Editing IPCC:AR6/WGII/Chapter-2 (section)

==== 2.5.1.3 Risk of Species’ Extinctions ====

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===== 2.5.1.3.1 Overview =====

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This assessment of current findings is of studies across a range of taxa and modelling techniques. Extinction risk estimates whether or not a particular species may be at risk of extinction over the coming decades if climatic trends continue, and usually does not take into account other human-induced stressors (e.g., invasive species or pollution). It is not a prediction that a species will definitely become extinct because, even when complete loss of a species’ range is projected, the scale of the model cannot estimate persistence in very small-scale micro-climatic refugia (that can be on the order of metres in size) ( [[#Suggitt--2015|Suggitt et al., 2015]] ; [[#Suggitt--2018|Suggitt et al., 2018]] ). Individuals and populations can survive after the conditions for successful reproduction are gone, leading to a lagged decline, called ‘extinction debt’ (see section 2.4.2.8) ( [[#Alexander--2018|Alexander et al., 2018]] ). Therefore, range loss is an established criterion for assessing endangerment status and risk of extinction. As a species range becomes smaller and occupied habitats become more isolated, the likelihood of a single stochastic event causing extinction increases. It is this combination of projected loss of climatically suitable space and additional stressors (especially LULCC of critical habitat) that is expected to drive future extinctions.

The IUCN Red List Criteria ( [[#IUCN--2019|IUCN, 2019]] ) classifies a species as ‘critically endangered’ if it has suffered a range loss of ≥80%, with a resulting likelihood of extinction of &gt;50% in the near term (10–100 yrs, depending upon generation length). A species is classified as ‘endangered’ if it has suffered a range loss of ≥50%, with a resulting likelihood of extinction of &gt;20% in the near term (10–100 years). In this assessment, a species that is projected to become classified as ‘endangered’ is deemed to be at ''‘high risk’'' of extinction, and becoming classified as ‘critically endangered’ is deemed at ''‘very high risk’'' of extinction.

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===== 2.5.1.3.2 Projections for freshwater biodiversity =====

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Because risk to freshwater species has been limited in past reports, this section provides details of freshwater risk. Lakes, rivers and freshwater wetlands cover approximately 7.7–9.1% of global land surface area; ( [[#Lehner--2008|Lehner et al., 2008]] ; [[#Fluet-Chouinard--2015|Fluet-Chouinard et al., 2015]] ; [[#Allen--2018|Allen and Pavelsky, 2018]] ) and hold 9.5% of the Earth’s described animals ( [[#Balian--2008|Balian et al., 2008]] ), with climate change indicated as a threat to 50–75% of fish ( [[#Xenopoulos--2005|Xenopoulos et al., 2005]] ; [[#Darwall--2015|Darwall and Freyhof, 2015]] ). Climate change is cited as a primary factor in species’ extinction risk due to changes in water temperatures, stream flow, loss of cold water habitat, increased variability of precipitation and increased disease risk from warming temperatures ( ''robust evidence'' , ''high agreement'' , ''high confidence'' ) ( [[#Knouft--2017|Knouft and Ficklin, 2017]] ; [[#Pletterbauer--2018|Pletterbauer et al., 2018]] ; [[#Jaric--2019|Jaric et al., 2019]] ; [[#Reid--2019|Reid et al., 2019]] ) adding to the stress of overexploitation and LULCC ( [[#Craig--2017|Craig et al., 2017]] ; [[#IPBES--2019|IPBES, 2019]] ).

Increased frequency of stream drying events, reducing hydrologic connectivity and limiting access of native fishes to spawning habitats is projected for RCP8.5 in Colorado, USA ( ''medium evidence'' , ''medium agreement'' ) ( [[#Jaeger--2014|Jaeger et al., 2014]] ). Cold-water habitats and associated obligate species are particularly vulnerable, and losses in these habitats have been both documented and projected, for example, in salmonids ( [[#Santiago--2016|Santiago et al., 2016]] ; [[#Fullerton--2017|Fullerton et al., 2017]] ; [[#Merriam--2017|Merriam et al., 2017]] ). River networks are projected to lose connections to cold tributary refugia, that are important thermal refuges for cold water species ( ''robust evidence'' , ''high agreement'' ) ( [[#Isaak--2016|Isaak et al., 2016]] ) during low flows ( [[#Merriam--2017|Merriam et al., 2017]] ).

