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

=== 2.6.2 Integrated pathways for climate change mitigation ===

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Land-based response options have the potential to interact, resulting in additive effects (e.g., climate co-benefits) or negating each other (e.g., through competition for land). They also interact with mitigation options in other sectors (such as energy or transport) and thus they need to be assessed collectively under different climate mitigation targets and in combination with other sustainability goals (Popp et al. 2017 <sup>[[#fn:r1866|1866]]</sup> ; Obersteiner et al. 2016 <sup>[[#fn:r1867|1867]]</sup> ; Humpenöder et al. 2018 <sup>[[#fn:r1868|1868]]</sup> ). IAMs with distinctive land-use modules are the basis for the assessment of mitigation pathways as they combine insights from various disciplines in a single framework and cover the largest sources of anthropogenic GHG emissions from different sectors (see also SR15 Chapter 2 and Technical Annex for more details). IAMs consider a limited, but expanding, portfolio of land- based mitigation options. Furthermore, the inclusion and detail of a specific mitigation measure differs across IAMs and studies (see also SR15 and Chapter 6). For example, the IAM scenarios based on the shared socio-economic pathways (SSPs) (Riahi et al. 2017 <sup>[[#fn:r1869|1869]]</sup> ) (Cross-Chapter Box 1 and Chapter 1) include possible trends in agriculture and land use for five different socioeconomic futures, but cover a limited set of land-based mitigation options: dietary changes, higher efficiency in food processing (especially in livestock production systems), reduction of food waste, increasing agricultural productivity, methane reductions in rice paddies, livestock and grazing management for reduced methane emissions from enteric fermentation, manure management, improvement of N-efficiency, 1st generation biofuels, reduced deforestation, afforestation, 2nd generation bioenergy crops and BECCS (Popp et al. 2017 <sup>[[#fn:r1870|1870]]</sup> ). However, many ‘natural climate solutions’ (Griscom et al. 2017 <sup>[[#fn:r1871|1871]]</sup> ), such as forest management, rangeland management, soil carbon management or wetland management, are not included in most of these scenarios. In addition, most IAMs neglect the biophysical effects of land-use such as changes in albedo or evapotranspiration with few exceptions (Kreidenweis et al. 2016 <sup>[[#fn:r1872|1872]]</sup> ).

Mitigation pathways, based on IAMs, are typically designed to find the least cost pathway to achieve a pre-defined climate target (Riahi et al. 2017 <sup>[[#fn:r1873|1873]]</sup> ). Such cost-optimal mitigation pathways, especially in RCP2.6 (broadly a 2°C target) and 1.9 scenarios (broadly a 1.5°C target), project GHG emissions to peak early in the 21st century, 2 strict GHG emission reduction afterwards and, depending on the climate target, net CDR from the atmosphere in the second half of the century (Chapter 2 of SR15; Tavoni et al. 2015 <sup>[[#fn:r1874|1874]]</sup> ; Riahi et al. 2017 <sup>[[#fn:r1875|1875]]</sup> ). In most of these pathways, land use is of great importance because of its mitigation potential as discussed in Section 2.7.1: these pathways are based on the assumptions that (i) large-scale afforestation and reforestation removes substantial amounts of CO <sub>2</sub> from the atmosphere, (ii) biomass grown on cropland or from forestry residues can be used for energy generation or BECCS substituting fossil fuel emissions and generating CDR, and (iii) non-CO <sub>2</sub> emissions from agricultural production can be reduced, even under improved agricultural management (Popp et al. 2017 <sup>[[#fn:r1876|1876]]</sup> ; Rogelj et al. 2018a <sup>[[#fn:r1877|1877]]</sup> ; Van Vuuren et al. 2018 <sup>[[#fn:r1878|1878]]</sup> , Frank et al. 2018 <sup>[[#fn:r1879|1879]]</sup> ).

