Editing IPCC:AR6/SRCCL/Chapter-2 (section)

==== 2.6.1.5 Bioenergy and bioenergy with carbon capture and storage ====

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An introduction and overview of bioenergy and bioenergy with carbon capture and storage (BECCS) can be found in Cross-Chapter Boxes 7 and 12, and Chapters 6 and 7. CCS technologies are discussed in SR15. The discussion below refers to modern bioenergy only (e.g., liquid biofuels for transport and the use of solid biofuels in combined heat and power plants).

The mitigation potential of bioenergy coupled with CCS (i.e., BECCS), is estimated to be between 0.4 and 11.3 GtCO <sub>2</sub> yr <sup>–1</sup> ( ''medium confidence'' ) based on studies that directly estimate mitigation for BECCS (not bioenergy) in units of CO <sub>2</sub> (not EJ) (McLaren 2012 <sup>[[#fn:r1758|1758]]</sup> ; Lenton 2014 <sup>[[#fn:r1759|1759]]</sup> ; Fuss et al. 2018 <sup>[[#fn:r1760|1760]]</sup> ; Turner et al. 2018b <sup>[[#fn:r1761|1761]]</sup> ; Lenton 2010 <sup>[[#fn:r1762|1762]]</sup> ; Koornneef et al. 2012 <sup>[[#fn:r1763|1763]]</sup> ; Powell and Lenton 2012 <sup>[[#fn:r1764|1764]]</sup> ). SR15 reported a potential of 1–85 GtCO <sub>2</sub> yr <sup>–1</sup> which they noted could be narrowed to a range of 0.5–5 GtCO <sub>2</sub> yr <sup>–1</sup> when taking account of sustainability aims (Fuss et al. 2018 <sup>[[#fn:r1765|1765]]</sup> ). The upper end of the SR15 range is considered as a theoretical potential. Previously, the IPCC Special Report on Renewable Energy Sources concluded the technical potential of biomass supply for energy (without BECCS) could reach 100–300 EJ yr–1 by 2050, which would be 2–15 GtCO <sub>2</sub> yr <sup>–1</sup> (using conversion factors 1 EJ = 0.02–0.05 GtCO <sub>2</sub> yr <sup>–1</sup> emission reduction, SR15). A range of recent studies including sustainability or economic constraints estimate that 50–244 EJ (1–12 GtCO <sub>2</sub> yr <sup>–1</sup> using the conversion factors above) of bioenergy could be produced on 0.1–13 Mkm <sup>2</sup> of land (Fuss et al. 2018 <sup>[[#fn:r1766|1766]]</sup> ; Chan and Wu 2015 <sup>[[#fn:r1767|1767]]</sup> ; Schueler et al. 2016 <sup>[[#fn:r1767|1767]]</sup> ; Wu et al. 2013 <sup>[[#fn:r1768|1768]]</sup> ; Searle and Malins 2015 <sup>[[#fn:r1769|1769]]</sup> ; Wu et al. 2019 <sup>[[#fn:r1770|1770]]</sup> ; Heck et al. 2018 <sup>[[#fn:r1771|1771]]</sup> ; Fritz et al. 2013 <sup>[[#fn:r1772|1772]]</sup> ).

There is ''high confidence'' that the most important factors determining future biomass supply for energy are land availability and land productivity (Berndes et al. 2013 <sup>[[#fn:r1773|1773]]</sup> ; Creutzig et al. 2015a <sup>[[#fn:r1774|1774]]</sup> ; Woods et al. 2015 <sup>[[#fn:r1775|1775]]</sup> ; Daioglou et al. 2019 <sup>[[#fn:r1776|1776]]</sup> ). Estimates of marginal/degraded lands currently considered available for bioenergy range from 3.2–14.0 Mkm2, depending on the adopted sustainability criteria, land class definitions, soil conditions, land mapping method and environmental and economic considerations (Campbell et al. 2008 <sup>[[#fn:r1778|1778]]</sup> ; Cai et al. 2011 <sup>[[#fn:r1779|1779]]</sup> ; Lewis and Kelly 2014 <sup>[[#fn:r1780|1780]]</sup> ).

