these figures, further research would be valuable. This is particularly true for the contribution of methane emissions, which is a factor not usually included in calculations but which can have a major impact. The studies reviewed in the 2015 article mentioned above, together with other estimates,19 show considerable variability in methane emissions from stored sawdust, chips and pellets, and this can also vary depending on the storage conditions, whether the pile is covered, the ambient temperature and so on.

Similar supply-chain emissions are associated with fossil fuel extraction, from mining or drilling, processing and transport, and these should be taken into account in comparing alternative fuel scenarios. Again estimates vary, but studies suggest that an additional 5–10 per cent greenhouse gas emissions should be added to the combustion emissions from coal and about 30–35 per cent to those from gas (the figure is higher for gas because of the methane released during production).20

These variations in the technology in which the fuel is used, and in the life-cycle assessments, explain much of the difference in the greenhouse gas emission levels cited in various studies. Converting an old coal station to a modern biomass station or a remote rural community transiting from diesel-fired electricity generators to a biomass CHP plant using locally sourced feedstock might reduce carbon emissions over the entire life cycle of the system (depending on factors such as the type of feedstock and its impact on the forest). But these are limited examples; in most circumstances, comparing technologies of similar ages, it can be assumed that the use of woody biomass for energy releases higher levels of emissions than coal, and considerably higher levels than gas, as shown by the emission levels from Drax and Schiller quoted above.

This is only part of the picture, however, of the climate impact of woody biomass. The impacts will also vary with the type of woody biomass used, with what would have happened to it if it had not been burnt for energy and with what happens to the forest from which it was sourced. These questions are explored in the sections below.

Biomass energy feedstocks

Several different types of wood are commonly burnt for energy. The impact of their use on net carbon emissions, and therefore on the climate, depends partly on what would otherwise have been done with them if they had not been burnt for energy.

Mill residues

Mill residues are sides, bark, shavings, sawdust, trim ends, offcuts and so on produced as waste in sawmills; they typically amount to 45–55 per cent of the volume of timber entering the mill. Many years ago these were often burnt as waste, or sometimes disposed of in landfill, but now they are generally in demand for fibre products such as particleboard (e.g. MDF) or for use in pulp mills or for energy, either on-site in the sawmill or in biomass energy facilities elsewhere.

If the mill residues would otherwise have been burnt as waste or landfilled, or left to decay, it makes sense to use them for energy as the carbon content of the residues would be released into the atmosphere anyway as carbon dioxide and methane. If they would otherwise have been used for wood products, however, using them for energy will result in increased carbon emissions equal to the difference between the emissions from combustion and the supply chain (collection, transport and processing such as pelletizing) and combustion and supply-chain emissions from the fossil fuels replaced (plus any impacts from the manufacturers using alternative sources of wood). A full life-cycle analysis would be needed to calculate the precise impact in any given scenario. Using mill residues locally for energy in the sawmill would have the lowest impact, as supply-chain emissions are minimized.

Forest residues

Forest residues (or ‘slash’) are the parts of harvested trees that are left in the forest after log products have been removed, including stumps, tops and small branches, and pieces too short or defective to be used. These can amount to as much as 40–60 per cent of the total tree volume. Sometimes forest residues may be burnt as waste, but more frequently they are left to rot in the forest or at the roadside. They can be used for energy and can be made into pellets, but this can cause problems in biomass plants (particularly when co-fired with coal) because of their high ash, dirt and alkali salt content, which accelerates corrosion of the boilers.

The impact on overall carbon emissions from using forest residues for energy depends partly on the rate at which they would have decayed and released carbon dioxide and methane into the atmosphere, which varies with factors such as the local climate, the type of soil and the amount of water present. All else being equal, decay rates tend to be faster in wet conditions. In the US, the majority of logging residue decay half-lives are 50 years or less. While under warm conditions (such as in much of the southeastern US) decay half-lives are generally less than 20 years, under cooler conditions half-lives of 100 years or longer have been reported.21 A study of forest-residue decay in Finland found significant differences between types of residue (branches decayed far more quickly than stumps, for example) and between the southern and northern (and much colder) parts of the country.22 The European Commission Joint Research Centre has reported decay rates varying between 40 per cent per year for needles and twigs, 11.5 per cent a year for branches in temperate climates and 2 per cent a year for coarse deadwood.23

Many studies have shown that the removal of forest residues reduces both soil carbon storage and nutrient availability, which in turn leads to a fall in site fertility and tree growth, thereby reducing carbon storage in tree biomass in the long term.

The slower the decay rate the larger will be the net increase in carbon emissions from the use of residues for energy in the short and medium term, as the carbon is released immediately on combustion rather than being trapped in the residue. The net impact gradually falls over time as the residues would have rotted and released carbon.

These decay rates by themselves understate the impact of using forest residues for energy, however, as their removal may also have significant impacts on levels of soil carbon and on rates of tree growth. Many studies have shown that the removal of forest residues reduces both soil carbon storage and nutrient availability, which in turn leads to a fall in site fertility and tree growth, thereby reducing carbon storage in tree biomass in the long term.24 The reduction in soil nutrients may also necessitate the use of fertilizers, with additional impacts on greenhouse gas emissions.25 If these impacts are taken into account, the use of forest residues for energy may result in much larger increases in net carbon emissions, though this will depend partly on the proportion of residues removed. It should also be noted that the dynamics of soil carbon, including the amount of carbon from residues sequestered in the soil over time, and how much may be released due to harvesting, are not yet fully understood, and further research would be helpful.26

Roundwood

Compared to residues, the burning of roundwood (i.e. wood in its natural state as felled, including stemwood – the wood above ground – and stumps, which are sometimes classified as residues) for energy, represents the removal of growing forest carbon stock. Some of this roundwood may derive from other harvesting operations, or from additional fellings specifically for use as energy (through, for example, an increase in the area harvested annually or an increase in the intensification of felling, including clear-cutting) or from the diversion of harvested wood from other uses.

As with other types of wood, the impact on carbon emissions depends on what would have happened to the roundwood in the absence of use for energy – whether it would have been left growing, or harvested for some other use, or burnt or left to rot as otherwise unmerchantable, i.e. not fit for sale, parts of a harvest. In general, however, the net increase in carbon emissions will be much higher than from the use of mill or forest residues, as it includes not only the higher volume of emissions from burning biomass compared to burning fossil fuels but also the carbon emissions that would otherwise have been sequestered by the growing tree. (See below in this chapter for a discussion of carbon absorption by mature trees.)

Thinnings – the removal of selected trees or rows to allow stronger growth of the remaining trees, or to reduce the risk of fire – is one source of roundwood, though in the southeastern US the volume of thinnings has fallen in the last 20 years as plantation management has tended towards planting at lower densities.27 However, studies suggest that the use of thinnings even from fire-prone forests do not reduce net greenhouse gas emissions for decades.28 One study found that the use of thinnings for energy reduced carbon stocks in the forest, compared to leaving the forest alone, over 50 years.29

The increase in carbon emissions will also be high if roundwood is diverted from use in wood products such as panels or furniture or construction timber, as the carbon is emitted immediately rather than being fixed for years or decades. The competition for the raw material may also tend to increase prices, which may lead to increased rates of harvesting, higher imports of wood products, substitution to non-wood products, and an increase in the rate of planting new forests. This depends, though, on the relative levels of demand; for example, there may be little competition in practice if the output of the competing industry is declining.

In 2015 a comprehensive review of the supply of woody biomass from the southeastern US to the EU found little evidence of any such diversion in practice, apart possibly for some sawmill residues.30 Similarly, in 2016 a European Commission state aid investigation into the UK government’s financial support for the conversion of the third unit at Drax from coal to biomass, triggered in part because of its potential impact on competition for wood, concluded that the increased demand from wood pellets ‘could be fulfilled by the market without undue negative side-effects’.31 Nevertheless, a number of wood-products industries have expressed concern over the distorting effect of subsidies for biomass energy on the market for the raw material on which they depend.32

Black liquor

Although black liquor is an important source of biomass energy in many countries, its climate impacts have received relatively little attention compared to those of other feedstocks. A waste product from the kraft pulping process, which digests pulpwood into paper pulp, black liquor comprises a solution of lignin residues, hemicellulose and the inorganic chemicals used in the process. Originally simply discharged into local watercourses (with major local environmental impacts), virtually all pulp and paper mills now burn black liquor in recovery boilers for energy, generating steam and recovering some of the chemicals used. Modern mills should be self-sufficient for energy; indeed, many produce a surplus of electricity for export to the local or national grid. New waste-to-energy methods involving gasification have the potential to achieve higher efficiencies than the conventional recovery boiler while also generating an energy-rich syngas, which can be used to generate electricity or be converted into methanol and other transport fuels.

Black liquor is very different from most other uses of biomass. It is in its entirety waste produced as a by-product of a wood-based industry, with no impact on forest carbon stock (separate from the impact of the pulp and paper industry). It is generated and used on-site, with no transport costs. If it was not burnt for energy, the pulp mills would face the task of disposing of a highly polluting substance. In general the use of black liquor should be economic without the need for subsidy, though in the US a tax loophole aimed at promoting alternative fuels has allowed paper companies to claim very substantial tax refunds for its use.33 One study of the life-cycle impact of black liquor recovery on climate change concluded that greenhouse gas emissions were approximately 90 per cent lower than those for a comparable fossil fuel-based system.34 From the point of view of analysis, it is highly regrettable that black liquor is often included alongside other types of solid biomass in reported statistics since its climate impact is clearly very different.