From the IAM scenarios available to this assessment, a set of feasible mitigation pathways has been identified which is illustrative of the range of possible consequences on land use and GHG emissions (presented in this chapter) and sustainable development (Chapter 6). Thus, the IAM scenarios selected here vary due to underlying socio- economic and policy assumptions, the mitigation options considered, long-term climate goals, the level of inclusion of other sustainability goals (such as land and water restrictions for biodiversity conservation or food production) and the models by which they are generated.

In the baseline case without climate change mitigation, global CO <sub>2</sub> emissions from land-use change decrease over time in most scenarios due to agricultural intensification and decreases in demand for agricultural commodities – some even turning negative by the end of the century due to abandonment of agricultural land and associated carbon uptake through vegetation regrowth. Median global CO <sub>2</sub> emissions from land-use change across 5 SSPs and 5 IAMs decrease throughout the 21st century: 3, 1.9 and –0.7 GtCO <sub>2</sub> yr <sup>–1</sup> in 2030, 2050 and 2100 respectively (Figure 2.25). In contrast, CH <sub>4</sub> and N <sub>2</sub> O emissions from agricultural production remain rather constant throughout the 21st century (CH <sub>4</sub> : 214, 231.7 and 209.1 MtCH <sub>4</sub> yr <sup>–1</sup> in 2030, 2050 and 2100 respectively; N <sub>2</sub> O: 9.1, 10.1 and 10.3 MtN <sub>2</sub> O yr <sup>–1</sup> in 2030, 2050 and 2100 respectively).

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<span id="land-based-global-ghg-emissions-and-removals-in-2030-2050-and-2100-for-baseline-rcp4.5-rcp2.6-and-rcp1.9-based-on-the-ssp.-source-popp-et-al.-2017-rogelj-et-al.-2018-riahi-et-al.-2017.-data-is-from-an-update-of-the-iamc-scenario-explorer-developed-for-the-sr15-huppmann-et-al.-2018-rogelj-et-al."></span>
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'''Land-based global GHG emissions and removals in 2030, 2050 and 2100 for baseline, RCP4.5, RCP2.6 and RCP1.9 based on the SSP. Source: Popp et al. (2017), Rogelj et al. (2018), Riahi et al. (2017). Data is from an update of the IAMC Scenario Explorer developed for the SR15 (Huppmann et al. 2018; Rogelj et al. […]'''
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[[File:f2ed1ae088217867f58da644c8d6a06e Figure-2.25-1024x859.jpg]]

Land-based global GHG emissions and removals in 2030, 2050 and 2100 for baseline, RCP4.5, RCP2.6 and RCP1.9 based on the SSP. Source: Popp et al. (2017) <sup>[[#fn:r2193|2193]]</sup> , Rogelj et al. (2018) <sup>[[#fn:r2194|2194]]</sup> , Riahi et al. (2017) <sup>[[#fn:r2195|2195]]</sup> . Data is from an update of the IAMC Scenario Explorer developed for the SR15 (Huppmann et al. 2018 <sup>[[#fn:r2196|2196]]</sup> ; Rogelj et al. 2018 <sup>[[#fn:r2197|2197]]</sup> ). Boxplots (Tukey style) show median (horizontal line), interquartile range (IQR box) and the range of values within 1.5 × IQR at either end of the box (vertical lines) across 5 SSPs and across 5 IAMs. Outliers (red crosses) are values greater than 1.5 × IQR at either end of the box. The categories CO <sub>2</sub> Land, CH <sub>4</sub> Land and N <sub>2</sub> O Land include GHG emissions from land-use change and agricultural land use (including emissions related to bioenergy production). In addition, the category CO <sub>2</sub> Land includes negative emissions due to afforestation. BECCS reflects the CO <sub>2</sub> emissions captured from bioenergy use and stored in geological deposits.

