Editing IPCC:AR6/SRCCL/Chapter-7 (section)

==== 7.2.3.2 Risk associated with land-based mitigation ====

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While historically land-use activities have been a net source of GHG emissions, in future decades the land sector will not only need to reduce its emissions, but also to deliver negative emissions through carbon dioxide removal (CDR) to reach the objective of limiting global warming to 2°C or below (Section 2.5).Although land-based mitigation in itself is a risk-reduction strategy aiming at abating climate change, it also entails risks to humans and ecosystems, depending on the type of measures and the scale of deployment. These risks fall broadly into two categories: risk of mitigation failure – due to uncertainties about mitigation potential, potential for sink reversal and moral hazard; and risks arising from adverse side effects – due to increased competition for land and water resources. This section focuses specifically on bioenergy and bioenergy with carbon capture and storage (BECCS) since it is one of the most prominent land-based mitigation strategies in future mitigation scenarios (along with large-scale forest expansion, which is discussed in Cross-Chapter Box 1 in Chapter 1). Bioenergy and BECCS is assessed in Chapter 6 as being, at large scales, the only response option with adverse side effects across all dimensions (adaptation, food security, land degradation and desertification) (Section 6.4.1).

'''Risk of mitigation failure.''' The mitigation potential from bioenergy and BECCS is highly uncertain, with estimates ranging from 0.4 to 11.3 GtCO <sub>2</sub> e yr <sup>–1</sup> for the technical potential, while consideration of sustainability constraints suggest an upper end around 5 GtCO <sup>2</sup> e yr <sup>–1</sup> (Chapter 2, Section 2.6). In comparison, IAM-based mitigation pathways compatible with limiting global warming at 1.5°C project bioenergy and BECCS deployment exceeding this range (Figure 2.24 in Chapter 2). There is ''medium confidence'' that IAMs currently do not reflect the lower end and exceed the upper end of bioenergy and BECCS mitigation potential estimates (Anderson and Peters 2016 <sup>[[#fn:r188|188]]</sup> ; Krause et al. 2018 <sup>[[#fn:r189|189]]</sup> ; IPCC 2018c <sup>[[#fn:r190|190]]</sup> ), with implications for the risk associated with reliance on bioenergy and BECCS deployment for climate mitigation.

In addition, land-based CDR strategies are subject to a risk of carbon sink reversal. This implies a fundamental asymmetry between mitigation achieved through fossil fuel emissions reduction compared to CDR. While carbon in fossil fuel reserves – in the case of avoided fossil fuel emissions – is locked permanently (at least over a time scale of several thousand years), carbon sequestered into the terrestrial biosphere – to compensate fossil fuel emissions – is subject to various disturbances, in particular from climate change and associated extreme events (Fuss et al. 2018 <sup>[[#fn:r191|191]]</sup> ; Dooley and Kartha 2018 <sup>[[#fn:r192|192]]</sup> ). The probability of sink reversal therefore increases with climate change, implying that the effectiveness of land-based mitigation depends on emission reductions in other sectors and can be sensitive to temperature overshoot ( ''high confidence'' ). In the case of bioenergy associated with CCS (BECCS), the issue of the long-term stability of the carbon storage is linked to technical and geological constraints, independent of climate change but presenting risks due to limited knowledge and experience (Chapter 6 and Cross-Chapter Box 7 in Chapter 6).

Another factor in the risk of mitigation failure, is the moral hazard associated with CDR technologies. There is medium evidence and medium agreement that the promise of future CDR deployment – bioenergy and BECCS in particular – can deter or delay ambitious emission reductions in other sectors (Anderson and Peters 2016 <sup>[[#fn:r193|193]]</sup> ; Markusson et al. 2018a <sup>[[#fn:r194|194]]</sup> ; Shue 2018a <sup>[[#fn:r195|195]]</sup> ). The consequences are an increased pressure on land with higher risk of mitigation failure and of temperature overshoot, and a transfer of the burden of mitigation and unabated climate change to future generations. Overall, there is therefore medium evidence and high agreement that prioritising early decarbonisation with minimal reliance on CDR decreases the risk of mitigation failure and increases intergenerational equity (Geden et al. 2019 <sup>[[#fn:r196|196]]</sup> ; Larkin et al. 2018 <sup>[[#fn:r197|197]]</sup> ; Markusson et al. 2018b <sup>[[#fn:r198|198]]</sup> ; Shue 2018b <sup>[[#fn:r199|199]]</sup> ).

