Editing IPCC:AR6/WGII/Chapter-14 (section)

=== 14.6.3 Cumulative Risk, Tipping Points, Thresholds and Limits ===

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Across North America, climate change poses a risk to social–ecological systems increasingly destabilised by compounding climate impacts and non-climate pressures ( ''high confidence'' ) (Sections 14.5.1–14.5.3) that erode the connectivity and redundancy underpinning system resilience (Sections 14.5.1–14.5.5; [[#Xiao--2017a|Xiao et al., 2017a]] ; [[#Koven--2020|Koven et al., 2020]] ; [[#Malhi--2020|Malhi et al., 2020]] ; [[#Turner--2020|Turner et al., 2020]] ). Accelerating climate change and increasingly severe hazards and shocks may induce abrupt changes or push systems, people and species to critical points–tipping points–where a small additional change causes a disproportionately large response, triggering feedbacks that lock systems into novel regimes ( [[#Scheffer--2001|Scheffer et al., 2001]] ; [[#Scheffer--2010|Scheffer, 2010]] ; [[#Anderies--2013|Anderies et al., 2013]] ; [[#Lenton--2013|Lenton, 2013]] ; [[#Iglesias--2020|Iglesias and Whitlock, 2020]] ; [[#Lenton--2020a|Lenton, 2020a]] ). Climate-change tipping points can compound and amplify climate impacts and risk, induce disparate climate burdens and benefits across human and ecological systems, and irreversibly restructure ecosystems and livelihoods (e.g., species extinctions, fisheries collapse, community-managed relocation) ( [[#Lynham--2017|Lynham et al., 2017]] ). Examples of systems with potential tipping points in North America include (a) permafrost and sea ice loss triggering transformation of ecological and human systems (including substantial shipping opportunities) in the Arctic that are permanent and irreversible except on geological timescales, and which are potentially underway ( ''high agreement, low evidence'' ) ( [[#14.6.2|Section 14.6.2]] ; see Box 14.3, CCP6), (b) mid-latitude forest ecosystems at low to middle elevations in western North America where wildfire and cumulative climate and non-climate pressures may restructure forests and succession in ways that promote transition to new vegetation types ( [[#14.5.1|Section 14.5.1]] ) and (c) agricultural communities in northern Mexico and the southwest USA where aridification and drought may interact with water resource policies, economic opportunities and pressures, and farm practices to induce either adaptation (via changes in irrigation practices) or farm abandonment, land-use transformation and livelihood changes (due to heat stress, soil deterioration or reduced economic viability) (Sections 14.5.3, 14.5.4, CCP6, [[#Yumashev--2019|Yumashev et al., 2019]] ; [[#Turner--2020|Turner et al., 2020]] ; [[#Heinze--2021|Heinze et al., 2021]] ).

Identification of critical thresholds, elements and connections within a system may also help identify potential positive tipping points, that is, focal components or processes in a system where a relatively small investment or intervention can induce a large benefit and enable self-reinforcing transformative adaptation ( [[#14.7|Section 14.7]] ; Chapter 17; [[#Tàbara--2018|Tàbara et al., 2018]] ; [[#Lenton--2020b|Lenton, 2020b]] ; [[#Otto--2020|Otto et al., 2020]] ). Under low-mitigation scenarios, compounding risks and higher-carbon-emission scenarios increase the potential that amplifying feedback loops and fatal synergies across sectors could lead to existential threats to the social–ecological systems of North America ( ''medium confidence'' ). Societal collapse has been linked to shifts in climate regimes, especially when societies have lost resilience due to slowly mounting social–ecological challenges, while other studies reveal that social continuity and flexibility enable historical climate resilience and prosperity under changing environments (FAQ 14.2; [[#Lenton--2019|Lenton et al., 2019]] ; [[#Otto--2020|Otto et al., 2020]] ; [[#Degroot--2021|Degroot et al., 2021]] ; [[#Richards--2021|Richards et al., 2021]] ).

Accounting for tipping points, interactions and reinforcing dynamics among ecological, social and climate processes is necessary for comprehensive analyses of climate-change risk, cost and urgency, as well as effective adaptation design and implementation ( [[#14.7|Section 14.7]] ; [[#Cai--2015|Cai et al., 2015]] ; Steffen and et al., 2018; [[#Lenton--2019|Lenton et al., 2019]] ; [[#Narita--2020|Narita et al., 2020]] ; [[#Dietz--2021|Dietz et al., 2021]] ). Multiple lines of evidence across sectors assessed in this chapter suggest that after mid-century and without carbon mitigation, climate-driven changes to ecological and social boundary conditions may rapidly push many systems into disequilibrium ( ''medium confidence'' ), emphasising the importance of prioritising adaptation actions with co-benefits for mitigation ( [[#14.5.4|Section 14.5.4]] ; see Box 14.3). Reducing climate hazards through mitigation and removing catalysts of system instability through adaptation measures that increase system resilience (e.g., ecosystem restoration) will help reduce the risk that systems move across a tipping point from a desirable to an alternate or undesirable state (Sections 14.5.4, 14.7; see Box 14.3; [[#Narita--2020|Narita et al., 2020]] ; [[#Turner--2020|Turner et al., 2020]] ; [[#Heinze--2021|Heinze et al., 2021]] ).

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