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

=== CCP1.2.1 Global Perspective ===

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==== CCP1.2.1.1 Observed Impacts ====

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===== CCP1.2.1.1.1 Observed climatic hazards =====

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Terrestrial and freshwater hotspots have been warming less over the last 50 years than non-hotspot areas, whereas marine hotspots have been warming more ( [[#Kocsis--2021|Kocsis et al., 2021]] ). The warming inside terrestrial hotspots is 0.91°C (Myers) and 1.04°C (G200), respectively, while for freshwater hotspots it is 0.89°C, compared to 1.08°C warming outside ( [[#Kocsis--2021|Kocsis et al., 2021]] ). In contrast, mean annual sea surface temperatures in the G200 marine biodiversity hotspots have warmed 41% more than the regions outside (0.53°C compared with 0.38°C) ( [[#Kocsis--2021|Kocsis et al., 2021]] ). Thus, terrestrial biodiversity hotspots have been warming slightly less, and marine hotspots considerably more than non-hotspots ( ''medium confidence'' ).

Climate velocity, the direction and pace of movement in climate variables (typically temperature) in space, is key to understanding the origin and fate of biodiversity hotspots under climate change ( [[#Loarie--2009|Loarie et al., 2009]] ; [[#Burrows--2011|Burrows et al., 2011]] ). Climate trajectories generally predict the direction and pace of past and future species range shifts ( [[#Pinsky--2013|Pinsky et al., 2013]] ; [[#Brito-Morales--2018|Brito-Morales et al., 2018]] ), although there are exceptions ( [[#Fuchs--2020|Fuchs et al., 2020]] ). Spatial patterns of climate trajectories show regions where species are expected to leave, pass through, and/or arrive under climate change ( [[#Burrows--2014|Burrows et al., 2014]] ). Regions of high climate velocities are those with low topographic relief on land, particularly flooded grasslands and deserts ( [[#Loarie--2009|Loarie et al., 2009]] ), and tropical as well as offshore and polar sea regions ( [[#Burrows--2011|Burrows et al., 2011]] ; [[#Burrows--2014|Burrows et al., 2014]] ; [[#García%20Molinos--2016|García Molinos et al., 2016]] ; [[#Brito-Morales--2018|Brito-Morales et al., 2018]] ; [[#Brito-Morales--2020|Brito-Morales et al., 2020]] ).

On millennial time scales, some areas of low climate velocity have more endemic species and can be considered climate refugia, at least on land ( [[#Sandel--2011|Sandel et al., 2011]] ) and, for marine species, around Antarctica (H213) ( [[#Costello--2010|Costello et al., 2010]] ). This suggests that, if these areas are subject to increased velocities, they will lose species that are not able to disperse fast enough to cope with the pace of climate change ( ''medium confidence'' ) ( [[#Sandel--2011|Sandel et al., 2011]] ; [[#Brito-Morales--2018|Brito-Morales et al., 2018]] ).

Climate velocities are 47% (Myers), 29% (G200, terrestrial) and 10% (G200, freshwater) lower inside biodiversity hotspots than outside, respectively ( [[#Kocsis--2021|Kocsis et al., 2021]] ), but are 69% higher inside marine hotspots than outside ( ''medium confidence'' ). Climate velocities from 1970 to 2019 ranged from 3–4 km per decade (terrestrial and freshwater) to ~11 km per decade in marine ( [[#Kocsis--2021|Kocsis et al., 2021]] ).

For terrestrial and freshwater hotspots, the highest climate velocities are in central South America, including the Amazon (H153, 154) (Figure CCP1.3). Terrestrial hotspots also have high velocities in the Arctic (H196, 214) and east of the Caspian Sea, while freshwater hotspots have low velocities in the eastern European Mediterranean and eastern Australia.

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[[File:c0bd1751c9399491c25b2ebc32c80076 IPCC_AR6_WGII_Figure_CCP1_003.png]]

'''Figure CCP1.3 |''' '''Climate velocities in terrestrial''' '''(a)''' ''', freshwater''' '''(b)''' '''and marine''''(c)''' '''hotspots between 1970–2019.''' Values are presented in kilometres per decade and derived using the analytical package VoCC ( [[#García%20Molinos--2019|García Molinos et al., 2019]] ) from gridded temperature data, sea surface temperatures for marine ( [[#Rayner--2003|Rayner et al., 2003]] ) and near-surface air temperatures on land and freshwater ( [[#Harris--2020|Harris et al., 2020]] ). Positive and negative velocities indicate warming and cooling, respectively.

