Sustainable biochar to mitigate global climate change

Abstract

Production of biochar (the carbon (C)-rich solid formed by pyrolysis of biomass) and its storage in soils have been suggested as a means of abating climate change by sequestering carbon, while simultaneously providing energy and increasing crop yields. Substantial uncertainties exist, however, regarding the impact, capacity and sustainability of biochar at the global level. In this paper we estimate the maximum sustainable technical potential of biochar to mitigate climate change. Annual net emissions of carbon dioxide (CO₂ ), methane and nitrous oxide could be reduced by a maximum of 1.8 Pg CO₂ -C equivalent (CO₂ -C_e ) per year (12% of current anthropogenic CO₂ -C_e emissions; 1 Pg=1 Gt), and total net emissions over the course of a century by 130 Pg CO₂ -C_e , without endangering food security, habitat or soil conservation. Biochar has a larger climate-change mitigation potential than combustion of the same sustainably procured biomass for bioenergy, except when fertile soils are amended while coal is the fuel being offset.

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Received 29 Oct 2009 | Accepted 14 Jul 2010 | Published 10 Aug 2010 DOI:10.1038/ncomms1053

Sustainable biochar to mitigate global climate change

Dominic Woolf1, James E. Amonette2, F. Alayne Street-Perrott1, Johannes Lehmann3 & Stephen Joseph4

Production of biochar (the carbon (C)-rich solid formed by pyrolysis of biomass) and its storage in soils have been suggested as a means of abating climate change by sequestering carbon, while simultaneously providing energy and increasing crop yields. Substantial uncertainties exist, however, regarding the impact, capacity and sustainability of biochar at the global level. In this paper we estimate the maximum sustainable technical potential of biochar to mitigate climate change. Annual net emissions of carbon dioxide (CO 2), methane and nitrous oxide could be reduced by a maximum of 1.8 Pg CO 2 -C equivalent (CO 2-Ce)per year (12 % of current anthropogenic CO 2-Ce emissions;1 Pg = 1 Gt),andtotalnetemissions over the course of a century by 130 Pg CO 2-Ce , without endangering food security, habitat or soil conservation. Biochar has a larger climate-change mitigation potential than combustion of the same sustainably procured biomass for bioenergy, except when fertile soils are amended while coal is the fuel being offset.

1 School of the Environment and Society, Swansea University, Singleton Park, Swansea SA2 8PP, UK.2 Chemical and Materials Sciences Division, Pacic Northwest National Laboratory, Richland, Washington 99352, USA.3 Department of Crop and Soil Sciences, College of Agriculture and Life Sciences, Cornell University, Ithaca, New York 14853, USA.4 The School of Materials Science and Engineering, University of New South Wales , Sydney,

New South Wales 2052 , Australia . Correspondence and requests for materials should be addressed to J.E.A. (email: [email protected]).

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Since 2000, anthropogenic carbon dioxide (CO 2) emissions have risen by more than 3 % annually 1, putting Earths ecosystems on a trajectory towards rapid climate change that is

both dangerous and irreversible 2. To change this trajectory, a timely and ambitious programme of mitigation measures is needed. Several studies have shown that, to stabilize global mean surface temperature, cumulative anthropogenic greenhouse-gas (GHG) emissions must be kept below a maximum upper limit, thus indicating that future net anthropogenic emissions must approach zero 26 . If humanity oversteps this threshold of maximum safe cumulative emissions (a limit that may already have been exceeded 7 ), no amount of emissions reduction will return the climate to within safe bounds. Mitigation strategies that draw down excess CO 2 from the atmosphere would then assume an importance greater than an equivalent reduction in emissions.

Production of biochar, in combination with its storage in soils, has been suggested as one possible means of reducing the atmospheric CO 2 concentration(refs813andseealso Supplementary

Note for a history of the concept and etymology of the term). Bio-char s climate-mitigation potential stems primarily from its highly recalcitrant nature 1416, which slows the rate at which photosynthetically xed carbon (C) is returned to the atmosphere. In addition, biochar yields several potential co-benets. It is a source of renew-

able bioenergy; it can improve agricultural productivity, particularly in low-fertility and degraded soils where it can be especially useful to the world s poorest farmers; it reduces the losses of nutrients and agricultural chemicals in run-o; it can improve the water-holding capacity of soils; and it is producible from biomass waste 17,18. Of the

possible strategies to remove CO 2 from the atmosphere, biochar is notable, if not unique, in this regard.

Biochar can be produced at scales ranging from large industrial facilities down to the individual farm 19 , and even at the domestic level 20, making it applicable to a variety of socioeconomic situations. Various pyrolysis technologies are commercially available that yield dierent proportions of biochar and bioenergy products, such as bio-oil and syngas. Th e gaseous bioenergy products are typically used to generate electricity; the bio-oil may be used directly for low-grade heating applications and, potentially, as a diesel substitute a er suitable treatment 21. Pyrolysis processes are classied into two major types, fast and slow, which refer to the speed at which the biomass is altered. Fast pyrolysis, with biomass residence times of a few seconds at most, generates more bio-oil and less biochar than slow pyrolysis, for which biomass residence times can range from hours to days.