Feedstocks in use

The discussion earlier highlights the critical influence of the type of wood product used as feedstock. In general the use of residues and wastes is likely to result in a much smaller net increase in carbon emissions, or in some circumstances a reduction, compared to the use of roundwood.

Many of the models contained in studies of the impacts of using wood for energy (discussed further below) assume that residues are the main feedstock. In the model used in a 2012 paper, residues supplied 65 per cent of the woody biomass projected to be used for energy in 2015, and remained important beyond that unless constrained by policy.35 Similarly, scenarios modelled in one 2015 study, which looked ahead to 2032, assumed that mill residues comprised 67 per cent of feedstock in a situation of low demand; additional harvesting (of pulpwood – debarked sections of stems 5–23 cm in diameter) provided 19 per cent.36 In a situation of high demand, however, that study assumed that the supply of mill residues would not be sufficient and would only provide 36 per cent of feedstock; the proportion provided by additional harvesting was estimated as 36 per cent. Two other papers in 2013 and 2015 argued for using residues more intensively.37 The second of these claimed a much greater potential for using forest residues in Sweden than the 20 per cent currently used for bioenergy.38 As discussed above, however, greater use of forest residues seems likely to release more soil carbon and to reduce forest growth, thus increasing net carbon emissions.

Information provided by the biomass energy industry, including wood pellet companies, tends to emphasize the use of residues. For example, in its supply report for 2014 Drax reported that its feedstock mix included 37 per cent sawdust and sawmill residues, 29 per cent forest residues (which it defined as including low-grade wood) and 24 per cent thinnings.39 For 2015–16 it reported 47 per cent sawmill residues, 26 per cent low-grade roundwood and forest residues, and 24 per cent thinnings.40 Enviva, the largest US pellet producer, stresses its use of low-grade wood fibre (wood that would otherwise have been rejected from lumber mills), tops and limbs, chips made by suppliers in the forest out of low-grade wood and waste materials and commercial thinnings, alongside mill waste and residues.41 Both companies tend to group ‘low-grade wood’ along with ‘forest residues’, though the impact on carbon emissions is not the same.

In contrast, however, in April 2015, in the prospectus accompanying its initial public offering, Enviva stated that:

Our primary source of wood fiber is traditional pulpwood, which has historically exhibited less pricing volatility than other sources of wood fiber…we also procure industrial residuals (sawdust and shavings) and forest residuals (wood chips and slash), which have been more volatile historically in terms of price and supply but occasionally represent lower cost alternative inputs.42

NGOs in the US have identified cases where biomass energy companies have stated either that they regard waste and forest residues as unsuitable feedstocks in terms of quantity or quality, or both, or classify whole trees or whole-tree chips as ‘waste’.43 The Vyborgskaya pellet plant in Russia sources only logs, according to a corporate presentation in 2013 that did not mention either mill or forest residues.44

The European Commission’s 2015 review of the supply of woody biomass from the southeastern US to the EU concluded that, while sawmill residues were in many ways the ideal source material for pellets, US mill residues were already almost entirely utilized by the biomass energy or other industries, and there was very limited room for expansion.45 As noted earlier, the use of forest residues can cause problems in biomass plants because of their high ash, dirt and alkali salt content. Partly for this reason, the European Commission concluded that residues such as tops, limbs and other unmerchantable materials ‘currently do not play a significant role’ in the woody biomass supply chain. Various types of roundwood, mainly pulpwood but also larger sizes, were therefore the main source – typically about three-quarters – of the feedstock volume of large industrial pellet facilities.46

These findings are supported by other studies. One 2015 study suggested that 76 per cent of feedstock used to produce pellets in the southern US was pulpwood while mill residues and forest residues accounted for 12 per cent each.47 A survey of forest resources in the US found that in 2011 less than 1 per cent of mill residues was not already used; 43 per cent was used for commercial fuel, 40 per cent for fibre products and the rest for other products.48

The question of the types of wood used for biomass energy has become one of the most bitterly contested issues in the debate over its impacts. NGOs have published reports claiming that pellet plants use whole trees extensively, including sourcing from harvesting specifically for energy use.49 Where these are hardwoods – which provide up to 100 per cent of the feedstock for some of Enviva’s pellet plants, according to information provided by the company in 2015 – this increases net carbon emissions over time, as hardwoods take much longer to grow back than softwoods.50 The pellet and biomass energy companies counter that where whole trees are used they tend only to be dead or diseased or otherwise unmerchantable trees that would have no other use – though trees that would not qualify as high-quality sawtimber could nevertheless be used for pulp, panels or laminated products.

This is important because of the significant difference these categories can make to the impact on net carbon emissions. As discussed above, the impacts from using mill or forest residues are much lower than those for material from growing trees harvested specifically for energy use, since in the latter case carbon absorption from growing trees is foregone (along with the higher carbon emissions from using biomass instead of fossil fuels). In 2015 an analysis of the feedstock sources from the southern US reported by Drax for 2014 (which differentiated between ‘forest residues’ and ‘low-grade wood’ – as noted, the two are combined in Drax’s figures) used the UK government’s BEaC scenarios (see below) to calculate net carbon emissions.51 This concluded that Drax’s emissions were at least 2,677 kg CO2-eq/MWh for a scenario in which 80 per cent of feedstock derived from additional biomass harvests in southeastern US hardwoods, with the remainder coming from sawmill or forest residues; or at least 1,227 kg CO2-eq/MWh for a scenario assuming 48 per cent of the feedstock derived from forest residues that would otherwise have decayed, with the remainder sourced from sawmill residues (17 per cent) and additional biomass harvests (35 per cent). In each case these emissions levels are significantly higher than those from coal. A Drax spokesperson commented that the study was based ‘on a mountain of assumptions… based on an outlandish scenario’ and insisted that the hardwood sourced by Enviva for its pellets was a residue of normal commercial operations.52

Part of the problem is the lack of clear definitions of the term ‘forest residues’. The EU Renewable Energy Directive, for example, does not define it. In the UK, the energy regulator, Ofgem, defines forestry residues as material ‘derived from “virgin wood”’, including:

all raw materials collected directly from the forest, whether or not as a result of thinning or logging activities. This may include (but is not limited to) materials such as tree tops, branches, brash, clippings, trimmings, leaves, bark, shavings, woodchips and saw dust from felling.53

‘Virgin wood’ is defined as:

timber from whole trees and the woody parts of trees including branches and bark derived from forestry works, woodland management, tree surgery and other similar operations. It does not include clippings or trimmings that consist primarily of foliage (though these may be forestry residues).54

These definitions are confusing and potentially overlapping: whole trees, or logs, could fall under the definition of forest residues or of virgin wood despite their very different impacts on emissions. Similarly, the definitions of logging residues by the US Forest Service and US Department of Energy can include whole trees. In one 2016 report the latter defined logging residues as ‘trees not meeting merchantable timber specifications and tree components, such as limbs, tops, and cull logs’.55 These imprecise definitions are not helpful in resolving the debate over climate impacts.

Biomass and the forest carbon cycle

It is not disputed that burning woody biomass for energy produces emissions of carbon dioxide and other greenhouse gases. But the argument is often made that since these carbon emissions are absorbed as part of the natural forest cycle of growth and regrowth, they should therefore be counted as zero at the point of combustion (in other words, that the discussion above about the climate impact of different types of feedstocks is irrelevant). Many studies of the benefits of biomass energy, including the ones cited above, assume just that. Similarly, national sustainability criteria for woody biomass that set minimum levels of greenhouse gas savings compared to the fossil fuels they replace ignore the emissions produced during combustion and consider only supply-chain emissions from harvest, processing and transport (see Chapter 3). This is what lies behind claims such as one about biomass representing an 80 per cent emissions saving compared to coal.56 The argument may also be used that, if waste (including residues) is used as the feedstock, emissions can be considered to be zero, since no additional harvesting is involved.

This argument takes various forms. The most extreme version is that woody biomass emissions should count as zero because carbon has already been absorbed during the growth of the trees that are logged and burnt. As one study argued in 2011, ‘Those trees have been gathering carbon (some of which is from the combustion of fossil fuels) for… 30 years… We have accrued a dividend. We can then derive a benefit from that dividend by using those trees for energy.’57 This argument implies that, once they have grown, what happens to trees later – whether they are left to grow further, or harvested and made into wood products, or harvested and burnt for energy – somehow makes no difference to carbon concentrations in the atmosphere. This is obviously not the case.

A similar argument is that, as long as the trees are harvested from a forest that is sustainably managed, their carbon emissions should be considered to be zero: effectively, forest growth, replacing the logged trees, cancels out the emissions released when burnt. The description of the IEA’s Bioenergy Task 38 on Climate Change Effects of Biomass and Bioenergy Systems, for example, includes the statement that:

Biomass fuels can have higher carbon emission rates (amount of carbon emitted per unit of energy) than fossil fuels (e.g. oil, or natural gas) due to generally lower energy density of biomass. This fact is only relevant, when biomass fuels are derived from unsustainable land-use practices (the carbon emissions from combustion of sustainable biomass are excluded from calculations because they are counterbalanced by the uptake of CO2 as the feedstock is grown i.e. the photosynthetic and combustion stages of the life cycle are carbon neutral).58

As mentioned earlier, this argument must assume that whatever happens to the trees after they are harvested (assuming sustainable management, i.e. that forest growth replaces the forest carbon lost when logged) makes no difference to carbon concentrations in the atmosphere: burning them for energy is the same as fixing the carbon in wood products. Again, as above, this is clearly wrong. Furthermore, this argument ignores the carbon sequestration forgone from harvesting the trees: they would have continued to grow and absorb carbon if left un-harvested, and the uptake of carbon therefore falls when they are logged, whether or not the forest is sustainably managed. This is not true only if the forest grows more slowly in the absence of logging for energy, or if harvesting promotes additional growth fast enough to replace the carbon emitted when burnt; both issues are discussed below.