Community turnovers are expected in freshwaters as cold-adapted species lose and warm-adapted species gain climatically suitable habitat ( [[#Domisch--2011|Domisch et al., 2011]] ; [[#Domisch--2013|Domisch et al., 2013]] ; [[#Shah--2014|Shah et al., 2014]] ). While a number of warm-adapted species may experience range expansions, the majority of species are predicted to lose climatically suitable areas by, on average, 38–44%, depending on the emission scenario (A2a and B2a) ( ''medium evidence'' ) ( [[#Domisch--2013|Domisch et al., 2013]] ).

Molluscs are projected to be the most at-risk group, given their limited dispersal capability ( [[#Woodward--2010|Woodward et al., 2010]] ). Mediterranean freshwater fish are especially susceptible to climate change due to increasing flood and drought events and the risk of surpassing critical temperature thresholds ( [[#Santiago--2016|Santiago et al., 2016]] ; [[#Jaric--2019|Jaric et al., 2019]] ). In southern Europe, aquatic insects (Ephemeroptera, Plecoptera and Trichoptera) are endangered by climate change ( [[#Conti--2014|Conti et al., 2014]] ). European protected areas are not expected to be sufficient under warming to provide habitat for the majority of rare molluscs and fish ( [[#Markovic--2014|Markovic et al., 2014]] ). Observed trends agree with model projections in direction, but magnitude remains uncertain ( ''medium evidence'' , ''medium agreement'' , ''medium confidence'' ) (see Figure 2.8 for extinction risk globally for dragonflies, amphibians and turtles).

Regional threats from climate change have been reported for 40% of amphibians in China, ( [[#Wu--2020|Wu, 2020]] ), 33% of European freshwater fish species ( [[#Janssen--2016|Janssen et al., 2016]] ) and 56–69% of odonates in Australia, ( [[#Bush--2014b|Bush et al., 2014b]] ). Assessment of site-specific extirpation for 88 aquatic insect taxa projected that climate change-induced hydrological alteration would result in a 30–40% loss of taxa in warmer, drier ecoregions and a 10–20% loss in cooler, wetter ecoregions ( ''medium evidence, medium agreement'' ) ( [[#Pyne--2017|Pyne and Poff, 2017]] ). In Africa’s Albertine Rift, 51% ( ''n'' = 551) of fish are expected to be impacted by climate change, with 5.5% at a high risk due to their sensitivity and poor adaptative capability ( ''medium evidence, high agreement'' ) ( [[#Carr--2013|Carr et al., 2013]] ).

The GLOBIO-Aquatic model ( [[#Janse--2015|Janse et al., 2015]] a) links models for demography, economy, LUCs, climate change, nutrient emissions, a global hydrological model and a global map of water bodies. It projects that changes in both water quality (eutrophication) and quantity (flow) will generate negative relations in freshwater ecosystems between the persistence of species originally present in each community and a constellation of stressors, including harmful algal blooms. Under a 4°C rise by 2050, mean abundance of species is projected to decline by 70% in running water and by 80% in standing water ( ''medium evidence'' , ''high agreement'' , ''medium confidence'' ) ( [[#Janse--2015|Janse et al., 2015]] a ).

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===== 2.5.1.3.3 Global projections of extinction risk =====

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In previous reports, risk assessed from the literature was generally based on estimates of overall range contractions with climate change. In AR4, extinction risk was carefully quantified: ‘There is ''medium confidence'' that approximately 20–30% of species assessed so far are ''likely'' to be at increased risk of extinction if increases in global average warming exceed 1.5–2.5°C (relative to mean temperatures from 1980–1999). As global average temperature increase exceeds about 3.5°C, model projections suggest significant extinctions (40–70% of species assessed) around the globe.’ These estimates approximately correspond to 50–80% reductions in range size (depending upon study), that this assessment equates with a ''‘high’'' and ''‘very high’'' extinction risk, respectively ( [[#IPCC--2007|IPCC, 2007]] ). AR5 stated: ‘a large fraction of terrestrial and freshwater species face increased extinction risk under projected climate change during and beyond the 21st century, especially as climate change interacts with other pressures ( ''high confidence'' )’ ( [[#Field--2014|Field et al., 2014]] ). A series of multi-species and global analyses have been published since AR5, using both statistical models and trait-based approaches.

