Editing IPCC:AR6/WGII/Chapter-6 (section)

=== 6.2.5 Compound and Cascading Risks in Urban Areas ===

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Compound events can be initiated via hazards such as single extreme events, or multiple coincident events overlapping and interacting with exposed urban systems or sectors as compound climate risks (Leonard et al., 2014; [[#IPCC--2019b|IPCC, 2019b]] ; Piontek et al., 2014). Hydrometeorological hazards, such as extreme precipitation from tropical cyclones, fronts and thunderstorms, often combine with storm surges and freshwater discharge leading to high compound risks at exposed settlements (Zheng, Westra and Sisson, 2013; [[#Chen--2014|Chen and Liu, 2014]] ; [[#Ourbak--2018|Ourbak and Magnan, 2018]] ; [[#Dowdy--2017|Dowdy and Catto, 2017]] ). The compounding effect between these hydrometeorological hazards suggest that the combined impact of these events are greater than each of these variables on its own, and can amplify risks in affected settlements (Kew et al., 2013; Vitousek et al., 2017). These risks are concentrated in coastal cities exposed to sea level rise and severe storms (van den Hurk et al., 2015; Wahl et al., 2015; Paprotny et al., 2018b; Lagmay et al., 2015), or in settlements located in valleys prone to slope failure, such as the 2013 Uttarakhand floods and landslides arising from extreme precipitation and glacial lake outbursts along the Mandakini river in India (Ziegler et al., 2016; Barata et al., 2018).

Cascading climate events occur when an extreme event triggers a sequence of secondary events within natural and human systems that causes additional physical, natural, social or economic disruption. The resulting impact can be significantly larger than the initial hazard ( [[#IPCC--2019b|IPCC, 2019b]] ). Each step in a risk cascade can generate direct (immediate impacts) and secondary (consequential impacts) losses. Risks from these cascading impacts are complex and multi-dimensional (Hao, Singh and Hao, 2018; [[#Zscheischler--2017|Zscheischler and Seneviratne, 2017]] ). For instance, combined droughts and heatwaves increases risks of urban water scarcity (Miralles et al., 2019; Gillner, Bräuning and Roloff, 2014; Gill et al., 2013), as well as increasing wildfire extent and lowering snowpack conditions that affected peri-urban settlements adjacent to forested areas, as observed in California during the 2014 drought (AghaKouchak et al., 2014). Similarly, heatwaves can increase the risk of mortality associated with air pollution (see [[IPCC:Wg2:Chapter:Chapter-7#7.2.2.5|Section 7.2.2.5]] ).

Urban areas and their infrastructure are susceptible to both compounding and cascading risks arising from interactions between severe weather from climate change and increasing urbanisation ( ''medium evidence'' , ''high agreement'' ) ( [[#Moretti--2018|Moretti and Loprencipe, 2018]] ; Markolf et al., 2019). Risks are complex and multi-dimensional, and can significantly amplify the impact of single events across space, scale and time. Impacts are determined by the magnitude of urban vulnerability and/or the interdependence of urban critical infrastructure ( [[#Pescaroli--2018|Pescaroli and Alexander, 2018]] ; Zuccaro, De Gregorio and Leone, 2018). Poorer and wealthier settlements and cities are then both at risk from compound and cascading risks though potentially through contrasting mechanisms. For richer and poorer cities, managing climate risk as part of compound and cascading risks that can also include technological, biological and political risks places renewed emphasis on investment in generic capabilities that reduce vulnerability and on risk monitoring capability to track and respond to impacts across infrastructures and places ( ''limited evidence'' , ''high agreement'' ). Considering climate risk and managing such risk as part of complex, compounding and/or cascading risks is in its infancy but rapidly being accepted as necessary, especially when considering the wider poverty and justice implications of climate change arising from differentiated vulnerability in cities.

Compound risks to key infrastructure in cities have increased from extreme weather ( ''medium evidence'' , ''high agreement'' ), such as from urban flooding from extreme precipitation and storm surges disrupting transport infrastructure and networks, for example Mehrotra et al. (2018), see also San Juan case study in this chapter), ICT networks, for example underground cables or transmission towers (Schwarze et al., 2018), and energy generation from power plants (Marcotullio et al., 2018).

The increased risk arises not just from greater exposure from climate events impacting cities, but is also magnified by low adaptive capacity that can arise from intra-urban variations in infrastructure quality. For instance, infrastructure within expanding informal settlements is associated with deficiency in materials, structural safety and a lack of accessibility. These areas are often located in the most risk-prone urban areas in developing nations that are vulnerable to compound hazards (Dawson et al., 2018). Further, these risks can be exacerbated from complications arising from local versus national governance and/or regulations related to hazard management ( [[#Garschagen--2016|Garschagen, 2016]] ; [[#Castán%20Broto--2017|Castán Broto, 2017]] ).

Projected global compound risks will increase in the future, with significant risks across energy, food and water sectors that likely overlap spatially and temporally while affecting increasing numbers of people and regions particularly in Africa and Asia ( ''high confidence'' ) (Hoegh-Guldberg et al., 2018). In cities, the prevalence of compounding risks therefore necessitates methodologies accounting for non-stationary risk factors.

