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

== Executive Summary ==

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<div id="Introduction" class="h4-container"></div>

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===== Introduction and framing =====

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This chapter synthesises observed climate change impacts ( [[#16.2|Section 16.2]] ), adaptation-related responses ( [[#16.3|Section 16.3]] ), limits to adaptation ( [[#16.4|Section 16.4]] ), and the key risks identified across sectors and regions ( [[#16.5|Section 16.5]] ). We consider how these risks accrue with increasing global average temperature, how they depend on future development and adaptation efforts and what this implies for the Sustainable Development Goals and the five main Reasons for Concern about climate change ( [[#16.6|Section 16.6]] ).

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===== Observed impacts =====

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'''The impacts of changes in climate-related systems have been identified in a wide range of natural, human and managed systems (''' '''''very high confidence''''' [[#footnote-002|1]] ''').''' Compared with the last Intergovernmental Panel on Climate Change (IPCC) 5th Assessment Report (AR5), there is more evidence for impacts of long-term changes in climate-related systems (including the atmosphere, ocean and cryosphere) on socioeconomic indicators and ''high confidence'' in the sensitivity of societies to weather conditions. There is also stronger evidence for impacts of long-term climate change on ecosystems, including the observed widespread mortality of warm water corals, far reaching shifts in phenology in marine and terrestrial ecosystems and the expansion of tropical species into the ranges of temperate species, and boreal species moving into Arctic regions ( ''high confidence'' ). {16.2.3, 16.2.3.1}

'''Increased rainfall intensity associated with tropical cyclones and rising sea levels have contributed to observed damages in local coastal systems (''' '''''medium confidence''''' ''').''' However, while the impact is expected to be widespread, formal attribution of damages to long-term changes in the climate-related systems is still limited by restricted knowledge about changes in exposure and vulnerability and the missing quantification of the contribution of sea level rise to the extent of flooded areas. {16.2.3.3}

'''Due to complex interactions with socioeconomic conditions, evidence on the impact of long-term climate change on crop prices and malnutrition is largely lacking''' , while the sensitivity of malnutrition to weather conditions has become more evident in some regions, particularly Africa ( ''medium'' to ''high confidence'' ). A negative impact of long-term climate change on crop yields has been identified in some regions (e.g., wheat yields in Europe) ( ''medium confidence'' ), while studies are still inconsistent in other regions. {16.2.3.4}

'''Climate change has increased observed heat-related mortality (''' '''''medium confidence''''' ''') and contributed to the observed latitudinal or altitudinal range expansion of vector-borne diseases into previously colder areas (''' '''''medium''''' '''to''' '''''high confidence''''' '''), while evidence on the impact of long-term climate change on water-borne diseases is largely lacking.''' Overall, there is extensive observational evidence that extreme ambient temperatures increase human mortality ( ''high confidence'' ) and that the occurrence of water- and vector-borne diseases is sensitive to weather conditions ( ''high confidence'' ). {16.2.3.5, 16.2.3.6, 16.2.3.7}

'''Extreme weather events not only cause substantial direct economic damage (''' '''''high confidence''''' '''), but also reduce economic growth in the short term (year of, and year after event) (''' '''''high confidence''''' ''') as well as in the long term (up to 15 years after the event) (''' '''''medium confidence''''' '''), with more severe impacts in developing than in industrialised economies (''' '''''high confidence''''' ''').''' Evidence has increased for all of these conclusions; however, evidence for impacts of long-term climate change is still limited. {16.2.3.7}

'''Climate variability and extremes are associated with increased prevalence of conflict, with more consistent evidence for low-intensity organised violence than for major armed conflict (''' '''''medium confidence''''' ''').''' Compared with other socioeconomic drivers, the link is relatively weak ( ''medium confidence'' ) and conditional on high population size, low socioeconomic development, high political marginalisation and high agricultural dependence ( ''medium confidence'' ). Literature also suggests a larger climate-related influence on the dynamics of conflict than on the likelihood of initial conflict outbreak ( ''low confidence'' ). There is insufficient evidence at present to attribute armed conflict to climate change. {16.2.3.8}

