Editing IPCC:AR6/SRCCL/Chapter-4 (section)

=== 4.5.3 Potential contributions of land-based CDR to reducing and reversing land degradation ===

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Although large-scale implementation of land-based CDR has significant potential risks, the need for negative emissions and the anticipated investments to implement such technologies can also create significant opportunities. Investments into land-based CDR can contribute to halting and reversing land degradation, to the restoration or rehabilitation of degraded and marginal lands (Chazdon and Uriarte 2016 <sup>[[#fn:r703|703]]</sup> ; Fritsche et al. 2017 <sup>[[#fn:r704|704]]</sup> ) and can contribute to the goals of LDN (Orr et al. 2017 <sup>[[#fn:r705|705]]</sup> ).

Estimates of the global area of degraded land range from less than 10 to 60 Mkm2 (Gibbs and Salmon 2015 <sup>[[#fn:r706|706]]</sup> ) (Section 4.3.1). Additionally, large areas are classified as marginal lands and may be suitable for the implementation of bioenergy and land-based CDR (Woods et al. 2015 <sup>[[#fn:r707|707]]</sup> ). The yield per hectare of marginal and degraded lands is lower than on fertile lands, and if CDR will be implemented on marginal and degraded lands, this will increase the area demand and costs per unit area of achieving negative emissions (Fritsche et al. 2017 <sup>[[#fn:r708|708]]</sup> ). The selection of lands suitable for CDR must be considered carefully to reduce conflicts with existing users, to assess the possible trade-offs in biodiversity contributions of the original and the CDR land uses, to quantify the impacts on water budgets, and to ensure sustainability of the CDR land use.

Land use and land condition prior to the implementation of CDR affect climate change benefits (Harper et al. 2018 <sup>[[#fn:r709|709]]</sup> ). Afforestation/ reforestation on degraded lands can increase carbon stocks in vegetation and soil, increase carbon sinks (Amichev et al. 2012 <sup>[[#fn:r710|710]]</sup> ), and deliver co-benefits for biodiversity and ecosystem services, particularly if a diversity of local species are used. Afforestation and reforestation on native grasslands can reduce soil carbon stocks, although the loss is typically more than compensated by increases in biomass and dead organic matter carbon stocks (Bárcena et al. 2014 <sup>[[#fn:r711|711]]</sup> ; Li et al. 2012 <sup>[[#fn:r712|712]]</sup> ; Ovalle-Rivera et al. 2015 <sup>[[#fn:r713|713]]</sup> ; Shi et al. 2013 <sup>[[#fn:r714|714]]</sup> ), and may impact on biodiversity (Li et al. 2012 <sup>[[#fn:r715|715]]</sup> ).

Strategic incorporation of energy crops into agricultural production systems, applying an integrated landscape management approach, can provide co-benefits for management of land degradation and other environmental objectives. For example, buffers of Miscanthus and other grasses can enhance soil carbon and reduce water pollution (Cacho et al. 2018 <sup>[[#fn:r716|716]]</sup> ; Odgaard et al. 2019 <sup>[[#fn:r717|717]]</sup> ), and strip-planting of short-rotation tree crops can reduce the water table where crops are affected by dryland salinity (Robinson et al. 2006 <sup>[[#fn:r718|718]]</sup> ). Shifting to perennial grain crops has the potential to combine food production with carbon sequestration at a higher rate than annual grain crops and avoid the trade-off between food production and climate change mitigation (Crews et al. 2018 <sup>[[#fn:r719|719]]</sup> ; de Olivera et al. 2018 <sup>[[#fn:r720|720]]</sup> ; Ryan et al. 2018 <sup>[[#fn:r721|721]]</sup> ) (Section 4.9.2).

Changes in land cover can affect surface reflectance, water balances and emissions of volatile organic compounds and thus the non-GHG impacts on the climate system from afforestation/reforestation or planting energy crops (Anderson et al. 2011 <sup>[[#fn:r722|722]]</sup> ; Bala et al. 2007 <sup>[[#fn:r723|723]]</sup> ; Betts 2000 <sup>[[#fn:r724|724]]</sup> ; Betts et al. 2007 <sup>[[#fn:r725|725]]</sup> ) (see Section 4.6 for further details). Some of these impacts reinforce the GHG mitigation benefits, while others offset the benefits, with strong local (slope, aspect) and regional (boreal vs. tropical biomes) differences in the outcomes (Li et al. 2015 <sup>[[#fn:r726|726]]</sup> ). Adverse effects on albedo from afforestation with evergreen conifers in boreal zones can be reduced through planting of broadleaf deciduous species (Astrup et al. 2018 <sup>[[#fn:r727|727]]</sup> ; Cai et al. 2011a <sup>[[#fn:r728|728]]</sup> ; Anderson et al. 2011 <sup>[[#fn:r729|729]]</sup> ).

Combining CDR technologies may prove synergistic. Two soil management techniques with an explicit focus on increasing the soil carbon content rather than promoting soil conservation more broadly have been suggested: addition of biochar to agricultural soils (Section 4.9.5) and addition of ground silicate minerals to soils in order to take up atmospheric CO <sub>2</sub> through chemical weathering (Taylor et al. 2017 <sup>[[#fn:r730|730]]</sup> ; Haque et al. 2019 <sup>[[#fn:r731|731]]</sup> ; Beerling 2017 <sup>[[#fn:r732|732]]</sup> ; Strefler et al. 2018 <sup>[[#fn:r733|733]]</sup> ). The addition of biochar is comparatively well understood and also field tested at large scale, see Section 4.9.5 for a comprehensive discussion. The addition of silicate minerals to soils is still highly uncertain in terms of its potential (from 95 GtCO <sub>2</sub> yr <sup>–1</sup> (Strefler et al. 2018) to only 2–4 GtCO <sub>2</sub> yr <sup>–1</sup> (Fuss et al. 2018 <sup>[[#fn:r734|734]]</sup> )) and costs (Schlesinger and Amundson 2018 <sup>[[#fn:r735|735]]</sup> ).

Effectively addressing land degradation through implementation of bioenergy and land-based CDR will require site-specific local knowledge, matching of species with the local land, water balance, nutrient and climatic conditions, ongoing monitoring and, where necessary, adaptation of land management to ensure sustainability under global change (Fritsche et al. 2017 <sup>[[#fn:r736|736]]</sup> ). Effective land governance mechanisms including integrated land-use planning, along with strong sustainability standards could support deployment of energy crops and afforestation/reforestation at appropriate scales and geographical contexts (Fritsche et al. 2017 <sup>[[#fn:r737|737]]</sup> ). Capacity-building and technology transfer through the international cooperation mechanisms of the Paris Agreement could support such efforts. Modelling to inform policy development is most useful when undertaken with close interaction between model developers and other stakeholders including policymakers to ensure that models account for real world constraints (Dooley and Kartha 2018 <sup>[[#fn:r738|738]]</sup> ).

International initiatives to restore lands, such as the Bonn Challenge (Verdone and Seidl 2017 <sup>[[#fn:r739|739]]</sup> ) and the New York Declaration on Forests (Chazdon et al. 2017 <sup>[[#fn:r740|740]]</sup> ), and interventions undertaken for LDN and implementation of NDCs (see Glossary) can contribute to NET objectives. Such synergies may increase the financial resources available to meet multiple objectives (Section 4.8.4).

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