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In the mitigation cases (RCP4.5, RCP2.6 and RCP1.9), most of the scenarios indicate strong reductions in CO <sub>2</sub> emissions due to (i) reduced deforestation and (ii) carbon uptake due to afforestation. However, CO <sub>2</sub> emissions from land use can occur in some mitigation scenarios as a result of weak land-use change regulation (Fujimori et al. 2017 <sup>[[#fn:r1880|1880]]</sup> ; Calvin et al. 2017 <sup>[[#fn:r1881|1881]]</sup> ) or displacement effects into pasture land caused by high bioenergy production combined with forest protection only (Popp et al. 2014 <sup>[[#fn:r1882|1882]]</sup> ). The level of CO <sub>2</sub> removal globally (median value across SSPs and IAMs) increases with the stringency of the climate target (RCP4.5, RCP2.6 and RCP1.9) for both afforestation (–1.3, –1.7 and –2.4 GtCO <sub>2</sub> yr <sup>–1</sup> in 2100) and BECCS (–6.5, –11 and –14.9 GtCO <sub>2</sub> yr <sup>–1</sup> in 2100) (Cross-Chapter Box 7 and Chapter 6). In the mitigation cases (RCP4.5, RCP2.6 and RCP1.9), CH <sub>4</sub> and N <sub>2</sub> O emissions are remarkably lower compared to the baseline case (CH4: 133.2, 108.4 and 73.5 MtCH <sub>4</sub> yr <sup>–1</sup> in 2100; N <sub>2</sub> O: 7.4, 6.1 and 4.5 MtN <sub>2</sub> O yr <sup>–1</sup> in 2100; see previous paragraph for CH <sub>4</sub> and N <sub>2</sub> O emissions in the baseline case). The reductions in the mitigation cases are mainly due to improved agricultural management such as improved nitrogen fertiliser management, improved water management in rice production, improved manure management (by, for example, covering of storages or adoption of biogas plants), better herd management and better quality of livestock through breeding and improved feeding practices. In addition, dietary shifts away from emission-intensive livestock products also lead to decreased CH <sub>4</sub> and N <sub>2</sub> O emissions especially in RCP2.6 and RCP1.9 scenarios. However, high levels of bioenergy production can result in increased N <sub>2</sub> O emissions due to nitrogen fertilisation of dedicated bioenergy crops.

Such high levels of CO <sub>2</sub> removal through mitigation options that require land conversion (BECCS and afforestation) shape the land system dramatically (Figure 2.26). Across the different RCPs, SSPs and IAMs, median change of global forest area throughout the 21st century ranges from about –0.2 to +7.2 Mkm <sup>2</sup> between 2010 and 2100, and agricultural land used for 2nd generation bioenergy crop production ranges from about 3.2–6.6 Mkm <sup>2</sup> in 2100 (Popp et al. 2017 <sup>[[#fn:r1883|1883]]</sup> ; Rogelj et al. 2018 <sup>[[#fn:r1884|1884]]</sup> ). Land requirements for bioenergy and afforestation for a RCP1.9 scenario are higher than for a RCP2.6 scenario and especially a RCP4.5 mitigation scenario. As a consequence of the expansion of mainly land-demanding mitigation options, global pasture land is reduced in most mitigation scenarios much more strongly than compared to baseline scenarios (median reduction of 0, 2.6, 5.1 and 7.5 Mkm <sup>2</sup> between 2010 and 2100 in baseline, RCP4.5, RCP2.6 and RCP1.9 respectively). In addition, cropland for food and feed production decreases with the stringency of the climate target (+1.2, +0.2, –1.8 and –4 Mkm <sup>2</sup> in 2100 compared to 2010 in baseline, RCP4.5, RCP2.6 and RCP1.9 respectively). These reductions in agricultural land for food and feed production are facilitated by agricultural intensification on agricultural land and in livestock production systems (Popp et al. 2017 <sup>[[#fn:r1885|1885]]</sup> ), but also by changes in consumption patterns (Fujimori et al. 2017 <sup>[[#fn:r1886|1886]]</sup> ; Frank et al. 2017b <sup>[[#fn:r1887|1887]]</sup> ).