The third version of the argument discounts any link between the trees, or parts of trees, burnt for energy and the forest stand, or the forest, from which they derive, and asserts that as long as the forest as a whole or forests in general are expanding, emissions from combustion can be ignored. Although globally deforestation is continuing, this is not the case in Europe or North America, which are currently the main sources of wood for energy in modern technologies and are seeing an increase in forest cover. This fact is sometimes cited as evidence that the use of wood from these areas for energy is sustainable: if total forest cover is increasing, more carbon is being absorbed, which offsets the additional carbon emitted to the atmosphere when wood from those areas is burnt.59

Again, this ignores the carbon absorption forgone when the trees are harvested and burnt as well as the counterfactual regarding what would have happened if the trees had not been harvested and burnt for energy. There is no automatic link between the increase in forest growth and burning wood for biomass – particularly when the argument depends on expansion in forests entirely unconnected to those from which the wood for energy is harvested – and there is no reason to assume that, globally, forests would grow more slowly in the absence of the biomass industry.

Carbon absorption, forest growth and forest age

The main argument for a positive impact of burning woody biomass is if the forest area expands as a direct result of harvesting wood for energy, and if the additional growth exceeds the emissions from combustion of biomass. Various models have predicted that this could be the case: that the additional income from selling wood for energy (even if this is only part of the harvest) may encourage forest owners to invest more in their forests and plant a greater area.60 These are models, however, rather than real-world observations, and it is not clear that this phenomenon is actually being observed. As can be seen in Table 3, the area of commercial timberland (i.e. forest land available for the production of forest products) in the five southeastern US states where most US wood pellet mills are found did not change significantly between 2011 and 2014, a period during which the wood pellet and biomass industries were both expanding.

Table 3: Timberland area of southeastern US states, 2011 and 2014

State	2011	2014	Change 2011–14
	Area of timberland (000 ha)
Alabama	9,279	9,320	+0.44%
Georgia	9,874	9,776	–0.99%
North Carolina	7,316	7,331	+0.21%
South Carolina	5,237	5,180	–1.10%
Virginia	6,198	6,228	+0.48%

Source: US Forest Service (undated), ‘Forest Inventory and Analysis – Southern Research Station’, http://srsfia2.fs.fed.us/states/state_information.shtml.

If anything, the evidence suggests the opposite. In 2014, for example, the US Forest Service reported that while forest hardwood inventories were expected to continue increasing to 2020, even as bioenergy demand increased, the rate of growth of forest carbon stocks would be lower as a result of demand for biomass for energy. It concluded: ‘Even assuming full utilisation of mill residues and increased utilisation of logging residues, harvest of pine and hardwood non-sawtimber feedstock increases… hardwood inventories continue to increase although these end at lower levels’ than without new bioenergy demand.61

In addition, the models always assume that younger trees grow faster and therefore absorb more carbon than older, more mature trees; as one study stated, ‘the CO2 uptake in old forests is low, and in very old stands the CO2 is even negative’ (because of the greater likelihood of carbon losses due to fire, storms or insects).62 Thus it is argued that harvesting mature trees and replanting will increase the rate of carbon uptake. Studies suggest, however, that this is not true, particularly in old-growth forests, though it may be in plantations (possibly because of lower soil nutrient availability in plantations compared to natural forests).

Many studies, particularly some conducted recently, have shown that mature trees absorb more carbon than younger trees, mainly because of their much higher number of leaves, which enable greater absorption of carbon dioxide from the atmosphere.63 As a 2014 study concluded:

for most species mass growth rate increases continuously with tree size. Thus, large, old trees do not act simply as senescent carbon reservoirs but actively fix large amounts of carbon compared to smaller trees; at the extreme, a single big tree can add the same amount of carbon to the forest within a year as is contained in an entire mid-sized tree.64

According to one 2008 study:

[the] commonly accepted and long-standing view that old-growth forests are carbon neutral… was originally based on ten years’ worth of data from a single site. It is supported by the observed decline of stand-level net primary production with age in plantations, but is not apparent in some ecoregions.65

Although the rate of carbon uptake does tend to decline with the age of the tree, it found that ‘in forests between 15 and 800 years of age, net ecosystem productivity (the net carbon balance of the forest including soils) is usually positive.’66 Several studies suggest that the rate of carbon uptake has accelerated in recent years with the increasing concentration of carbon dioxide in the atmosphere. Since trees are prone to disease and pests, the high rate of carbon uptake of older trees is somewhat offset by their higher mortality rates, but only partially, and it should be possible to reduce this by management for conservation (e.g. removing diseased or dead trees).

This conclusion is supported by other studies suggesting that, far from accelerating carbon uptake, harvesting may in fact bring it to a temporary halt. One reviewing the impacts of forest disturbances (including harvesting, fires, storms and insect infestation) throughout the US concluded that in most cases the forest did not return to its status as a carbon sink for at least 10, and sometimes as much as 20, years, partly due to the large soil carbon losses associated with the event.67 Similarly, a model-based study of forest carbon storage in the northeastern US compared different types of forest management and concluded that the highest rate of carbon uptake and storage was achieved simply by leaving the forest alone:

The results supported both our first hypothesis that passive management sequesters more carbon than active management, as well as our second hypothesis that management practices favoring lower harvesting frequencies and higher structural retention sequester more carbon than intensive forest management.68

Most of the models assuming that the production of wood for energy accelerates carbon uptake also assume that much of the rapid growth is achieved by replacing old-growth forests with plantations, most commonly of relatively fast-growing pine species.69 As well as causing higher carbon emissions from the loss of mature trees, at the point of harvest and in terms of foregone future carbon sequestration, this is also highly likely to have negative impacts on biodiversity and habitats.70 This reinforces the need to protect old-growth forests, not only for their value for biodiversity and amenity but also for their role as a significant carbon sink.

The temporal dimension: the carbon payback period

A different way of looking at the climate impacts of biomass energy is to consider the temporal dimension of the issue. It can be argued that the carbon dioxide emitted by burning woody biomass for energy is indeed absorbed from the atmosphere by forest growth, but this takes place only over time, a factor ignored b y the arguments discussed earlier.

Following this argument, the carbon dioxide (and other greenhouse gases) released by the burning of woody biomass for energy, along with their associated life-cycle emissions, create what is termed a ‘carbon debt’ – i.e. the additional emissions caused by burning biomass instead of the fossil fuels it replaces, plus the emissions absorption foregone from the harvesting of the forests.71 Over time, regrowth of the harvested forest removes this carbon from the atmosphere, reducing the carbon debt. The period until carbon parity is achieved (i.e. the point at which the net cumulative emissions from biomass use are equivalent to those from a fossil fuel plant generating the same amount of energy) is usually termed the ‘carbon payback period’. After this point, as regrowth continues biomass may begin to yield ‘carbon dividends’ in the form of atmospheric greenhouse gas levels lower than would have occurred if fossil fuels had been used. Eventually carbon levels in the forest return to the level at which they would have been if they had been left unharvested. (Some of the literature employs the term ‘carbon payback period’ to describe this longer period, but it is more commonly used to mean the time to parity with fossil fuels; this meaning is used in this paper.)

The factors affecting the length of the carbon payback period are the same as those discussed above: the level of emissions produced during harvesting, collecting, processing, transporting and burning the biomass compared to the fossil fuels that it replaces, together with the counterfactual about what would have happened to the wood if it had not been used for energy and to the forest from which it was sourced.72

Following the discussion earlier, the carbon payback period for mill residues can be assumed to be very low as no additional felling is involved. If the residues would otherwise have been burnt as waste the payback period may be zero. The carbon payback period for forest residues is more complex, depending on the rate at which they would decay if left to rot in the forest, and on the impacts on forest growth of the removal of residues; but again no additional felling is involved. In neither case is there any additional regrowth of forests; the carbon debt is repaid over time from the lower emissions from the residues not being burnt as waste or decaying.

Since the burning of roundwood in general represents the removal of growing forest carbon stock, the carbon payback period will be longer as it includes the foregone future absorption of carbon emissions. This is particularly the case in forest systems with relatively slow growth rates – such as hardwoods, common in the southeastern US – and will also vary depending on the age of the trees, whether they are natural growth or plantations and the extent to which the forest has been managed before the harvest.73 As discussed above, harvesting may also release significant volumes of soil carbon.74

If wood is diverted from alternative uses, such as construction or wood panels or paper, the carbon payback period may be very high as carbon can be fixed in some of these products for decades – though, as discussed above, there is little evidence of this taking place so far.

Many attempts have been made to estimate average payback periods.75 Eight different studies carried out between 2009 and 2012 in Europe and North America, summarized in a 2012 report, produced estimated payback periods between zero (for the use of fellings residues to replace coal for electricity) to 459 years (for the use of wood from old-growth forests to produce ethanol for transport fuel).76 The scenarios using residues, branches, thinnings or stumps all showed payback periods between zero and 74 years, with most less than 25 years. Where old-growth or second-growth trees were assumed to be used, the payback period was much longer.