'''There is''' '''''high confidence''''' '''that anthropogenic climate forcing has had an impact on internal displacement, given the observed impact of anthropogenic climate forcing on the occurrence of weather extremes (''' '''''high confidence''''' ''', Table SM16.21) and the strong contribution of weather extremes to observed displacement (''' '''''high confidence''''' ''').''' However, the link between long-term changes in the climate-related systems has not been demonstrated systematically, and so far there is no attribution of observed trends in displacement to long-term changes in the climate-related systems. Links between weather fluctuations (including extreme events) and human mobility are complex and conditional on socioeconomic situations; for example, poor populations may more often be involuntarily displaced or ‘trapped’ and not be able to migrate. {16.2.3.9}

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===== Observed adaptation in ecosystems =====

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'''While species are increasingly responding to climate change, these responses may not be adaptive or sufficient to cope with the rate of climate change (''' '''''high confidence''''' ''').''' Responses have been documented in a range of species, including, for example, changes in the timing of breeding and migration. It is unclear whether these responses reflect long-term evolutionary adaptation or short-term coping mechanisms. Existing assessments indicate that some species’ responses will be insufficient to avert extinction. {16.3.1}

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===== Observed adaptation-related responses in human systems =====

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'''Responses across all sectors and regions reported in the scientific literature are dominated by minor modifications to usual practices or measures for dealing with extreme weather events, while evidence of transformative adaptation in human systems is low (''' '''''high confidence''''' ''').''' Responses have accelerated in both developed and developing regions since AR5, with some examples of regression. Despite this, there is negligible evidence in the scientific literature documenting responses that are simultaneously widespread and rapid, and that challenge norms and adaptation limits. {16.3.2.3}

'''There is negligible evidence that existing responses are adequate to reduce climate risk (''' '''''high confidence''''' ''').''' There is some evidence of global vulnerability reduction, particularly for mortality and economic losses due to flood risk and extreme heat ( [[#16.3.2.4|Section 16.3.2.4]] ). Evidence on the effectiveness of specific adaptations remains limited. There is negligible ''robust evidence'' to assess the overall adequacy of the global adaptation response to address the scale of climate risk. No studies have systematically assessed the adequacy and effectiveness of adaptation at a global scale, across nations or sectors, or for different levels of warming. {16.3.2.3}

'''Adaptation responses are showing co-benefits, for mitigation and other societal goals (''' '''''high confidence''''' ''').''' There is increasing evidence of co-benefits of adaptation responses. Co-benefits are most frequently linked to changes in agricultural practices (e.g., conservation agriculture), land use management (e.g., agroforestry), building technologies (e.g., building efficiency standards) and urban design (e.g., walkable neighbourhoods). {16.3.2.3}

'''Evidence of maladaptation is increasing (''' '''''high confidence''''' ''')''' , that is, adaptation that increases climate risk or creates new risks in other systems or for other actors. Globally, maladaptation has been reported most frequently in the context of agriculture and migration in the Global South. {16.3.2.6}

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===== Limits to adaptation across natural and human systems =====

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'''There is increasing evidence on limits to adaptation which result from the interaction of adaptation constraints and can be differentiated into soft and hard limits (''' '''''high confidence''''' ''').''' Soft limits may change over time as additional adaptation options become available. Hard limits will not change over time as no additional adaptive actions are possible. Evidence focuses on constraints that may lead to limits at some point of the adaptation process, with less information on how limits may be related to different levels of socioeconomic or climatic change ( ''high confidence'' ). {16.4.1, 16.4.2, 16.4.3}

'''Limits to adaptation have been identified for terrestrial and aquatic species and ecosystems, coastal communities, water security, agricultural production, and human health and heat (''' '''''high confidence''''' ''').''' Beginning at 1.5°C, autonomous and evolutionary adaptation responses by terrestrial and aquatic species and ecosystems face hard limits, resulting in biodiversity decline, species extinction and loss of related livelihoods ( ''high confidence'' ). Beginning at 3°C, hard limits are projected for water management measures, leading to decreased water quality and availability, negative impacts on health and well-being, economic losses in water and energy-dependent sectors and potential migration of communities ( ''medium confidence'' ). Adaptation to address risks of heat stress, heat mortality and reduced capacities for outdoor work for humans face soft and hard limits across regions beginning at 1.5°C, and are particularly relevant for regions with warm climates ( ''high confidence'' ). {16.4.2, 16.4.3}

'''Soft limits are currently being experienced by individuals and households along the coast and by small-scale farmers (''' '''''medium confidence''''' ''').''' As sea levels rise and extreme events intensify, coastal communities face soft limits due to financial, institutional and socioeconomic constraints reducing the efficacy of coastal protection and accommodation approaches and resulting in loss of life and economic damages ( ''medium confidence'' ). {16.4.2, 16.4.3}

