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

====== Lead Authors ======

* Rachel Warren (United Kingdom)
* Marcos Buckeridge (Brazil)
* Sabine Fuss (Germany)
* Markku Kanninen (Finland)
* Joeri Rogelj (Austria, Belgium)
* Sonia I. Seneviratne (Switzerland)
* Raphael Slade (United Kingdom)

<div id="section-3-6-2-3-block-1"></div>

Climate and land form a complex system characterized by multiple feedback processes and the potential for non-linear responses to perturbation. Climate determines land cover and the distribution of vegetation, affecting above-and below-ground carbon stocks. At the same time, land cover influences global climate through altered biogeochemical processes (e.g., atmospheric composition and nutrient flow into oceans), and regional climate through changing biogeophysical processes including albedo, hydrology, transpiration and vegetation structure (Forseth, 2010) <sup>[[#fn:r1327|1327]]</sup> .

Greenhouse gas (GHG) fluxes related to land use are reported in the ‘agriculture, forestry and other land use’ sector (AFOLU) and comprise about 25% (about 10–12 GtCO <sub>2eq</sub> yr <sup>–1</sup> ) of anthropogenic GHG emissions (P. Smith et al., 2014) <sup>[[#fn:r1328|1328]]</sup> . Reducing emissions from land use, as well as land-use change, are thus an important component of low-emissions mitigation pathways (Clarke et al., 2014) <sup>[[#fn:r1329|1329]]</sup> , particularly as land-use emissions can be influenced by human actions such as deforestation, afforestation, fertilization, irrigation, harvesting, and other aspects of cropland, grazing land and livestock management (Paustian et al., 2006; Griscom et al., 2017; Houghton and Nassikas, 2018) <sup>[[#fn:r1330|1330]]</sup> .

In the IPCC Fifth Assessment Report, the vast majority of scenarios assessed with a 66% or better chance of limiting global warming to 2°C by 2100 included carbon dioxide removal (CDR) – typically about 10 GtCO <sub>2</sub> yr <sup>–1</sup> in 2100 or about 200–400 GtCO <sub>2</sub> over the course of the century (Smith et al., 2015; van Vuuren et al., 2016) <sup>[[#fn:r1331|1331]]</sup> . These integrated assessment model (IAM) results were predominately achieved by using bioenergy with carbon capture and storage (BECCS) and/or afforestation and reforestation (AR). Virtually all scenarios that limit either peak or end-of-century warming to 1.5°C also use land-intensive CDR technologies (Rogelj et al., 2015; Holz et al., 2017; Kriegler et al., 2017; Fuss et al., 2018; van Vuuren et al., 2018) <sup>[[#fn:r1332|1332]]</sup> . Again, AR (Sections 2.3 and 4.3.7) and BECCS (Sections 4.3.2. and 4.3.7) predominate. Other CDR options, such as the application of biochar to soil, soil carbon sequestration, and enhanced weathering (Section 4.3.7) are not yet widely incorporated into IAMs, but their deployment would also necessitate the use of land and/or changes in land management.

Integrated assessment models provide a simplified representation of land use and, with only a few exceptions, do not include biophysical feedback processes (e.g., albedo and evapotranspiration effects) (Kreidenweis et al., 2016) <sup>[[#fn:r1333|1333]]</sup> despite the importance of these processes for regional climate, in particular hot extremes (Section 3.6.2.2; Seneviratne et al., 2018c) <sup>[[#fn:r1334|1334]]</sup> . The extent, location and impacts of large-scale land-use change described by existing IAMs can also be widely divergent, depending on model structure, scenario parameters, modelling objectives and assumptions (including regarding land availability and productivity) (Prestele et al., 2016; Alexander et al., 2017; Popp et al., 2017; Seneviratne et al., 2018c) <sup>[[#fn:r1335|1335]]</sup> . Despite these limitations, IAM scenarios effectively highlight the extent and nature of potential land-use transitions implicit in limiting warming to 1.5°C.

