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

== Cross-Chapter Box 7: Land-Based Carbon Dioxide Removal in Relation to1.5°C of Global Warming ==

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====== 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)

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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'' ).

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<!-- 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 -->

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=== 3.6.3 Implications Beyond the End of the Century ===

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==== 3.6.3.1 Sea ice ====

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Sea ice is often cited as a tipping point in the climate system (Lenton, 2012) <sup>[[#fn:r1356|1356]]</sup> . Detailed modelling of sea ice (Schröder and Connolley, 2007; Sedláček et al., 2011; Tietsche et al., 2011) <sup>[[#fn:r1357|1357]]</sup> , however, suggests that summer sea ice can return within a few years after its artificial removal for climates in the late 20th and early 21st centuries. Further studies (Armour et al., 2011; Boucher et al., 2012; Ridley et al., 2012) <sup>[[#fn:r1358|1358]]</sup> modelled the removal of sea ice by raising CO <sub>2</sub> concentrations and studied subsequent regrowth by lowering CO <sub>2</sub> . These studies suggest that changes in Arctic sea ice are neither irreversible nor exhibit bifurcation behaviour. It is therefore plausible that the extent of Arctic sea ice may quickly re-equilibrate to the end-of-century climate under an overshoot scenario.

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==== 3.6.3.2 Sea level ====

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Policy decisions related to anthropogenic climate change will have a profound impact on sea level, not only for the remainder of this century but for many millennia to come (Clark et al., 2016) <sup>[[#fn:r1359|1359]]</sup> . On these long time scales, 50 m of sea level rise (SLR) is possible (Clark et al., 2016) <sup>[[#fn:r1360|1360]]</sup> . While it is ''virtually certain'' that sea level will continue to rise well beyond 2100, the amount of rise depends on future cumulative emissions (Church et al., 2013) <sup>[[#fn:r1361|1361]]</sup> as well as their profile over time (Bouttes et al., 2013; Mengel et al., 2018) <sup>[[#fn:r1362|1362]]</sup> . Marzeion et al. (2018) <sup>[[#fn:r1363|1363]]</sup> found that 28–44% of present-day glacier volume is unsustainable in the present-day climate and that it would eventually melt over the course of a few centuries, even if there were no further climate change. Some components of SLR, such as thermal expansion, are only considered reversible on centennial time scales (Bouttes et al., 2013; Zickfeld et al., 2013) <sup>[[#fn:r1364|1364]]</sup> , while the contribution from ice sheets may not be reversible under any plausible future scenario (see below).

Based on the sensitivities summarized by Levermann et al. (2013) <sup>[[#fn:r1365|1365]]</sup> , the contributions of thermal expansion (0.20–0.63 m °C <sup>–1</sup> ) and glaciers (0.21 m °C <sup>–1</sup> but falling at higher degrees of warming mostly because of the depletion of glacier mass, with a possible total loss of about 0.6 m) amount to 0.5–1.2 m and 0.6–1.7 m in 1.5°C and 2°C warmer worlds, respectively. The bulk of SLR on greater than centennial time scales will therefore be caused by contributions from the continental ice sheets of Greenland and Antarctica, whose existence is threatened on multi-millennial time scales.

For Greenland, where melting from the ice sheet’s surface is important, a well-documented instability exists where the surface of a thinning ice sheet encounters progressively warmer air temperatures that further promote melting and thinning. A useful indicator associated with this instability is the threshold at which annual mass loss from the ice sheet by surface melt exceeds mass gain by snowfall. Previous estimates put this threshold at about 1.9°C to 5.1°C above pre-industrial temperatures (Gregory and Huybrechts, 2006) <sup>[[#fn:r1366|1366]]</sup> . More recent analyses, however, suggest that this threshold sits between 0.8°C and 3.2°C, with a best estimate at 1.6°C (Robinson et al., 2012) <sup>[[#fn:r1367|1367]]</sup> . The continued decline of the ice sheet after this threshold has been passed is highly dependent on the future climate and varies between about 80% loss after 10,000 years to complete loss after as little as 2000 years (contributing about 6 m to SLR). Church et al. (2013) <sup>[[#fn:r1368|1368]]</sup> were unable to quantify a ''likely'' range for this threshold. They assigned ''medium confidence'' to a range greater than 2°C but less than 4°C, and had ''low confidence'' in a threshold of about 1°C. There is insufficient new literature to change this assessment.