The pace of projected land-use change over the coming decades in ambitious mitigation scenarios goes well beyond historical changes in some instances (Turner et al. (2018b) <sup>[[#fn:r2207|2207]]</sup> , see also SR15). This raises issues for societal acceptance, and distinct policy and governance for avoiding negative consequences for other sustainability goals will be required (Humpenöder et al. 2018 <sup>[[#fn:r1888|1888]]</sup> ; Obersteiner et al. 2016 <sup>[[#fn:r1889|1889]]</sup> ; Calvin et al. 2014 <sup>[[#fn:r1890|1890]]</sup> ) (Chapters 6 and 7).

Different mitigation strategies can achieve the net emissions reductions that would be required to follow a pathway that limits global warming to 2°C or 1.5°C, with very different consequences on the land system.

Figure 2.27 shows six alternative pathways (archetypes) for achieving ambitious climate targets (RCP2.6 and RCP1.9), highlighting land- based strategies and GHG emissions. All pathways are assessed by different models but are all based on the SSP2 (Riahi et al. 2017 <sup>[[#fn:r1891|1891]]</sup> ), with all based on an RCP 1.9 mitigation pathway expect for Pathway 1, which is RCP2.6. All scenarios show land-based negative emissions, but the amount varies across pathways, as do the relative contributions of different land-based CDR options, such as afforestation/reforestation and BECCS.

Pathway 1 RCP2.6 ‘Portfolio’ (Fricko et al. 2017 <sup>[[#fn:r1892|1892]]</sup> ) shows a strong near-term decrease of CO <sub>2</sub> emissions from land-use change, mainly due to reduced deforestation, as well as slightly decreasing N <sub>2</sub> O and CH4 emissions after 2050 from agricultural production due to improved agricultural management and dietary shifts away from emissions-intensive livestock products. However, in contrast to CO <sub>2</sub> emissions, which turn net-negative around 2050 due to afforestation/ reforestation, CH <sub>4</sub> and N <sub>2</sub> O emissions persist throughout the century due to difficulties of eliminating these residual emissions based on existing agricultural management methods (Stevanović et al. 2017 <sup>[[#fn:r1893|1893]]</sup> ; Frank et al. 2017b <sup>[[#fn:r1894|1894]]</sup> ). In addition to abating land related GHG emissions as well as increasing the terrestrial sink, this example also shows the importance of the land sector in providing biomass for BECCS and hence CDR in the energy sector. In this scenario, annual BECCS-based CDR is about three times higher than afforestation-based CDR in 2100 (–11.4 and –3.8 GtCO <sub>2</sub> yr <sup>–1</sup> respectively). Cumulative CDR throughout the century amounts to –395 GtCO <sub>2</sub> for BECCS and –73 GtCO <sub>2</sub> for afforestation. Based on these GHG dynamics, the land sector turns GHG emission neutral in 2100. However, accounting also for BECCS- based CDR taking place in the energy sector, but with biomass provided by the land sector, turns the land sector GHG emission neutral already in 2060, and significantly net-negative by the end of the century.

Pathway 2 RCP1.9 ‘Increased Ambition’ (Rogelj et al. 2018 <sup>[[#fn:r1895|1895]]</sup> ) has dynamics of land-based GHG emissions and removals that are very similar to those in Pathway 1 (RCP2.6) but all GHG emission reductions as well as afforestation/reforestation and BECCS-based CDR start earlier in time at a higher rate of deployment. Cumulative CDR throughout the century amounts to –466 GtCO <sub>2</sub> for BECCS and –117 GtCO <sub>2</sub> for afforestation.