In this chapter, risk to species, with implications for ecosystems, is assessed using three different approaches. First is an assessment of the geographic distributions of local species’ losses at different levels of GSAT warming, termed ‘local biodiversity loss’, measured as the proportion of species within a given location becoming classified as “endangered” or worse (sensu IUCN), and so at ''high'' ''risk'' of local population losses (local population extinctions) (Figure 2.6). This measure provides the best estimates of which sites are at most risk of losing substantial numbers of species locally, leading to degradation of that ecosystem’s ability to function.

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[[File:e2574f66c47e123775e3a9fbd8d549a1 IPCC_AR6_WGII_Figure_2_006.png]]

'''Figure 2.6 | Biodiversity loss for different areas at increasing levels of climate change.''' The higher the percentage of species projected to lose suitable climate in a given area, the higher the risk to ecosystem integrity, functioning and resilience to climate change. Warming levels are based on global levels (GSAT) above pre-industrial temperatures. Colour shading represents proportion of species for which the climate is projected to become sufficiently unsuitable that the species becomes locally ‘endangered’ and at ''high risk'' of local extinction within a given pixel across their current distributions at a given GSAT warming level, based on underlying data ( [[#Warren--2018|Warren et al., 2018]] ) (modelled ''n'' = 119,813 species globally, with no dispersal, averaged over 21 CMIP5 climate models). Areas shaded in deep orange and red represent a significant risk of biodiversity loss (areas where climates become sufficiently unsuitable that it renders &gt;50% and &gt;75% of species at ''high ris'' k of becoming locally extinct, respectively). The maps of species richness remaining have been overlaid with a landcover layer (2015) from the European Space Agency (ESA) Climate Change Initiative. This landcover layer leaves habitats classified by the ESA as natural as transparent. Areas with a landcover identified as agriculture are 5% transparent, such that the potential species richness remaining if the land had not been converted for agriculture shows as pale shading of the legend colours (very pale yellow to very pale red). These paler areas represent biodiversity loss due to habitat destruction, but with a potential to be restored, with yellow shading having the potential for restoration to greater species richness than orange or red shading.

Second is assessment of the proportions of species becoming endangered globally (not just locally), so at ''high'' ''risk'' of global extinction of the species, termed ‘global biodiversity loss’ (Figure 2.8b). This metric (losing &gt; 50% of suitable climate space across the species’ entire range) also serves to estimate a species’ becoming sufficiently rare that the species no longer fully contributes to ecosystem functioning, a state that often occurs decades before complete extinction (death of the last individual). The proportions of species becoming at ''high'' ''risk'' of global extinction is the foundation for the burning embers diagram on global biodiversity loss in Table 2.5 and Figure 2.11.

Third is an assessment of risk of the proportions of species becoming at ''very high'' ''risk'' of extinction globally at different levels of GSAT warming, measured using the IUCN criteria for ‘critically endangered’, and termed ‘species’ extinction risk’ (Figure 2.7 and Figure 2.8a). This measure is closest to assessing the complete loss of a species in the wild and can be used to compare to past (palaeo) extinction rates. These three approaches provide complementary information of the overall risks to individual species, to biodiversity at the community scale, and to ecosystem integrity and functioning at different levels of warming.

