Editing IPCC:AR6/SRCCL/Chapter-6 (section)

==== 6.4.4.2 Interlinkages and response options in future scenarios ====

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This section assesses more than 80 articles quantifying the effect of various response options in the future, covering a variety of response options and land-based challenges. These studies cover spatial scales ranging from global (Popp et al. 2017 <sup>[[#fn:r1046|1046]]</sup> ; Fujimori et al. 2019 <sup>[[#fn:r1047|1047]]</sup> ) to regional (Calvin et al. 2016a <sup>[[#fn:r1048|1048]]</sup> ; Frank et al. 2015 <sup>[[#fn:r1049|1049]]</sup> ) to country level (Gao and Bryan 2017; Pedercini et al. 2018 <sup>[[#fn:r1050|1050]]</sup> ). This section focuses on models that can quantify interlinkages between response options, including agricultural economic models, land system models, and Integrated Assessment Models (IAMs). The IAM and non-IAM literature, however, is also categorised separately to elucidate what is and is not included in global mitigation scenarios, like those included in the SR15. Results from bottom-up studies and models (e.g., Griscom et al. 2017 <sup>[[#fn:r1274|1274]]</sup> ) are assessed in Sections 6.2–6.3.

''Response options in future scenarios''

More than half of the 40 land-based response options discussed in this chapter are represented in global IAMs models used to develop and analyse future scenarios, either implicitly or explicitly (Table 6.76). For example, all IAMs include improved cropland management, either explicitly through technologies that improve nitrogen use efficiency (Humpenöder et al. 2018 <sup>[[#fn:r1051|1051]]</sup> ) or implicitly through marginal abatement cost curves that link reductions in nitrous oxide emissions from crop production to carbon prices (most other models).

However, the literature discussing the effect of these response options on land-based challenges is more limited (Table 6.76). There are 57 studies (43 IAM studies) that articulate the effect of response options on mitigation, with most including bioenergy and BECCS or a combination of reduced deforestation, reforestation, and afforestation; 37 studies (21 IAM studies) discuss the implications of response options on food security, usually using food price as a metric. While a small number of non-IAM studies examine the effects of response options on desertification (three studies) and land degradation (five studies), no IAM studies were identified. However, some studies quantify these challenges indirectly using IAMs, either via climate outputs from the representative concentration pathways (RCPs) (Huang et al. 2016 <sup>[[#fn:r1052|1052]]</sup> ) or by linking IAMs to other land and ecosystem models (Ten Brink et al. 2018 <sup>[[#fn:r1275|1275]]</sup> ; UNCCD 2017 <sup>[[#fn:r1053|1053]]</sup> ).

For many of the scenarios in the literature, land-based response options are included as part of a suite of mitigation options (Popp et al. 2017 <sup>[[#fn:r1054|1054]]</sup> ; Van Vuuren et al. 2015). As a result, it is difficult to isolate the effect of an individual option on land-related challenges. A few studies focus on specific response options (Calvin et al. 2014 <sup>[[#fn:r1055|1055]]</sup> ; Popp et al. 2014 <sup>[[#fn:r1056|1056]]</sup> ; Kreidenweis et al. 2016 <sup>[[#fn:r1057|1057]]</sup> ; Humpenöder et al. 2018 <sup>[[#fn:r1058|1058]]</sup> ), quantifying the effect of including an individual option on a variety of sustainability targets.

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'''Table 6.76'''

<span id="number-of-iam-and-non-iam-studies-including-specific-response-options-rows-and-quantifying-particular-land-challenges-columns."></span>
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'''Number of IAM and non-IAM studies including specific response options (rows) and quantifying particular land challenges (columns).'''

Thethird column shows how many IAM models include the individual response option. The remaining columns show challenges related to climate change (C), mitigation (M), adaptation (A), desertification (D), land degradation (L), food security (F), and biodiversity/ecosystem services/sustainable development (B). Additionally, counts of total (left value) and IAM-only (right value) studies are included. Some IAMs include agricultural economic models, which can also be run separately; these models are not counted as IAM literature when used on their own. Studies using a combination of IAMs and non-IAMs are included in the total only. A complete list of studies is included in the Appendix.
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''Interactions and interlinkages between response options''

The effect of response options on desertification, land degradation, food security, biodiversity, and other SDGs depends strongly on which options are included, and the extent to which they are deployed. For example, Sections 2.6 and 6.3.6, and Cross-Chapter Box 7 note that bioenergy and BECCS has a large mitigation potential, but could potentially have adverse side effects for land degradation, food security, and other SDGs. Global modelling studies demonstrate that these effects are dependent on scale. Increased use of bioenergy can result in increased mitigation (Figure 6.8, panel A) and reduced climate change, but can also lead to increased energy cropland expansion (Figure 6.8, panel B), and increased competition for land, resulting in increased food prices (Figure 6.8, panel C). However, the exact relationship between bioenergy deployment and each sustainability target depends on a number of other factors, including the feedstock used, the underlying socio-economic scenario, assumptions about technology and resource base, the inclusion of other response options, and the specific model used (Calvin et al. 2014 <sup>[[#fn:r1059|1059]]</sup> ; Clarke et al. 2014 <sup>[[#fn:r1060|1060]]</sup> ; Popp et al. 2014, 2017 <sup>[[#fn:r1061|1061]]</sup> ; Kriegler et al. 2014 <sup>[[#fn:r1062|1062]]</sup> ).

