Editing IPCC:AR6/SR15/Chapter-3 (section)

=== 3.6.2 Non-CO2 Implications and Projected Risks of Mitigation Pathways ===

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==== 3.6.2.1 Risks arising from land-use changes in mitigation pathways ====

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In mitigation pathways, land-use change is affected by many different mitigation options. First, mitigation of non-CO <sub>2</sub> emissions from agricultural production can shift agricultural production between regions via trade of agricultural commodities. Second, protection of carbon-rich ecosystems such as tropical forests constrains the area for agricultural expansion. Third, demand-side mitigation measures, such as less consumption of resource-intensive commodities (animal products) or reductions in food waste, reduce pressure on land (Popp et al., 2017; Rogelj et al., 2018) <sup>[[#fn:r1272|1272]]</sup> . Finally, carbon dioxide removal (CDR) is a key component of most, but not all, mitigation pathways presented in the literature to date which constrain warming to 1.5°C or 2°C. Carbon dioxide removal measures that require land include bioenergy with carbon capture and storage (BECCS), afforestation and reforestation (AR), soil carbon sequestration, direct air capture, biochar and enhanced weathering (see Cross-Chapter Box 7 in this chapter). These potential methods are assessed in Section 4.3.7.

In cost-effective integrated assessment modelling (IAM) pathways recently developed to be consistent with limiting warming to 1.5°C, use of CDR in the form of BECCS and AR are fundamental elements (Chapter 2; Popp et al., 2017; Hirsch et al., 2018; Rogelj et al., 2018; Seneviratne et al., 2018c) <sup>[[#fn:r1273|1273]]</sup> . The land-use footprint of CDR deployment in 1.5°C-consistent pathways can be substantial (Section 2.3.4, Figure 2.11), even though IAMs predominantly rely on second-generation biomass and assume future productivity increases in agriculture.

A body of literature has explored potential consequences of large-scale use of CDR. In this case, the corresponding land footprint by the end of the century could be extremely large, with estimates including: up to 18% of the land surface being used (Wiltshire and Davies-Barnard, 2015) <sup>[[#fn:r1274|1274]]</sup> ; vast acceleration of the loss of primary forest and natural grassland (Williamson, 2016) <sup>[[#fn:r1275|1275]]</sup> leading to increased greenhouse gas emissions (P. Smith et al., 2013, 2015) <sup>[[#fn:r1276|1276]]</sup> ; and potential loss of up to 10% of the current forested lands to biofuels (Yamagata et al., 2018) <sup>[[#fn:r1277|1277]]</sup> . Other estimates reach 380–700 Mha or 21–64% of current arable cropland (Section 4.3.7). Boysen et al. (2017) <sup>[[#fn:r1278|1278]]</sup> found that in a scenario in which emissions reductions were sufficient only to limit warming to 2.5°C, use of CDR to further limit warming to 1.7°C would result in the conversion of 1.1–1.5 Gha of land – implying enormous losses of both cropland and natural ecosystems. Newbold et al. (2015) <sup>[[#fn:r1279|1279]]</sup> found that biodiversity loss in the Representative Concentration Pathway (RCP)2.6 scenario could be greater than that in RCP4.5 and RCP6, in which there is more climate change but less land-use change. Risks to biodiversity conservation and agricultural production are therefore projected to result from large-scale bioenergy deployment pathways (P. Smith et al., 2013; Tavoni and Socolow, 2013) <sup>[[#fn:r1280|1280]]</sup> . One study explored an extreme mitigation strategy encouraging biofuel expansion sufficient to limit warming to 1.5°C and found that this would be more disruptive to land use and crop prices than the impacts of a 2°C warmer world which has a larger climate signal and lower mitigation requirement (Ruane et al., 2018) <sup>[[#fn:r1281|1281]]</sup> . However, it should again be emphasized that many of the pathways explored in Chapter 2 of this report follow strategies that explore how to reduce these issues. Chapter 4 provides an assessment of the land footprint of various CDR technologies (Section 4.3.7).

The degree to which BECCS has these large land-use footprints depends on the source of the bioenergy used and the scale at which BECCS is deployed. Whether there is competition with food production and biodiversity depends on the governance of land use, agricultural intensification, trade, demand for food (in particular meat), feed and timber, and the context of the whole supply chain (Section 4.3.7, Fajardy and Mac Dowell, 2017; Booth, 2018; Sterman et al., 2018) <sup>[[#fn:r1282|1282]]</sup> .

