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

=== CCP7.3.5 Projected Impacts of Climate Change on Tropical Forest ===

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Climate change projections indicate increased warming and changes in rainfall patterns in the tropical region as elsewhere globally (IPCC, 2021, AR6 WGI). These would have impacts on carbon stocks ( [[#Mitchard--2018|Mitchard, 2018]] ; Hubau et al., 2020), water availability (Tamoffo et al., 2019), and structure and diversity (Malhi et al., 2014; McDowell et al., 2020) in tropical forests, amplified by deforestation (CCP7.3.6).

Tropical forests are critical repositories of global carbon; living tropical trees are estimated to hold 200–300 Pg C or about one-third of the levels in the atmosphere ( [[#Mitchard--2018|Mitchard, 2018]] ). CMIP5 and CMIP6 Earth System Models (ESM) project an increasing future tropical carbon sink, which is particularly strong in the scenarios with more pronounced increases in atmospheric CO 2 concentration (Koch et al., 2021). However, major uncertainties regarding the ecophysiological processes governing carbon turnover and tree mortality under a changing climate ( [[#Hartmann--2015|Hartmann et al., 2015]] ; [[#Pugh--2020|Pugh et al., 2020]] ), and the ecosystem-level responses of tropical forests to elevated atmospheric CO 2 ( [[#Körner--2009|Körner, 2009]] ) explain the contrast between observational data and modelling results ( [[#Rammig--2021|Rammig and Lapola, 2021]] ). Observational data show that structurally intact old-growth tropical forests have been net sinks of atmospheric carbon in recent decades, but there is evidence that the capacity of such intact tropical forests to build up carbon stock may be limited as biomass peaked during the 1990s and has since weakened by 30% in the Amazon since the 1990s ( ''high confidence'' ), mainly due to increased tree mortality and faster carbon turnover, and the African tropical forest sink following this trend since about 2010 ( [[#Hubau--2020|Hubau et al., 2020]] ; [[#Gatti--2021|Gatti et al., 2021]] ). From a peak pan-tropical (Amazonia, Africa and Southeast Asia) forest sink of 1.26 Pg C yr −1 during the 1990s, it is projected to decline to an uptake of only 0.29 Pg C yr −1 , reaching zero in the Amazon, during the 2030s ( [[#Hubau--2020|Hubau et al., 2020]] ). This decline will possibly be driven by the reduced rates of forest carbon uptake from the weakening global CO 2 fertilisation effect mediated by limiting soil nutrient, and reduced water availability and higher temperatures during extreme droughts ( [[#Qie--2017|Qie et al., 2017]] ; [[#Fleischer--2019|Fleischer et al., 2019]] ; [[#Wang--2020|Wang et al., 2020]] ), reinforced by deforestation and forest degradation [IPCC SRCCL, 2019].

Offline (uncoupled) vegetation model simulations indicate that the extensive tropical and subtropical forests of the Americas could gradually transit towards a savanna-like vegetation, with the most pronounced shifts (of up to 600 km northward) from relatively stable forests to savanna-forest transitions occurring in the eastern Amazonian region (Huntingford et al., 2013; Anadon et al., 2014; Nobre et al., 2016) depending largely on the yet uncertain strength of the CO 2 fertilisation effect and future dry season length, with important feedbacks on the flux of moisture from the forest to the atmosphere (Zemp et al., 2017). More limited simulations for Central American rainforests under RCP 4.5 and 8.5 also support a transition in some areas to lower biomass tropical dry forest and savanna-like vegetation (Lyra et al., 2017). Such transitions from one biome type to another will cause major changes in forest structure, species compositions and overall biodiversity. Additionally, the difficulty of species to migrate through highly fragmented tropical forested regions (such as West Africa or South and Southeast Asia) and ‘non-analogue climates’, under a climate change scenario, poses extra pressure on tropical biodiversity to adapt and survive (Pörtner et al., 2021). Even in expansive tracts of forests, such as in the Amazon, climate change is expected to become more important than deforestation by 2050 in causing the loss of tree species (Gomes et al., 2019). Tropical mountain biodiversity hotspots (e.g., Andes, Himalayas) are particularly vulnerable to species loss due to elevation range shifts (Sekercioglu et al., 2008). Under a 2°C increase scenario, a substantial reduction of tropical montane cloud forest in Kenya is estimated (Los et al., 2019).

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