Editing IPCC:AR6/WGII/Chapter-16 (section)

==== 16.6.3.1 Unique and Threatened Systems (RFC1) ====

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This RFC addresses the potential for increased damage to or irreversible loss of a wide range of physical, biological and human systems that are unique (i.e., restricted to relatively narrow geographical ranges and have high endemism or other distinctive properties) and are threatened by future changes in climate ( [[#Smith--2001|Smith et al., 2001]] ; [[#Smith--2009|Smith et al., 2009]] ; [[#Oppenheimer--2014|Oppenheimer et al., 2014]] ). The specific examples of such systems given in previous IPCC assessment reports has remained broadly consistent, with AR4 including ‘coral reefs, tropical glaciers, endangered species, unique ecosystems, biodiversity hotspots, small island states, and indigenous communities’ (Smith 2009), AR5 including ‘a wide range of physical, biological, and human systems that are restricted to relatively narrow geographical ranges’ and ‘are threatened by future changes in climate’ ( [[#Smith--2001|Smith et al., 2001]] ), and SR15 [[IPCC:Wg2:Chapter:Chapter-3|Chapter 3]] including ‘ecological and human systems that have restricted geographic ranges constrained by climate related conditions and have high endemism or other distinctive properties. Examples include coral reefs, the Arctic and its Indigenous People, mountain glaciers and biodiversity hotspots’. In this cycle, we retain the definition used in SR15 as most explicit and inclusive of the previous definitions.

AR5 ( [[#Oppenheimer--2014|Oppenheimer et al., 2014]] ) assessed the transition from undetectable to moderate risk for RFC1 to lie below recent global temperatures (1986–2005, which at the time was considered to correspond to a global warming level of 0.6°C above pre-industrial levels; AR6 WGI now considers this time period of 1986–2005 to correspond to a global warming or approximately 0.7°C). At that time, there was at least ''medium confidence'' in attribution of a major role for climate change for impacts on at least one each of ecosystems, physical systems and human systems within this RFC. SR15 [[IPCC:Wg2:Chapter:Chapter-3#3.5.2|Section 3.5.2.1]] ( [[#Hoegh-Guldberg--2018b|Hoegh-Guldberg et al., 2018b]] ), concurred with ''high confidence'' that the transition to moderate risk had already occurred before the time of writing.

The transitions here are informed by these assessments, along with the assessment in [[IPCC:Wg2:Chapter:Chapter-2|Chapter 2]] on species high extinction risk and on ecosystem transitions. It also draws substantially from information in [https://www.ipcc.ch/chapter/cross-chapter-paper-1 Cross-Chapter Paper 1] and Table SM16.22 on risks to unique and threatened biological systems. Some unique and threatened systems, such as coral reefs and sea-ice-dependent ecosystems, were already showing attributable impacts with ''high confidence'' (see Table SM16.22 , [https://www.ipcc.ch/chapter/cross-chapter-paper-1 Cross-Chapter Paper 1] and Chapter 2) based on data collected in the mid to latter 20th century, when global warming of 0.5°C above pre-industrial levels had taken place, as noted already in AR3. In this AR6 assessment, the temperature range for the transition from undetectable to moderate risk is still located at a median value of 0.5°C above pre-industrial levels, with ''very high confidence.'' Since impacts were first detected in coral reef systems in the 1980s when warming of ~0.4°C of global warming had occurred (SR15 Chapter 3), this provides the temperature at which the transition begins. The September Arctic sea ice volume has declined by 55–65% between 1979 and 2010 (AR6 WGI, [[#Schweiger--2019|Schweiger et al., 2019]] ) as global warming increased from around 0.36°C in 1979 to around 0.9°C in 2010. These provide evidence of a start to the transition from undetectable to moderate risk at 0.4°C above pre-industrial levels. Recent evidence of observed impacts on mountaintop ecosystems and sea-ice-dependent species, and of range shifts in multiple ecosystems during 1990–2000, which AR6 WGI now assesses as corresponding to a global warming of 0.69°C (see WGI AR6 Cross-Chapter Box 2.3, Figure 1, [[#Gulev--2021|Gulev et al., 2021]] ), provides evidence for an upper limit to this transition of 0.7°C with ''very high confidence'' . Overall, the transition is located at a median of 0.5°C with lower and upper limits of 0.4°C and 0.7°C, respectively, with ''very high confidence.''