'''Risk from adverse side-effects.''' At large scales, bioenergy (with or without CCS) is expected to increase competition for land, water resources and nutrients, thus exacerbating the risks of food insecurity, loss of ES and water scarcity (Chapter 6 and Cross-Chapter Box 7 in Chapter 6). Figure 7.3 shows the risk level (from undetectable to very high, aggregating risks of food insecurity, loss of ES and water scarcity) as a function of the global amount of land (million km <sup>2</sup> ) used for bioenergy, considering second generation bioenergy. Two illustrative future Socio-economic Pathways (SSP1 and SSP3; see Section 7.2.2 for more details) are depicted: in SSP3 the competition for land is exacerbated compared to SSP1 due to higher food demand resulting from larger population growth and higher consumption of meat-based products. The literature used in this assessment is based on IAM and non-IAM-based studies examining the impact of bioenergy crop deployment on various indicators, including food security (food prices or population at risk of hunger with explicit consideration of exposure and vulnerability), SDGs, ecosystem losses, transgression of various planetary boundaries and water consumption (see Supplementary Material). Since most of the assessed literature is centred around 2050, prevailing demographic and economic conditions for this year are used for the risk estimate. An aggregated risk metric including risks of food insecurity, loss of ES and water scarcity is used because there is no unique relationship between bioenergy deployment and the risk outcome for a single system. For instance, bioenergy deployment can be implemented in such a way that food security is prioritised at the expense of natural ecosystems, while the same scale of bioenergy deployment implemented with ecosystem safeguards would lead to a fundamentally different outcome in terms of food security (Boysen et al. 2017a <sup>[[#fn:r200|200]]</sup> ). Considered as a combined risk, however, the possibility of a negative outcome on either food security, ecosystems or both can be assessed with less ambiguity and independently of possible implementation choices.

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'''Figure 7.3'''

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'''Risks associated with bioenergy crop deployment as a land-based mitigation strategy under two SSPs (SSP1 and SSP3). The assessement is based on literature investigating the consequences of bioenergy expansion for food security, ecosystem loss and water scarcity. These risk indicators were aggregated as a single risk metric in the figure. In this context, very high […]'''
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Risks associated with bioenergy crop deployment as a land-based mitigation strategy under two SSPs (SSP1 and SSP3). The assessement is based on literature investigating the consequences of bioenergy expansion for food security, ecosystem loss and water scarcity. These risk indicators were aggregated as a single risk metric in the figure. In this context, very high risk indicates that important adverse consequences are expected for all these indicators (more than 100 million people at risk of hunger, major ecosystem losses and severe water scarcity issues). The climate scenario considered is a mitigation scenario consistent with limiting global warming at 2°C (RCP2.6), however some studies considering other scenarios (e.g., no climate change) were considered in the expert judgement as well as results from other SSPs (e.g., SSP2). The literature supporting the assessment is provided in Table SM7.3.

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In SSP1, there is ''medium confidence'' that 1 to 4 million km <sup>2</sup> can be dedicated to bioenergy production without significant risks to food security, ES and water scarcity. At these scales of deployment, bioenergy and BECCS could have co-benefits for instance by contributing to restoration of degraded land and soils (Cross-Chapter Box 7 in Chapter 6). Although currently degraded soils (up to 20 million km <sup>2</sup> ) represent a large amount of potentially available land (Boysen et al. 2017a <sup>[[#fn:r201|201]]</sup> ), trade-offs would occur already at smaller scale due to fertiliser and water use (Hejazi et al. 2014 <sup>[[#fn:r202|202]]</sup> ; Humpenöder et al. 2017 <sup>[[#fn:r203|203]]</sup> ; Heck et al. 2018a <sup>[[#fn:r204|204]]</sup> ; Boysen et al. 2017b <sup>[[#fn:r205|205]]</sup> ). There is ''low confidence'' that the transition from moderate to high risk is in the range 6–8.7 million km <sup>2</sup> . In SSP1, (Humpenöder et al. 2017 <sup>[[#fn:r206|206]]</sup> ) found no important impacts on sustainability indicators at a level of 6.7 million km <sup>2</sup> , while (Heck et al. 2018b <sup>[[#fn:r207|207]]</sup> ) note that several planetary boundaries (biosphere integrity; land-system change; biogeochemical flows; freshwater use) would be exceeded above 8.7 million km <sup>2</sup> .

There is very ''high confidence'' that all the risk transitions occur at lower bioenergy levels in SSP3, implying higher risks associated with bioenergy deployment, due to the higher competition for land in this pathway. In SSP3, land-based mitigation is therefore strongly limited by sustainability constraints such that moderate risk occur already between 0.5 and 1.5 million km <sup>2</sup> ( ''medium confidence'' ). There is ''medium confidence'' that a bioenergy footprint beyond 4 to 8 million km <sup>2</sup> would entail very high risk with transgression of most planetary boundaries (Heck et al. 2018b <sup>[[#fn:r208|208]]</sup> ), strong decline in sustainability indicators (Humpenöder et al. 2017 <sup>[[#fn:r209|209]]</sup> ) and increase in the population at risk of hunger well above 100 million (Fujimori et al. 2018a <sup>[[#fn:r210|210]]</sup> ; Hasegawa et al. 2018b <sup>[[#fn:r211|211]]</sup> ).

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