Editing IPCC:AR6/WGII/Cross-Chapter-Paper-7 (section)

=== CCP7.3.6 Climate Responses to Tropical Deforestation and Links to Forest Resilience ===

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Since AR5, there has been meaningful advancement in understanding the climate effects of deforestation and concomitant changes in forest ecosystem resilience. The IPCC Special Report on Climate Change and Land (Jia et al., 2019) and IPCC AR6 WGI (Douville et al., 2021) both describe significant climate-related changes resulting from tropical deforestation ( ''high confidence'' ).

Deforestation generally reduces rainfall and enhances temperatures and landscape dryness; effects that increase with the scale of forest loss, whereas reforestation and afforestation generally reverses these effects ( ''high confidence'' ) ( [[#Lawrence--2015|Lawrence and Vandecar, 2015]] ; [[#Alkama--2016|Alkama and Cescatti, 2016]] ; Khanna et al., 2017; Jia et al., 2019; Staal et al., 2020; Douville et al., 2021; Hofmann et al., 2021; Leite-Filho et al., 2021). There is also ''medium evidence'' from observations and modelling that deforestation enhances surface runoff (Douville et al., 2021). Whereas quantitative information is much more limited for other tropical regions, past deforestation in the Amazon has led to a small reduction in rainfall of −2.3% to −1.3%, shortening and delay of the wet season, and an estimated 4% increase in dryness (Leite-Filho et al., 2020; Staal et al., 2020; Douville et al., 2021).

Modelling studies estimate that large-scale tropical deforestation will contribute to average warming of the deforested areas with +0.61 ± 0.48°C and will lead to large changes in diurnal temperature ranges owing to a reduction of nocturnal cooling ( ''medium confidence'' ) (Jia et al., 2019). Large-scale deforestation will also strongly decrease average regional precipitation and evapotranspiration and further delay the onset of the wet season, enhancing the chance of dry spells and intensifying dry seasons, but the magnitude of the decline depends on the scale and type of land-cover change ( ''high confidence'' ) (Zemp et al., 2017; Jia et al., 2019; Douville et al., 2021; Gatti et al., 2021).

Continued forest landscape drying and fragmentation in connection with deforestation may also enhance surface flow variability (Farinosi et al., 2019; Souza et al., 2019) and will aggravate the risk of forest dieback (Zemp et al., 2017), elevate forest flammability (Alencar et al., 2015) and increase fire incidence ( ''high confidence'' ) (Aragão et al., 2018; Jia et al., 2019; Silveira et al., 2020; dos Reis et al., 2021), ultimately leading to savannisation of many tropical rainforests (Sales et al., 2020). However, compositional heterogeneity and diversity of forest assemblages increases resilience against climate-enhanced forest degradation (Réjou-Méchain et al., 2021).

For the Amazon, deforestation (ca. 40% of the region) in combination with climate change will raise the prospect of passing a tipping point leading to large-scale savannisation of the rainforest biome, but uncertainty remains whether this will take place in the 21st century (Nobre et al., 2016; Jia et al., 2019; Douville et al., 2021). However, considering that the Amazon has already lost ca. 20% of its forests (Nobre et al., 2016), crossing the tipping point may not only create savannas of the deforested parts but may also result in precipitation reductions of 40% in non-deforested parts of the western Amazon due to a breakdown of the South American monsoonal circulation and the subsequent western cascade of precipitation and evapotranspiration (Boers et al., 2017). Other effects of forest degradation include loss of ecosystem services, biodiversity, carbon storage and Indigenous culture (Watson et al., 2018; Strassburg et al., 2019; Gatti et al., 2021), as well as potentially reduced hydropower capacity and agricultural production (Sumila et al., 2017), and increases in tropical diseases (Husnina et al., 2019).

The dearth of data for tropical forest regions other than the Amazon makes assessments of deforestation-related changes in temperature, precipitation and streamflow difficult ( ''high confidence'' ), and hampers estimates of tropical forest ecosystem health, biodiversity loss and vulnerability to current and future climatic and other pressures ( ''high confidence'' ). There is, hence, a strong need for increased investment in relevant data and research to narrow the knowledge gaps (Davison et al., 2021). Nonetheless, conclusions based on a newly developed tropical vulnerability index synthesising remotely sensed land use and climate information indicate that forests in the Americas are already reaching critical levels to multiple stressors, while forests in Asia reveal vulnerability primarily to land-use change and African forests still show relative resilience to climate change (Saatchi et al., 2021).

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