Bioenergy in the IPCC Assessments

Acknowledgements The input of PS contributes to the UK NERC-funded Soils-R-GGREAT project (NE/P019455/1), the UK EPSRC-funded project MAGLUE (EP/M013200/1) and the UKERC-funded Assess-BECCS project.

From the First (in 1990) to the Fifth (in 2014) IPCC Assessment Reports (which denote as AR1 to AR5 here, though at the time they were designated as FAR, SAR, TAR and then AR4 and AR5), the level of detail and quantification of potentials and impacts of bioenergy have increased significantly, and this largely reflects the increase in available literature on the topic. Figure 1 shows the papers published on Web of Knowledge database between 1990 and 2017 on bioenergy/biofuels and the subject areas in which these articles were published. With~6000 papers published each year on bioenergy since the last assessment report (Fig. 1a), the task of synthesizing this vast literature, across a diverse range of disciplines ( Fig. 1b), has become ever more challenging.
Below, we describe how the treatment of bioenergy and biofuels has changed over the history of the IPCC, as tracked through AR1 to AR5, including the Special Report on Renewable Energy (SRREN). The Special Report on Land Use, Land-Use Change and Forestry (2000) was also considered, but does not cover bioenergy. Table 1 shows the number of pages dedicated to bioenergy/biofuels in IPCC reports and the percentage of total pages in each volume dedicated to bioenergy/ biofuels. The number of pages dedicated to bioenergy/ biofuels has increased between AR1 and AR5.
More space has been dedicated to bioenergy/biofuels in subsequent IPCC Assessments, with <0.04% of total volume pages in AR1, rising to 1.5% of all pages for AR5 (Table 1). For the SRREN (which only deals with renewable energy), one whole chapter (~10% of total report pages) was dedicated to bioenergy (Table 1). In AR1 and AR2, the term biofuel was used to denote both liquid and solid forms of bioenergy, while from AR3 onwards, biofuel tended to be used to mean liquid transport fuels, while bioenergy was used as the more generic term to cover all energy from biomass.
Perhaps, the most important changes between AR1 and AR5 are (i) the degree of quantification of the global mitigation potential bioenergy and (ii) the increase in diversity of bioenergy options considered, for example from simple consideration of wood for energy in AR1 to consideration of bioenergy with carbon capture and storage (BECCS) in AR5. AR1 WGII (Parry et al., 1990) noted that short rotation forestry might be a form of alternative energy. The potential use of biomass to replace fossil fuels for energy generation is referred to in AR1 WGIII (Kupfer & Karimanzira, 1990), and biofuel plantations are mentioned twice, but there is no quantification of potential.
In AR2, WGII covered bioenergy in the chapter on Agricultural Options for Mitigation of Greenhouse Gas Emissions (Cole et al., 1995). Around three pages were used to discuss dedicated biofuel crops, bioethanol and biodiesel, and crop residues, and the first estimates of global potentials were included, at 300-1300 MtC yr À1 of fossil fuel carbon offsets for biofuels, with an additional 100-200 MtC yr À1 from crop residues, giving a total (in CO 2 e) of 1470-5500 MtCO 2 e yr À1 .
Unlike AR1 and AR2, the WGIII volume of AR3 had no sectoral chapters and included economic assessments of mitigation potential for the first time. Discussion of bioenergy occurred in two chapters, Ch3 on Technological and Economic Potential of Greenhouse Gas Emissions Reduction (Moomaw et al., 2001) in section 3.6 on agriculture and energy cropping and in Ch4 on Technological and Economic Potential of Options to Enhance, Maintain, and Manage Biological Carbon Reservoirs and Geo-engineering (Kauppi et al., 2001) in section 4.3.3 on Opportunities in agricultural land. Energy cropping was estimated to have a global potential (in 2020 at 0-100 US$ per tCeq.) of 1300-2750 MtCO 2 eq yr À1 (converted from original units of MtC yr À1 ).