AR5 assessed the transition from moderate to high risk to lie around 1°C above 1986–2005 levels (which corresponded at that time to 1.6°C above pre-industrial levels but has been reassessed by AR6 WGI to correspond to 1.7°C) to reflect projected ‘increasing risk to unique and threatened systems, including Arctic sea ice and coral reefs, as well as threatened species as temperature increases over this range.’ SR15 relocated the transition slightly from 1.6°C to 1.5°C, owing to increased literature projecting the effects of climate change upon Arctic sea ice and new literature assessing projected impacts of climate change on biodiversity at 1.5°C warming.

In this AR6 assessment, the transition from moderate to high is based on the high level of observed impacts, and the areas projected to begin undergoing major transformations by 1.5°C (see Cross-Chapter Paper 1, [[IPCC:Wg2:Chapter:Chapter-2|Chapter 2]] and SR15 ( [[#IPCC--2018a|IPCC, 2018a]] )). A substantial number of unique and threatened systems are assessed to be in a high risk state owing to the influence of anthropogenic climate change by the 2000–2010 period, when global warming had reached approximately 0.85°C (range 0.7–1°C) (see WGI AR6 Cross-Chapter Box 2.3, [[#Gulev--2021|Gulev et al., 2021]] ) using the 1995–2014 figure as a proxy for 2000–2010).

The most prominent example of a system assessed to be already in a high risk state is that of coral reefs, which are already degrading rapidly. Observed impacts on coral reefs increased significantly during 2014–2017 (Table SM16.22 , corresponding to a global warming of about 0.9°C). This includes mass bleaching in the Indian Ocean in 1998, 2010, 2015 and 2016 when bleaching intensity exceeded 20% in surveyed locations in the western Indian Ocean, eastern Indian Ocean and western Indonesia. In the tropical Pacific Ocean, climate-driven mass bleaching was reported in all countries in the region, with most bleaching reports coinciding with 2014–2017 marine heatwaves. Fifty percent of coral within shallow-water reefs of the northern and central two-thirds of the Great Barrier Reef were killed in 2015/2016. Subsequent coral recruitment in 2018 was reduced to only 11% of the long-term average, representing an unprecedented shift in the ecology of the northern and middle sections of the reef system to a highly degraded state. A second key example are sea-ice-dependent systems in the Arctic. During August to October of 2010–2019, corresponding to a global warming of about 0.9°C, average Arctic sea ice area has declined in area by 25% relative to 1979–1988 ( ''high confidence'' , AR6 WGI, Figure 9.13). September Arctic sea ice volume has declined by about 72% between 1979 and 2016, with the latter deemed a conservative estimate (AR6 WGI, [[#Schweiger--2019|Schweiger et al., 2019]] ).