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'''Figure 2.7 | Global assessment of species’ extinction risks under different levels of warming.''' Graph shows a synthesis of climate-driven models of individual species projected to become at ''very high'' ''risk'' of extinction globally (i.e, becoming “critically endandered” sensu IUCN by losing &gt;80% of their suitable climate space or through estimates of extinction risk from process-based models). The relationship between modelled projections of extinction (expressed as a proportion of species at a risk of extinction assessed in individual studies) and GSAT increase above the pre-industrial average. Data (global sample size ''n'' = 178 modelled estimates) were taken from a number of sources, including digitization of data points in Figure 2 in the synthetic analysis of ( [[#Urban--2015|Urban, 2015]] ) (note: unweighted for sample size) ''n'' = 126; Table 4.1 of AR4 WGII [https://www.ipcc.ch/report/ar6/wg2/chapter/chapter-2 Chapter 2] ( [[#Fischlin--2007|Fischlin et al., 2007]] ), ''n'' = 40; ( [[#Hannah--2020|Hannah et al., 2020]] ) ''n'' = 6; and ( [[#Warren--2018|Warren et al., 2018]] ) ''n'' = 6. The quantile regression (which is robust to the non-normal distribution of the response variable, and less sensitive to data outliers) was chosen as a descriptive statistic to fit quantile estimates for levels relevant to informing ''likely'' (between the 0.17 and 0.83 quantiles, shaded in orange) and ''very likely'' ranges (between the 0.05 and 0.95 quantiles, shaded in green) relating extinction risk to GSAT increase (quantile regression implemented using the Barrodale and Roberts algorithm in XLSTAT). The roughly equivalent estimate of this risk as expressed in AR4 ( [[#Fischlin--2007|Fischlin et al., 2007]] ) is indicated by the dotted block indicating the ''medium confidence'' statement ‘Approximately 20–30% of plant and animal species assessed so far (in an unbiased sample) are likely to be at increasingly high risk of extinction as global mean temperatures exceed a warming of 2–3°C above pre-industrial levels ( ''medium confidence'' ) ''.'' ’ This box is open on the right side because AR4 estimates stipulated temperatures at or exceeding the given levels. Thick dark horizontal bars show the median values of percent of species at ''very high risk'' of extinction at 1.5°C, 2°C, 3°C, 4°C and 5°C, indicating that half of the data points lie above the bar and half below for a given level of global warming.

Risk of local biodiversity loss, estimated as the proportion of species in a given area projected to become endangered (sensu IUCN), and therefore at ''high'' ''risk'' of extinction, is projected to affect a greater number of regions experiencing increasing warming. About one-third of land area risks more than 50% of species becoming “endangered” by 4.0° GSAT warming (Figure 2.6). That is, the deep orange and red areas in Figure 2.6 are those areas for which &gt;50% of species currently inhabiting those ecosystems are projected to lose &gt;50% of their climatically suitable habitat. Species’ losses are projected to be worst in northern South America, southern Africa, most of Australia and at northern high latitudes ( ''medium confidence'' ) (Figure 2.6).

For risk of global biodiversity loss, at 1.58°C global warming (median estimate), &gt;10% of species are projected to become “endangered”, and so at ''high'' ''risk'' of extinction (sensu IUCN). At 2.07°C (median) &gt;20% of species are projected to become endangered. Ten-twenty percent losses represent ''high'' and ''very high'' ''risk'' of biodiversity losses, respectively, substantial enough to reduce ecosystem integrity and functioning ( ''medium confidence'' ) (Figure 2.8b) (see [[#2.5.4|Section 2.5.4]] ; Figure 2.11; Table 2.5, Table SM2.5).

Risk of global biodiversity loss differs among taxonomic groups. The percent of species projected at ''high risk'' of extinction was 49% for all insects, 44% for all plants and 26% for all vertebrates at ~3°C global rise in temperature (Figure 2.8b) ( [[#Warren--2018|Warren et al., 2018]] ). These estimates dropped considerably at lower levels of warming, down to 18%, 16% and 8% at 2°C; and 6%, 8% and 4% at 1.5°C (Figure 2.8b) ( [[#Warren--2018|Warren et al., 2018]] ), so not entirely dissimilar to the numbers in AR4 (Figure 2.7).

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'''Figure 2.8 | Percent of species of different groups classified as being at risk of extinction.'''

'''(a)''' Species groups listed projected to be at a ''very high'' ''risk'' of extinction, corresponding to the IUCN Red List criteria for a species classified as ‘critically endangered’ (v3.1) by losing &gt;80% of its climatically suitable range area.

