Editing IPCC:AR6/WGII/Cross-Chapter-Paper-1 (section)

==== CCP1.2.1.3 Compounding and Cascading Effects ====

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All biodiversity hotspots are already impacted, to differing degrees, by human activities ( ''high confidence'' ) (Table CCP1.1, Figures CCP1.1, CCP1.2, [[#Myers--2000|Myers et al., 2000]] ; [[#Le%20Roux--2019|Le Roux et al., 2019]] ). At present, over three billion people live within terrestrial and (catchments of) freshwater biodiversity hotspots, many of which border marine hotspots (Figures CCP1.1; CCP1.2; Table SMCCP1.1; Gutiérrez et al., 2021). Thus, climate change impacts on biodiversity hotspots are compounded by other anthropogenic impacts, increasing the vulnerability and reducing the resilience of biodiversity to climate change ( ''very high confidence'' ). Projections of changing climate alone may overestimate or underestimate the impacts on biodiversity ( ''medium evidence, high agreement'' ). The additional risk of the combined effects of climate change and other impacts (e.g., land use change, overhunting, pollution and invasive species) on species has been raised since the Third Assessment Report. The terrestrial hotspots projected to be most affected by global warming are, in general, those already being impacted by loss of habitat due to land use change (Figure CCP1.4; Table SMCCP1.1) ( [[#Warren--2018a|Warren et al., 2018a]] ). This remains a trend in the recent literature, although most studies still address only one stressor ( [[#Titeux--2016|Titeux et al., 2016]] ). For example, [[#Mantyka-Pringle--2015|Mantyka-Pringle et al. (2015)]] show that when the interaction between projected climate change and habitat loss is taken into account, the extinction risk of birds and mammals in 15–32% of terrestrial biodiversity hotspots changes. Similarly, [[#Bellard--2014b|Bellard et al. (2014b)]] found different results when examining the impact of climate change, invasive species and land use change independently, as opposed to synergistically. When combining those three impacts they identified the Atlantic Forest (H47), Cape Floristic Region (H65) and Polynesia-Micronesia (H1, 2, 138, 139, 142) as particularly vulnerable.

In a global assessment of the threat of climate change to river fish biodiversity, [[#Tedesco--2013|Tedesco et al. (2013)]] projected that current extinction rates of species may be 7% greater due to climate change. The main threat is due to the effects of drought and reduced river flows, which would be 18 times greater than without climate change. However, just 20 of the 110 river basins studied would experience sufficient climate-driven water loss to cause fish extinctions by 2090. Moreover, the present rates of species loss due to human activities are 130 times greater than those projected under future climate change ( ''medium confidence'' ) ( [[#Tedesco--2013|Tedesco et al., 2013]] ).

Marine systems are also vulnerable to cumulative human impacts, which can be direct (e.g., pollution, overfishing) and indirect (altered food webs) ( ''very high confidence'' ) ( [[#Halpern--2008|Halpern et al., 2008]] ; [[#Halpern--2015|Halpern et al., 2015]] ). The marine hotspots most currently threatened by non-climate-related human impacts are all situated in the Northern Hemisphere, specifically along the northern European, Mediterranean and Asian coasts, where the overlap of overfishing and pollution is especially large (Figure CCP1.2 b; [[#Halpern--2008|Halpern et al., 2008]] ; [[#Halpern--2015|Halpern et al., 2015]] ; [[#Ramírez--2018|Ramírez et al., 2018]] ). Although there is a strong overlap of non-climatic and climatic impacts in marine ecosystems ( [[#Blowes--2019|Blowes et al., 2019]] ; [[#Bowler--2020|Bowler et al., 2020]] ), the effects suggest that climate change impacts are most severe in tropical and northern high-latitude seas ( ''high confidence'' ) ( [[#Doney--2012|Doney et al., 2012]] ; [[#Gattuso--2015|Gattuso et al., 2015]] ; [[#Cheung--2018|Cheung et al., 2018]] ; [[#IPCC--2019b|IPCC, 2019b]] ). Temperature-driven range shifts and range expansions are projected to also lead to cascading effects on marine biodiversity through ecological interactions ( ''high confidence'' ) ( [[#Pecl--2017|Pecl et al., 2017]] ; [[#Vergés--2019|Vergés et al., 2019]] ). Cascading effects may be especially pronounced in temperate reefs, where tropicalisation could lead to the arrival of herbivorous fish and predators previously absent ( [[#Vergés--2019|Vergés et al., 2019]] ). However, how these indirect effects of climate change on species may change food webs and ecosystem function, including carbon sequestration, is unknown. Direct and indirect human impacts due to fisheries and pollution can also lead to cascading effects that may be additive to climate impacts on biodiversity. Destruction of marine biogenic habitats due to trawling and dredging and loss of large proportions of marine megafauna, particularly fish, mammals, birds and reptiles, alter food webs and reduce resilience to additional disturbances, such as those caused by climate change ( ''medium evidence, high agreement'' ) ( [[#Brander--2007|Brander, 2007]] ; [[#Wernberg--2011|Wernberg et al., 2011]] ; [[#Ramírez--2017|Ramírez et al., 2017]] ; [[#Cheung--2018|Cheung et al., 2018]] ; [[#Bates--2019|Bates et al., 2019]] ; [[#Costello--2021|Costello, 2021]] ).

The following sections report observed and projected climate change impacts on terrestrial, freshwater and marine environments.

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