Cross-Chapter Box 7 Table 1 presents a comparison of the five CDR options assessed in this report. This illustrates that if BECCS and AR were to be deployed at a scale of 12 GtCO <sub>2</sub> yr <sup>–1</sup> in 2100, for example, they would have a substantial land and water footprint. Whether this footprint would result in adverse impacts, for example on biodiversity or food production, depends on the existence and effectiveness of measures to conserve land carbon stocks, limit the expansion of agriculture at the expense of natural ecosystems, and increase agriculture productivity (Bonsch et al., 2016; Obersteiner et al., 2016; Bertram et al., 2018; Humpenöder et al., 2018) <sup>[[#fn:r1336|1336]]</sup> . In comparison, the land and water footprints of enhanced weathering, soil carbon sequestration and biochar application are expected to be far less per GtCO <sub>2</sub> sequestered. These options may offer potential co-benefits by providing an additional source of nutrients or by reducing N <sub>2</sub> O emissions, but they are also associated with potential side effects. Enhanced weathering would require massive mining activity, and providing feedstock for biochar would require additional land, even though a proportion of the required biomass is expected to come from residues (Woolf et al., 2010; Smith, 2016) <sup>[[#fn:r1337|1337]]</sup> . For the terrestrial CDR options, permanence and saturation are important considerations, making their viability and long-term contributions to carbon reduction targets uncertain.

The technical, political and social feasibility of scaling up and implementing land-intensive CDR technologies (Cross-Chapter Box 3 in Chapter 1) is recognized to present considerable potential barriers to future deployment (Boucher et al., 2013a; Fuss et al., 2014, 2018; Anderson and Peters, 2016; Vaughan and Gough, 2016; Williamson, 2016; Minx et al., 2017, 2018; Nemet et al., 2018; Strefler et al., 2018; Vaughan et al., 2018) <sup>[[#fn:r1338|1338]]</sup> . To investigate the implications of restricting CDR options should these barriers prove difficult to overcome, IAM studies (Section 2.3.4) have developed scenarios that limit – either implicitly or explicitly – the use of BECCS and bioenergy (Krey et al., 2014; Bauer et al., 2018; Rogelj et al., 2018) <sup>[[#fn:r1339|1339]]</sup> or the use of BECCS and afforestation (Strefler et al., 2018) <sup>[[#fn:r1340|1340]]</sup> . Alternative strategies to limit future reliance on CDR have also been examined, including increased electrification, agricultural intensification, behavioural change, and dramatic improvements in energy and material efficiency (Bauer et al., 2018; Grubler et al., 2018; van Vuuren et al., 2018) <sup>[[#fn:r1341|1341]]</sup> . Somewhat counterintuitively, scenarios that seek to limit the deployment of BECCs may result in increased land use, through greater deployment of bioenergy, and afforestation (Chapter 2, Box 2.1; Krey et al., 2014; Krause et al., 2017; Bauer et al., 2018; Rogelj et al., 2018) <sup>[[#fn:r1342|1342]]</sup> . Scenarios aiming to minimize the total human land footprint (including land for food, energy and climate mitigation) also result in land-use change, for example by increasing agricultural efficiency and dietary change (Grubler et al., 2018) <sup>[[#fn:r1343|1343]]</sup> .

The impacts of changing land use are highly context, location and scale dependent (Robledo-Abad et al., 2017) <sup>[[#fn:r1344|1344]]</sup> . The supply of biomass for CDR (e.g., energy crops) has received particular attention. The literature identifies regional examples of where the use of land to produce biofuels might be sustainably increased (Jaiswal et al., 2017) <sup>[[#fn:r1345|1345]]</sup> , where biomass markets could contribute to the provision of ecosystem services (Dale et al., 2017) <sup>[[#fn:r1346|1346]]</sup> , and where bioenergy could increase the resilience of production systems and contribute to rural development (Kline et al., 2017) <sup>[[#fn:r1347|1347]]</sup> . However, studies of global biomass potential provide only limited insight into the local feasibility of supplying large quantities of biomass on a global scale (Slade et al., 2014) <sup>[[#fn:r1348|1348]]</sup> . Concerns about large-scale use of biomass for CDR include a range of potential consequences including greatly increased demand for freshwater use, increased competition for land, loss of biodiversity and/or impacts on food security (Section 3.6.2.1; Heck et al., 2018) <sup>[[#fn:r1349|1349]]</sup> . The short-versus long-term carbon impacts of substituting biomass for fossil fuels, which are largely determined by feedstock choice, also remain a source of contention (Schulze et al., 2012; Jonker et al., 2014; Booth, 2018; Sterman et al., 2018) <sup>[[#fn:r1350|1350]]</sup> .

