Editing IPCC:AR6/SR15/Chapter-4 (section)

==== 4.3.7.2 Afforestation and reforestation (AR) ====

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Afforestation implies planting trees on land not forested for a long time (e.g., over the last 50 years in the context of the Kyoto Protocol), while reforestation implies re-establishment of forest formations after a temporary condition with less than 10% canopy cover due to human-induced or natural perturbations. Houghton et al. (2015) <sup>[[#fn:r626|626]]</sup> estimate about 500 Mha could be available for the re-establishment of forests on lands previously forested, but not currently used productively. This could sequester at least 3.7 GtCO <sub>2</sub> yr <sup>−1</sup> for decades. The full literature range gives 2050 potentials of 1–7 GtCO <sub>2</sub> yr <sup>−1</sup> ( ''low evidence, medium agreement'' ), narrowed down to 0.5–3.6 GtCO <sub>2</sub> yr <sup>−1</sup> based on a number of constraints (Fuss et al., 2018) <sup>[[#fn:r627|627]]</sup> . Abatement costs are estimated to be low compared to other CDR options, 5–50 USD tCO <sub>2</sub> -eq <sup>−1</sup> ( ''robust evidence, high agreement'' ). Yet, realizing such large potentials comes at higher land and water footprints than BECCS, although there would be a positive impact on nutrients and the energy requirement would be negligible (Smith et al., 2016b <sup>[[#fn:r628|628]]</sup> ; Cross-Chapter Box 7 in Chapter 3). The 2030 estimate by Griscom et al. (2017) <sup>[[#fn:r629|629]]</sup> is up to 17.9 GtCO <sub>2</sub> yr <sup>−1</sup> for reforestation with significant co-benefits (Cross-Chapter Box 7 in Chapter 3).

Biogenic storage is not as permanent as emission reductions by geological storage. In addition, forest sinks saturate, a process which typically occurs in decades to centuries compared to the thousands of years of residence time of CO <sub>2</sub> stored geologically (Smith et al., 2016a) <sup>[[#fn:r630|630]]</sup> and is subject to disturbances that can be exacerbated by climate change (e.g., drought, forest fires and pests) (Seidl et al., 2017) <sup>[[#fn:r631|631]]</sup> . Handling these challenges requires careful forest management. There is much practical experience with AR, facilitating upscaling but with two caveats: AR potentials are heterogeneously distributed (Bala et al., 2007) <sup>[[#fn:r632|632]]</sup> , partly because the planting of less reflective forests results in higher net absorbed radiation and localised surface warming in higher latitudes (Bright et al., 2015; Jones et al., 2015) <sup>[[#fn:r633|633]]</sup> , and forest governance structures and monitoring capacities can be bottlenecks and are usually not considered in models (Wang et al., 2016; Wehkamp et al., 2018b) <sup>[[#fn:r634|634]]</sup> . There is ''medium agreement'' on the positive impacts of AR on ecosystems and biodiversity due to different forms of afforestation discussed in the literature: afforestation of grassland ecosystems or diversified agricultural landscapes with monocultures or invasive alien species can have significant negative impacts on biodiversity, water resources, etc. (P. Smith et al., 2014) <sup>[[#fn:r635|635]]</sup> , while forest ecosystem restoration (forestry and agroforestry) with native species can have positive social and environmental impacts (Cunningham et al., 2015; Locatelli et al., 2015; Paul et al., 2016 <sup>[[#fn:r636|636]]</sup> ; See Section 4.3.2).

Synergies with other policy goals are possible (see also Section 4.5.4); for example, land spared by diet shifts could be afforested (Röös et al., 2017) <sup>[[#fn:r637|637]]</sup> or used for energy crops (Grubler et al., 2018) <sup>[[#fn:r638|638]]</sup> . Such land-sparing strategies could also benefit other land-based CDR options.

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