Other important examples of observed impacts on unique ecosystems that indicate that risks are already at a high level (Table SM16.22) include mass tree mortalities, now well recorded in multiple unique forest and woodland ecosystems around the world. Sections 2.4.3.3 and 2.4.5 report that, between 1945 and 2007, drought-induced tree mortality (sometimes associated with insect damage and wildfire) has caused the mortality of up to 20% of trees in western North America, the African Sahel, and North Africa, linked to a warming of 0.3–0.9°C above pre-industrial levels, and is implicated in more than 100 other cases of drought-induced tree mortality in Africa, Asia, Australia, Europe, and North and South America ( ''high confidence'' ). Species in biodiversity hotspots already show changes in response to climate change (CCP1, ''high confidence'' ). [[#Román-Palacios--2020|Román-Palacios and Wiens (2020)]] attribute local extinctions of several taxonomic groups between the latter 20th century and 2003–2012, (corresponding to warming of less than 0.85°C) to climate-change-related temperature extremes for up to 44% (0–75%) of species. Widespread declines of up to 35% in the species richness of the unique pollinator group, bumble bees, between 1901–1974 and 2000–2014 are also attributed to climate change, via increasing exceedance of their thermal tolerance limits across Europe and North America ( [[#Soroye--2020|Soroye et al., 2020]] ). The first extinctions attributed to climate change have been now detected with the present 1.2°C warming, including that of the Bramble Cay melomys ( ''Melomys rubicola'' ), a sub-species of the lemuroid ringtail possum ( ''Hemibelideus lemuroides'' ), and golden toad ( ''Incilius periglenes'' ) (Chapter 2). An increasing frequency or unprecedented occurrence of mass animal mortality due to climate-change-enhanced heatwaves has also been observed in recent years on more than one continent, including temperature-vulnerable terrestrial birds and mammals in South Africa and Australia ( [[#Ratnayake--2019|Ratnayake et al., 2019]] ; [[#McKechnie--2021|McKechnie et al., 2021]] ). There have also been 90% declines in sea-ice-dependent species such as sea lions and penguins in the Antarctic (Table SM16.22 ). A strong effect of climate change on the observed contraction of ranges of polar fish species and strong expansion of ranges of arcto-boreal or boreal fish was observed between 2004 and 2012 (Frainer et al.., 2017). Even if current human-driven habitat loss is excluded, many hotspots are projected to cease to be refugia (i.e., to remain climatically suitable for &gt;75% of the species they contain which have been modelled), at 1.0–1.5°C (Cross-Chapter Paper 1).

Based on observed and modelled impacts to unique and threatened systems, including in particular coral reefs, sea-ice-dependent systems and biodiversity hotspots, AR6 assesses that the transition to high risks for RFC1 have already occurred at a median level of 0.9°C, with a lower bound at 0.7°C and an upper bound at the present-day level of global warming of 1.2°C ( [[#WMO--2020|WMO, 2020]] ) ( ''very high confidence'' ).

Identification of the transition to very high risk is associated by definition with the reaching of limits to natural and/or societal adaptation. Adaptation which occurs naturally is already included in the risk assessment, but experts also discussed the effect of additional human-planned adaptation in reducing risk levels in RFC1. This additional adaptation could help species to survive ''in situ'' despite a changing climate (for example, by reducing current anthropogenic stresses such as over-harvesting), or facilitate the ability of species to shift geographic range in response to changes in climate, and the potential benefits of nature-based solutions and restoration (see Cross-Chapter Box NATURAL, [[IPCC:Wg2:Chapter:Chapter-2#2.6.5.1|Section 2.6.5.1]] ).