Th e sustainable-biochar concept is summarized in Figure 1 . CO 2 is removed from the atmosphere by photosynthesis. Sustainably

CO2 removedby photosynthesis

CO2 returned

INPUTS

OUTPUTS APPLICATIONS

IMPACT

Biofuel CO2 emissions

Rice

Other cereals

Manures

Agroforestry

Felling loss

Agricultural

residues

Energy

Soil Stored C

Process heat

Bio-oil Syngas

Avoided

Biochar

Avoided

Avoided CH4/N2O

Sugar cane

PROCESS

Avoided

avoided emissions

biomass decay soil

amendment

fossil CO2

emissions

Biomass crops

Pyrolysis

Biomasscrops,

agroforestry

Oxidation, soil C, tillage, transport

Net

Enhanced primary productivity

Figure 1 | Overview of the sustainable biochar concept. The gure shows inputs, process, outputs, applications and impacts on global climate . Within each of these categories, the relative proportions of the components are approximated by the height / width of the coloured elds. CO 2 is removed from the atmosphere by photosynthesis to yield biomass. A sustainable fraction of the total biomass produced each year, such as agricultural residues, biomass crops and agroforestry products, is converted by pyrolysis to yield bio-oil, syngas and process heat, together with a solid product, biochar, which is a recalcitrant form of carbon and suitable as a soil amendment. The bio-oil and syngas are subsequently combusted to yield energy and CO 2. This energy and the process heat are used to offset fossil carbon emissions, whereas the biochar stores carbon for a signicantly longer period than would have occurred if the original biomass had been left to decay. In addition to fossil energy offsets and carbon storage, some emissions of methane and nitrous oxide are avoided by preventing biomass decay (see Supplementary Table S5 for example) and by amending soils with biochar. Additionally, the removal of CO 2 by photosynthesis is enhanced by biochar amendments to previously infertile soils, thereby providing a positive feedback. CO 2 is returned to the atmosphere directly through combustion of bio-oil and syngas, through the slow decay of biochar in soils, and through the use of machinery to transport biomass to the pyrolysis facility, to transport biochar from the same facility to its disposal site and to incorporate biochar into the soil. In contrast to bioenergy, in which all CO 2 that is xed in the biomass by photosynthesis is returned to the atmosphere quickly as fossil carbon emissions are offset, biochar has the potential for even greater impact on climate through its enhancement of the productivity of infertile soils and its effects on soil GHG uxes.

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procured crop residues, manures, biomass crops, timber and forestry residues, and green waste are pyrolysed by modern technology to yield bio-oil, syngas, process heat and biochar. As a result of pyrolysis, immediate decay of these biomass inputs is avoided. Th e outputs of the pyrolysis process serve to provide energy, avoid emissions of GHGs such as methane (CH 4 ) and nitrous oxide (N 2 O), and amend agricultural soils and pastures. The bioenergy is used to oset fossil-fuel emissions, while returning about half of the C xed by photosynthesis to the atmosphere. In addition to the GHG emissions avoided by preventing decay of biomass inputs, soil emissions of GHGs are also decreased by biochar amendment to soils. Th e biochar stores carbon in a recalcitrant form that can increase soil water- and nutrient-holding capacities, which typically result in increased plant growth. This enhanced productivity is a positive feedback that further enhances the amount of CO 2 removed from the atmosphere. Slow decay of biochar in soils, together with tillage and transport activities, also returns a small amount of CO 2 to the atmosphere. A schematic of the model used to calculate the magnitudes of these processes is shown as Supplementary Figure S1 .

Even under the most zealous investment programme, biochar production will ultimately be limited by the rate at which biomass can be extracted and pyrolysed without causing harm to the biosphere or to human welfare. Globally, human activity is responsible for the appropriation of 16 Pg C per year from the biosphere, which correspondsto 24%of potentialterrestrial net primary productivity (NPP) 22 . Higher rates of appropriation will increase pressure on global ecosystems, exacerbating a situation that is already unsustainable 23.

Th e main aim of this study is to provide an estimate of the theoretical upper limit, under current conditions, to the climate-change mitigation potential of biochar when implemented in a sustainable manner. Th is limit, which we term the maximum sustainable technical potential (MSTP), represents what can be achieved when the portion of the global biomass resource that can be harvested sustainably (that is, without endangering food security, habitat or soil conservation) is converted to biochar by modern high-yield, low-emission, pyrolysis methods. Th e fraction of the MSTP that is actually realized will depend on a number of socioeconomic factors, including the extent of government incentives and the relative emphasis placed on energy production relative to climate-change mitigation. Aside from assuming a maximum rate of capital investment that is consistent with that estimated to be required for climate-change mitigation 24, this study does not take into account any economic, social or cultural barriers that might further limit the adoption of biochar technology.