Similarly, a 2013 survey of studies of the replacement of fossil fuel-generated electricity reported payback periods between zero and 400 years.77 The use of residues and slash saw payback periods between zero and 44 years, with the lowest periods for the replacement of coal and the highest for natural gas. The lowest payback periods for the use of roundwood was between zero and 105 years in the case of additional fellings in previously unmanaged forests, or 12–46 years for the use of thinnings and additional fellings from existing plantations with a 20–25 year rotation, in each case replacing coal. A 2014 study found some greenhouse gas benefits from the use of forest residues with payback periods up to 25 years, while the use of whole trees, whether from thinnings, reduced-impact logging or short-rotation forestry, saw little or no savings over 50 years.78

In 2014 the UK Department of Energy and Climate Change published a comprehensive assessment of the climate impacts of imports of biomass from the US (the main source of woody biomass for UK consumption) – the Bioenergy Emissions and Counterfactuals (BEaC) calculator.79 Of the 29 scenarios analysed, those that involved utilizing residues that would otherwise have been burnt as waste, or newly established tree plantations on low-carbon land resulted in low net carbon emissions and short payback periods. In contrast, scenarios that involved harvesting additional roundwood from naturally growing forests or converting forests into plantations resulted in high or very high emissions (depending on the rotation length and hence carbon stocks of the forests and plantations). Of the 29 scenarios, 11 resulted in net emissions higher than using natural gas, and five of those had net emissions higher than using coal. For some types of biomass, such as additional fellings in already managed forests, the carbon payback period was many decades, perhaps even centuries.

The BEaC report was criticized by industry. For example, a spokesperson for Drax claimed that the model was ‘not a very accurate way of estimating carbon changes in forests and its scenarios were “hypothetical”’.80 In 2015 the Department of Energy and Climate Change commissioned a further study, including an assessment of the likelihood of the high-emission scenarios, an analysis of the factors determining harvest rates as well as consideration of whether harvest rotation lengths had changed in response to the demand for biomass, whether UK demand for biomass could divert pulpwood, thinnings or sawmill residues from other users, and whether whole trees were used in pellet manufacture and if so, the carbon stock impacts.81 At the time of publication the report is still awaited.

The evidence suggests that mature trees continue to absorb carbon (at least in old-growth forests) and that harvesting not only removes mature trees, thus substantially reducing total carbon uptake, but in the short term also increases carbon losses from soil disturbance.

The concepts of carbon debt and carbon payback have proved helpful in focusing attention on the range of factors that influence their magnitudes, and therefore the impact of different types of biomass feedstock on the climate. The approach is not, however, without its problems. It depends partly on the hypothesis that the higher levels of carbon emitted from burning woody biomass are compensated over time by faster growth of the forest from which it is sourced. This implictly accepts the argument that mature forests do not absorb carbon, and that harvesting and replacing old (carbon-neutral) trees with young (carbon-absorbing) trees increases the rate of carbon uptake in the forest, thereby offsetting the biomass-related emissions.

This is an essential part of the approach: if carbon absorption carries on at the same (or a lower) rate after harvesting as before, the carbon debt cannot be repaid. As discussed above, however, the evidence suggests that mature trees continue to absorb carbon (at least in old-growth forests) and that harvesting not only removes mature trees, thus substantially reducing total carbon uptake, but in the short term also increases carbon losses from soil disturbance. If this is correct, harvesting biomass for energy permanently reduces the rate of carbon uptake: the carbon debt can never be paid back and the carbon payback period is infinite. At the very least, if forest carbon uptake eventually stops (after perhaps 800 years, according to one of the studies cited above), the carbon payback period is extremely long. This may not be the case in plantations, where carbon absorption does appear to plateau, but the disturbance caused by harvesting, plus the fact the young trees absorb far less carbon than older trees, suggest long payback periods even there.

The carbon payback period and climate targets

Despite these reservations, the carbon payback approach has gained relatively wide acceptance (including in the impact assessment published by the European Commission to accompany the new draft Renewable Energy Directive in November 2016 – see further in Chapter 3). So how much does the length of the carbon payback period matter? Payback periods in the hundreds of years will counteract efforts to limit climate change over any reasonable timeframe, but what is a suitable time horizon over which to measure the impact?

Opinions on this question vary. One study considers 2050 to be an appropriate reference point, since energy systems (fossil and bioenergy) have lifetimes of typically 20 to 30 years. Of the scenarios it surveyed, only the use of residues that would otherwise have been burnt as waste or left to decay, replacing coal or oil-fired electricity (not gas), had payback period ranges falling wholly before 2050. Some of the roundwood scenarios would fall before 2050 only at the bottom end of their estimated payback ranges.

Some analysts prefer longer time horizons. A 2016 study looking at Swedish forests chose a 100-year time horizon, mirroring the Swedish Forests Agency’s 100-year forest impact assessments.82

Other studies prefer not to specify any particular timeframe. A 2014 one drew attention to the IPCC’s conclusion that it is cumulative greenhouse gas emissions that matter, not the timeframe within which these emissions are released: ‘The concept of cumulative carbon also implies that higher initial emissions can be compensated by a faster decline in emissions later or by negative emissions.’83 For carbon dioxide, the longest-lived of the greenhouse gases, it was cumulative emissions over the entire century that ‘to a first approximation determine the CO2 concentration at the end of the century, and therefore no individual year’s emissions are critical’.84 The study concluded that it is more important, therefore, to avoid lock-in of high-carbon technologies and infrastructure – such as coal – than to worry about short-term or even medium-term increases in carbon emissions, particularly if there could later be a carbon dividend from the use of biomass energy.

There are two main reasons, however, for thinking that short-term increases in carbon emissions matter. First, there is increasing concern over the possible existence of ‘climate tipping points’, when global temperature rise triggers a possibly irreversible change in the global climate from one stable state to another at a higher temperature. Examples include boreal forest dieback, Amazon rainforest dieback, the loss of Arctic and Antarctic sea ice and the melting of the Greenland and Antarctic ice sheets, disruption to the Indian and West African monsoon, and the loss of permafrost leading to potential Arctic methane release.85 Although in 2013 the IPCC concluded that there was as yet no evidence for global-scale tipping points (though there was possibly evidence for regional-scale tipping points, particularly in the Arctic),86 more recent studies have contested this, concluding that the probability is much higher than previously thought.87 If this is true, the risks of increasing carbon emissions in the short or medium term are accordingly higher than considered by the IPCC in 2013.

The second reason is the global climate targets adopted at the Paris climate conference in 2015, which committed signatory countries to hold ‘the increase in the global average temperature to well below 2°C above pre-industrial levels and to pursue efforts to limit the temperature increase to 1.5°C above pre-industrial levels’.88 The IPCC is scheduled to produce a special report on the implications of the 1.5°C target in 2018, but preliminary analyses suggest that achieving this target may require emissions levels to peak very soon, perhaps as early as 2020, and then fall – though there is still considerable uncertainty over this, and longer timescales for peaking emissions have also been suggested.89 Achieving the 1.5°C target is therefore likely to limit the use of biomass for energy to the shortest carbon payback periods.

Bioenergy with carbon capture and storage

Bioenergy with carbon capture and storage (BECCS) is a technology – as yet unproven – in which the carbon emissions from the burning of biomass for energy are captured before release into the atmosphere and permanently stored, thus removing them from the atmosphere and preventing their contribution to global warming. If it is assumed that biomass energy is carbon-neutral, BECCS generates negative carbon emissions.

The concept of BECCS emerged in the late 1990s and early 2000s.90 In 2007, the IPCC identified BECCS as a potential option for stabilizing emissions or as a rapid-response prevention strategy for abrupt climate change. It cautioned, however, that:

To date, detailed analysis of large-scale biomass conversion with CO2 capture and storage is scarce… further research is necessary to characterise biomass’s long-term mitigation potential… and opportunity costs… In particular, present studies are relatively poor in representing land competition with food supply and timber production, which has a significant influence on the economic potential of bio-energy crops.91

In 2011 a study published by the IEA reviewed the potential of BECCS in different forms, including dedicated biomass stations with CCS, co-firing with coal with CCS and liquid biofuel production with CCS.92 It concluded that the technical potential existed for negative greenhouse gas emissions of up to 10 GtCO2-eq annually (in comparison, total global emissions in 2012 were about 43 GtCO2-eq), the largest reductions coming from dedicated biomass power generation with CCS. The report identified the immaturity of the technology, uncertainty over the availability of sustainable biomass supply and secure and permanent carbon dioxide storage, and negative public perceptions (local opposition to CCS projects) as important barriers, though it considered that the association of CCS with biomass, as a renewable energy technology, could help overcome public resistance.

In 2014 the IPCC was more positive about the potential for BECCS than in its previous assessment report. Of the 116 scenarios it reviewed aiming to achieve stabilization of carbon at 430–480 parts per million (the level considered necessary to limit global warming to 2°C), 101 involved some form of negative emissions – either through BECCS or afforestation. Every scenario aiming to limit global warming to 1.5°C included BECCS.93 The IPCC viewed BECCS as necessary in particular to compensate for residual emissions from sectors where mitigation was more expensive, or to return to the target emissions level after an overshoot. The synthesis report concluded that: ‘Many models could not limit likely warming to below 2°C if bioenergy, CCS, and their combination (BECCS) are limited (high confidence).’94 Similarly, the full mitigation report observed that ‘CDR [carbon dioxide removal] technologies such as BECCS are fundamental to many scenarios that achieve low-CO2-eq concentrations, particularly those based on substantial overshoot as might occur if near-term mitigation is delayed’.95

Overall, models reported by the IPCC estimated that the global technical potential for BECCS varied from three to more than 10 GtCO2/year, while cost estimates ranged from around $60 to $250/tonne CO2. Important limiting factors included land availability, a sustainable supply of biomass and storage capacity, and possible competition for biomass from other uses of bioenergy. The IPCC cautioned that:

The potential role of BECCS will be influenced by the sustainable supply of large-scale biomass feedstock and feasibility of capture, transport, and long-term underground storage of CO2 as well as the perceptions of these issues. The use of BECCS faces large challenges in financing, and currently no such plants have been built and tested at scale.96

As of the autumn of 2016, only one commercial BECCS project was under way: Archer Daniels Midland’s corn ethanol plant in Decatur, Illinois, in the US.97 During its pilot phase in 2011–14 the plant sequestered one million tonnes of carbon dioxide from fermenting corn, which was injected into local porous sandstone formations lying beneath three layers of dense shale. With US government funding, the next phase (which was due to start in late 2016) aims to capture and store 2.26 million tonnes over two and half years. However, given the emissions produced from the energy needed to run the plant as well as to capture and store the carbon emissions, plus the carbon emitted when the ethanol itself is burnt, it is not clear whether the plant has in fact produced negative emissions. In addition, one of the aims of the project is to use some of the captured carbon dioxide for enhanced oil recovery, increasing the financial returns but further contributing to greenhouse gas emissions. Abengoa’s ethanol plant in Rotterdam in the Netherlands has been capturing carbon dioxide since 2011 (about 100,000 tonnes a year), but this is used in nearby greenhouses rather than stored.98

Overall, there are three main problems with the vision of BECCS as a major contributor to negative emissions.

