Editing IPCC:AR6/WGI/Chapter-12 (section)

==== 12.4.10.1 Hotspots of Biodiversity (Land, Coasts and Oceans) ====

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Hotspots of biodiversity are defined by the AR6 WGII as ‘geographic areas with exceptionally high richness of species, including rare (endemic) species’ (WGII Cross-Chapter Paper 1). The AR6 assessment is based on 238 distinctive regions often called the ‘Global 200 ecoregions’ ( [[#Olson--2002|Olson and Dinerstein, 2002]] ).

Mean temperature increase is a major climatic impact-driver for biodiversity hotspots, and it is ''very likely'' that it will affect all hotspot areas identified in the literature, at various rates in all climate scenarios, except those located in the North Atlantic where warming is uncertain (see Chapter 4). Terrestrial ecosystems will experience an enhanced warming compared to ocean ecosystems, because land temperatures are warming faster than ocean temperatures (Chapter 4). Marine ecoregions will experience ocean acidification and temperatures that increase faster in high latitudes ( ''high confidence'' ), but critical temperature and oxygen thresholds are projected to be crossed earlier (by mid-century RCP8.5) in tropical areas ( [[#Hughes--2017a|Hughes et al., 2017a]] ; [[#Bruno--2018|Bruno et al., 2018]] ). A warming trend is also expected for freshwater ecosystems, with different local magnitudes due to combined effects of groundwater system inertia as well as hydrology changes ( [[#Knouft--2017|Knouft and Ficklin, 2017]] ). In tropical land areas, because interannual temperature variability is weak compared to mean changes, the temperature distribution range is ''likely'' to be shifted to a very different range in all projection scenarios, with unprecedented values relative to pre-industrial conditions. High climate velocities are particularly noteworthy for biodiversity hotspots given complex ecosystem dynamics and niche climates not easily replicated under shifted geographies ( [[#Burrows--2014|Burrows et al., 2014]] ; [[#Halpern--2015|Halpern et al., 2015]] ; [[#Dobrowski--2016|Dobrowski and Parks, 2016]] ). In some regions (e.g., Central Africa, Amazon, South East Asia) the mean temperature change is already beyond the normal range of variations as it has reached levels higher than three (and up to six) times larger than the standard deviation of the interannual variations ( [[#Hawkins--2020|Hawkins et al., 2020]] ). Together with global warming, land and marine heatwaves are ''very likely'' to increase in the future climate in biodiversity hotspots (Sections 12.4.1–12.4.7).

There is ''low confidence'' in broad patterns of future drying or wet trends across the land and freshwater biodiversity hotspots in the humid tropics, although drying trends have been detected and predicted in parts of the Amazon ( [[#Fu--2013|Fu et al., 2013]] ; [[#Boisier--2015|Boisier et al., 2015]] ). There is ''medium confidence'' ( ''limited evidence'' , ''high agreement'' ) that in several regions the length of the dry season has already increased and is projected to further increase in some parts of the Mediterranean, Amazonia and sub-Saharan Africa ( [[#Debortoli--2015|Debortoli et al., 2015]] ; [[#Dunning--2018|Dunning et al., 2018]] ; [[#Hochman--2018|Hochman et al., 2018]] ; [[#Saeed--2018|Saeed et al., 2018]] ). Longer dry seasons also extend the seasonal length and geographical extent of fire weather in all future scenarios ( ''medium confidence'' ) ( [[#Jolly--2015|Jolly et al., 2015]] ; [[#Abatzoglou--2019|Abatzoglou et al., 2019]] ).

'''In conclusion, biodiversity hotspots around the world will each face unique challenges as climatic impact-drivers change. However, heat, drought and length of dry season, fire weather, sea surface temperature and deoxygenation are relevant drivers to terrestrial, freshwater and marine ecosystems, and have marked increasing trends.'''

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