The Antarctic ice sheet, in contrast, loses the mass gained by snowfall as outflow and subsequent melt to the ocean, either directly from the underside of floating ice shelves or indirectly by the melting of calved icebergs. The long-term existence of this ice sheet will also be affected by a potential instability (the marine ice sheet instability, MISI), which links outflow (or mass loss) from the ice sheet to water depth at the grounding line (i.e., the point at which grounded ice starts to float and becomes an ice shelf) so that retreat into deeper water (the bedrock underlying much of Antarctica slopes downwards towards the centre of the ice sheet) leads to further increases in outflow and promotes yet further retreat (Schoof, 2007) <sup>[[#fn:r1369|1369]]</sup> . More recently, a variant on this mechanism was postulated in which an ice cliff forms at the grounding line and retreats rapidly though fracture and iceberg calving (DeConto and Pollard, 2016) <sup>[[#fn:r1370|1370]]</sup> . There is a growing body of evidence (Golledge et al., 2015; DeConto and Pollard, 2016) <sup>[[#fn:r1371|1371]]</sup> that large-scale retreat may be avoided in emissions scenarios such as Representative Concentration Pathway (RCP)2.6 but that higher-emissions RCP scenarios could lead to the loss of the West Antarctic ice sheet and sectors in East Antarctica, although the duration (centuries or millennia) and amount of mass loss during such a collapse is highly dependent on model details and no consensus exists yet. Schoof (2007) <sup>[[#fn:r1372|1372]]</sup> suggested that retreat may be irreversible, although a rigorous test has yet to be made. In this context, overshoot scenarios, especially of higher magnitude or longer duration, could increase the risk of such irreversible retreat.

Church et al. (2013) <sup>[[#fn:r1373|1373]]</sup> noted that the collapse of marine sectors of the Antarctic ice sheet could lead to a global mean sea level (GMSL) rise above the ''likely'' range, and that there was ''medium confidence'' that this additional contribution ‘would not exceed several tenths of a metre during the 21st century’.

The multi-centennial evolution of the Antarctic ice sheet has been considered in papers by DeConto and Pollard (2016) <sup>[[#fn:r1374|1374]]</sup> and Golledge et al. (2015) <sup>[[#fn:r1375|1375]]</sup> . Both suggest that RCP2.6 is the only RCP scenario leading to long-term contributions to GMSL of less than 1.0 m. The long-term committed future of Antarctica and the GMSL contribution at 2100 are complex and require further detailed process-based modelling; however, a threshold in this contribution may be located close to 1.5°C to 2°C of global warming.

In summary, there is ''medium confidence'' that a threshold in the long-term GMSL contribution of both the Greenland and Antarctic ice sheets lies around 1.5°C to 2°C of global warming relative to pre-industrial; however, the GMSL associated with these two levels of global warming cannot be differentiated on the basis of the existing literature.

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==== 3.6.3.3 Permafrost ====

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The slow rate of permafrost thaw introduces a lag between the transient degradation of near-surface permafrost and contemporary climate, so that the equilibrium response is expected to be 25–38% greater than the transient response simulated in climate models (Slater and Lawrence, 2013) <sup>[[#fn:r1376|1376]]</sup> . The long-term, equilibrium Arctic permafrost loss to global warming was analysed by Chadburn et al. (2017) <sup>[[#fn:r1377|1377]]</sup> . They used an empirical relation between recent mean annual air temperatures and the area underlain by permafrost coupled to Coupled Model Intercomparison Project Phase 5 (CMIP5) stabilization projections to 2300 for RCP2.6 and RCP4.5. Their estimate of the sensitivity of permafrost to warming is 2.9–5.0 million km <sup>2</sup> °C <sup>–1</sup> (1 standard deviation confidence interval), which suggests that stabilizing climate at 1.5°C as opposed to 2°C would reduce the area of eventual permafrost loss by 1.5 to 2.5 million km <sup>2</sup> (stabilizing at 56–83% as opposed to 43–72% of 1960–1990 levels). This work, combined with the assessment of Collins et al. (2013) <sup>[[#fn:r1378|1378]]</sup> on the link between global warming and permafrost loss, leads to the assessment that permafrost extent would be appreciably greater in a 1.5°C warmer world compared to in a 2°C warmer world ( ''low to medium confidence'' ).

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