Our analysis shows that sustainable global implementation of biochar can potentially oset a maximum of 12 % of current anthropogenic CO 2-C equivalent (CO2-C e ) emissions (that is, 1.8 Pg CO 2-

C e per year of the 15.4 Pg CO 2-C e emitted annually), and that over the course of a century, the total net oset from biochar would be 130Pg CO 2-C e . We also show that conversion of all sustainably obtained biomass to maximize bioenergy, rather than biochar, production can oset a maximum of 10 % of the current anthropogenic CO 2-C e emissions. Th e relative climate-mitigation potentials of bio-char and bioenergy depend on the fertility of the soil amended and the C intensity of the fuel being oset, as well as the type of biomass. Locations at which the soil fertility is high and coal is the fuel being oset are best suited for bioenergy production. The climate-mitigation potential of biochar (with combined energy production) is higher for all other situations.

Results

Sustainable biomass-feedstock availability. To ensure that our

estimates represent a sustainable approach, we use a stringent set of criteria to assess potential feedstock availability for biochar

production. Of primary importance is the conversion of land to generate feedstock. In addition to its negative eects on ecosystem conservation, land clearance to provide feedstock may also release carbon stored in soils and biomass, leading to unacceptably high carbon-payback times before any net reduction in atmospheric CO 2 is achieved (ref. 25, Supplementary Methods and Supplementary Fig. S2 ). For example, we nd that a land-use change carbon debt greater than 22MgCha1 (an amount that would be exceeded by conversion of temperate grassland to annual crops 25 ) will result in a carbon-payback time that is greater than 10 years. Clearance of rainforests to provide land for biomass-crop production leads to carbon payback times in excess of 50 years. Where rainforest on peatland is converted to biomass-crop production, carbon-payback times may be in the order of 325 years. We therefore assume that no land clearance will be used to provide biomass feedstock, nor do we include conversion of agricultural land from food to biomass-crop production as a sustainable source of feedstock, both because of the negative consequences for food security and because it may indirectly induce land clearance elsewhere 26 . Some dedicated biomass-crop production on abandoned, degraded agricultural soil has been included in this study as this will not adversely aect food security 27

and can improve biodiversity 28,29 . We further assume that extraction rates of agricultural and forestry residues are suffi ciently low to preclude soil erosion or loss of soil function, and that no industrially treated waste biomass posing a risk of soil contamination will be used.

Other constraints on biochar production methods arise because emissions of CH 4, N2O, soot or volatile organic compounds combined with low biochar yields (for example, from traditional charcoal kilns or smouldering slash piles) may negate some or all of the carbon-sequestration benets, cause excessive carbon-payback times or be detrimental to health. Th erefore, we do not consider any biochar production systems that rely on such technologies, and restrict our analysis to systems in which modern, high-yield, low-emission pyrolysis technology can feasibly be used to produce high-quality biochar.

Within these constraints, we derived a biomass-availability scenario for our estimate of MSTP, as well as two additional scenarios, Alpha and Beta, which represent lower demands on global biomass resources ( Table 1 ). Attainment of the MSTP would require substantial alteration to global biomass management, but would not endanger food security, habitat or soil conservation. Th e Alpha scenario restricts biomass availability to residues and wastes available using current technology and practices, together with a moderate amount of agroforestry and biomass cropping. All three scenarios represent fairly ambitious projects, and require progressively greater levels of political intervention to promote greater adoption of sustainable land-use practices and increase the quantity of uncontaminated organic wastes available for pyrolysis. We do not consider any scenarios that are not ambitious in this study, as the intention is to investigate whether biochar could make a substantial contribution to climate-change mitigation an aspiration that certainly will not be accomplished by half-hearted measures. Th e range of mitigation results reported thus refers only to the scenarios considered and does not encompass the full range of less-eective outcomes corresponding to varying levels of inaction. Th e scenarios are based on current biomass availability ( Supplementary Methods and Supplementary Tables S1 and S2 ), the composition and energy contents of dierent types of biomass and the biochar derived from each ( Supplementary Tables S3 and S4 ), and the rate of adoption of biochar technology ( Supplementary Fig. S3 ). How this biomass resource base changes over the course of 100 years will depend on the potential eects of changing climate, atmospheric CO 2 , sea level, land use, agricultural practices, technology, population, diet and economic development. Some of these factors may increase biomass availability and some may decrease it. A full assessment of the wide range of possible future scenarios within plausible ranges of these factors remains outside the scope of this study.

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Table 1 | Annual globally sustainable biomass feedstock availability.