The more recent literature reviewed in Chapter 2 explores pathways which limit warming to 2°C or below and achieve a balance between sources and sinks of CO <sub>2</sub> by using BECCS that relies on second-generation (or even third-generation) biofuels, changes in diet or more generally, management of food demand, or CDR options such as forest restoration (Chapter 2; Bajželj et al., 2014) <sup>[[#fn:r1283|1283]]</sup> . Overall, this literature explores how to reduce the issues of competition for land with food production and with natural ecosystems (in particular forests) (Cross-Chapter Box 1 in Chapter 1; van Vuuren et al., 2009; Haberl et al., 2010, 2013; Bajželj et al., 2014; Daioglou et al., 2016; Fajardy and Mac Dowell, 2017) <sup>[[#fn:r1284|1284]]</sup> .

Some IAMs manage this transition by effectively protecting carbon stored on land and focusing on the conversion of pasture area into both forest area and bioenergy cropland. Some IAMs explore 1.5°C-consistent pathways with demand-side measures such as dietary changes and efficiency gains such as agricultural changes (Sections 2.3.4 and 2.4.4), which lead to a greatly reduced CDR deployment and consequently land-use impacts (van Vuuren et al., 2018) <sup>[[#fn:r1285|1285]]</sup> . In reality, however, whether this CDR (and bioenergy in general) has large adverse impacts on environmental and societal goals depends in large part on the governance of land use (Section 2.3.4; Obersteiner et al., 2016; Bertram et al., 2018; Humpenöder et al., 2018) <sup>[[#fn:r1286|1286]]</sup> .

Rates of sequestration of 3.3 GtC ha–1 require 970 Mha of afforestation and reforestation (Smith et al., 2015) <sup>[[#fn:r1287|1287]]</sup> . Humpenöder et al. (2014) <sup>[[#fn:r1288|1288]]</sup> estimated that in least-cost pathways afforestation would cover 2800 Mha by the end of the century to constrain warming to 2°C. Hence, the amount of land considered if least-cost mitigation is implemented by afforestation and reforestation could be up to three to five times greater than that required by BECCS, depending on the forest management used. However, not all of the land footprint of CDR is necessarily to be in competition with biodiversity protection. Where reforestation is the restoration of natural ecosystems, it benefits both carbon sequestration and conservation of biodiversity and ecosystem services (Section 4.3.7) and can contribute to the achievement of the Aichi targets under the Convention on Biological Diversity (CBD) (Leadley et al., 2016) <sup>[[#fn:r1289|1289]]</sup> . However, reforestation is often not defined in this way (Section 4.3.8; Stanturf et al., 2014) <sup>[[#fn:r1290|1290]]</sup> and the ability to deliver biodiversity benefits is strongly dependent on the precise nature of the reforestation, which has different interpretations in different contexts and can often include agroforestry rather than restoration of pristine ecosystems (Pistorious and Kiff, 2017) <sup>[[#fn:r1291|1291]]</sup> . However, ‘natural climate solutions’, defined as conservation, restoration, and improved land management actions that increase carbon storage and/or avoid greenhouse gas emissions across global forests, wetlands, grasslands and agricultural lands, are estimated to have the potential to provide 37% of the cost-effective CO <sub>2</sub> mitigation needed by southern Europe and the Mediterranean by 2030 – in order to have a &gt;66% chance of holding warming to below 2°C (Griscom et al., 2017) <sup>[[#fn:r1292|1292]]</sup> .

Any reductions in agricultural production driven by climate change and/or land management decisions related to CDR may (e.g., Nelson et al., 2014a; Dalin and Rodríguez-Iturbe, 2016) <sup>[[#fn:r1293|1293]]</sup> or may not (Muratori et al., 2016) <sup>[[#fn:r1294|1294]]</sup> affect food prices. However, these studies did not consider the deployment of second-generation (instead of first-generation) bioenergy crops, for which the land footprint can be much smaller.

Irrespective of any mitigation-related issues, in order for ecosystems to adapt to climate change, land use would also need to be carefully managed to allow biodiversity to disperse to areas that become newly climatically suitable for it (Section 3.4.1) and to protect the areas where the future climate will still remain suitable. This implies a need for considerable expansion of the protected area network (Warren et al., 2018b) <sup>[[#fn:r1295|1295]]</sup> , either to protect existing natural habitat or to restore it (perhaps through reforestation, see above). At the same time, adaptation to climate change in the agricultural sector (Rippke et al., 2016) <sup>[[#fn:r1296|1296]]</sup> can require transformational as well as new approaches to land-use management; in order to meet the rising food demand of a growing human population, it is projected that additional land will need to be brought into production unless there are large increases in agricultural productivity (Tilman et al., 2011) <sup>[[#fn:r1297|1297]]</sup> . However, future rates of deforestation may be underestimated in the existing literature (Mahowald et al., 2017a) <sup>[[#fn:r1298|1298]]</sup> , and reforestation may therefore be associated with significant co-benefits if implemented to restore natural ecosystems ( ''high confidence'' ).