'''Hard limits for coastal communities reliant on nature-based coastal protection will be experienced beginning at 1.5°C (''' '''''medium confidence''''' ''').''' Soft and hard limits for agricultural production are related to water availability and the uptake and effectiveness of climate-resilient crops which are constrained by socioeconomic and political challenges ( ''medium confidence'' ). {16.4.2, 16.4.3}

'''Across regions and sectors, the most significant determinants of soft limits are financial, governance, institutional and policy constraints (''' '''''high confidence''''' ''').''' The ability of actors to overcome these socioeconomic constraints largely influence whether additional adaptation is able to be implemented and prevent soft limits from becoming hard. While the rate, extent and timing of climate hazards largely determine hard limits of biophysical systems, these factors appear to be less influential in determining soft limits for human systems ( ''medium confidence'' ). {16.4.2, 16.4.3}

'''Financial constraints are important determinants of limits to adaptation, particularly in low-to-middle-income countries (''' '''''high confidence''''' ''').''' Impacts of climate change may increase financial constraints ( ''high confidence'' ) and contribute to soft limits to adaptation being reached ( ''medium confidence'' ). Global and regional evidence shows that climate impacts may limit the availability of financial resources, stunt national economic growth, result in higher levels of losses and damages and thereby increase financial constraints. {16.4.3.2, 16.4.3.3}

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===== Key risks across climate and development pathways =====

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'''Regional and sectoral chapters of this report identified over 120 key risks (KRs) that could become severe under particular conditions of climate hazards, exposure, and vulnerability. These key risk are represented in eight so-called Representative Key Risks (RKRs) clusters of key risks relating to low-lying coastal systems; terrestrial and ocean ecosystems; critical physical infrastructure, networks and services; living standards; human health; food security; water security; and peace and human mobility (''' '''''high confidence''''' ''').''' A key risk is defined as a potentially ‘severe’ risk, that is, one that is relevant to the interpretation of dangerous anthropogenic interference (DAI) with the climate system. Key risks cover scales from the local to the global, are especially prominent in particular regions or systems, and are particularly large for vulnerable subgroups, especially low-income populations, and already at-risk ecosystems ( ''high confidence'' ). The conditions under which RKRs would become severe have been assessed along levels for warming, exposure/vulnerability, and adaptation: for warming, high refers to climate outcomes consistent with RCP8.5 or higher, low refers to climate outcomes consistent with RCP2.6 or lower, and medium refers to outcomes for scenarios between RCPs 2.6 and 8.5; exposure/vulnerability levels are relative to the range of future conditions considered in the literature; for adaptation, high refers to near maximum potential and low refers to the continuation of today’s trends (and Sections 6.5.2.1, 16.5.2.2, SM16.7.4).

'''For most Representative Key Risks (RKRs), potentially global and systemically pervasive risks become severe in the case of high warming, combined with high exposure/vulnerability, low adaptation, or both (''' '''''high confidence''''' ''').''' Under these conditions, there would be severe and pervasive risks to critical infrastructure ( ''high confidence'' ) and to human health from heat-related mortality, to low-lying coastal areas, aggregate economic output, and livelihoods (all ''medium confidence'' ), of armed conflict ( ''low confidence),'' and to various aspects of food security (with different levels of confidence). Severe risks interact through cascading effects, potentially causing amplification of RKRs over the course of this century ( ''limited evidence'' , ''high agreement'' ). {16.5.2.3, 16.5.2.4, 16.5.4, Figure 16.10}

'''For some RKRs, potentially global and systemically pervasive risks would become severe even with medium to low warming (i.e., 1.5–2°C) if exposure/vulnerability is high and/or adaptation is low (''' '''''medium''''' '''to''' '''''high confidence''''' ''').''' Under these conditions, there would be severe and pervasive risks associated with water scarcity and water-related disasters ( ''high confidence'' ), poverty, involuntary mobility, and insular ecosystems and biodiversity hotspots (all ''medium confidence'' ). {16.5.2.3, 16.5.2.4}

