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 effects 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 find that a land-use change carbon debt greater than 22 Mg C ha−1 (an amount that would be exceeded by conversion of temperate grassland to annual crops25) 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 elsewhere26. Some dedicated biomass-crop production on abandoned, degraded agricultural soil has been included in this study as this will not adversely affect food security27 and can improve biodiversity28,29. We further assume that extraction rates of agricultural and forestry residues are sufficiently 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 , N 2 O, 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 benefits, cause excessive carbon-payback times or be detrimental to health. Therefore, 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. The 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. The range of mitigation results reported thus refers only to the scenarios considered and does not encompass the full range of less-effective outcomes corresponding to varying levels of inaction. The scenarios are based on current biomass availability (Supplementary Methods and Supplementary Tables S1 and S2), the composition and energy contents of different 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 effects 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.

Table 1 Annual globally sustainable biomass feedstock availability. Full size table

Avoided GHG emissions

Results for the three scenarios are expressed below as a range from the Alpha scenario first to the MSTP last. The model predicts that maximum avoided emissions of 1.0–1.8 Pg CO 2 -C e per year are approached by mid-century and that, after 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 2 O. Cumulative and annual avoided emissions for the individual gases CO 2 , CH 4 and N 2 O are given in Supplementary Figures S4, S5 and S6.

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 for the 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 fills up and alternative disposal options are needed. Full size image

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 (18–39 Pg CO 2 -C e ).

Figure 3: Breakdown of cumulative avoided GHG emissions (Pg CO 2 -C e ) from sustainable biochar production. The data are for the three model scenarios over 100 years by feedstock and factor. The left side of the figure 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 2 O from biomass decay, (4) avoided N 2 O 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. Full size image

Of the beneficial 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 fields (see Supplementary Table S5 for estimate of the mean CH 4 emission factor). The next largest positive feedbacks, in order of decreasing magnitude, arise from biochar-enhanced NPP on cropland, which contributes 9–16 Pg CO 2 -C e to the net avoided emissions (if these increased crop residues are converted to biochar), followed by reductions in soil N 2 O emissions (4.0–6.2 Pg CO 2 -C e ), avoided N 2 O emissions during biomass decomposition (1.8–3.3 Pg CO 2 -C e ) and enhanced CH 4 oxidation by dry soils (0.44–0.8 Pg CO 2 -C e ).

Of the adverse feedbacks, biochar decomposition is the largest (8–17 Pg 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 (1.3–1.9 Pg 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 (0.2–0.3 Pg CO 2 -C e ) are negligible (although their financial costs may not be).

The 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 benefits 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).

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 for each 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. Full size image

The 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 find (in agreement with previous work30) 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 millennial range (see Supplementary Methods, Supplementary Table S6 and refs 8,15,16,318, 15, 16, 31), the contribution of its decay to the net GHG balance over centennial timescales is likely to be small.

The 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.

After carbon yield, the next largest sensitivity is to the carbon intensity of the fuel offset by pyrolysis energy production, with net avoided emissions varying by −4% from the baseline assumption when natural gas is the fuel being offset and by +15% when coal is offset.

Varying the impact of biochar amendment on soil N 2 O 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 left 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 work33,34. This would increase the net avoided GHG emissions by up to 8%.

Uncertainty in the response of crop yields to biochar amendment results in estimated range of −6% to +7% in the impact of enhanced NPP of cropland on net avoided GHG emissions.

Sensitivities to the pyrolysis energy efficiency (±5%), to the half-life of the biochar's labile fraction (−4% to +1%) and to its impact on soil CH 4 oxidation (± 1%) are small.

The net effect 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 effects of biochar additions on soil GHG fluxes, sensitivity within realistic ranges of these parameters is small, resulting in an estimated uncertainty of ±8 to 10% (±1 s.d.) in the cumulative 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 offset energy sources11. At our baseline carbon intensity (17.5 kg C GJ−1; see Methods section), the model predicts that, on an average, the mitigation impact of biochar is 27–22% (14–23 Pg CO 2 -C e ) larger than the 52–107 Pg CO 2 -C e predicted if the same sustainably procured biomass were combusted to extract the maximum amount of energy (Fig. 2). This advantage of biochar over bioenergy is largely attributable to the beneficial feedbacks from enhanced crop yields and soil GHG fluxes (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). The carbon intensity of offset energy varies from near-zero for renewable and nuclear energy to 26 kg C GJ−1 for coal combustion35. Mean cumulative avoided emissions from biochar and biomass combustion are equal in our scenarios when the carbon intensity of offset energy is 26–24 kg C GJ−1 (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 benefit of biochar remains higher than that of bioenergy, even when 100% coal is offset. The cumulative avoided emissions from both strategies decrease as the carbon intensity of the offset 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 .

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 M b 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. Full size image

Given that much of the increased climate mitigation from biochar relative to biomass combustion stems from the beneficial 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. S9 and Supplementary Tables S8–S11 for 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. The amount of biomass produced in soils of different 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 offsets are considered. The relative benefit of producing biochar compared with biomass combustion is greatest when biochar is added to marginal lands and the energy produced by pyrolysis is used to offset natural gas, renewable or nuclear energy. When biochar is added to the most infertile cropland to offset the current global primary energy mix (M w ), which has a carbon intensity of 16.5 kg C GJ−1, the relative benefit from biochar is as much as 79–64% greater than that from bioenergy (Fig. 7, Supplementary Fig. S10). This net benefit diminishes as more coal is offset 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 offsetting coal emissions (in which bioenergy yields up to 16–22% greater mitigation impact than biochar), biochar shows a greater climate-mitigation potential than bioenergy. The relative benefit 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.

Figure 6: Soil-fertility constraints to cropland productivity (5′ 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). Full size image