First, as discussed above, the burning of biomass is not necessarily carbon-neutral at the point of combustion or even over the short or medium term – although, as discussed, it may be over the longer term depending on the carbon payback period. The surveys and models of the potential for BECCS, including those reviewed by the IPCC, simply assume that all bioenergy is carbon-neutral (provided that basic sustainability standards are in place, e.g. no conversion of forests to bioenergy crops). A 2015 survey was unable to find a single study that had calculated the potential for negative emissions based on any type of life-cycle greenhouse gas assessment that could have taken into account changes in the forest carbon stock as a result of harvesting for bioenergy.99 The IPCC in 2014 acknowledged the potential for significant emissions from land-use change and increased nitrous oxide emissions from greater fertilizer use, but did not consider any of the wider factors discussed above.100 In reality, since BECCS assumes that forests are planted specifically for use as energy, carbon payback periods are likely to be at the higher end of those discussed above, though it can be assumed that much of the new forest would be fast-growing softwood plantations, for which the carbon payback period is rather lower (depending partly on what the forest replaced).

The technology has proved more expensive and less effective than originally expected and, as in other areas, the falling prices of renewable energy technologies, particularly solar PV and wind, have undercut the appeal of CCS as a low-carbon option and accelerated the complete phase-out of coal.

Second, CCS technology is proving more difficult to commercialize and deploy than originally predicted. By the spring of 2016, there were 15 large-scale CCS projects in operation worldwide, capturing 28 million tonnes of carbon dioxide a year. By the end of 2017, this was projected to increase to 22 projects capturing about 40 million tonnes a year.101 While significant, this is far off the trajectory needed to satisfy the IEA’s 2015 prediction that CCS would capture two billion tonnes a year by 2030. Furthermore, most of the projects currently operating are producing carbon dioxide for enhanced oil recovery rather than permanent storage. In general, the technology has proved more expensive and less effective than originally expected and, as in other areas, the falling prices of renewable energy technologies, particularly solar PV and wind, have undercut the appeal of CCS as a low-carbon option and accelerated the complete phase-out of coal, thus removing one of the sources of fossil fuels CCS was intended to operate alongside. CCS equipment can be fitted to gas-fired power plants and industrial processes, but the benefits in terms of reducing carbon emissions are lower, and therefore the cost per tonne of carbon captured is higher. Further technological development can be expected, but it is difficult not to conclude that the current speed of development and deployment of CCS is too low to justify the reliance placed on BECCS by the IPCC.

Third, as noted by the IPCC and others, the availability of land for bioenergy is a limiting factor. The highest estimates of BECCS assume that 15–18 GtCO2 could be removed per year, with energy production of 200–400 EJ per year. This comprises 80–100 EJ/year from the by-products of agriculture and forest industries, and the remaining 180–300 EJ/year from dedicated energy crops.102 (These are very large quantities; in comparison, world energy production was roughly 575 EJ in total in 2014.)103 A review in 2015 calculated that production of 100 EJ/year could require up to 500 million hectares of land (assuming an average biomass yield of 10 tonnes of dry biomass per hectare annually). The top end of the projections for BECCS would therefore require two billion hectares – an area greater than the total global land area currently planted with agricultural crops (about 1.5 billion hectares in 2015) and about half the total global forest area (about 4 billion hectares ).104 Scenarios like this also tend to assume radical changes in behaviour, including a major shift away from eating meat (releasing much of the land currently used for pasture, about 3.4 billion hectares), together with rapid increases in food yields (sufficient to meet global food demand, which is projected to double over the next 50 years). Neither of these developments seems at all likely.

Another study that focused on using switchgrass for feedstock estimated that 200 million hectares (about half the total cropland of the US) would be needed to remove 3.7 GtCO2 per year (about one-fifth of the volume estimated in the highest projections for BECCS).105 The process would also consume 20 per cent of global fertilizer production and require 4,000 km3/year of water, equal to current global water withdrawals for irrigation.

For all these reasons, the prospects for the development of BECCS at scale seem highly unlikely; and, in any case, its impacts on the climate would not necessarily be positive in the short term. The reliance on BECCS of so many of the climate-mitigation scenarios reviewed by the IPCC is of major concern, potentially distracting attention from other mitigation options and encouraging decision-makers to lock themselves into high-carbon options in the short term on the assumption that the emissions thus generated can be compensated for in the long term.106

Conclusions and recommendations

Changes in the forest carbon stock must be fully accounted for in assessing the climate impact of the use of woody biomass for energy. It is not valid to claim that because trees absorb carbon as they grow, the emissions from burning them can be ignored. This is true whether or not the forest from which the biomass is sourced is sustainably managed, or whether it is growing in size, or whether forests as a whole are expanding. All these approaches either treat what happens to the trees after they are harvested as irrelevant, or ignore the carbon sequestration forgone when the trees are harvested, or both. As the European Commission Joint Research Centre concluded:

in order to assess the climate change mitigation potential of forest bioenergy pathways, the assumption of biogenic carbon neutrality is not valid under policy relevant time horizons (in particular for dedicated harvest of stemwood for bioenergy only) if carbon stock changes in the forest are not accounted for.107

Along with changes in forest carbon stock, a full analysis of the impact on the climate of using woody biomass for energy needs to take into account the emissions from combustion (which are generally higher than those for fossil fuels) and the supply-chain emissions from harvesting, collection, processing and transport. There is still some uncertainty over some of these factors, including levels of supply-chain emissions, the impact on soil carbon and tree growth of using forest residues, and levels of methane emissions produced during the storage of wood pellets and wood chips. The rate of carbon absorption by mature trees is routinely ignored by many of the models used to predict climate impacts. More research into all these issues would be helpful.

There is also uncertainty over market dynamics. While it may be the case that the growth of the woody biomass industry could lead to greater investment in forests, and therefore a higher rate of tree planting, which can help to offset higher emissions from combustion, the evidence for this happening is so far largely lacking. In any case, the models that predict this often assume that old-growth forests are replaced by fast-growing plantations, which in itself leads to higher carbon emissions, together with negative impacts on biodiversity.

Notwithstanding all this, harvesting of whole trees for energy will in almost all circumstances increase net carbon emissions very substantially compared to using fossil fuels, because of the loss of future carbon sequestration from the growing trees and because of the loss of soil carbon consequent upon the disturbance. This is particularly true for mature trees in old-growth forests, whose rate of carbon absorption can be very high.

The use of sawmill residues for energy has lower impacts, because it involves no additional harvesting as it is waste from other wood industry operations. The impact will be most positive for the climate if they are burnt on-site for energy without any associated transport or processing emissions. However, mill residues can also be used for wood products such as particleboard; if diverted instead to energy, this will raise carbon concentrations in the atmosphere. The current high levels of use of mill residues mean that this source is unlikely to provide much additional feedstock for the biomass energy industry in the future (or, if it does, it will be at the expense of other wood-based industries). Black liquor, a waste from the pulp and paper industry, can also be burnt on-site for energy and has no other use; in many ways it is the ideal feedstock for biomass energy.

The use of forest residues for energy also implies no additional harvesting, so its impacts on net carbon emissions can be low. This depends mainly on the rate at which the residues would decay and release carbon if left in the forest, which can vary substantially. If slow-decaying residues are burnt, the impact would be an increase in net carbon emissions, potentially for decades. In addition, removing residues from the forest can adversely affect soil carbon and nutrient levels as well as tree growth rates.

The carbon payback approach argues that, while they are higher than using fossil fuels, carbon emissions from burning woody biomass can be absorbed by forest regrowth. The time this takes – the carbon payback period before which carbon emissions return to the level they would have been at if fossil fuels had been used – is of crucial importance. There are problems with this approach, but it does help to highlight the range of factors that affect the impact of biomass, and focuses attention on the very long payback periods of some feedstocks, particularly whole trees, which is a matter of considerable concern given the potential existence of climate tipping points and the near-term targets for carbon emission reductions agreed in Paris in 2015.

For all these reasons, the provision of financial or regulatory support to biomass energy on the grounds of its contribution to mitigating climate change needs to be strictly controlled. Only those feedstocks that reduce carbon emissions over the short term should be eligible. This topic is considered further in Chapter 3.

Finally, while interest is growing in BECCS, its future development at scale seems highly unlikely, given the slow rate of commercialization of CCS technology and likely limits on the availability of land. In addition, the studies of options for BECCS almost always assume that biomass is zero-carbon at the point of combustion – which, as argued above, is not a valid assumption. The reliance on BECCS of so many of the climate-mitigation scenarios reviewed by the IPCC is, accordingly, of major concern, potentially distracting attention from other mitigation options and encouraging decision-makers to lock themselves into high-carbon options in the short term on the assumption that the emissions thus generated can be compensated for in the long term.

