Editing IPCC:AR6/WGIII/Chapter-7 (section)

==== 7.4.2.2 Afforestation, Reforestation and Forest Ecosystem Restoration ====

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'''Activities, co-benefits, risks and implementation opportunities and barriers.''' Afforestation and reforestation (A/R) are activities that convert land to forest, where reforestation is on land that has previously contained forests, while afforestation is on land that historically has not been forested (Box 7.2). Forest restoration refers to a form of reforestation that gives more priority to ecological integrity as well, even though it can still be a managed forest. Depending on the location, scale, and choice and management of tree species, A/R activities have a wide variety of co-benefits and trade-offs. Well-planned, sustainable reforestation and forest restoration can enhance climate resilience and biodiversity, and provide a variety of ecosystem services including water regulation, microclimatic regulation, soil erosion protection, as well as renewable resources, income and livelihoods ( [[#Locatelli--2015|Locatelli et al. 2015]] ; [[#Stanturf--2015|Stanturf et al. 2015]] ; [[#Ellison--2017|Ellison et al. 2017]] ; [[#Verkerk--2020|Verkerk et al. 2020]] ). Afforestation, when well planned, can help address land degradation and desertification by reducing runoff and erosion and lead to cloud formation however, when not well planned, there are localised trade-offs such as reduced water yield or biodiversity ( [[#Teuling--2017|Teuling et al. 2017]] ; [[#Ellison--2017|Ellison et al. 2017]] ). The use of non-native species and monocultures may have adverse impacts on ecosystem structure and function, and water availability, particularly in dry regions ( [[#Ellison--2017|Ellison et al. 2017]] ). A/R activities may change the surface albedo and evapotranspiration regimes, producing net cooling in the tropical and subtropical latitudes for local and global climate and net warming at high latitudes ( [[#7.4.2|Section 7.4.2]] ). Very large-scale implementation of A/R may negatively affect food security since an increase in global forest area can increase food prices through land competition ( [[#Kreidenweis--2016|Kreidenweis et al. 2016]] ).

'''Conclusions from AR5 and IPCC Special Reports (SR1.5, SROCC and SRCCL); mitigation potential, costs, and pathways.''' The AR5 did not provide a new specification of A/R potential, but referred to IPCC AR4 mostly for forestry measures ( [[#Nabuurs--2007|Nabuurs et al. 2007]] ). The AR5 did view the feasible A/R potential from a diets change scenario that released land for reforestation and bioenergy crops. The AR5 provided top-down estimates of costs and potentials for forestry mitigation options – including reduced deforestation, forest management, afforestation, and agroforestry, estimated to contribute between 1.27 and 4.23 GtCO 2 yr –1 of economically viable abatement in 2030 at carbon prices up to USD100 tCO 2 -eq –1 (Smith et al. 2014).

The SRCCL remained with a reported wide range of mitigation potential for A/R of 0.5–10.1 GtCO 2 yr –1 by 2050 ( ''medium confidence'' ) ( [[#Kreidenweis--2016|Kreidenweis et al. 2016]] ; [[#Griscom--2017|Griscom et al. 2017]] ; [[#Hawken--2017|Hawken 2017]] ; [[#Fuss--2018|Fuss et al. 2018]] ; [[#Roe--2019|Roe et al. 2019]] ) (SRCCL Chapters 2 and 6). The higher estimate represents a technical potential of reforesting all areas where forests are the native cover type (reforestation), constrained by food security and biodiversity considerations, considering above and below-ground carbon pools and implementation on a rather theoretical maximum of 678 Mha of land ( [[#Griscom--2017|Griscom et al. 2017]] ; [[#Roe--2019|Roe et al. 2019]] ). The lower estimates represent the minimum range from an Earth System Model and a sustainable global CDR potential ( [[#Fuss--2018|Fuss et al. 2018]] ). Climate change will affect the mitigation potential of reforestation due to impacts in forest growth and composition, as well as changes in disturbances including fire. However, none of the mitigation estimates included in the SRCCL account for climate impacts.