When considering planned adaptation, the main option often considered in terrestrial ecosystems is the expansion of the protected area network, which is broadly beneficial in increasing the resilience of ecosystems to climate change (e.g., [[#Hannah--2020|Hannah et al., 2020]] ). However, this action is not effective if the unique and threatened systems in question reach a hard limit to adaptation (as in the case of the loss of Arctic summer sea ice, the submergence of a small island, the contraction and elimination of a species’ climatic niche from a mountaintop, or the degradation of a coral reef) ( [[#16.4|Section 16.4]] ). Furthermore, adaptation benefits deriving from restoration rapidly diminish with increasing temperature (Cross-Chapter Paper 1). One study quantifies how land management (in terms of protecting existing ecosystems or restoring lost ones) might reduce extinctions in biodiversity hotspots or globally significant terrestrial biodiversity areas more generally ( [[#Warren--2018b|Warren et al., 2018b]] ). While the latter suggests that substantial benefits can result globally in terrestrial systems, allowing less unique systems to persist at higher levels of warming but only under a high adaptation scenario in which globally applied terrestrial ecosystem restoration and protected area expansion takes place, this is less likely for many of the unique and threatened terrestrial systems which are more vulnerable than the globally significant biodiversity areas treated in that study (which excludes coral reefs and Arctic sea-ice-dependent systems). Such high levels of adaptation globally are likely infeasible owing to competition for land use with food production ( [[#Pörtner--2021|Pörtner et al., 2021]] ). Novel targeted adaptation interventions for coral reefs such as artificial upwelling and local radiation management show some promise for reducing the adverse effects of thermal stress and resulting coral bleaching ( [[#Condie--2021|Condie et al., 2021]] ), but are far from implementation ( [[#Sawall--2020|Sawall et al., 2020]] ; [[#Kleypas--2021|Kleypas et al., 2021]] ). Larger benefits in this RFC could theoretically accrue only if adaptation action became ubiquitous and extensive, which experts considered infeasible at the scales required. Small island communities are confronted by socio-ecological limits to adaptation well before 2100, especially those reliant on coral reef systems for their livelihoods, even for a low-emissions pathway (Chapter 3) ( ''high confidence'' ). At warming levels beyond 1.5°C, the potential to reach biophysical limits to adaptation due to limited water resources are reported for small islands ( ''medium confidence'' ) and unique systems dependent on glaciers and snowmelt (Chapter 4) ( ''medium confidence'' ).

AR5 assessed with ''high confidence'' that the transition from high to very high risks for RFC1 lies around 2°C above 1986–2005 levels (then considered to correspond to 2.6°C above pre-industrial levels) to reflect the very high risk to species and ecosystems projected to occur beyond that level as well as limited ability to adapt to impacts on coral reef systems and in Arctic sea-ice-dependent systems. Using the additional literature which became available on projected risks to Arctic sea ice, biodiversity and ecosystems at 1.5°C versus 2°C warming above pre-industrial levels, SR15 assessed that the transition from high to very high risks in RFC1 lay between 1.5°C and 2°C above pre-industrial levels.

In AR6, risks are considered to start to transition from high to very high risks above 1.2°C warming (present day, [[#WMO--2020|WMO, 2020]] ), with a median value of 1.5°C, owing in particular to the observation of a present-day onset of ecosystem degradation in coral reefs, which are projected in the SR15 report ‘to decline by a further 70–90% at 1.5°C ( ''very high confidence'' )’ . The literature for projected increases in risk to other unique and threatened systems and their limited ability to adapt above 2°C warming is substantial and robust, and the confidence level in very high risk remains high. At 2°C, 18% of 34,000 insects are projected to lose &gt;50% climatically determined geographic range, as compared with 6% at 1.5°C ( [[#Warren--2018a|Warren et al., 2018a]] ). The risk of species extinction increases with warming in all climate change projections, for all native species studied in biodiversity hotspots (Cross-Chapter Paper 1, ''high confidence'' ), being roughly threefold greater for endemic than more widespread species for global warming of 3°C above pre-industrial levels than 1.5°C) ( [[#Manes--2021|Manes et al., 2021]] , Cross-Chapter Paper 1) ( ''medium confidence'' ). The Arctic is projected to be practically ice free in September in some years for global warming of between 1.5°C and 2°C (WGI AR6 [[IPCC:Wg2:Chapter:Chapter-9#9.3.1|Section 9.3.1.1]] , [[#Fox-Kemper--2021|Fox-Kemper et al., 2021]] ), undermining the persistence of ice-dependent species such as polar bears, ringed seals and walrus ( [[#Meredith--2019|Meredith et al., 2019]] ), and adversely affecting Indigenous communities. Warming of 1.5°C is also assessed (Chapter 3) to reduce the habitability of small islands, due to the combined impacts of several key risks ( ''high confidence'' ). Hence, the transition from high to very high risk in these systems is assessed to occur with ''high confidence'' beginning at 1.2°C, passing through a median value of 1.5°C, and completing (i.e., reaching its upper bound) at 2°C warming.

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