8 Griffiths, J. (2016), Scoping Dialogue on Sustainable Woody Biomass for Energy, p. 8, New Haven, CT: The Forests Dialogue, http://theforestsdialogue.org/sites/default/files/files/TFD%20Bankground%20Paper%20Scoping%20dialogue%20Sustainable%20Woody%20Biomass%20DRAFT%2020%2022%20June%202016(1).pdf (accessed 30 Dec. 2016).

9 Strange Olesen, A. et al. (2015), Environmental Implications of Increased Reliance of the EU on Biomass from the South East US, p. 8, Brussels: European Commission, http://www.aebiom.org/wp-content/uploads/2016/08/DG-ENVI-study-imports-from-US-Final-report-July-2016.pdf (accessed 30 Dec. 2016).

10 As noted in, for example, Agostini, A., Giuntoli, J. and Boulamanti, A. (2013), Carbon accounting of forest bioenergy: Conclusions and recommendations from a critical literature review, p.16, European Commission Joint Research Centre, http://publications.jrc.ec.europa.eu/repository/bitstream/JRC70663/eur25354en_online.pdf (accessed 30 Dec. 2016); and IEA Bioenergy Task 38 on Climate Change Effects of Biomass and Bioenergy Systems (2013), ‘Description of IEA Task 38’, http://www.task38.org/publications/task38_description_2013.pdf (accessed 30 Dec. 2016).

11 European Commission Joint Research Centre (2006), Integrated Pollution Prevention and Control Reference Document on Best Available Techniques for Large Combustion Plants, http://eippcb.jrc.ec.europa.eu/reference/BREF/lcp_bref_0706.pdf (accessed 30 Dec. 2016).

12 Biomass Availability and Sustainability Information System (BASIS) (2015), Report on conversion efficiency of biomass, Version 2. http://www.basisbioenergy.eu/fileadmin/BASIS/D3.5_Report_on_conversion_efficiency_of_biomass.pdf (accessed 30 Dec. 2016).

13 Partnership for Policy Integrity (2012), uploaded data: ‘EPA’s non-cogen egrid data for 2012’, http://www.pfpi.net/epas-non-cogen-egrid-data-for-2012 (accessed 20 Feb. 2017).

14 DONG Energy (undated), ‘Avedøre Power Station’, http://www.dongenergy.com/en/our-business/bioenergy-thermal-power/where-we-operate (accessed 30 Dec. 2016).

15 As noted in Wang, W. et al. (2015), ‘Carbon savings with transatlantic trade in pellets: accounting for market-driven effects’, pp. 4–5, Environmental Research Letters, 10, doi:10.1088/1748-9326/10/11/114019 (accessed 30 Dec. 2016).

16 Jonker J. G. G., Junginger, M. and Faaij, A. (2014), ‘Carbon payback period and carbon offset parity point of wood pellet production in the South-eastern United States’, p. 375, GCB Bioenergy, 6:4, DOI: 10.1111/gcbb.12056 (accessed 30 Dec. 2016).

17 Galik, C. S. and Abt, R. C. (2016), ‘Sustainability guidelines and forest market response: an assessment of European Union pellet demand in the southeastern United States’, GCB Bioenergy, p. 6, 8:3, DOI: 10.1111/gcbb.12273 (accessed 30 Dec. 2016).

18 Röder, M., Whittaker, C. and Thornley, P. (2015), ‘How certain are greenhouse gas reductions from bioenergy? Life cycle assessment and uncertainty analysis of wood pellet-to-electricity supply chains from forest residues’, Biomass and Bioenergy, 79, http://dx.doi.org/10.1016/j.biombioe.2015.03.030 (accessed 30 Dec. 2016). The study considered the impact of a wide range of factors, including different fuels used for drying.

19 See, for example, Svedberg, U., Samuelsson, J. and Melin, S. (2008), ‘Hazardous Off-Gassing of Carbon Monoxide and Oxygen Depletion during Ocean Transportation of Wood Pellets’, Annals of Occupational Hygiene, pp. 259–66 (which showed methane concentrations in the holds of ships transporting pellets varying between 216 and 956 parts per million (ppm), 52:4, DOI: 10.1093/annhyg/men013 (accessed 30 Dec. 2016); and Zilkha Biomass Energy (2013), ‘Cofiring Zilkha Black® Pellets’, presentation at 3rd IEA CCC Cofiring Biomass with Coal Workshop, June 2013, which included figures of 275 ppm for white pellets (compared to about 50 ppm for black pellets) after 20 days’ storage in laboratory jars.

20 See, for example, Spath, P. L., Mann, M. K. and Kerr, D. R. (1999), Life Cycle Assessment of Coal-fired Power Production, Golden, CO: US National Renewable Energy Laboratory, DOI: 10.2172/12100 (accessed 30 Dec. 2016); Fulton, M. et al. (2011), Comparing Life-Cycle Greenhouse Gas Emissions from Natural Gas and Coal, Deutsche Bank, https://www.db.com/cr/en/docs/Natural_Gas_LCA_Update_082511.pdf (accessed 27 Dec. 2016).

21 Miner, R. A. et al. (2014), ‘Forest Carbon Accounting Considerations in US Bioenergy Policy’, Journal of Forestry, 112:6, p. 9, https://doi.org/10.5849/jof.14-009 (accessed 27 Dec. 2016).

22 Repo, A. (2015), Climate impacts of bioenergy from forest harvest residues, Aalto University. https://aaltodoc.aalto.fi/bitstream/handle/123456789/15923/isbn9789526061887.pdf? (accessed 27 Dec. 2016).

23 Marelli, L. and Giuntoli, J. (2016), ‘Assessing climate change mitigation potential of bioenergy technologies’, presentation to European Commission bioenergy stakeholder conference, Brussels, 12 May 2016.

24 See, for example, Buchholz, T. et al. (2014), ‘Mineral soil carbon fluxes in forests and implications for carbon balance assessments’, GCB Bioenergy, 6:4, DOI: 10.1111/gcbb.12044 (accessed 27 Dec. 2016); Achat, D. L. et al. (2015), ‘Quantifying consequences of removing harvesting residues on forest soils and tree growth – A meta-analysis’, Forest Ecology and Management, 348, http://dx.doi.org/10.1016/j.foreco.2015.03.042 (accessed 27 Dec. 2016); Achat, D. L. et al. (2015), ‘Forest soil carbon is threatened by intensive biomass harvesting’, Nature Scientific Reports, 5, DOI:10.1038/srep15991 (accessed 27 Dec. 2016).

25 Schulze, E.-D. et al. (2012), ‘Large-scale bioenergy from additional harvest of forest biomass is neither sustainable nor greenhouse gas neutral’, GCB Bioenergy, 4:6, DOI: 10.1111/j.1757-1707.2012.01169.x (accessed 27 Dec. 2016).

26 See, for example, Lamers, P. and Junginger, M. (2013), ‘The “debt” is in the detail: A synthesis of recent temporal forest carbon analyses on woody biomass for energy’, Biofuels Bioproducts and Biorefining, 7:4, DOI: 10.1002/bbb.1407 (accessed 27 Dec. 2016); ‘Re: Burning wood from Southern US forests to generate electricity in Europe’ Letter from US academics to European Commissioner for Energy Günther Oettinger, 30 August 2013, https://www.nrdc.org/sites/default/files/ene_13090603a.pdf (accessed 27 Dec. 2016).

27 Strange Olesen, A. et al. (2015), Environmental Implications of Increased Reliance of the EU on Biomass from the South East US, p. 40.

28 See, for example, Hudiburg, T. et al. (2011), ‘Regional carbon dioxide implications of forest bioenergy production’, Nature Climate Change, 1, DOI:10.1038/nclimate1264 (accessed 27 Dec. 2016).

29 Clark, J. et al. (2011), Impacts of thinnings on carbon stores in the PNW: A plot level analysis, Corvallis, OR: Oregon State University, https://www.nrdc.org/sites/default/files/ene_13041704a.pdf (accessed 27 Dec. 2016).

30 Strange Olesen, A. et al. (2015), Environmental Implications of Increased Reliance of the EU on Biomass from the South East US, pp. 141–44.

31 European Commission (2016), ‘State aid: Commission authorises UK support to convert unit of Drax power plant from coal to biomass’, 19 December 2016, http://europa.eu/rapid/press-release_IP-16-4462_en.htm (accessed 27 Dec. 2016).

32 See, for example, American Forest and Paper Association (undated), ‘Biomass and Renewable Energy Mandates’, http://www.afandpa.org/issues/issues-group/biomass-and-renewable-energy-mandates (accessed 27 Dec. 2016); and RISI (2015), An Analysis of UK Biomass Power Policy, US South Pellet Production and Impacts on Wood Fiber Markets, Bedford, MA: RISI, http://docplayer.net/25281897-An-analysis-of-uk-biomass-power-policy-us-south-pellet-production-and-impacts-on-wood-fiber-markets-prepared-for-the-american-forest-paper.html (accessed 27 Dec. 2016).

33 Hoffman, W. (2014), ‘Black Liquor: The Loophole That Won’t Quit’, Tax Analysts, 9 April 2014 http://www.taxanalysts.org/content/black-liquor-loophole-wont-quit (accessed 27 Dec. 2016).