Bioenergy production systems can lead to net emissions in the short term that can be ‘paid-back’ over time, with multiple harvest cycles and fossil fuel substitution, unlike fossil carbon emissions (Campbell et al. 2008 <sup>[[#fn:r1781|1781]]</sup> ; Cai et al. 2011 <sup>[[#fn:r1782|1782]]</sup> ; Lewis and Kelly 2014 <sup>[[#fn:r1783|1783]]</sup> ; De Oliveira Bordonal et al. 2015 <sup>[[#fn:r1784|1784]]</sup> ). Stabilising bioenergy crops in previous high carbon forestland or peatland results in high emissions of carbon that may take from decades to more than a century to be re-paid in terms of net CO <sub>2</sub> emission savings from replacing fossil fuels, depending on previous forest carbon stock, bioenergy yields and displacement efficiency (Elshout et al. 2015 <sup>[[#fn:r1785|1785]]</sup> ; Harper et al. 2018 <sup>[[#fn:r1786|1786]]</sup> ; Daioglou et al. 2017 <sup>[[#fn:r1787|1787]]</sup> ). In the case of bioenergy from managed forests, the magnitude and timing of the net mitigation benefits is controversial as it varies with differences due to local climate conditions, forest management practice, fossil fuel displacement efficiency and methodological approaches (Hudiburg et al. 2011 <sup>[[#fn:r1788|1788]]</sup> ; Berndes et al. 2013 <sup>[[#fn:r1789|1789]]</sup> ; Guest et al. 2013 <sup>[[#fn:r1790|1790]]</sup> ; Lamers and Junginger 2013 <sup>[[#fn:r1791|1791]]</sup> ; Cherubini et al. 2016 <sup>[[#fn:r1792|1792]]</sup> ; Cintas et al. 2017 <sup>[[#fn:r1793|1793]]</sup> ; Laurance et al. 2018 <sup>[[#fn:r1794|1794]]</sup> ; Valade et al. 2018 <sup>[[#fn:r1795|1795]]</sup> ; Baker et al. 2019 <sup>[[#fn:r1796|1796]]</sup> ). Suitable bioenergy crops can be integrated in agricultural landscapes to reverse ecosystem carbon depletion (Creutzig et al. 2015a <sup>[[#fn:r1797|1797]]</sup> ; Robertson et al. 2017 <sup>[[#fn:r1798|1798]]</sup> ; Vaughan et al. 2018 <sup>[[#fn:r1799|1799]]</sup> ; Daioglou et al. 2017 <sup>[[#fn:r1800|1800]]</sup> ). Cultivation of short rotation woody crops and perennial grasses on degraded land or cropland previously used for annual crops typically accumulate carbon in soils due to their deep root systems (Don et al. 2012 <sup>[[#fn:r1801|1801]]</sup> ; Robertson et al. 2017 <sup>[[#fn:r1802|1802]]</sup> ). The use of residues and organic waste as bioenergy feedstock can mitigate land use change pressures associated with bioenergy deployment, but residues are limited and the removal of residues that would otherwise be left on the soil could lead soil degradation (Chum et al. 2011 <sup>[[#fn:r1803|1803]]</sup> ; Liska et al. 2014 <sup>[[#fn:r1804|1804]]</sup> ; Monforti et al. 2015 <sup>[[#fn:r1805|1805]]</sup> ; Zhao et al. 2015 <sup>[[#fn:r1806|1806]]</sup> ; Daioglou et al. 2016 <sup>[[#fn:r1807|1807]]</sup> ).

The steps required to cultivate, harvest, transport, process and use biomass for energy generate emissions of GHGs and other climate pollutants (Chum et al. 2011 <sup>[[#fn:r1808|1808]]</sup> ; Creutzig et al. 2015b <sup>[[#fn:r1809|1809]]</sup> ; Staples et al. 2017 <sup>[[#fn:r1810|1810]]</sup> ; Daioglou et al. 2019 <sup>[[#fn:r1811|1811]]</sup> ). Life-cycle GHG emissions of modern bioenergy alternatives are usually lower than those for fossil fuels ( ''robust evidence, medium agreement'' ) (Chum et al. 2011 <sup>[[#fn:r1812|1812]]</sup> ; Creutzig et al. 2015b <sup>[[#fn:r1813|1813]]</sup> ). The magnitude of these emissions largely depends on location (e.g., soil quality, climate), prior land use, feedstock used (e.g., residues, dedicated crops, algae), land use practice (e.g., soil management, fertiliser use), biomass transport (e.g., distances and transport modes) and the bioenergy conversion pathway and product (e.g., wood pellets, ethanol). Use of conventional food and feed crops as a feedstock generally provides the highest bioenergy yields per hectare, but also causes more GHG emissions per unit energy compared to agriculture residues, biomass from managed forests and lignocellulosic crops such as short-rotation coppice and perennial grasses (Chum et al. 2011 <sup>[[#fn:r1814|1814]]</sup> ; Gerbrandt et al. 2016 <sup>[[#fn:r1815|1815]]</sup> ) due to the application of fertilisers and other inputs (Oates et al. 2016 <sup>[[#fn:r1816|1816]]</sup> ; Rowe et al. 2016 <sup>[[#fn:r1817|1817]]</sup> ; Lai et al. 2017 <sup>[[#fn:r1818|1818]]</sup> ; Robertson et al. 2017 <sup>[[#fn:r1819|1819]]</sup> ).