Biomass available in scenario (Pg C per year)

Alpha Beta

Maximum sustainable technical potential

Rice 0.22 0.25 0.28

Rice husks and 70 % of paddy rice straw not used for animal feed

Rice husks and 90 % of paddy rice straw not used for animal feed

Other cereals 0.072 0.13 0.188% of total straw and stover (assumes25 % extraction rate of crop residues minus quantity used as animal feed)

Rice husks and 80 % of paddy rice straw not used for animal feed

20 % of total straw and stover (45 % extraction rate minus animal feed)

Sugar cane 0.09 0.11 0.13Waste bagasse plus 25% of eld trash Waste bagasse plus 50 % of eld trash Waste bagasse plus 75 % of eld trash

Manures 0.10 0.14 0.1912.5% of cattle manure plus 50% of pig and poultry manure

14 % of total straw and stover (35 % extraction rate minus animal feed)

25 % of cattle manure plus 90 % of pig and poultry manure

Biomass crops 0.30 0.45 0.6050% of potential production of abandoned, degraded cropland that is not in other use

19 % of cattle manure plus 70 % of pig and poultry manure

100 % of potential production of abandoned, degraded cropland that is not in other use

Forestry residues 0.14 0.14 0.14

44 % of difference between reported fellings and extraction

Agroforestry 0.06 0.34 0.6217Mha of tropical silvopasture 85Mha of tropical grass pasture converted to silvopasture

75 % of potential production of abandoned, degraded cropland that is not in other use

170 Mha of tropical grass pasture converted to silvopasture

Green/wood waste 0.029 0.085 0.1475% of low-end estimate of yard-trimmings production and wood-milling residues

Beta plus high-end estimate of global yard trimmings and food waste, including 80 % of waste sawn wood

Total 1.01 1.64 2.27

Alpha plus mid-range estimate of yard trimmings plus urban food waste, including 40 % of waste sawnwood (legislation required to ensure that this fraction of waste wood is free of harmful contaminants)

Avoided GHG emissions . Results for the three scenarios are expressed below as a range from the Alpha scenario rst to the MSTP last. Th e model predicts that maximum avoided emissions of 1.01.8PgCO 2-C e per year are approached by mid-century and that, a er a century, the cumulative avoided emissions are 66 130 Pg CO 2-C e ( Fig. 2 ). Half of the avoided emissions are due to the net carbon sequestered as biochar, 30 % to replacement of fossil-fuel energy by pyrolysis energy and 20 % to avoided emissions of CH 4 and N 2O.

Cumulative and annual avoided emissions for the individual gases CO 2, CH4 and N 2O are given in Supplementary Figures S4 S6 .

A detailed breakdown of the sources of cumulative avoided GHG emissions over 100 years is given in Figure 3 . The two most important factors contributing to the avoided emissions from biochar are carbon stored as biochar in soil (43 94 Pg CO 2-C e ) and fossil-fuel offsets from coproduction of energy (1839 Pg CO 2-C e).

Of the benecial feedbacks, the largest is due to avoided CH 4 emissions from biomass decomposition (14 17 Pg CO 2-C e),predominantly arising from the diversion of rice straw from paddy elds (see Supplementary Table S5 for estimate of the mean CH 4 emission factor). Th e next largest positive feedbacks, in order of decreasing magnitude, arise from biochar-enhanced NPP on cropland,whichcontributes916 PgCO 2-C e to the net avoided emissions (if these increased crop residues are converted to bio-char), followed by reductions in soil N 2Oemissions (4.06.2 Pg

CO 2-C e ), avoided N 2 O emissions during biomass decomposition (1.83.3 Pg CO 2-C e ) and enhanced CH 4 oxidation by dry soils (0.440.8 Pg CO 2-C e).

Of the adverse feedbacks, biochar decomposition is the largest (817Pg CO 2-C e), followed by loss of soil organic carbon due to diversion of biomass from soil into biochar production (6 10 Pg CO 2-C e), and transport (1.31.9Pg CO 2-C e, see Supplementary Fig. S7 ). Contributions to the overall GHG budget from tillage (0.03 0.044 Pg CO 2-

C e) and reduced N-fertilizer production (0.20.3Pg CO 2-C e ) are negligible (although their nancial costs may not be).

Th e relative importance of all these factors to the GHG budget varies considerably among feedstocks. Notably, rice residues, green waste and manure achieve the highest ratios of avoided CO 2-C e emissions per unit of biomass-carbon (1.2 1.1, 0.9 and 0.8 CO 2-C e/C, respectively) because of the benets of avoided CH 4 emissions.

Sensitivity and Monte Carlo analyses. Sensitivity and Monte Carlo analyses with respect to reasonable values of key variables were used to estimate the uncertainty of the model results; they suggest areas in which future research is most needed and provide guidance on how biochar production systems might be optimized ( Fig. 4 ).

Th e strongest sensitivity is to the half-life of the recalcitrant fraction of biochar (see also Supplementary Table S6 ). Net avoided GHG emissions vary by 22% to +4% from that obtained using the baseline assumption of 300 years. However, most of this variation occurs for half-life < 100 years, in which range we nd (in agreement with previous work 30 ) that sensitivity to this factor is high. Conversely, for a more realistic half-life of the recalcitrant fraction ( > 100 years), sensitivity to this factor is low because biochar can be produced much more rapidly than it decays. As currently available data suggest that the half-life of biochar s recalcitrant fraction in soil is in the

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2.0

Annual

1.5

1.0

Annual net avoided emissions (Pg CO 2-C e)

0.5

0

150

Cumulative

100

MSTP-Biochar MSTP-Combustion Beta-Biochar Beta-Combustion Alpha-Biochar Alpha-Combustion

50

0

0 20 40 60 80 100

Time (y)

Figure 2 | Net avoided GHG emissions. The avoided emissions are attributable to sustainable biochar production or biomass combustion over 100 years, relative to the current use of biomass. Results are shown forthe three model scenarios, with those for sustainable biochar represented by solid lines and for biomass combustion by dashed lines. The top panel shows annual avoided emissions; the bottom panel, cumulative avoided emissions. Diamonds indicate transition period when biochar capacity of the top 15 cm of soil lls up and alternative disposal options are needed.

millennial range (see Supplementary Methods , Supplementary Table S6 and refs 8, 15, 16, 31), the contribution of its decay to the net GHG balance over centennial timescales is likely to be small.