The previous sections have examined the effects of individual land-response options on multiple challenges. A number of studies using global modelling and analyses have examined interlinkages and interaction effects among land response options by incrementally adding or isolating the effects of individual options. Most of these studies focus on interactions with bioenergy and BECCS (Table 6.77). Adding response options that require land (e.g., reforestation, afforestation, reduced deforestation, avoided grassland conversion, or biodiversity conservation) results in increased food prices (Calvin et al. 2014 <sup>[[#fn:r1063|1063]]</sup> ; Humpenöder et al. 2014 <sup>[[#fn:r1064|1064]]</sup> ; Obersteiner et al. 2016 <sup>[[#fn:r1065|1065]]</sup> ; Reilly et al. 2012 <sup>[[#fn:r1066|1066]]</sup> ) and potentially increased temperature through biophysical climate effects (Jones et al. 2013 <sup>[[#fn:r1067|1067]]</sup> ). However, this combination can result in reduced water consumption (Hejazi et al. 2014b <sup>[[#fn:r1068|1068]]</sup> ), reduced cropland expansion (Calvin et al. 2014 <sup>[[#fn:r1069|1069]]</sup> ; Humpenöder et al. 2018 <sup>[[#fn:r1070|1070]]</sup> ), increased forest cover (Calvin et al. 2014 <sup>[[#fn:r1071|1071]]</sup> ; Humpenöder et al. 2018 <sup>[[#fn:r1072|1072]]</sup> ; Wise et al. 2009 <sup>[[#fn:r1073|1073]]</sup> ) and reduced biodiversity loss (Pereira et al. 2010 <sup>[[#fn:r1074|1074]]</sup> ), compared to scenarios with bioenergy and BECCS alone. While these options increase total mitigation, they reduce mitigation from bioenergy and BECCS as they compete for the same land (Wu et al. 2019 <sup>[[#fn:r1075|1075]]</sup> ; Baker et al. 2019 <sup>[[#fn:r1076|1076]]</sup> ; Calvin et al. 2014 <sup>[[#fn:r1077|1077]]</sup> ; Humpenöder et al. 2014 <sup>[[#fn:r1078|1078]]</sup> ).

The inclusion of land-sparing options (e.g., dietary change, increased food productivity, reduced food waste, management of supply chains) in addition to bioenergy and BECCS results in reduced food prices, reduced agricultural land expansion, reduced deforestation, reduced mitigation costs, reduced water use, and reduced biodiversity loss (Bertram et al. 2018 <sup>[[#fn:r1276|1276]]</sup> ; Wu et al. 2019 <sup>[[#fn:r1079|1079]]</sup> ; Obersteiner et al. 2016 <sup>[[#fn:r1080|1080]]</sup> ; Stehfest et al. 2009 <sup>[[#fn:r1081|1081]]</sup> ; Van Vuuren et al. 2018). These options can increase bioenergy potential, resulting in increased mitigation than from bioenergy and BECCS alone (Wu et al. 2019 <sup>[[#fn:r1082|1082]]</sup> ; Stehfest et al. 2009 <sup>[[#fn:r1083|1083]]</sup> ; Favero and Massetti 2014 <sup>[[#fn:r1084|1084]]</sup> ).

Other combinations of land response options create synergies, alleviating land pressures. The inclusion of increased food productivity and dietary change can increase mitigation, reduce cropland use, reduce water consumption, reduce fertiliser application, and reduce biodiversity loss (Springmann et al. 2018 <sup>[[#fn:r1085|1085]]</sup> ; Obersteiner et al. 2016 <sup>[[#fn:r1086|1086]]</sup> ). Similarly, improved livestock management, combined with increased food productivity, can reduce agricultural land expansion (Weindl et al. 2017 <sup>[[#fn:r1087|1087]]</sup> ). Reducing disturbances (e.g., fire management) in combination with afforestation can increase the terrestrial carbon sink, resulting in increased mitigation potential and reduced mitigation cost (Le Page et al. 2013 <sup>[[#fn:r1088|1088]]</sup> ).

Studies including multiple land response options often find that the combined mitigation potential is not equal to the sum of individual mitigation potential as these options often share the same land. For example, including both afforestation and bioenergy and BECCS results in a cumulative reduction in GHG emissions of 1200 GtCO 2 between 2005 and 2100, which is much lower than the sum of the contributions of bioenergy (800 GtCO 2 ) and afforestation (900 GtCO 2 ) individually (Humpenöder et al. 2014 <sup>[[#fn:r1089|1089]]</sup> ). More specifically, Baker et al. (2019) <sup>[[#fn:r1090|1090]]</sup> find that woody bioenergy and afforestation are complementary in the near term, but become substitutes in the long term, as they begin to compete for the same land. Similarly, the combined effect of increased food productivity, dietary change and reduced waste on GHG emissions is less than the sum of the individual effects (Springmann et al. 2018 <sup>[[#fn:r1091|1091]]</sup> ).