Bioenergy from dedicated crops are in some cases held responsible for GHG emissions resulting from indirect land use change (iLUC), that is the bioenergy activity may lead to displacement of agricultural or forest activities into other locations, driven by market-mediated effects. Other mitigation options may also cause iLUC. At a global level of analysis, indirect effects are not relevant because all land-use emissions are direct. iLUC emissions are potentially more significant for crop-based feedstocks such as corn, wheat and soybean, than for advanced biofuels from lignocellulosic materials (Chum et al. 2011 <sup>[[#fn:r1820|1820]]</sup> ; Wicke et al. 2012 <sup>[[#fn:r1821|1821]]</sup> ; Valin et al. 2015 <sup>[[#fn:r1822|1822]]</sup> ; Ahlgren and Di Lucia 2014 <sup>[[#fn:r1823|1823]]</sup> ). Estimates of emissions from iLUC are inherently uncertain, widely debated in the scientific community and are highly dependent on modelling assumptions, such as supply/demand elasticities, productivity estimates, incorporation or exclusion of emission credits for coproducts and scale of biofuel deployment (Rajagopal and Plevin 2013 <sup>[[#fn:r1824|1824]]</sup> ; Finkbeiner 2014 <sup>[[#fn:r1825|1825]]</sup> ; Kim et al. 2014 <sup>[[#fn:r1826|1826]]</sup> ; Zilberman 2017 <sup>[[#fn:r1827|1827]]</sup> ). In some cases, iLUC effects are estimated to result in emission reductions. For example, market-mediated effects of bioenergy in North America showed potential for increased carbon stocks by inducing conversion of pasture or marginal land to forestland (Cintas et al. 2017 <sup>[[#fn:r1828|1828]]</sup> ; Duden et al. 2017 <sup>[[#fn:r1829|1829]]</sup> ; Dale et al. 2017 <sup>[[#fn:r1830|1830]]</sup> ; Baker et al. 2019 <sup>[[#fn:r1831|1831]]</sup> ). There is a wide range of variability in iLUC values for different types of biofuels, from –75–55 gCO <sub>2</sub> MJ <sup>–1 </sup> (Ahlgren and Di Lucia 2014 <sup>[[#fn:r1832|1832]]</sup> ; Valin et al. 2015 <sup>[[#fn:r1833|1833]]</sup> ; Plevin et al. 2015 <sup>[[#fn:r1834|1834]]</sup> ; Taheripour and Tyner 2013 <sup>[[#fn:r1835|1835]]</sup> ; Bento and Klotz 2014 <sup>[[#fn:r1836|1836]]</sup> ). There is ''low confidence'' in attribution of emissions from iLUC to bioenergy.

Bioenergy deployment can have large biophysical effects on regional climate, with the direction and magnitude of the impact depending on the type of bioenergy crop, previous land use and seasonality ( ''limited evidence, medium agreement'' ). A study of two alternative future bioenergy scenarios using 15 Mkm2 of intensively used managed land or conversion of natural areas showed a nearly neutral effect on surface temperature at global levels (considering biophysical effects and CO <sub>2</sub> and N <sub>2</sub> O fluxes from land but not substitution effects), although there were significant seasonal and regional differences (Kicklighter et al. 2013 <sup>[[#fn:r1837|1837]]</sup> ). Modelling studies on biofuels in the US found the switch from annual crops to perennial bioenergy plantations like Miscanthus could lead to regional cooling due to increases in evapotranspiration and albedo (Georgescu et al. 2011 <sup>[[#fn:r1838|1838]]</sup> ; Harding et al. 2016 <sup>[[#fn:r1839|1839]]</sup> ), with perennial bioenergy crop expansion over suitable abandoned and degraded farmlands causing near-surface cooling up to 5°C during the growing season (Wang et al. 2017b <sup>[[#fn:r1840|1840]]</sup> ). Similarly, growing sugarcane on existing cropland in Brazil cools down the local surface during daytime conditions up to –1°C, but warmer conditions occur if sugar cane is deployed at the expense of natural vegetation (Brazilian Cerrado (Loarie et al. 2011 <sup>[[#fn:r1841|1841]]</sup> ). In general, bioenergy crops (as for all crops) induce a cooling of ambient air during the growing season, but after harvest the decrease in evapotranspiration can induce warming (Harding et al. 2016 <sup>[[#fn:r1842|1842]]</sup> ; Georgescu et al. 2013 <sup>[[#fn:r1843|1843]]</sup> ; Wang et al. 2017b <sup>[[#fn:r1844|1844]]</sup> ). Bioenergy crops were found to cause increased isoprene emissions in a scenario where 0.69 Mkm2 of oil palm for biodiesel in the tropics and 0.92 Mkm2 of short rotation coppice (SRC) in the mid-latitudes were planted, but effects on global climate were negligible (Ashworth et al. 2012 <sup>[[#fn:r1845|1845]]</sup> ).

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