Th e next largest sensitivity is to the pyrolysis carbon yield ( 9 % to +11%), indicating the importance of engineering to optimize for high yields of biochar rather than for energy production. This will be constrained, however, by the sensitivity to the labile fraction of the biochar (7% to +4%), which indicates the importance of optimizing for production of recalcitrant biochar rather than for higher yields of lower-quality biochar.

Aer carbon yield, the next largest sensitivity is to the carbon intensity of the fuel oset by pyrolysis energy production, with net avoidedemissions varyingby 4%from thebaselineassumption when natural gas is the fuel being osetandby +15%whencoal is oset.

Varying the impact of biochar amendment on soil N 2O emissions from zero to the largest reported reduction (80 % ; ref. 32) produces a sensitivity of 4% to +11%. Further variability in the impact of biochar on N 2 O emissions arises from adjusting the fraction of biomass-N that (if le to decompose) would be converted to N 2 O-N, from the Intergovernmental Panel on Climate Change

default values assumed in this study up to the higher rate of 5 % suggested by more recent work 33,34. Th is would increase the net avoided GHG emissions by up to 8 % .

Uncertainty in the response of crop yields to biochar amendment resultsinestimatedrange of 6%to +7%intheimpact of enhanced NPP of cropland on net avoided GHG emissions.

Sensitivities to the pyrolysis energy effi ciency ( 5%), to the half-life of the biochars labile fraction (4% to +1%) and to its impact on soil CH 4 oxidation ( 1%) are small.

Th e net eect of covariance of the above factors was assessed using the Monte Carlo analysis ( n=1,000, Supplementary Table S7). Despite limited data on the decomposition rate of biochar in soils and the eects of biochar additions on soil GHG uxes, sensitivity within realistic ranges of these parameters is small, resulting in an estimated uncertainty of 8to 10%(1s.d.)inthecumulative avoided GHG emissions for the three scenarios.

Comparison of biochar and bioenergy approaches. The mitigation impact of the renewable energy obtained from both biochar production and biomass combustion depends on the carbon intensity (that is, the mass of carbon emitted per unit of total energy produced) of the oset energy sources 11. At our baseline carbon intensity (17.5kgCGJ1; see Methods section), the model predicts that, on an average,themitigation impact of biocharis2722%(1423Pg CO 2-C e) larger than the 52107Pg CO 2-C e predicted if the same sustainably procured biomass were combusted to extract the maximum amount of energy ( Fig. 2 ). Th is advantage of biochar over bioenergy is largely attributable to the benecial feedbacks from enhanced crop yields and soil GHG uxes (Fig. 3, Supplementary Fig. S8).

Because the principal contribution of biomass combustion to avoided GHG emissions is the replacement of fossil fuels ( Fig. 3 ), the bioenergy approach shows a considerably higher sensitivity to carbon intensity than does biochar ( Fig. 5 ). Th e carbon intensity of oset energy varies from near-zero for renewable and nuclear energy to 26kgCGJ1 for coal combustion 35. Mean cumulative avoided emissions from biochar and biomass combustion are equal in our scenarios when the carbon intensity of osetenergy is2624kgCGJ1

( Fig. 5 ). In the MSTP scenario, this corresponds to an energy mix to which coal combustion contributes about 80 % , whereas in the Alpha scenario, the mean mitigation benet of biochar remains higher than that of bioenergy, even when 100 % coal is oset. The cumulative avoided emissions from both strategies decrease as the carbon intensity of the oset energy mix decreases, but the rate of decrease for biomass combustion is 2.5 2.7 times greater than that for biochar. As expected, the cumulative avoided emissions for biomass combustion are essentially zero when the carbon intensity of the energy mix is also zero. In contrast, the cumulative avoided emissions for biochar are still substantial at 48 91 Pg CO 2-C e.

Given that much of the increased climate mitigation from bio-char relative to biomass combustion stems from the benecial feedbacks of adding biochar to soil, and that these feedbacks will be greatest on the least fertile soils, the relative mitigation potentials will vary regionally with soil type (see Supplementary Methods,Supplementary Fig.S9andSupplementaryTablesS8S11for an account of how these feedbacks are calculated). The distribution of soils of varying fertility on global cropland is shown in Figure 6 . Globally, 0.31 Gha of soils with no fertility constraints are in use as cropland, as well as 0.29 Gha of cropland with few fertility constraints, 0.21 Gha with slight constraints, 0.32 Gha with moderate constraints, 0.18 Gha with severe constraints, 0.13 Gha with very severe constraints and 0.09 Gha of cropland on soils categorized as unsuitable for crop production. Th e amount of biomass produced in soils of dierent fertilities is shown in Supplementary Table S12 .