'''All potentially severe risks that apply to particular sectors or groups of people at more specific regional and local levels require high exposure/vulnerability or low adaptation (or both), but do not necessarily require high warming (''' '''''high confidence''''' ''').''' Under these conditions, there would be severe, specific risks to low-lying coastal systems, to people and economies from critical infrastructure disruption, economic output in developing countries, livelihoods in climate-sensitive sectors, waterborne diseases especially in children in low- and middle-income countries, water-related impacts on traditional ways of life, and involuntary mobility for example in small islands and low-lying coastal areas ( ''medium'' to ''high confidence'' ). {16.5.2.3, 16.5.2.4}

'''Some severe impacts are already occurring (''' '''''high confidence''''' ''') and will occur in many more systems before mid-century (''' '''''medium confidence''''' ''').''' Tropical and polar low-lying coastal human communities are experiencing severe impacts today ( ''high confidence'' ), and abrupt ecological changes resulting from mass population-level mortality are already observed following climate extreme events. Some systems will experience severe risks before the end of the century ( ''medium confidence'' ), for example critical infrastructure affected by extreme events ( ''medium confidence'' ). Food security for millions of people, particularly low-income populations, also faces significant risks with moderate to high warming or high vulnerability, with a growing challenge by 2050 in terms of providing nutritious and affordable diets ( ''high confidence'' ). {16.5.2.3, 16.5.3}

'''In specific systems already marked by high exposure and vulnerability, high adaptation efforts will not be sufficient to prevent severe risks from occurring under high warming (''' '''''limited evidence''''' ''',''' '''''medium agreement''''' ''').''' This is particularly the case for some ecosystems and water-related risks (from water scarcity and to indigenous and traditional cultures and ways of life). {16.5.2.3, 16.5.2.4, 16.5.3}

'''Interconnectedness and globalisation establish pathways for the transmission of climate-related risks across sectors and borders, for instance through trade, finance, food and ecosystems (''' '''''high confidence''''' ''').''' Examples include semiconductors, global investments, major food crops like wheat, maize and soybean, and transboundary fish stocks. There are knowledge gaps on the need for, effectiveness of, and limits to adaptation to such inter-regional risks. { [[#cross-chapter-box-intereg|Cross-Chapter Box INTERREG]] in this Chapter }

'''Key risks increase the challenges in achieving global sustainability goals (''' '''''high confidence''''' ''').''' The greatest challenges will be from risks to water (RKR-G), living standards (RKR-D), coastal socio-ecological systems (RKR-A) and peace and human mobility (RKR-H). The most relevant goals are zero hunger (Sustainable Development Goal [SDG] 2), sustainable cities and communities (SDG11), life below water (SDG14), decent work and economic growth (SDG8), and no poverty (SDG1). Priority areas for regions are indicated by the intersection of hazards, risks and challenges, where, in the near term, challenges to SDGs indicate probable systemic vulnerabilities and issues in responding to climatic hazards ( ''high confidence'' ). {16.6.1}

The scale and nature of climate risks is partly determined by the responses to climate change, not only in how they reduce risk, but also how they may create other risks (sometimes inadvertently, and sometimes to others than those who implement the response, in other places, or later in time).

'''Solar radiation modification (SRM) approaches have potential to offset warming and ameliorate other climate hazards, but their potential to reduce risk or introduce novel risks to people and ecosystems is not well understood (''' '''''high confidence''''' ''').''' SRM effects on climate hazards are highly dependent on deployment scenarios, and substantial residual climate change or overcompensating change would occur at regional scales and seasonal time scales ( ''high confidence'' ). Due in part to limited research, there is ''low confidence'' in projected benefits or risks to crop yields, economies, human health or ecosystems. Large negative impacts are projected from rapid warming for a sudden and sustained termination of SRM in a high-CO 2 scenario ( ''high confidence'' ). SRM would not stop CO 2 from increasing in the atmosphere or reduce resulting ocean acidification under continued anthropogenic emissions ( ''high confidence'' ). There is ''high agreement'' in the literature that for addressing climate change risks SRM is, at best, a supplement to achieving sustained net zero or net negative CO 2 emission levels globally. Co-evolution of SRM governance and research provides a chance for responsibly developing SRM technologies with broader public participation and political legitimacy, guarding against potential risks and harms relevant across a full range of scenarios. { [[#cross-working-group-box-srm|Cross-Working Group Box SRM]] }