Marine hotspots have a wider range of climate velocities than terrestrial and freshwater environments (Figure CCP1.3), being faster in equatorial, Mediterranean (H216), Baltic (H215), North and Okhotsk (H229), and Arctic hotspots (H196, 214), and slow in the Antarctic hotspot (H213). Marine species tend to follow climate velocities more closely than terrestrial species ( ''high confidence'' ) ( [[#Sunday--2012|Sunday et al., 2012]] ; [[#Pinsky--2019|Pinsky et al., 2019]] ; [[#Lenoir--2020|Lenoir et al., 2020]] ). The reasons may be smaller thermal safety margins in the seas or greater human impacts on land impeding species range shifts. Climate velocities are particularly fast in equatorial seas (Figure CCP1.3; ( [[#Burrows--2011|Burrows et al., 2011]] ), which are therefore expected to be source areas for species shifting their ranges towards the subtropics ( [[#Burrows--2014|Burrows et al., 2014]] ). The subtropics are then source areas of species that shift to temperate latitudes and so forth, such that observed impacts in marine biodiversity hotspots are largely attributable to species range shifts ( ''high confidence'' ) ( [[#Pecl--2017|Pecl et al., 2017]] ). Because marine climate velocities are significantly greater within than outside hotspots, marine hotspots are especially prone to species redistributions ( ''medium confidence'' ) (Figure CCP1.3; ( [[#Kocsis--2021|Kocsis et al., 2021]] ).

While species from lower latitudes may shift their geographic ranges to higher latitudes to adapt to changing climate, there are no species to replace low latitude species. Thus, as already observed in the oceans around the equator, the loss of species in low latitudes will continue with future climate warming ( ''high confidence'' ) ( [[#Yasuhara--2020|Yasuhara et al., 2020]] ; [[#Chaudhary--2021|Chaudhary et al., 2021]] ). The issue also extends to altitudinal ranges in terrestrial environments, with species moving to higher elevations where surface area generally declines with increasing elevation; mountaintop species may have nowhere to go ( [[#Flousek--2015|Flousek et al., 2015]] ; [[#Freeman--2018|Freeman et al., 2018]] ; [[#Kidane--2019|Kidane et al., 2019]] ).

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===== CCP1.2.1.1.2 Observed impacts on biodiversity =====

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Although conservation status has only been assessed globally for about 6% of all species ( [[#Costello--2019|Costello, 2019]] ) and most confirmed extinctions and threatened species are terrestrial, a higher proportion of freshwater species are threatened. This is reflected in the higher proportion of freshwater hotspots impacted by humans ( [[#Collen--2014|Collen et al., 2014]] ; [[#Costello--2015|Costello, 2015]] ; [[#Harrison--2018|Harrison et al., 2018]] ). The rate of species endemicity is exceptionally high in freshwater biogeographic realms (i.e., large regions of distinct species composition and endemicity), at 89–96% for fish in all but one realm, compared to 11–98% for terrestrial vertebrate groups ( [[#Leroy--2019|Leroy et al., 2019]] ) and 17–84% for marine realms ( [[#Costello--2017|Costello et al., 2017]] ). Already, one-third of wetlands have been lost and 9000 freshwater species are threatened with extinction without considering the effects of climate change ( [[#Darwall--2018|Darwall et al., 2018]] ), and only 13% of world rivers were recently classified as least impacted ( [[#Su--2021|Su et al., 2021]] ).

Globally, observed climate-driven changes in biodiversity are typically of species distributions shifting to higher latitudes ( ''virtually certain'' ) ( [[#Lenoir--2020|Lenoir et al., 2020]] , Ch.2, Ch. 3.4). Since the 1950s, marine species richness has shifted poleward in the Northern Hemisphere, increased in mid-latitudes and declined at the equator in concert with ocean warming ( ''medium confidence'' ) ( [[#Chaudhary--2021|Chaudhary et al., 2021]] ). Climate-driven altitudinal shifts are common on land ( ''high confidence'' ) ( [[#Lenoir--2015|Lenoir and Svenning, 2015]] ; [[#Steinbauer--2018|Steinbauer et al., 2018]] ), and depth shifts in the ocean may occur but are little studied ( ''low confidence'' ) ( [[#Burrows--2019|Burrows et al., 2019]] ; [[#Jorda--2020|Jorda et al., 2020]] ). While climate-induced range expansions can be viewed as opportunities for increasing regional biodiversity, range contractions adversely affect biodiversity through regional extirpations ( ''high confidence'' ) ( [[#Cahill--2013|Cahill et al., 2013]] ; [[#Chaudhary--2021|Chaudhary et al., 2021]] ).

Both of the two climate change associated global species extinctions to date support the predictions that endemic species on mountains and islands are at the greatest risk of extinction ( [[#Manes--2021|Manes et al., 2021]] ). The golden toad ( ''Bufo periglenes'' ) became extinct after some years of decline associated with changes in climate warming and precipitation in the Talamancan-Isthmian Pacific Forests biodiversity hotspot (H22) ( [[#Pounds--1999|Pounds et al., 1999]] ; [[#Cahill--2013|Cahill et al., 2013]] , WGII Ch2.4.2.2). The Bramble Cay melomys ( ''Melomys rubicola'' ) '','' a rodent endemic to an island between Australia and Papua New Guinea and closely related to a mainland Australian species, became extinct due to habitat loss arising from climate change-related sea level rise and cyclone activity ( [[#Fulton--2017|Fulton, 2017]] ; [[#Roycroft--2021|Roycroft et al., 2021]] , WGII Ch.11).

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==== 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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==== 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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