Figure 7 shows how the climate mitigation from biochar varies relative to biomass combustion when both soil fertility and the carbon intensity of energy osets are considered. Th e relative benet of

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CH4 Biomass CH4 Soil N2O Biomass N2O Soil Fossil fuel offsetC in Biochar Soil organic C Transportation and tillage Biochar decomposition

200

40 Biochar

MSTP

32

160

24

120

16

80

8

40

Cumulative avoided emissions (Pg CO 2-C e)

Cumulative avoided emissions (Pg CO 2-C e)

0

0

-8

-40

Beta

32

160

24

120

16

80

8

40

0

0

32

Alpha

160

24

Biochar

Combustion

120

16

80

8

40

0

0

-8

-40

Rice

Totals

Other cereals

Sugar cane

Manures

Biomass cropsForestry residues

Agroforestry

Green/wood waste

Enhanced NPP

Figure 3 | Breakdown of cumulative avoided GHG emissions (Pg CO2-Ce) from sustainable biochar production. The data are for the three model scenarios over 100 years by feedstock and factor. The left side of the gure displays results for each of eight feedstock types and the additional biomass residues that are attributed to NPP increases from biochar amendments; the right side displays total results by scenario for both biochar (left column) and biomass combustion (right column). For each column, the total emission-avoiding and emission-generating contributions are given, respectively, by the height of the columns above and below the zero line. The net avoided emissions are calculated as the difference between these two values. Within each column, the portion of its contribution caused by each of six emission-avoiding mechanisms and three emission-generating mechanisms is shown by a different colour. These mechanisms (from top to bottom within each column) are (1) avoided CH 4 from biomass decay, (2) increased CH 4 oxidation by soil biochar, (3) avoided N 2O from biomass decay, (4) avoided N 2O caused by soil biochar, (5) fossil fuel offsets from pyrolysis energy production, (6) avoided CO 2 emissions from carbon stored as biochar, (7) decreased carbon stored as soil organic matter caused by diversion of biomass to biochar, (8) CO 2 emissions from transportation and tillage activities and (9) CO 2 emissions from decomposition of biochar in soil.

producing biochar compared with biomass combustion is greatest when biochar is added to marginal lands and the energy produced by pyrolysis is used to oset natural gas, renewable or nuclear energy. When biochar is added to the most infertile cropland to oset the current global primary energy mix ( Mw), which has a carbon intensity of 16.5kgCGJ1, the relative benet from biochar is as much as 79 64 % greater than that from bioenergy ( Fig. 7 , Supplementary Fig. S10 ). This net benet diminishes as more coal is oset and as biochar is added to soils with higher fertility. Nevertheless, with the exception of those geographical regions having both naturally high soil fertility and good prospects for osetting coal emissions (in which bioenergy yields up to 16 22 % greater mitigation impact than biochar), biochar shows a greater climate-mitigation potential than bioenergy. Th e relative benet of producing biochar compared with bioenergy is greatest when biomass crops are used as

feedstocks ( Fig. 7b ), because avoided CH 4 emissions from the use of manure, green waste and rice residues occur regardless of whether these other feedstocks are used for energy or biochar.

Discussion

Our analysis demonstrates that sustainable biochar production (with addition to soils) has the technical potential to make a substantial contribution to mitigating climate change. Maximum avoided emissions of the order of 1.8 Pg CO 2-C e annually, and of 130 Pg

CO 2-C e over the course of a century, are possible at current levels of feedstock availability, while preserving biodiversity, ecosystem stability and food security.

Th e biochar scenarios described here, with their very high levels of biomass utilization, are not compatible with simultaneous implementation of an ambitious biomass energy strategy. The opportunity

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Mb

100 200

Half-life recalcitrant C (y)

50

300 1000

150

MSTP

Pyrolysis C yield (%)

C Intensity of fuel offset (kg C GJ-1)

Decrease in soil N2O emissions (%)

Cropland NPP (% yield response)

Labile-C fraction (%)

Global N2O emission factor (%)

Pyrolysis energy efficiency (%)

Half-life labile C (y)

49 18

25 100 15

75 20

100

40

15

0 50

301.05 65

1 0

62

Biochar

Combustion

Cumulative net avoided emissions (Pg CO 2-C e)

26

80

Beta

100

5

150

Alpha

85

5

50

25

200

Soil CH4 oxidation (mg CH4 per m2 per year)

Renewables

Gas

Oil

Coal

-20 -10 0 10 20 Deviation from reported estimate (%)

0

5 10 15 20 25 30

Figure 4 | Sensitivity of the model to key variables. Sensitivity is expressed as a percentage deviation from the reported value of cumulative net avoided GHG emissions over 100 years for each scenario. Top (blue), middle (yellow) and bottom (red) bars for each variable correspond to Alpha, Beta and MSTP scenarios. Minimum and maximum values foreach variable are at the ends of the bars (with additional sensitivities to recalcitrant carbon half-life of 100 and 200 years shown); baseline values of the key variables used in this study correspond to 0 % deviation.

