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

==== CCP1.2.1.2 Projected Impacts ====

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===== CCP1.2.1.2.1 Projected climatic hazards =====

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Comparison of climate warming projected for air and sea temperature shows biodiversity hotspots will continue to experience the greatest net increases in temperature at higher Northern Hemisphere latitudes, particularly in tundra regions (Figures CCP1.4; CCP1.5; Table CCP1.1). Generally, terrestrial and freshwater hotspots are projected to continue to warm more than marine (Figure CCP1.3). Modelled temperatures are projected to continue to be the highest in the tropics, indicating where there are more thermally stressful conditions for more species ( ''high confidence'' ) ( [[#Stuart-Smith--2015|Stuart-Smith et al., 2015]] ; [[#Stuart-Smith--2017|Stuart-Smith et al., 2017]] ; [[#Foster--2018|Foster et al., 2018]] ; [[#Waldock--2019|Waldock et al., 2019]] ). By the end of this century, all terrestrial biodiversity hotspots in Central and South America, Africa, India and southern and eastern Asia (including the Indo–West Pacific islands) are projected to experience climates unprecedented in their species’ evolutionary history ( ''medium confidence'' ) ( [[#Williams--2007|Williams et al., 2007]] ).

Based on WGI ''Interactive Atlas'' data (Gutiérrez et al., 2021), global warming is projected to affect terrestrial hotspots less than non-hotspot areas: 80% less for Myers and 95–96% less for G200 terrestrial and freshwater hotspots at global warming of 1.5°C–3°C ( ''medium confidence'' ) ( [[#Kocsis--2021|Kocsis et al., 2021]] ). In contrast, warming is projected to be 12–13% greater inside than outside marine hotspots ( ''medium confidence'' ) ( [[#Kocsis--2021|Kocsis et al., 2021]] ). Precipitation is generally projected to increase more in terrestrial and freshwater biodiversity hotspots compared to outside them ( ''low confidence'' ) ( [[#Kocsis--2021|Kocsis et al., 2021]] ). The exception is Myers hotspots, which are projected to have, on average, ~28% less precipitation at 1.5°C warming, but ~33% more at 2°C and ~65% more at 3°C ( ''low confidence'' ). However, precipitation changes are often difficult to assess as many hotspots cover large areas, with some areas projected to be wetter and some drier with wide differences between different climate models.

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'''Figure CCP1.4 |''' '''Projected loss of climatically suitable area in terrestrial biodiversity hotspots for a global average of 1.''' '''5°C (upper row, a–b), 2°C (middle, c–d) and 3°C (lower, e–f).''' Left-hand column displays the projected human impact using the five equal 20% categories of present-day impact (Figure CCP1.1). The right-hand column indicates the changes of impact categories compared to present-day impact. See Table SMCCP1.1 for more details.

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'''Figure CCP1.5 |''' '''Projected future warming in degrees Celsius for freshwater (left column, near-surface air temperature, panels''' '''a''' ''',''' '''c''' '''and''' '''e''' ''') and marine (right column as sea surface temperature, panels''' '''b''' ''',''' '''d''' '''and''' '''f''' ''')''' '''hotspots for a global average warming of +1.''' '''5°C (a, b), +2°C (c, d) and +3°C''' '''(e, f) compared to pre-industrial conditions.''' Values in text boxes in the figures indicate temperature increase from present-day (2005–2014) settings. Projected temperatures were calculated with averages of multi-model, yearly means across Shared Socioeconomic Pathways (SSP) 1.26 (only for +1.5°C), SSP2-45, SSP3-70 and SSP5-85.

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===== CCP1.2.1.2.2 Projected impacts on biodiversity =====

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Biodiversity hotspots are expected to be especially vulnerable to climate change because their endemic species have smaller geographic ranges ( ''high confidence'' ) ( [[#Sandel--2011|Sandel et al., 2011]] ; [[#Brown--2020|Brown et al., 2020]] ; [[#Manes--2021|Manes et al., 2021]] ). [[#Manes--2021|Manes et al. (2021)]] reviewed over 8000 projections of climate change impacts on biodiversity in 232 studies, including 6116 projections on endemic, native and introduced species in terrestrial (200 studies), freshwater (14 studies) and marine (34 studies) environments in biodiversity hotspots. Only half of the hotspots had studies on climate change impacts. All measures of biodiversity were found to be negatively impacted by projected climate change, namely, species abundance, diversity, area, physiology and fisheries catch potential ( ''medium confidence'' ). However, introduced species’ responses were neutral to positive ( ''medium confidence'' ). Land areas were projected to be more negatively affected by climate warming than marine. Land plants, insects, birds, reptiles and mammals were all projected to be negatively affected ( ''medium confidence'' ), as well as fish, coral reef, benthic, planktonic and other marine species ( ''medium confidence'' ).

Of the 6116 projections for more than 2,700 species assessed in biodiversity hotspots, ~44% were found to be at high extinction risk, and ~24% at very high extinction risk due to climate change ( [[#Manes--2021|Manes et al., 2021]] ) ( ''medium confidence'' ). Risks of extinction were estimated based on the projections for all warming levels combined, showing that endemic species were about 2.7 times more at very high risk of extinction compared to non-endemic native species ( [[#Manes--2021|Manes et al., 2021]] ). Extinction risks were highest for endemic species of both land and ocean ( ''medium confidence'' ), and were higher for those living on islands (~100%, ''medium confidence'' ) and mountains (~84%, ''medium confidence'' ) than in the ocean (~54%, ''low evidence, medium agreement'' ; ''low confidence'' ) and on continents (~12%, ''robust evidence, medium agreement, medium confidence'' ) (Figure CCP1.6). Extinction risks for non-endemic natives were ~20% for both terrestrial and marine species, with introduced species projected to become more rather than less invasive. At 1.5°C warming, ~2% of both terrestrial and marine species and at 3°C, ~20% and ~32% respectively, were projected to be at very high risk of extinction in the hotspots (Figure CCP1.6). Thus, a doubling of warming results in a roughly 10-fold increase in species at very high extinction risk.

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'''Figure CCP1.6 |''' '''A summary of''' '''the projected risks of species extinction at global warming levels of &lt;1''' '''.''' 5°C, 1.5–2.0°C and &gt;3°C in terrestrial and marine biodiversity hotspots. Data from [[#Manes--2021|Manes et al. (2021)]] .

[[#Manes--2021|Manes et al. (2021)]] found that any benefits to species (e.g., range or abundance increase) were projected to be localised and transient (e.g., Arctic, H196, 214). This and previous assessments indicate that, while climate change varies spatially and taxa may respond differently, a loss of biodiversity is projected across all terrestrial hotspots ( ''high confidence'' ) ( [[#Foden--2013|Foden et al., 2013]] ; [[#Warren--2018a|Warren et al., 2018a]] ; [[#Manes--2021|Manes et al., 2021]] ). Abrupt changes across species assemblages may occur under all scenarios: in 9% of assemblages at 1.75°C and 35% at 4.4°C on both land and sea ( [[#Trisos--2020|Trisos et al., 2020]] ). However, species losses may be reduced if species have thermal microclimate refugia and behavioural thermoregulation, or greater due to extreme events, such as heatwaves.

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