In AR4, bioenergy was considered in the chapter on Agriculture (Smith et al., 2007). Global bioenergy mitigation potential (from fossil fuel substitution) was estimated to be 70-1260 MtCO 2 -eq yr À1 at up to 20 US$ per tCO 2 -eq, and 560-2320 MtCO 2 -eq yr À1 at up to 50 US$ per tCO 2 -eq. A potential of 2720 MtCO 2 e yr À1 was reported for prices above 100 US$ per tCO 2 -eq. Bioenergy received thorough treatment in the SRREN (Chum et al., 2011) with 92 pages dedicated to the topic. BECCS appears for the first time, but with only 27 lines on the chapter dedicated to the topic and no estimated potential for BECCS. The SRREN notes that electricity generation from biomass could reach 1220 Mt CO 2 eq by 2030, much of which at costs <~20 US$ per tCO 2 eq. The estimates of mitigation potential for bioenergy from the energy systems models are the same as those reported 10th Anniversary Editorial/Perspective. in AR4, that is 70-1260 Mt CO 2 eq yr À1 for costs of <~200 US$ per tCO 2 eq. and 560-2320 Mt CO 2 eq yr À1 at 50 per t CO 2 eq. Of these totals, the overall mitigation from biomass energy from the forest sector was estimated to reach 400 MtCO 2 yr À1 to 2030 (Chum et al., 2011). In AR5, the WGIII chapter on Agriculture, Forestry and Other Land Use (Smith et al., 2014) had a dedicated section on bioenergy, totalling 15 pages of that chapter. Total mitigation potentials were not given, although the estimates for energy generation of 95 EJ yr À1 in 2030 and 245 EJ yr À1 in 2050 equate roughly to 5100-13 200 MtCO 2 e yr À1 using conversion factors of Hall & Scrase (1998). BECCS also featured strongly in the chapter Assessing Transformation Pathways (Clarke et al., 2014) which included the modelled scenarios of pathways to achieve climate stabilization at 2°C above pre-industrial levels. In assessing the overall potential for BECCs by analysis of the IPCC AR5 WGIII scenarios database, Smith et al. (2016) reported a mean level of implementation of BECCS at 12 100 MtCO 2 e yr À1 in 2100 for scenarios consistent with a 2°C target, with Clarke et al. (2014) reporting the full range as 0-22 000 MtCO 2 e yr À1 . The consideration of BECCS, and the specific focus on scenarios showing a very high level of mitigation ambition (to meet a 2°C target), means that the potential range reported in AR5 was much larger than in any previous assessment report or the SRREN.
Estimates of ranges of global mitigation potentials for bioenergy from AR1 to AR5 (including SRREN) are summarized in Figure 2.
While quantification has improved since AR1, wide ranges for estimated mitigation potential of bioenergy remain, as there are many sources of variation, which all contribute to the overall uncertainty (e.g. assumptions about land area available, yield and technology improvements, improvements in conversion technologies, speed of development of infrastructure; Fig. 2). While uncertainty is difficult to quantify numerically, it has been possible since AR4 to attach uncertainty language statements to most components of the bioenergy mitigation potential estimates. Looking forward to the IPCC AR6 cycle (including the Special Report on 1.5 Degrees, the Special Report on Climate Change and Land, and AR6 itself), key emerging issues are likely to be (i) trade-offs between the use of land for bioenergy production, food and fibre production and conservation of ecosystem integrity and (ii) the codelivery of bioenergy based climate change mitigation (with or without CCS and the UN Sustainable Development Goals. The Special Report on 1.5 Degrees will be delivered in 2018 and will include a critical appraisal of the feasibility of using bioenergy (specifically BECCS) in delivering the 1.5°C target, and the Special Report on Climate Change and Land, which will be delivered in 2019, will consider an even wider range of issues, including how bioenergy and BECCS impact and are impacted by, desertification, land degradation and food securityand the cobenefits and adverse side effects with climate mitigation, adaptation and sustainable land management. The two Special Reports will Global mitigation potential for Bioenergy (MtCO 2 /yr) IPCC report Fig. 2 Estimates of total global mitigation potential of bioenergy in AR1 to AR5 including SRREN (after conversion to common units), showing quoted global potentials. Ranges are shown where given (for different carbon prices from AR3 onwards), and the point in the range for AR5 corresponds to the mean potential for BECCS in 2100 for scenarios consistent with a 2°C target from the IPCC AR5 WGIII scenario database (Smith et al., 2016). Note that the AR2 values had no target year, AR3 values were for 2020, AR4 and SRREN values were for 2030, and AR5 values were for 2030 and 2050. The range and mean value for AR5 are for BECCS, while for all other reports, the mitigation potential is derived from fossil fuel offsets available from bioenergy. set the scene for the AR6 report, which is due to be delivered in 2021. Bioenergy continues to feature prominently in the AR6 cycle, but if anything, has become ever more controversial since it was first treated qualitatively in AR1.