See also Supplementary Table S7 .

C Intensity of fuel offset (kg C GJ-1)

Figure 5 | Cumulative mitigation potential (100 years) of biochar and biomass combustion as a function of carbon intensity of the type of energy being offset. The black vertical dashed line labelled Mb on the upper x axis refers to the carbon intensity of the baseline energy mix assumed in this study. Grey vertical dashed lines at 15, 19 and 26 kg C GJ 1

denote the carbon intensity of natural gas, oil and coal, respectively. The carbon intensity of renewable forms of energy is close to 0 kg C GJ 1.

cost of this forgone energy resource must be taken into account in an economic comparison of the two strategies. However, in terms of their potentials for climate-change mitigation, the mitigation impact of biochar is about one-fourth larger, on an average, than that obtained if the same biomass were combusted for energy. Regional deviations from this average are large because of dierences in soil fertility and available biomass. Our model predicts that the relative climate-mitigation benet of biochar compared with bioenergy is greatest in regions in which poor soils growing biomass crops can benet most from biochar additions. In contrast, biomass combustion leads to a greater climate-mitigation impact in regions with fertile soils where coal combustion can be eectively oset by biomass energy production. Th e global climate-mitigation potential achievable from the use of terrestrial biomass may thus be maximized by a mixed strategy favouring bioenergy in those regions with fertile soils where coal emissions can be oset, and biochar elsewhere. Nevertheless, we have included biochar production in fertile, coal-intensive regions in our scenarios because other potential benets of

biochar, such as its potential for more effi cient use of water and crop nutrients 3638, may favour its use even in such regions.

We emphasize that the results presented here assume that future biochar production follows strict sustainability criteria. Land-use changes that incur high carbon debts and biochar production using technologies with poorly controlled emissions lead to both large reductions in avoided emissions and excessively long carbon-payback times, during which net emissions are increased before any net reduction is observed. Biochar production and use, therefore, must be guided by well-founded and well-enforced sustainability protocols if its potential for mitigating climate change is to be realized.

Methods

Overall approach . A model (BGRAM version 1.1) to calculate the net avoided GHG emissions attributable to sustainable biochar production as a function of time was developed and applied to the three scenarios. Th is model includes the eects of

feedstock procurement, transport, pyrolysis, energy production, soil incorporation, soil GHG ux, soil fertility and fertilizer use (see also Supplementary Table S13 ), and biomass and biochar decomposition (see also Supplementary Fig. S11 ). The net avoided GHG emissions due to biochar were calculated as the dierence between the CO 2-equivalent emissions from biochar production and those that would have occurred as the biomass decomposed by other means had it not been converted to biochar. All emissions (actual or avoided) were calculated with time dependency. Wherever possible, conservative assumptions were used to provide a high degree of condence that our results represent a conservative estimate of the avoided GHG emissions achievable in each scenario. A detailed account of both the model and the three scenarios is given in Supplementary Methods .

Sustainability criteria . Biochar can be produced sustainably or unsustainably. Our criteria for sustainable biochar production require that biomass procured from agricultural and silvicultural residues be extracted at a rate and in a manner that does not cause soil erosion or soil degradation; crop residues currently in use as animal fodder not be used as biochar feedstock; minimal carbon debt be incurred from land-use change or use of feedstocks with a long life expectancy; no new lands be converted into biomass production and no agricultural land be taken out of food production; no biomass wastes that have a high probability of contamination, which would be detrimental to agricultural soils, be used; and biomass crop production be limited to production on abandoned agricultural land that has not subsequently been converted to pasture, forest or other uses. We further require that biochar be manufactured using modern technology that eliminates soot,CH 4 and N2O emissions while recovering some of the energy released during the pyrolysis process for subsequent use.

Greenhouse gases . We consider three GHGs in this analysis: CO 2, CH4 and N2O (see Supplementary Table S14 for a summary of estimated global warming potentials for these GHGs). Although the dierent atmospheric lifetimes of these gases ensure that there is no equivalence among them in any strict sense, we nevertheless adopt the common practice of normalizing each gas to a CO 2-C equivalent using the estimated radiative forcing produced by the emission of each gas, integrated over a 100-year period following emission, using Intergovernmental Panel on Climate Change 100-year global warming potentials 39 of 23 for CH 4and 296 for N 2O.

Comparison with bioenergy . To compare the net avoided GHG emissions stemming from biochar with those from bioenergy production, we apply the same model and sustainability criteria, but assume complete combustion to liberate the maximum possible energy, rather than slow pyrolysis, as the conversion technology.