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'''Table 6.77'''

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'''Interlinkages between bioenergy and BECCS and other response options.'''

Table indicates the combined effects of multiple land-response options on climate change (C), mitigation (M), adaptation (A), desertification (D), land degradation (L), food security (F), and biodiversity/ecosystem services/sustainable development (O). Each cell indicates the implications of adding the option specified in the row in addition to bioenergy and BECCS. Blue colours indicate positive interactions (e.g., including the option in the second column increases mitigation, reduces cropland area, or reduces food prices relative to bioenergy and BECCS alone). Yellow indicates negative interactions; grey indicates mixed interactions (some positive, some negative). Note that only response option combinations found in the assessed literature are included in the interest of space.
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Land-related response options can also interact with response options in other sectors. For example, limiting deployment of a mitigation response option will either result in increased climate change or additional mitigation in other sectors. A number of studies have examined limiting bioenergy and BECCS. Some such studies show increased emissions (Reilly et al. 2012 <sup>[[#fn:r1097|1097]]</sup> ). Other studies meet the same climate goal, but reduce emissions elsewhere ''via'' reduced energy demand (Grubler et al. 2018 <sup>[[#fn:r1098|1098]]</sup> ; Van Vuuren et al. 2018 <sup>[[#fn:r1277|1277]]</sup> ), increased fossil carbon capture and storage (CCS), nuclear energy, energy efficiency and/or renewable energy (Van Vuuren et al. 2018 <sup>[[#fn:r1278|1278]]</sup> ; Rose et al. 2014 <sup>[[#fn:r1099|1099]]</sup> ; Calvin et al. 2014 <sup>[[#fn:r1100|1100]]</sup> ; Van Vuuren et al. 2017b <sup>[[#fn:r1279|1279]]</sup> ), dietary change (Van Vuuren et al. 2018 <sup>[[#fn:r1280|1280]]</sup> ), reduced non-CO 2 emissions (Van Vuuren et al. 2018 <sup>[[#fn:r1281|1281]]</sup> ), or lower population (Van Vuuren et al. 2018 <sup>[[#fn:r1282|1282]]</sup> ). The co-benefits and adverse side effects of non-land mitigation options are discussed in SR15, Chapter 5. Limitations on bioenergy and BECCS can result in increases in the cost of mitigation (Kriegler et al. 2014 <sup>[[#fn:r1101|1101]]</sup> ; Edmonds et al. 2013 <sup>[[#fn:r1102|1102]]</sup> ). Studies have also examined limiting CDR, including reforestation, afforestation, and bioenergy and BECCS (Kriegler et al. 2018a <sup>[[#fn:r1282|1282]]</sup> ,b <sup>[[#fn:r1283|1283]]</sup> ). These studies find that limiting CDR can increase mitigation costs, increase food prices, and even preclude limiting warming to less than 1.5°C above pre-industrial levels (Kriegler et al. 2018a,b; Muratori et al. 2016 <sup>[[#fn:r1103|1103]]</sup> ).

In some cases, the land challenges themselves may interact with land-response options. For example, climate change could affect the production of bioenergy and BECCS. A few studies examine these effects, quantifying differences in bioenergy production (Calvin et al. 2013 <sup>[[#fn:r1104|1104]]</sup> ; Kyle et al. 2014 <sup>[[#fn:r1105|1105]]</sup> ) or carbon price (Calvin et al. 2013 <sup>[[#fn:r1106|1106]]</sup> ) as a result of climate change. Kyle et al. (2014) <sup>[[#fn:r1107|1107]]</sup> find increase in bioenergy production due to increases in bioenergy yields, while Calvin et al. (2013) <sup>[[#fn:r1108|1108]]</sup> find declines in bioenergy production and increases in carbon price due to the negative effects of climate on crop yield.

''Gaps in the literature''

Not all of the response options discussed in this chapter are included in the assessed literature, and many response options are excluded from the IAM models. The included options (e.g., bioenergy and BECCS; reforestation) are some of the largest in terms of mitigation potential (see Section 6.3). However, some of the options excluded also have large mitigation potential. For example, biochar, agroforestry, restoration/avoided conversion of coastal wetlands, and restoration/ avoided conversion of peatland all have mitigation potential of about 1 GtCO 2 yr –1 (Griscom et al. 2017 <sup>[[#fn:r1109|1109]]</sup> ). Additionally, quantifications of and response options targeting land degradation and desertification are largely excluded from the modelled studies, with a few notable exceptions (Wolff et al. 2018 <sup>[[#fn:r1110|1110]]</sup> ; Gao and Bryan 2017 <sup>[[#fn:r1111|1111]]</sup> ; Ten Brink et al. 2018 <sup>[[#fn:r1112|1112]]</sup> ; UNCCD 2017 <sup>[[#fn:r1113|1113]]</sup> ). Finally, while a large number of papers have examined interactions between bioenergy and BECCS and other response options, the literature examining other combinations of response options is more limited.

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