'''Developments since AR5 and IPCC Special Reports (SR1.5, SROCC and SRCCL).''' Since SRCCL, additional studies have been published on A/R mitigation potential by [[#Bastin--2019|Bastin et al. (2019)]] , [[#Lewis--2019|Lewis et al. (2019)]] , Doelman et al. (2019), [[#Favero--2020|Favero et al. (2020)]] and [[#Austin--2020|Austin et al. (2020)]] . These studies are within the range reported in the SRCCL stretching the potentials at the higher range. The rising public interest in nature-based solutions, along with high profile initiatives being launched (UN Decade on Restoration announced in 2019, the Bonn challenge on 150 million ha of restored forest in 2020 and the one trillion trees campaign launched by the World Economic Forum in 2020), has prompted intense discussions on the scale, effectiveness, and pitfalls of A/R and tree planting for climate mitigation ( [[#Luyssaert--2018|Luyssaert et al. 2018]] ; [[#Bond--2019|Bond et al. 2019]] ; [[#Anderegg--2020|Anderegg et al. 2020]] ; [[#Heilmayr--2020|Heilmayr et al. 2020]] ; [[#Holl--2020|Holl and Brancalion 2020]] ). The sometimes sole attention on afforestation and reforestation '''–''' suggesting it may solve the climate problem to large extent, in combination with the very high estimates of potentials '''–''' have led to polarisation in the debate, resulting in criticism to these measures or an emphasis on nature restoration only ( [[#Lewis--2019|Lewis et al. 2019]] ). Our assessment based on most recent literature produced regional economic mitigation potential at USD100 tCO 2 –1 estimate of 100–400 MtCO 2 yr –1 in Africa, 210–266 MtCO 2 yr –1 in Asia and Pacific, 291 MtCO 2 -eq yr –1 in Developed Countries (87% in North America), 30 MtCO 2 -eq yr –1 in Eastern Europe and West-Central Asia, and 345–898 MtCO 2 -eq yr –1 in Latin America and Caribbean ( [[#Roe--2021|Roe et al. 2021]] ), which totals to about 1200 MtCO 2 yr –1 , leaning to the lower range of the potentials in earlier IPCC reports. A recent global assessment of the aggregate costs for afforestation and reforestation suggests that at USD100 tCO 2 –1 , 1.6 GtCO 2 yr –1 could be sequestered globally for an annual cost of USD130 billion ( [[#Austin--2020|Austin et al. 2020]] ). Sectoral studies that are able to deal with local circumstances and limits estimate A/R potentials at 20 MtCO 2 yr –1 in Russia (Eastern Europe and West-Central Asia) ( [[#Romanovskaya--2020|Romanovskaya et al. 2020]] ) and 64 MtCO 2 yr –1 in Europe ( [[#Nabuurs--2017|Nabuurs et al. 2017]] ). ( [[#Domke--2020|Domke et al. 2020]] ) estimated for the USA an additional 20% sequestration rate from tree planting to achieve full stocking capacity of all understocked productive forestland, in total reaching 187 MtCO 2 yr –1 sequestration. A new study on costs in the USA estimates 72–91 MtCO 2 yr –1 could be sequestered between now and 2050 for USD100 tCO 2 –1 (Wade et al. 2019). The tropical and subtropical latitudes are the most effective for forest restoration in terms of carbon sequestration because of the rapid growth and lower albedo of the land surface compared with high latitudes ( [[#Lewis--2019|Lewis et al. 2019]] ). Costs may be higher if albedo is considered in North America, Russia, and Africa ( [[#Favero--2017|Favero et al. 2017]] ). In addition, a wide variety of sequestration rates have been collected and published in the IPCC Good Practice Guidance for the AFOLU sector ( [[#IPCC--2006|IPCC 2006]] ).

'''Critical assessment and conclusion.''' There is ''medium confidence'' that the global technical mitigation potential of afforestation and reforestation activities by 2050 is 3.9 (0.5–10.1) GtCO 2 yr –1 , and the economic mitigation potential (&lt;USD100 tCO 2 –1 ) is 1.6 (0.5–3.0) GtCO 2 yr –1 (requiring about 200 Mha). Per hectare a long (about 100 year) sustained effect of 5–10 t CO 2 ha –1 yr –1 is realistic with ranges between 1–20 t(CO 2 ) ha –1 yr –1 . Not all sectoral studies rely on economic models that account for leakage ( [[#Murray--2004|Murray et al. 2004]] ; Sohngen and Brown 2004), suggesting that technical potential may be overestimated.

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