'''(b)''' Species groups listed projected to be at a ''high'' ''risk'' of extinction, corresponding to the IUCN Red List criteria for a species classified as ‘endangered’ (v3.1) by losing &gt;50% of its climatically suitable range area. For (a) and (b), values were calculated from the underlying data in ( [[#Warren--2018|Warren et al., 2018]] ). Values for each temperature are the mean values across 21 CMIP5 models. The grey band represents the high end of extinction risk from the 10th percentile of the climate models to show the maximum range of values, while the low end (90th percentile, 1.5°C) is not shown as it is too small to appear on the plots. Taxa marked with * represent potential benefits from adaptation, specifically dispersal at realistic rates ( [[#Warren--2018|Warren et al., 2018]] ); those with no * have dispersal rates that are essentially not detected in the spatial resolution of the models (20 km). See ( [[#Warren--2018|Warren et al., 2018]] ) for caveats and more details. Sample size for each group is as follows: 1) fungi (16187 species); 2) all plants (72399 species), broken down into sub-groups of plants: flowering plants (52310 species), timber species (1328 species), grasses (3389 species) and pines (340 species); 3) all invertebrates (33,949 species), broken down into sub-groups of invertebrates: annelid worms (155 species), flies (4809 species), beetles (7630 species), moths (6910 species), true bugs (1728 species), spiders (2212 species), all pollinators (1755 species), butterflies (1684 species), ants/bees/wasps (5914 species), dragonflies (599 species); 4) Chordates (12642 species), broken down into major groups: 4i) all amphibians (1055 species), broken down into sub-groups of amphibians: frogs (887 species) and salamanders (163 species); 4ii) reptiles (1850 species), snakes (1741 species) and turtles (94 species); 4iii) all mammals (1769 species), broken down into sub-groups of mammals: ungulates (80 species), bats (500 species), carnivores (107 species), 4iv) all birds (7968 species), broken down into sub-groups of birds: passeriforme birds (4744 species), and non-passeriforme birds (3224 species).

‘Species’ extinction risk’, estimated as at ''very high'' ''risk'' of extinction globally, i.e. becoming “critically endangered” (sensu IUCN) is shown in Figures 2.7 across 178 studies and in Figure 2.8a split by taxonomic group. The percentage of species at ''very high'' ''risk'' of extinction (median estimates and maximum ''likely'' range) will be 9% (max. 14%) at 1.5°C, 10% (max. 18%) at 2°C, 12% (max. 29%) at 3.0°C, 13% (max. 39%) at 4°C and 15% (max. 48%) at 5°C (Figure 2.7). Maximum estimates of species at ''very high'' ''risk'' of extinction reach 60% within the 95% quartiles, ie the ''very likely'' range, for 5°C GSAT warming. Among the groups containing the largest numbers of species at a ''very high risk'' of extinction for mid-levels of projected warming (3.2°C rise in GSAT) are: invertebrates (15%), specifically pollinators (12%), amphibians (11%, but 24% for salamanders) and flowering plants (10%) (Figure 2.8a). All groups fare substantially better at 2°C, with extinction projections reducing to &lt;3% for all groups, except salamanders at 7% ( ''medium confidence'' ) (Figure 2.8a) ''.''

Figure 2.8 also shows the benefits of dispersal in reducing extinction risk in birds, mammals, butterflies, moths and dragonflies (depicted with an asterix). While dispersal may benefit individual species, it poses additional risks to communities and ecosystems that species are moving into, as interactions between species are changed or eliminated.

Projected species extinctions at future global warming levels are in accord with projections from AR4, assessed on much larger numbers of species with much greater geographic coverage and a broader range of climate models. (Figure 2.7; Figure 2.8a). Even the lowest estimates of species extinction (median of 9% at 1.5°C warming, Figure 2.7) are 1000 times the natural background rates ( [[#De%20Vos--2015|De Vos et al., 2015]] ).

Using data from geological timescales, ( [[#Song--2021|Song et al., 2021]] ) predicted that a warming of 5.2°C above pre-industrial levels would result in a mass extinction comparable to that of the five mass extinctions over the past 540 Myr, on the order of 70–85% of species becoming extinct, in the absence of non-climatic stressor. ( [[#Mathes--2021|Mathes et al., 2021]] ) found evidence in the geological record that short-term rapid warming, on top of long-term warming trends, increases extinction risk by up to 40% over that expected from the long-term trend alone, with a biodiversity ‘memory’ of up to 60 Myr, indicating an additonal risk of multi-decadal overshoot.

Most of the large-scale studies that have been performed are for losses based on climate alone (Figures 2.6, 2.7, 2.8). However, climate is rarely the only stressor affecting species survival. Habitat loss is currently the largest driver of range loss and extinction risk for most species ( [[#IUCN--2019|IUCN, 2019]] ). Communities in different regions are becoming more similar to each other as species tolerant of human activities prosper and spread, with many rare and endemic species already having been driven to extinction, primarily by LULCC ( [[#Pimm--2006|Pimm et al., 2006]] ). Thus, it will likely be the interaction of climate change and habitat conversion (often driven by climate change) that will ultimately determine the risk and ability of many species to survive over the next century.

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