34 Gaudreault, C. et al. (2012), ‘Life cycle greenhouse gases and non-renewable energy benefits of kraft black liquor recovery’, Biomass and Bioenergy, 46, http://dx.doi.org/10.1016/j.biombioe.2012.06.027 (accessed 27 Dec. 2016).

35 Daigneault, A., Sohngen, B. and Sedjo, R. (2012), ‘Economic approach to assess the forest carbon implications of biomass energy’, Environmental Science and Technology, p. 5668, 46:11, DOI: 10.1021/es2030142 (accessed 27 Dec. 2016).

36 Wang, W. et al. (2015), ‘Carbon savings with transatlantic trade in pellets’, pp. 5–6.

37 Lamers, P. and Junginger, M. (2013), ‘The “debt” is in the detail’, p. 382; Gustavsson, L. et al. (2015), ‘Climate effects of bioenergy from forest residues in comparison with fossil energy’, Applied Energy, 138.

38 Gustavsson, L. et al. (2015), ‘Climate effects of bioenergy from forest residues in comparison with fossil energy’.

39 Drax (2015), Biomass Supply 2014, p. 6, DOI: 10.1016/j.apenergy.2014.10.013 (accessed 27 Dec. 2016).

40 Drax (undated), ‘Drax feedstock mix by fibre type for compliance year 2015–16’, http://www.drax.com/sustainability/sustainability-reporting (accessed 27 Dec. 2016).

41 Enviva (undated), ‘Wood Fiber Resources’, http://www.envivabiomass.com/wp-content/uploads/Enviva-Wood-Fiber-Resources.pdf (accessed 27 Dec. 2016).

42 Enviva (2015), ‘Prospectus’, 28 April 2015, p. 131, http://www.sec.gov/Archives/edgar/data/1592057/000119312515155449/d808391d424b4.htm (accessed 27 Dec. 2016).

43 See, for example, Booth, M. and Bitov, K. (2013), Analysis of Risks and Corporate Disclosures Regarding Environmental and Climate Considerations in the Biomass Power Sector, Partnership for Policy Integrity, http://www.pfpi.net/wp-content/uploads/2013/11/PFPI-report-to-SEC-on-bioenergy-Nov-20-2013.pdf (accessed 27 Dec. 2016); Partnership for Policy Integrity and Dogwood Alliance (2016), Carbon Emissions and Climate Change Disclosure by the Wood Pellet Industry – A Report to the SEC on Enviva Partners LP, https://www.dogwoodalliance.org/wp-content/uploads/1999/11/Report-to-SEC-on-Enviva-March-14-2016.pdf (accessed 27 Dec. 2016).

44 Dale, A. (2013), ‘Wood Pellets from Russia’, presentation to Wood Pellet Association of Canada, 18–20 November 2013, http://www.pellet.org/images/21_-_Arnold_Dale_-_From_Russia_with_Love_2013.pdf (accessed 27 Dec. 2016).

45 Strange Olesen, A. et al. (2015), Environmental Implications of Increased Reliance of the EU on Biomass from the South East US, pp. 95–96.

46 Ibid.

47 RISI (2015), An Analysis of UK Biomass Power Policy, US South Pellet Production and Impacts on Wood Fiber Markets, p. 20.

48 Oswalt, S. N. et al. (2014), Forest Resources of the United States, 2012: a technical document supporting the Forest Service 2015 update of the RPA Assessment, p. 21, Washington, DC: US Department of Agriculture, Forest Service, https://www.srs.fs.usda.gov/pubs/gtr/gtr_wo091.pdf (accessed 27 Dec. 2016).

49 See, for example, reports produced by Dogwood Alliance, https://www.dogwoodalliance.org/campaigns/bioenergy/bioenergy-reports/), the Natural Resources Defense Council, https://www.nrdc.org/issues/support-renewable-energy-protects-wild, and the Southern Environmental Law Center, https://www.southernenvironment.org/cases-and-projects/biomass-energy-in-the-south.

50 Partnership for Policy Integrity and Dogwood Alliance (2016), Carbon Emissions and Climate Change Disclosure by the Wood Pellet Industry, p. 29.

51 Buchholz, T. and Gunn, J. (2015), Carbon Emission Estimates for Drax biomass powerplants in the UK sourcing from Enviva Pellet Mills in U.S. Southeastern Hardwoods using the BEAC model, Pleasanton, CA: Spatial Informatics Group.

52 ENDS Waste and Bioenergy (2015), ‘Drax rejects carbon criticism’, 3 June 2015, http://www.endswasteandbioenergy.com/article/1349937/drax-rejects-carbon-criticism (accessed 27 Dec. 2016).

53 Ofgem (2016), Renewables Obligation: Sustainability Criteria Guidance, Table 11, pp. 83–84, https://www.ofgem.gov.uk/system/files/docs/2016/03/ofgem_ro_sustainability_criteria_guidance_march_16.pdf (accessed 27 Dec. 2016).

54 Ibid, Table 10, pp. 81–82.

55 US Department of Energy (2016), 2016 Billion-Ton Report: Advancing Domestic Resources for a Thriving Bioeconomy, Volume 1: Economic Availability of Feedstocks, Washington, DC: US Department of Energy, p. 127 https://energy.gov/sites/prod/files/2016/12/f34/2016_billion_ton_report_12.2.16_0.pdf (accessed 27 Dec. 2016).

56 For example, according to a Drax spokesperson, ‘Using the latest biomass technology has resulted in an over 80 per cent carbon saving compared to coal. This independently verified data factors in the full carbon costs from across the whole supply chain – including harvesting, processing and transportation.’ Timperley, J. (2016), ‘Is biomass really more polluting than coal?’, Business Green, 17 October 2016, http://www.businessgreen.com/bg/analysis/2474217/is-biomass-really-more-polluting-than-coal (accessed 27 Dec. 2016).

57 Strauss, W. (2011), ‘How Manomet Got It Backwards: Challenging the ‘Debt-Then-Dividend’ Axiom’, Biomass Magazine, 22 June 2011, http://biomassmagazine.com/articles/5621/how-manomet-got-it-backwards-challenging-the-undefineddebt-then-dividendundefined-axiom (accessed 27 Dec. 2016).

58 IEA Bioenergy Task 38 (2013), ‘Description of IEA Task 38’.

59 Evans, S. (2015), ‘Investigation: Does the UK’s biomass burning help solve climate change?’, Carbon Brief, 11 May 2015, https://www.carbonbrief.org/investigation-does-the-uks-biomass-burning-help-solve-climate-change (accessed 27 Dec. 2016).

60 See, for example, Daigneault, A., Sohngen, B. and Sedjo, R. (2012), ‘Economic approach to assess the forest carbon implications of biomass energy’; Miner, R. A. et al. (2014), ‘Forest Carbon Accounting Considerations in US Bioenergy Policy’; Wang, W. et al. (2015), ‘Carbon savings with transatlantic trade in pellets’; Hektor, B., Backéus, S. and Andersson, K. (2016), ‘Carbon balance for wood production from sustainably managed forests’, Biomass and Bioenergy, 93, DOI: 10.1016/j.biombioe.2016.05.025 (accessed 27 Dec. 2016).

61 Abt, K. L., et al. (2014), Effect of Policies on Pellet Production and Forests in the U.S. South: A Technical Document Supporting the Forest Service Update of the 2010 RPA Assessment, Washington, DC: US Department of Agriculture, https://www.srs.fs.usda.gov/pubs/gtr/gtr_srs202.pdf (accessed 27 Dec. 2016).

62 Hektor, B., Backéus, S. and Andersson, K. (2016), ‘Carbon balance for wood production from sustainably managed forests’, p. 3.

63 See, for example, Luyssaert, S. et al. (2008), ‘Old-growth forests as global carbon sinks’, Nature, 455, DOI:10.1038/nature07276 (accessed 27 Dec. 2016); Lewis, S. et al. (2009), ‘Increasing carbon storage in intact African tropical forests’, Nature, 457, 19 February 2009, DOI:10.1038/nature07771 (accessed 27 Dec. 2016); Bellassen, V. and Luyssaert, S. (2014), ‘Carbon sequestration: Managing forests in uncertain times’, Nature, 506, 12 February 2014, DOI: 10.1038/506153a (accessed 27 Dec. 2016); Stephenson, N. L. et al. (2014), ‘Rate of tree carbon accumulation increases continuously with tree size’, Nature 507, DOI:10.1038/nature12914 (accessed 27 Dec. 2016); Craggs, G. (2016), The Role of Old-Growth Forests in Carbon Sequestration, Dalkeith: Future Directions International, http://www.futuredirections.org.au/publication/role-old-growth-forests-carbon-sequestration (accessed 27 Dec. 2016). Over 60 studies showing the same phenomenon are summarized in CO2 Science (2014), ‘Forests (Growth Rates of Old vs. Young Trees) – Summary’, http://www.co2science.org/subject/f/summaries/forestold.php (accessed 27 Dec. 2016).

64 Stephenson, N. L. et al. (2014), ‘Rate of tree carbon accumulation increases continuously with tree size’.

65 Luyssaert, S. et al. (2008), ‘Old-growth forests as global carbon sinks’, p. 213.

66 Ibid.

67 Amiro, B. D. et al. (2010), ‘Ecosystem carbon dioxide fluxes after disturbance in forests of North America’, Journal of Geophysical Research, 115:G4, DOI: 10.1029/2010JG001390 (accessed 27 Dec. 2016).

68 Nunery, J.S., and Keeton, W.S. (2010), ‘Forest carbon storage in the northeastern United States: Net effects of harvesting frequency, post-harvest retention, and wood products’, Forest Ecology and Management, 259:8, http://dx.doi.org/10.1016/j.foreco.2009.12.029 (accessed 27 Dec. 2016).