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Figure 6 | Soil-fertility constraints to cropland productivity (5[H11032] resolution). Soil fertility is indicated by hue, whereas the percentage of the gridcell currently being used as cropland is indicated by colour saturation (with white indicating the absence of cropland in a grid cell).

Figure 7 | Cumulative mitigation potential of biochar relative to bioenergy. The mitigation potential is reported as a function of both soil fertility and carbon intensity of the type of energy being offset (in the MSTP scenario). Points Mew,

Mw and Mb on the upper x axis refer to the carbon intensity of the current world electricity mix, the current world primary energy mix and the baseline energy mix assumed in our scenarios, respectively. Carbon intensity values for natural gas, oil and coal are also indicated. The relative mitigation is calculated as cumulative avoided emissions for biochar minus those for bioenergy, expressed as a fraction of the avoided emissions for bioenergy (for example, a value of 0.1 indicates that the cumulative mitigation impact of biochar is 10 % greater than that of bioenergy, a value of 0.1 indicates that it is 10 % lower and a value of zero indicates that they have the same mitigation impact). The soil-fertility classications marked on the vertical axis correspond to the soil categories mapped in Figure. 6 . Panel a (Residues) includes agricultural and forestry residues, together with green waste, as biomass inputs; Panel b (Biomass crops) includes both dedicated biomass crops and agroforestry products as biomass inputs. Panel c (Manures), includes bovine, pig and poultry manure as biomass inputs. Panel d (Total) includes all sources of biomass inputs in the proportions assumed in our model. An analogous gure for the Alpha scenario is shown as Supplementary Figure S10 .

Soil application and fertility classication. Maximum biochar application to the top 0.15m of agricultural soils was assumed to be 50MgCha1. It was assumed that only 20 % of pasture soils will receive these application rates because of constraints from terrain, accessibility, re and wind. See Supplementary Methods and Supplementary Figure S12 .

Soil-fertility classications were taken from ref. 40. Th ese were combined with a 5-minute resolution map of global cropland distribution 41 to produce a global map of cropland, categorized by the severity of soil-fertility constraints ( Fig. 6 ).

Carbon intensity of fuel offsets. Th e baseline carbon intensity of the fuel osets( Mb) used here is 17.5kgCGJ1. Th e current world primary-energy mix ( Mw) has a carbon intensity of 16.5kgCGJ1 and the current world electricity-generation mix ( Mew) has a carbon intensity of 15 kg C GJ 1 (ref. 42). See Supplementary Methods for the derivation of these carbon intensities.

Technology adoption rate. Th e rate at which installed biochar production capacity approaches its maximum is constrained by simple economic considerations. Data for estimated capital costs are shown in Supplementary Table S15 . These are implemented in the model using a Gompertz curve ( Supplementary Methods) .

Th e model allows for a lead time of 5 years, during which little plant capacity is commissioned. Slow-to-moderate investment for the remainder of the rst decade and rapid adoption over the following three decades at a rate of capital investment consistent with the 2 % of global gross domestic product that Lord Stern estimates to be required for climate-change mitigation 24 culminate in near-maximal biochar production rates a er a total of four decades. Net avoided GHG emissions over the rst decade are negligible, because of a combination of initially slow adoption and carbon-debt payback.

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Acknowledgments

D.W. and F.A.S.-P. acknowledge support from the United Kingdom s Natural Environment Research Council (NERC) and Economic and Social Research Council (ESRC). J.E.A. acknowledges support from the United States Department of Energy (USDOE) Offi ce of Science, Offi ce of Biological and Environmental Research, Climate and Environmental Science Division, Mitigation Science Focus Area and from the USDOE Offi ce of Fossil Energy, Terrestrial Carbon Sequestration Program. The Pacic Northwest National Laboratory is operated for the USDOE by Battelle Memorial Institute under contract DE-AC05-76RL01830. J.L. acknowledges support from the Cooperative State Research Service of the U.S. Department of Agriculture and from the New York State Energy Research and Development Authority. S.J. acknowledges support from VenEarth Group LLC.

Author contributions

D.W. and J.E.A. produced the model and wrote the paper. F.A.S.-P. supervised the NERC / ESRC-funded PhD project of which D.W s contribution formed a part. J.L. and S.J. provided specialist advice. All authors commented on the paper.

Additional information

Supplementary Information accompanies this paper on http://www.nature.com/ naturecommunications

Competing nancial interests: D.W., J.E.A., F.A.S.-P. and J.L. declare no competing nancial interests. S.J. is Chairman of Anthroterra, a company conducting research into the development of a biochar mineral complex to replace conventional fertilizers. Th is company plans to manufacture and sell portable pyrolysers.

Reprints and permission information is available online at http://npg.nature.com/ reprintsandpermissions/

How to cite this article: Woolf, D. et al. Sustainable biochar to mitigate global climate change. Nat. Commun. 1:56 doi: 10.1038 / ncomms1053 (2010).

Licence: Th is work is licensed under a Creative Commons Attribution-NonCommercial-NoDerivative Works 3.0 Unported License. To view a copy of this license, visit http://creativecommons.org/licenses/by-nc-nd/3.0/

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