Pathway 3 RCP 1.9 ‘Only BECCS’, in contrast to Pathway 2, includes only BECCS-based CDR (Kriegler et al. 2017 <sup>[[#fn:r1896|1896]]</sup> ). As a consequence, CO <sub>2</sub> emissions are persistent much longer, predominantly from indirect land-use change due to large-scale bioenergy cropland expansion into non-protected natural areas (Popp et al. 2017 <sup>[[#fn:r1897|1897]]</sup> ; Calvin et al. 2014 <sup>[[#fn:r1898|1898]]</sup> ). While annual BECCS CDR rates in 2100 are similar to Pathways 1 and 2 (–15.9 GtCO <sub>2</sub> yr <sup>–1</sup> ), cumulative BECCS-based CDR throughout the century is much larger (–944 GtCO <sub>2</sub> ).

Pathway 4 RCP1.9 ‘Early CDR’ (Bertram et al. 2018 <sup>[[#fn:r1899|1899]]</sup> ) indicates that a significant reduction in the later century in the BECCS-related CDR as well as CDR in general can be achieved with earlier and mainly terrestrial CDR, starting in 2030. In this scenario, terrestrial CDR is based on afforestation but could also be supported by soil organic carbon sequestration (Paustian et al. 2016 <sup>[[#fn:r1900|1900]]</sup> ) or other natural climate solutions, such as rangeland or forest management (Griscom et al. 2017 <sup>[[#fn:r1901|1901]]</sup> ). This scenario highlights the importance of the timing for CDR- based mitigation pathways (Obersteiner et al. 2016 <sup>[[#fn:r1902|1902]]</sup> ). As a result of near-term and mainly terrestrial CDR deployment, cumulative BECCS- based CDR throughout the century is limited to –300 GtCO <sub>2</sub> , while cumulative afforestation-based CDR amounts to –428 GtCO <sub>2</sub> .

In Pathway 5 RCP1.9 ‘Low residual emissions’ (van Vuuren et al. 2018 <sup>[[#fn:r1903|1903]]</sup> ), land-based mitigation is driven by stringent enforcement of measures and technologies to reduce end-of-pipe non-CO <sub>2</sub> emissions and by introduction of in-vitro (cultured) meat, reducing residual N <sub>2</sub> O and CH <sub>4</sub> emissions from agricultural production. In consequence, much lower amounts of CDR from afforestation and BECCS are needed with much later entry points to compensate for residual emissions. Cumulative CDR throughout the century amounts to –252 GtCO <sub>2</sub> for BECCS and –128 GtCO <sub>2</sub> for afforestation. Therefore, total cumulative land-based CDR in Pathway 5 is substantially lower compared to Pathways 2–4 (380 GtCO <sub>2</sub> ).

Finally, Pathway 6 RCP1.9 ‘Low Energy’ (Grubler et al. 2018 <sup>[[#fn:r1904|1904]]</sup> ), equivalent to Pathway LED in SR15, indicates the importance of other sectoral GHG emission reductions for the land sector. In this example, rapid and early reductions in energy demand and associated drops in energy- related CO <sub>2</sub> emissions limit overshoot and decrease the requirements for negative emissions technologies, especially for land-demanding CDR, such as biomass production for BECCS and afforestation. While BECCS is not used at all in Pathway 6, cumulative CDR throughout the century for afforestation amounts to –124 GtCO <sub>2</sub> .

Besides their consequences on mitigation pathways and land consequences, those archetypes can also affect multiple other sustainable development goals that provide both challenges and opportunities for climate action (Chapter 6).

