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

== CCP1.1 Point of Departure ==

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Biodiversity hotspots are geographic areas with an exceptionally high richness of species, including rare and endemic species. Such hotspots have deep evolutionary roots and are concentrated in areas where past climatic variability was moderate ( [[#Enquist--2019|Enquist et al., 2019]] ; [[#Brown--2020|Brown et al., 2020]] ; [[#Trew--2021|Trew and Maclean, 2021]] ). An important limitation of the biodiversity hotspot concept is that there may be species highly threatened with extinction that do not occur within what has traditionally been classified as a hotspot ( [[#Grenyer--2006|Grenyer et al., 2006]] ). Thus, biodiversity hotspot assessments need to be paralleled by assessments of highly endangered species, and the threats they face.

Many studies have proposed biodiversity hotspots based on different criteria, taxa and geographic contexts (e.g., [[#Myers--2000|Myers et al., 2000]] ; [[#Mittermeier--2004|Mittermeier et al., 2004]] ; [[#Mittermeier--2011|Mittermeier et al., 2011]] ; [[#Williams--2011|Williams et al., 2011]] ; [[#Noss--2015|Noss et al., 2015]] ; [[#Asaad--2017|Asaad et al., 2017]] ). A coherent comparative assessment of all such schemes is beyond the scope of this chapter but is provided in recent reviews ( [[#Asaad--2017|Asaad et al., 2017]] ; Jefferson and [[#Costello--2019|Costello, 2019]] ). We base this assessment on the ‘WWF Global 200’ areas of conservation importance ( [[#Olson--2002|Olson and Dinerstein, 2002]] ). These 238 ecoregions have been used in a previous climate risk assessment ( [[#Warren--2018b|Warren et al., 2018b]] ) and cover terrestrial, freshwater and marine environments (Table CCP1.1; Figures CCP1.1; CCP1.2). In addition, we included terrestrial ‘biodiversity hotspots’ as defined by [[#Myers--2000|Myers et al. (2000)]] and extended later ( [[#Mittermeier--2011|Mittermeier et al., 2011]] ; [[#Williams--2011|Williams et al., 2011]] ; [[#Noss--2015|Noss et al., 2015]] ). This assessment thus covers the ‘Global 200’ (hereafter G200) and ‘Myers’ biodiversity hotspots, rather than particular species or ecological systems, such as rainforests, coral reefs or the deep sea. Such systems are assessed in Chapters 2 and 3. Chapters 2 and 3 also cover observed and projected impacts, changes in ecosystem functioning, and species extinction risks at a global level.

Biodiversity hotspots were not explicitly covered in the Working Group II (WGII) Fifth Assessment Report ( [[#IPCC--2014|IPCC, 2014]] ) (hereafter AR5). Thus, the point of departure is WGII AR4 ( [[#IPCC--2007|IPCC, 2007]] ), which assessed that climate change exacerbates biodiversity risks in hotspots and that 15–40% of endemic species (species only occurring in one region) were projected to become extinct at 3.5°C global warming ( [[#Fischlin--2007|Fischlin et al., 2007]] ). Risks of extinction are assessed using the guidelines in Chapter 2, with species projected to lose 80% of their range or abundance being classified as at very high risk of extinction and those projected to lose 50% being at high risk of extinction (Figure 2.8 for definitions and a global overview).

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