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==== 3.6.2.2 Biophysical feedbacks on regional climate associated with land-use changes ====

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Changes in the biophysical characteristics of the land surface are known to have an impact on local and regional climates through changes in albedo, roughness, evapotranspiration and phenology, which can lead to a change in temperature and precipitation. This includes changes in land use through agricultural expansion/intensification (e.g., Mueller et al., 2016) <sup>[[#fn:r1299|1299]]</sup> , reforestation/revegetation endeavours (e.g., Feng et al., 2016; Sonntag et al., 2016; Bright et al., 2017) <sup>[[#fn:r1300|1300]]</sup> and changes in land management (e.g., Luyssaert et al., 2014; Hirsch et al., 2017) <sup>[[#fn:r1301|1301]]</sup> that can involve double cropping (e.g., Jeong et al., 2014; Mueller et al., 2015; Seifert and Lobell, 2015) <sup>[[#fn:r1302|1302]]</sup> , irrigation (e.g., Lobell et al., 2009; Sacks et al., 2009; Cook et al., 2011; Qian et al., 2013; de Vrese et al., 2016; Pryor et al., 2016; Thiery et al., 2017) <sup>[[#fn:r1303|1303]]</sup> , no-till farming and conservation agriculture (e.g., Lobell et al., 2006; Davin et al., 2014) <sup>[[#fn:r1304|1304]]</sup> , and wood harvesting (e.g., Lawrence et al., 2012) <sup>[[#fn:r1305|1305]]</sup> . Hence, the biophysical impacts of land-use changes are an important topic to assess in the context of low-emissions scenarios (e.g., van Vuuren et al., 2011b) <sup>[[#fn:r1306|1306]]</sup> , in particular for 1.5°C warming levels (see also Cross-Chapter Box 7 in this chapter).

The magnitude of the biophysical impacts is potentially large for temperature extremes. Indeed, changes induced both by modifications in moisture availability and irrigation and by changes in surface albedo tend to be larger (i.e., stronger cooling) for hot extremes than for mean temperatures (e.g., Seneviratne et al., 2013; Davin et al., 2014; Wilhelm et al., 2015; Hirsch et al., 2017; Thiery et al., 2017) <sup>[[#fn:r1307|1307]]</sup> . The reasons for reduced moisture availability are related to a strong contribution of moisture deficits to the occurrence of hot extremes in mid-latitude regions (Mueller and Seneviratne, 2012; Seneviratne et al., 2013) <sup>[[#fn:r1308|1308]]</sup> . In the case of surface albedo, cooling associated with higher albedo (e.g., in the case of no-till farming) is more effective at cooling hot days because of the higher incoming solar radiation for these days (Davin et al., 2014) <sup>[[#fn:r1309|1309]]</sup> . The overall effect of either irrigation or albedo has been found to be at the most in the order of about 1°C–2°C regionally for temperature extremes. This can be particularly important in the context of low-emissions scenarios because the overall effect is in this case of similar magnitude to the response to the greenhouse gas forcing (Figure 3.22; Hirsch et al., 2017; Seneviratne et al., 2018a, c) <sup>[[#fn:r1310|1310]]</sup> .

'' '' In addition to the biophysical feedbacks from land-use change and land management on climate, there are potential consequences for particular ecosystem services. This includes climate change-induced changes in crop yield (e.g., Schlenker and Roberts, 2009; van der Velde et al., 2012; Asseng et al., 2013, 2015; Butler and Huybers, 2013; Lobell et al., 2014) <sup>[[#fn:r1311|1311]]</sup> which may be further exacerbated by competing demands for arable land between reforestation mitigation activities, crop growth for BECCS (Chapter 2), increasing food production to support larger populations, and urban expansion (see review by Smith et al., 2010) <sup>[[#fn:r1312|1312]]</sup> . In particular, some land management practices may have further implications for food security, for instance throughincreases or decreases in yield when tillage is ceased in some regions (Pittelkow et al., 2014) <sup>[[#fn:r1313|1313]]</sup> .

We note that the biophysical impacts of land use in the context of mitigation pathways constitute an emerging research topic. This topic, as well as the overall role of land-use change in climate change projections and socio-economic pathways, will be addressed in depth in the upcoming IPCC Special Report on Climate Change and Land Use due in 2019.