69 Including Hektor, B., Backéus, S. and Andersson, K. (2016), ‘Carbon balance for wood production from sustainably managed forests’; Jonker J. G. G., Junginger, M. and Faaij, A. (2014), ‘Carbon payback period and carbon offset parity point of wood pellet production in the South-eastern United States’.

70 See, for example, Strange Olesen, A. et al. (2015), Environmental Implications of Increased Reliance of the EU on Biomass from the South East US, pp. 127–31.

71 ‘Carbon debt’ is not a precise term. It is sometimes used instead to refer to the period it takes for growing trees to recapture the emissions released from an equivalent amount of carbon. The meaning used here is taken from Mitchell, S. R., Harmon, M. E. and O’Connell, K. E. B. (2012), ‘Carbon debt and carbon sequestration parity in forest bioenergy production’, GCB Bioenergy, 4:6, DOI: 10.1111/j.1757-1707.2012.01173.x (accessed 27 Dec. 2016).

72 For an overview of many of these factors, see Agostini, A., Giuntoli, J. and Boulamanti, A. (2013), Carbon accounting of forest bioenergy.

73 Gunn, J. S., Ganz, D. V. and Keeton, W S. (2012), ‘Biogenic vs. geologic carbon emissions and forest biomass energy production’, GCB Bioenergy, 4:3, DOI: 10.1111/j.1757-1707.2011.01127.x (accessed 27 Dec. 2016).

74 Buchholz T. et al. (2014), ‘Mineral soil carbon fluxes in forests and implications for carbon balance assessments’.

75 See, for example, Walker, T. et al. (2010), Biomass Sustainability and Carbon Policy Study, Brunswick, ME: Manomet Center for Conservation Sciences, http://www.mass.gov/eea/docs/doer/renewables/biomass/manomet-biomass-report-full-hirez.pdf (accessed 27 Dec. 2016); Searchinger, T. (2012), ‘Global Consequences of the Bioenergy Greenhouse Gas Accounting Error’, in Inderwildi, O. and King, D. (eds) (2012), Energy, Transport and the Environment, London: Springer-Verlag; and Bowyer, C. et al. (2012), The GHG Emissions Intensity of Bioenergy: Does bioenergy have a role to play in reducing Europe’s GHG emissions?, London: Institute for European Environmental Policy, http://www.ieep.eu/assets/1008/IEEP_-_The_GHG_Emissions_Intensity_of_Bioenergy_-_October_2012.pdf (accessed 27 Dec. 2016).

76 See Agostini, A., Giuntoli, J. and Boulamanti, A. (2013), Carbon accounting of forest bioenergy, pp. 43–44.

77 Lamers, P. and Junginger, M. (2013), ‘The “debt” is in the detail’.

78 Baral, A. and Malins, C. (2014), Comprehensive Carbon Accounting for Identification of Sustainable Biomass Feedstocks, Washington, DC: International Council on Clean Transportation, http://www.theicct.org/sites/default/files/publications/ICCT_carbonaccounting-biomass_20140123.pdf (accessed 27 Dec. 2016).

79 Stephenson, A. and Mackay, D. (2014), Life Cycle Impacts of Biomass Electricity in 2020: Scenarios for Assessing the Greenhouse Gas Impacts and Energy Input Requirements of Using North American Woody Biomass for Electricity Generation in the UK, London: Department of Energy and Climate Change, https://www.gov.uk/government/uploads/system/uploads/attachment_data/file/349024/BEAC_Report_290814.pdf (accessed 27 Dec. 2016).

80 Quoted in Evans, S. (2015), ‘Investigation: Does the UK’s biomass burning help solve climate change?’.

81 Ibid.

82 Gustavsson, L. et al. (2017), ‘Climate change effects of forestry and substitution of carbon-intensive materials and fossil fuels’, Renewable & Sustainable Energy Reviews, 67, http://dx.doi.org/10.1016/j.rser.2016.09.056 (accessed 27 Dec. 2016).

83 Miner, R. A. et al. (2014), ‘Forest Carbon Accounting Considerations in US Bioenergy Policy’.

84 Ibid.

85 See, for example, Lenton, T. M. et al. (2008), ‘Tipping Points in the Earth’s Climate System’, Proceedings of the National Academy of Science, 105:6, DOI: 10.1073/pnas.0705414105 (accessed 27 Dec. 2016).

86 Miner, R. A. et al. (2014), ‘Forest Carbon Accounting Considerations in US Bioenergy Policy’.

87 See, for example, Drijfhout, S. et al. (2015), ‘Catalogue of abrupt shifts in Intergovernmental Panel on Climate Change climate models’, Proceedings of the National Academy of Science, 112:43, DOI: 10.1073/pnas.1511451112 (accessed 27 Dec. 2016); Cai, Y., Lenton, T. M. and Lontzek, T. S. (2016), ‘Risk of multiple interacting tipping points should encourage rapid CO2 emission reduction’, Nature Climate Change, 6, DOI:10.1038/nclimate2964 (accessed 27 Dec. 2016).

88 Paris Agreement, Article 2 (1) (a), https://unfccc.int/files/meetings/paris_nov_2015/application/pdf/paris_agreement_english_.pdf.

89 See, for example, Carbon Brief (2016), ‘Analysis: Only five years left before 1.5C carbon budget is blown’, 19 May 2016, https://www.carbonbrief.org/analysis-only-five-years-left-before-one-point-five-c-budget-is-blown (accessed 27 Dec. 2016); Hare, B. (2016), ‘No time to lose: the 1.5°C limit in the Paris Agreement’, Berlin: Climate Analytics, 10 August 2016, http://climateanalytics.org/blog/2016/the-1-5c-limit-in-the-paris-agreement-why-there-is-no-time-to-lose.html (accessed 27 Dec. 2016); Carbon Brief (2016), ‘Highlights: Day two at the 1.5C conference on climate change in Oxford’ 22 September 2016, https://www.carbonbrief.org/day-two-at-the-1-5-c-conference-on-climate-change-in-oxford (accessed 27 Dec. 2016).

90 See Hickman, L. (2016), ‘Timeline: How BECCS became climate change’s “saviour” technology’, Carbon Brief, 13 April 2016, https://www.carbonbrief.org/beccs-the-story-of-climate-changes-saviour-technology (accessed 28 Dec. 2016).

91 IPCC (2007), Climate Change 2007: Mitigation of Climate Change, Cambridge: Cambridge University Press, p. 211, https://www.ipcc.ch/pdf/assessment-report/ar4/wg3/ar4_wg3_full_report.pdf (accessed 28 Dec. 2016).

92 Ecofys (2011), Potential for Biomass and Carbon Dioxide Capture and Storage, Paris: International Energy Agency, http://www.eenews.net/assets/2011/08/04/document_cw_01.pdf (accessed 28 Dec. 2016).

93 IPCC (2014), Climate Change 2014: Mitigation of Climate Change. Also see Fuss, S. (2016), ‘The role of BECCS in climate change mitigation: potentials and limits’, presentation to IEA BECCS Specialist Meeting, London, 23 June 2016.

94 IPCC (2015), Climate Change 2014: Synthesis Report, Geneva: IPCC, p. 97, https://www.ipcc.ch/pdf/assessment-report/ar5/syr/SYR_AR5_FINAL_full_wcover.pdf (accessed 28 Dec. 2016).

95 IPCC (2014), Climate Change 2014: Mitigation of Climate Change, Cambridge: Cambridge University Press, p. 480.

96 Ibid., pp. 485–86.

97 See Carbon Brief (2016), ‘Analysis: Negative emissions tested at world’s first major BECCS facility’, 31 May 2016, https://www.carbonbrief.org/analysis-negative-emissions-tested-worlds-first-major-beccs-facility (accessed 28 Dec. 2016).

98 Kemper, J. (2016), ‘Status of biomass with carbon capture and storage (BECCS/Bio-CCS)’, presentation to IEA BECCS Specialist Meeting, London, 23 June 2016.

99 Ernsting, A. and Munnion, O. (2015), Last-ditch Climate Option or Wishful Thinking? Bioenergy with carbon capture and storage, Biofuelwatch, http://www.biofuelwatch.org.uk/wp-content/uploads/BECCS-report-web.pdf [Accessed 28 Dec. 2016).

100 IPCC (2014), Climate Change 2014.

101 Global CCS Institute (undated), ‘Large Scale CCS Projects’, http://www.globalccsinstitute.com/projects/large-scale-ccs-projects.

102 US National Research Council (2015), Climate Intervention: Carbon Dioxide Removal and Reliable Sequestration, p. 54, Washington, DC: National Academies Press, http://dels.nas.edu/resources/static-assets/materials-based-on-reports/reports-in-brief/climate-intervention-brief-final.pdf (accessed 28 Dec. 2016).

103 IEA (2016), Key World Energy Trends 2016, Paris: IEA, https://www.iea.org/publications/freepublications/publication/KeyWorldEnergyTrends.pdf (accessed 28 Dec. 2016).

104 Food and Agriculture Organization United Nations (FAO) (2016), State of the World’s Forests 2016, Forests and agriculture: land-use challenges and opportunities, Rome: FAO, http://www.fao.org/3/a-i5588e.pdf (accessed 28 Dec. 2016).

105 Reviewed in US National Research Council (2015), Climate Intervention.

106 See also Anderson, K. and Peters, G. (2016), ‘The trouble with negative emissions’, Science, 14 October 2016, DOI: 10.1126/science.aah4567 (accessed 28 Dec. 2016).

107 Agostini, A., Giuntoli, J. and Boulamanti, A. (2013), Carbon accounting of forest bioenergy, p. 77.

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