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'''Global change of major land cover types by 2030, 2050 and 2100 relative to 2010 for baseline, RCP4.5, RCP2.6 and RCP1.9 based on the SSP. Source: Popp et al. (2017), Rogelj et al. (2018), Riahi et al. (2017). Data is from an update of the IAMC Scenario Explorer developed for the SR15 (Huppmann et al. […]'''
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Global change of major land cover types by 2030, 2050 and 2100 relative to 2010 for baseline, RCP4.5, RCP2.6 and RCP1.9 based on the SSP. Source: Popp et al. (2017) <sup>[[#fn:r2208|2208]]</sup> , Rogelj et al. (2018) <sup>[[#fn:r2209|2209]]</sup> , Riahi et al. (2017) <sup>[[#fn:r2210|2210]]</sup> . Data is from an update of the IAMC Scenario Explorer developed for the SR15 (Huppmann et al. 2018 <sup>[[#fn:r2211|2211]]</sup> ; Rogelj et al. 2018 <sup>[[#fn:r2212|2212]]</sup> ). Boxplots (Tukey style) show median (horizontal line), interquartile range IQR (box) and the range of values within 1.5 × IQR at either end of the box (vertical lines) across 5 SSPs and across 5 IAMs. Outliers (red crosses) are values greater than 1.5 × IQR at either end of the box. In 2010, total land cover at global scale was estimated 15–16 Mkm <sup>2</sup> for cropland, 0–0.14 Mkm <sup>2</sup> for bioenergy, 30–35 Mkm <sup>2</sup> for pasture and 37–42 Mkm <sup>2</sup> for forest, across the IAMs that reported SSP pathways (Popp et al. 2017 <sup>[[#fn:r2198|2198]]</sup> ).

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'''Figure 2.27'''

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'''Evolution and breakdown of global land-based GHG emissions and removals under six alternative mitigation pathways.This figure illustrates the differences in timing and magnitude of land-based mitigation approaches including afforestation and BECCS. All pathways are based on different IAM realisations of SSP2. Pathway 1 is based on RCP 2.6, while all other pathways are based on […]'''
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[[File:732d49f4d14bd8c0f5b1861b5db42972 Figure-2.27-1024x683.jpg]]

Evolution and breakdown of global land-based GHG emissions and removals under six alternative mitigation pathways.This figure illustrates the differences in timing and magnitude of land-based mitigation approaches including afforestation and BECCS. All pathways are based on different IAM realisations of SSP2. Pathway 1 is based on RCP 2.6, while all other pathways are based on RCP 1.9. Pathway 1: MESSAGE-GLOBIOM (Fricko et al. 2017 <sup>[[#fn:r2199|2199]]</sup> ); Pathway 2: MESSAGE-GLOBIOM (Rogelj et al. 2018 <sup>[[#fn:r2200|2200]]</sup> ); Pathway 3: REMIND-MAgPIE (Kriegler et al. 2017 <sup>[[#fn:r2201|2201]]</sup> ); Pathway 4: REMIND-MAgPIE (Bertram et al. 2018 <sup>[[#fn:r2202|2202]]</sup> ); Pathway 5: IMAGE (van Vuuren et al. 2018 <sup>[[#fn:r2203|2203]]</sup> ); Pathway 6: MESSAGE-GLOBIOM (Grubler et al. 2018 <sup>[[#fn:r2204|2204]]</sup> ). Data is from an update of the IAMC Scenario Explorer developed for the SR15 (Rogelj et al. 2018 <sup>[[#fn:r2205|2205]]</sup> ). The categories CO <sub>2</sub> Land, CH <sub>4</sub> Land and N <sub>2</sub> O Land include GHG emissions from land-use change and agricultural land use (including emissions related to bioenergy production). In addition, the category CO <sub>2</sub> Land includes negative emissions due to afforestation. BECCS reflects the CO <sub>2</sub> emissions captured from bioenergy use and stored in geological deposits. Solid lines show the net effect of all land based GHG emissions and removals (CO <sub>2</sub> Land, CH <sub>4</sub> Land, N <sub>2</sub> O Land and BECCS), while dashed lines show the net effect excluding BECCS. CH <sub>4</sub> and N <sub>2</sub> O emissions are converted to CO <sub>2</sub> -eq using GWP factors of 28 and 265 respectively.

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