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'''Figure 3.22'''

<span id="regional-temperature-scaling-with-carbon-dioxide-co-2-concentration-ppm-from-1850-to-2099-for-two-different-regions-defined-in-the-special-report-on-managing-the-risks-of-extreme-events-and-disasters-to-advance-climate-change-adaptation-srex-for-central-europe-ceu-a-and-central-north-america-cna-b."></span>
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'''Regional temperature scaling with carbon dioxide (CO <sub>2</sub> ) concentration (ppm) from 1850 to 2099 for two different regions defined in the Special Report on Managing the Risks of Extreme Events and Disasters to Advance Climate Change Adaptation (SREX) for central Europe (CEU) (a) and central North America (CNA) (b).'''
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[[File:1af916f9bc505370d0033b90a956974b Figure_3.22-1024x522.png]]

Solid lines correspond to the regional average annual maximum daytime temperature (TXx) anomaly, and dashed lines correspond to the global mean temperature anomaly, where all temperature anomalies are relative to 1850–1870 and units are degrees Celsius. The black line in all panels denotes the three-member control ensemble mean, with the grey shaded regions corresponding to the ensemble range. The coloured lines represent the three-member ensemble means of the experiments corresponding to albedo +0.02 (cyan), albedo +0.04 (purple), albedo + 0.08 (orange), albedo +0.10 (red), irrigation (blue), and irrigation with albedo +0.10 (green). Adapted from Hirsch et al. (2017) <sup>[[#fn:r1314|1314]]</sup> .

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==== 3.6.2.3 Atmospheric compounds (aerosols and methane) ====

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There are multiple pathways that could be used to limit anthropogenic climate change, and the details of the pathways will influence the impacts of climate change on humans and ecosystems. Anthropogenic-driven changes in aerosols cause important modifications to the global climate (Bindoff et al., 2013a; Boucher et al., 2013b; P. Wu et al., 2013; Sarojini et al., 2016; H. Wang et al., 2016) <sup>[[#fn:r1315|1315]]</sup> . Enforcement of strict air quality policies may lead to a large decrease in cooling aerosol emissions in the next few decades. These aerosol emission reductions may cause a warming comparable to that resulting from the increase in greenhouse gases by mid-21st century under low CO <sub>2</sub> pathways (Kloster et al., 2009; Acosta Navarro et al., 2017) <sup>[[#fn:r1316|1316]]</sup> . Further background is provided in Sections 2.2.2 and 2.3.1; Cross Chapter Box 1 in Chapter 1). Because aerosol effects on the energy budget are regional, strong regional changes in precipitation from aerosols may occur if aerosol emissions are reduced for air quality reasons or as a co-benefit from switches to sustainable energy sources (H. Wang et al., 2016) <sup>[[#fn:r1317|1317]]</sup> . Thus, regional impacts, especially on precipitation, are very sensitive to 1.5°C-consistent pathways (Z. Wang et al., 2017) <sup>[[#fn:r1318|1318]]</sup> .

Pathways which rely heavily on reductions in methane (CH <sub>4</sub> ) instead of CO <sub>2</sub> will reduce warming in the short term because CH <sub>4</sub> is such a stronger and shorter-lived greenhouse gas than CO <sub>2</sub> , but will lead to stronger warming in the long term because of the much longer residence time of CO <sub>2</sub> (Myhre et al., 2013; Pierrehumbert, 2014) <sup>[[#fn:r1319|1319]]</sup> . In addition, the dominant loss mechanism for CH <sub>4</sub> is atmospheric photo-oxidation. This conversion modifies ozone formation and destruction in the troposphere and stratosphere, therefore modifying the contribution of ozone to radiative forcing, as well as feedbacks on the oxidation rate of methane itself (Myhre et al., 2013) <sup>[[#fn:r1320|1320]]</sup> . Focusing on pathways and policies which both improve air quality and reduce impacts of climate change can provide multiple co-benefits (Shindell et al., 2017) <sup>[[#fn:r1321|1321]]</sup> . These pathways are discussed in detail in Sections 4.3.7 and 5.4.1 and in Cross-Chapter Box 12 in Chapter 5.

Atmospheric aerosols and gases can also modify the land and ocean uptake of anthropogenic CO <sub>2</sub> ; some compounds enhance uptake while others reduce it (Section 2.6.2; Ciais et al., 2013) <sup>[[#fn:r1322|1322]]</sup> . While CO <sub>2</sub> emissions tend to encourage greater uptake of carbon by the land and the ocean (Ciais et al., 2013) <sup>[[#fn:r1323|1323]]</sup> , CH <sub>4</sub> emissions can enhance ozone pollution, depending on nitrogen oxides, volatile organic compounds and other organic species concentrations, and ozone pollution tends to reduce land productivity (Myhre et al., 2013; B. Wang et al., 2017) <sup>[[#fn:r1324|1324]]</sup> . Aside from inhibiting land vegetation productivity, ozone may also alter the CO <sub>2</sub> , CH <sub>4</sub> and nitrogen (N <sub>2</sub> O) exchange at the land–atmosphere interface and transform the global soil system from a sink to a source of carbon (B. Wang et al., 2017) <sup>[[#fn:r1325|1325]]</sup> . Aerosols and associated nitrogen-based compounds tend to enhance the uptake of CO <sub>2</sub> in land and ocean systems through deposition of nutrients and modification of climate (Ciais et al., 2013; Mahowald et al., 2017b) <sup>[[#fn:r1326|1326]]</sup> .

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