'''Recent global estimates of the economic cost of climate impacts exhibit significant spread and generally increase with global average temperature, as well as vary by other drivers, such as income, population and composition of the economy (''' '''''high confidence''''' ''').''' The wide variation across disparate methodologies does not allow a robust range of damage estimates to be identified with confidence, though the spread of estimates increases with warming in all methodologies, indicating higher risk (in terms of economic costs) at higher temperatures ( ''high confidence'' ). Reconciling methodological variance is a priority for facilitating use of different lines of evidence; however, that some new estimates are higher than the AR5 range indicates that global aggregate economic impacts could be higher than previously assessed ( ''low confidence'' due to the lack of robustness and comparability across methodologies). { [[#cross-working-group-box-economic|Cross-Working Group Box ECONOMIC]] in Chapter 16 }

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===== Reasons for Concern across scales =====

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The five major Reasons for Concern (RFCs), describing risks associated with (1) unique and threatened systems, (2) extreme weather events, (3) distribution of impacts, (4) global aggregate impacts, and (5) large-scale singular events, were updated using expert elicitation. RFC risk levels were assessed with no or low adaptation, but limits to adaptation are a factor in the identification of very high risk levels.

'''Compared with AR5 and SR15, risks increase to high and very high levels at lower global warming levels for all five RFCs (''' '''''high confidence''''' '''), and transition ranges are assigned with greater confidence. Transitions from high to very high risk emerge in all five RFCs, compared with just two RFCs in AR5 (''' '''''high confidence''''' '''). {16.6.3, Figure 16.15}'''

* For unique and threatened systems (RFC1), as before, levels of risk at a given level of warming are higher than for the other RFCs. Risks are already (at current warming of 1.1°C) in the transition from moderate to high ( ''very high confidence'' ), compared with moderate in AR5 and SR15, based on observed and modelled impacts. The transition to very high risk occurs between 1.2°C and 2.0°C warming ( ''high confidence'' ) ''.'' {16.6.3.1}
* For risks from extremes (RFC2), the transition to high risk is between 1.0°C and 1.5°C ( ''high confidence'' ) and to very high risk (new in AR6) between 1.8°C and 2.5°C ( ''medium confidence'' ). {16.6.3.2}
* For risks disproportionately affecting particularly vulnerable societies and socio-ecological systems, including disadvantaged people and communities in countries at all levels of development (RFC3), current risk is moderate ( ''high confidence'' ) and the transition to high risk is between 1.5°C and 2.0°C warming ( ''medium confidence'' ). The transition to very high risk occurs at between 2.0°C and 3.5°C warming ( ''medium confidence'' ). {16.6.3.3}
* The risk of global aggregate impacts, including monetary damages, lives affected, species lost or ecosystem degradation at a global scale (RFC4), has begun to transition to moderate risk ( ''medium confidence'' ), with a transition to high risk between 1.5°C and 2.5°C ( ''medium confidence'' ) and to very high risk (new in AR6) at between 2.5°C and 4.5°C ( ''medium confidence'' ). {16.6.3.4}
* Present-day risks associated with large-scale singular events (sometimes called tipping points or critical thresholds) (RFC5) are already moderate ( ''high confidence'' ), with a transition to high risk between 1.5°C and 2.5°C ( ''medium confidence'' ) and to very high risk (new in AR6) between 2.5°C and 4°C ( ''low confidence'' ). {16.6.3.5}

'''Limiting global warming to 1.5°C would ensure risk levels remain moderate for RFC3, RFC4 and RFC5 (''' '''''medium confidence''''' '''), but risk for RFC2 would have transitioned to a high risk at 1.5°C and RFC1 would be well into the transition to very high risk (''' '''''high confidence''''' '''). Remaining below 2°C warming (but above 1.5°C) would imply that risk for RFC3 through 5 would be transitioning to high, and risk for RFC1 and RFC2 would be transitioning to very high (''' '''''high confidence''''' ''').''' By 2.5°C warming, RFC1 will be in very high risk ( ''high confidence'' ) and all other RFCs will have begun their transitions to very high risk ( ''medium confidence'' for RFC2, RFC3 and RFC4, ''low confidence'' for RFC5).

RFC1, RFC2 and RFC5 include risks that are irreversible, such as species extinction, coral reef degradation, loss of cultural heritage, or loss of a small island due to sea level rise. Once such risks materialise, as is expected at very high risk levels, the impacts would persist even if global temperatures would subsequently decline to levels associated with lower levels of risk in an ‘overshooting’ scenario ( ''high confidence'' ). {16.6.3}

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