Afforestation and reforestation can also present trade-offs between biodiversity, carbon sequestration and water use, and these strategies have a higher land footprint per tonne of CO <sub>2</sub> removed (Cunningham, 2015; Naudts et al., 2016; Smith et al., 2018) <sup>[[#fn:r1351|1351]]</sup> . For example, changing forest management to strategies favouring faster growing species, greater residue extraction and shorter rotations may have a negative impact on biodiversity (de Jong et al., 2014) <sup>[[#fn:r1352|1352]]</sup> . In contrast, reforestation of degraded land with native trees can have substantial benefits for biodiversity (Section 3.6). Despite these constraints, the potential for increased carbon sequestration through improved land stewardship measures is considered to be substantial (Griscom et al., 2017) <sup>[[#fn:r1353|1353]]</sup> .

Evaluating the synergies and trade-offs between mitigation and adaptation actions, resulting land and climate impacts, and the myriad issues related to land-use governance will be essential to better understand the future role of CDR technologies. This topic will be addressed further in the IPCC Special Report on Climate Change and Land (SRCCL) due to be published in 2019.

'''Key messages:'''

Cost-effective strategies to limit peak or end-of-century warming to 1.5°C all include enhanced GHG removals in the AFOLU sector as part of their portfolio of measures ( ''high confidence'' ).

Large-scale deployment of land-based CDR would have far-reaching implications for land and water availability ( ''high confidence'' ). This may impact food production, biodiversity and the provision of other ecosystem services ( ''high confidence'' ).

The impacts of deploying land-based CDR at large scales can be reduced if a wider portfolio of CDR options is deployed, and if increased mitigation effort focuses on strongly limiting demand for land, energy and material resources, including through lifestyle and dietary changes ( ''medium confidence'' ).

Afforestation and reforestation may be associated with significant co-benefits if implemented appropriately, but they feature large land and water footprints if deployed at large scales ( ''medium confidence'' ).

<div id="section-3-6-2-3-block-2"></div>

<span id="cross-chapter-box-7-table-1"></span>
<!-- START TABLE -->
'''Cross-Chapter Box 7, Table 1'''

<span id="comparison-of-land-based-carbon-removal-options"></span>
'''Comparison of land-based carbon removal options'''

Sources: <sup>A</sup> assessed ranges by Fuss et al. (2018), see Figures in Section 4.3.7 for full literature range; <sup>B</sup> based on the 2100 estimate for mean potentials by Smith et al. (2015). Note that biophysical impacts of land-based CDR options besides albedo changes (e.g., through changes in evapotranspiration related to irrigation or land cover/use type) are not displayed.

<!-- TABLE -->
{| class="wikitable"
|-
! Option
! Potentials <sup>B</sup>
! Cost <sup>A</sup>
! Required land <sup>B</sup>
! Required water <sup>B</sup>
! Impact on nutrients <sup>B</sup>
! Impact on<br />
albedo <sup>B</sup>
! Saturation<br />
and permanence <sup>A</sup>
|-
|
| ''GtCO <sub>2</sub> y <sup>−1</sup>''
| ''$ tCO <sub>2</sub> <sup>−1</sup>''
| ''Mha GtCO <sub>2</sub> <sup>−1</sup>''
| ''km <sup>3</sup> GtCO <sub>2</sub> <sup>−1</sup>''
| ''Mt N, P, K y <sup>−1</sup>''
| ''No units''
|-
| BECCS
| 0.5–5
| 100–200
| 31–58
| 60
| Variable
| Variable; depends on source of biofuel (higher albedo for crops than for forests) and on land management (e.g., no-till farming for crops)
| Long-term governance of storage; limits on rates of bioenergy production and carbon sequestration
|-
| Afforestation<br />
&amp; reforestation
| 0.5–3.6
| 5–50
| 80
| 92
| 0.5
| Negative, or reduced GHG benefit where not negative
| Saturation of forests; vulnerable to disturbance; post-AR forest management essential
|-
| Enhanced<br />
weathering
| 2–4
| 50–200
| 3
| 0.4
| 0
| Saturation of soil; residence time from months to geological timescale
|-
| Biochar
| 0.3–2
| 30–120
| 16–100
| 0
| N: 8.2,
P: 2.7,

K: 19.1

| 0.08–0.12
| Mean residence times between decades to centuries, depending on soil type, management and environmental conditions
|-
| Soil carbon<br />
sequestration
| 2.3–5
| 0–100
| 0
| N: 21.8,
P: 5.5,

K: 4.1

| 0
| Soil sinks saturate and can reverse if poor management practices resume
|}
<!-- END TABLE -->

<span id="implications-beyond-the-end-of-the-century"></span>