Secondary impacts occurring sequentially after an extreme hazard can severely affect disaster management, especially in complex urban systems ( ''robust evidence'' , ''high agreement'' ). Over time, relatively small perturbations can cascade outward from a primary failure, triggering further failures in other dependent parts of the network some distance away from the primary failure (Penny et al., 2018). In some cities, such as those prone to compound flood hazards, these dependent network parts can be dams, levees or other critical flood protection infrastructure that are essential for managing these cascading risks ( [[#Serre--2018|Serre and Heinzlef, 2018]] ; [[#Fekete--2019|Fekete, 2019]] ). Failure of these infrastructure systems can result in sequential failures in urban transport ( [[#Zaidi--2018|Zaidi, 2018]] ), energy networks ( [[#Sharifi--2016|Sharifi and Yamagata, 2016]] ), urban biodiversity ( [[#Solecki--2013|Solecki and Marcotullio, 2013]] ) and so-called na-tech disasters; when natural hazards trigger technological disasters (Girgin, Necci and Krausmann, 2019). This risk cascade can propagate more widely by stopping flows of people, goods and services, with economic consequences beyond urban areas ( [[#Wilbanks--2014|Wilbanks and Fernandez, 2014]] ).

Compound and cascading climate risks require a different way of accounting for cumulative hazard impacts in urban areas ( ''medium evidence'' , ''high agreement'' ). There is emerging literature calling for analysis on interactions between individual and inter-related climate extremes with complex urban systems, so as to ascertain how urban and key infrastructural vulnerabilities can be identified and managed in a warming world (Butler, Deyle and Mutnansky, 2016; Gallina et al., 2016; Moftakhari et al., 2017; Zscheischler et al., 2018; Baldwin et al., 2019; [[#Pescaroli--2018|Pescaroli and Alexander, 2018]] ; Yin et al., 2017; AghaKouchak et al., 2020), as well as in managing adaptation for present and future pandemics, for example COVID-19 (Pelling et al., 2021; Phillips et al., 2020).

In terms of policy, case studies from London’s resilience planning process stressed the need for intermodal coordination, hazard risk and infrastructure mapping, clarifying tipping points and acceptable levels of risk, training citizens, strengthening emergency preparedness, identifying relevant data sources, and developing scenarios and contingency plans ( [[#Pescaroli--2018|Pescaroli, 2018]] ). Others also note the utility of a systems approach to analysing risks and benefits, including considerations of potential cascading ecological effects, full lifecycle environmental impacts, and unintended consequences, as well as possible co-benefits of responses (Ingwersen et al., 2014). Lowering these risks requires urban stakeholders to reduce urban vulnerability by going beyond linear approaches to risk management ( ''medium evidence'' , ''high agreement'' ).

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'''Box 6.3 | Climate Change Adaptation for Cities in Fragile and Conflict Affected States'''
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Larger cities may be the most stable administrative entities in states affected by conflict. Even here, ability to plan and deliver adaptation can be hampered. Extending into urban areas within stable states, alienation and loss of trust between local populations and the state can be exacerbated by top-down adaptation planning and delivery; socially and spatially uneven adaptation investment; and in the economic and administrative limits of government that can lead to some places being excluded from formal planned investment ( ''high confidence'' ) (see Sections 6.3 and 6.4). These pathways for exclusion can combine among already marginalised and low-income populations where trust in government agencies may already be low (Rodrigues, 2021).

Climate change can be a threat multiplier in cities and urban regions, exacerbating existing human security tension ( ''limited evidence'' , ''medium agreement'' ) ( [[#Froese--2019|Froese and Schilling, 2019]] ; Flörke, Schneider and McDonald, 2018; [[#Rajsekhar--2017|Rajsekhar and Gorelick, 2017]] ). Where conflict or administrative tensions extend beyond cities, adapting regional infrastructure systems that underpin urban life is challenging, for example where elements of networked infrastructure are under the control of conflicting political interests. This has been noted for the water sector (Tänzler, Maas and Carius, 2010). Coordinating political processes is a major challenge even for industrialised countries with adequate administrative capacity. In post-conflict societies, the difficulties of coordination for urban planning are disproportionately greater (Sovacool, Tan-Mullins and Abrahamse, 2018).

In planning adaptation measures in cities, conflict-sensitive approaches to ensure participatory methods (Bobylev et al., 2021) can avoid adaptation being a polarising activity (Tänzler, Maas and Carius, 2010; [[#Tänzler--2017|Tänzler, 2017]] ). Adaptation can provide a common goal reaching across political differences and be a part of building political trust and local cooperation between alienated communities (Tänzler, Maas and Carius, 2010). Peacebuilding programmes led by government or civil society are typically concerned with the short-term and framed by socioeconomic policy, integrating the longer-term view and engineering-technical expertise for adaptation is a challenge ( ''limited evidence'' , ''medium agreement'' ) ( [[#Ishiwatari--2021|Ishiwatari, 2021]] ).

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