Editing IPCC:AR6/WGIII/Chapter-8 (section)

== 8.2 Co-benefits and Trade-offs of Urban Mitigation Strategies ==

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Co-benefits are ‘the positive effects that a policy or measure aimed at one objective might have on other objectives, thereby increasing the total benefits to the society or environment’ ( [[#IPCC--2018b|IPCC 2018b]] ). AR5 WGIII [[IPCC:Wg3:Chapter:Chapter-12|Chapter 12]] reported a range of co-benefits associated with urban climate change mitigation strategies, including public savings, air quality and associated health benefits, and productivity increases in urban centres (Seto et al. 2014). Since AR5, evidence continues to mount on the co-benefits of urban mitigation. Highlighting co-benefits could make a strong case for driving impactful mitigation action ( [[#Bain--2016|Bain et al. 2016]] ), especially in developing countries, where development benefits can be the argument for faster implementation ( [[#Sethi--2018|Sethi and Puppim de Oliveira 2018]] ). Through co-benefits, urban areas can couple mitigation, adaptation, and sustainable development while closing infrastructure gaps ( [[#Thacker--2019|Thacker et al. 2019]] ; [[#Kamiya--2020|Kamiya et al. 2020]] ).

The urgency of coupling mitigation and adaptation is emphasised through a special Cross-Working Group Box on ‘Cities and Climate Change’ ( [[#8.2.3|Section 8.2.3]] and Cross-Working Group Box 2 in this chapter). This section further addresses synergies and trade-offs for sustainable development with a focus on linkages with the SDGs and perspectives for economic development, competitiveness, and equity.

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=== 8.2.1 Sustainable Development ===

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Sustainable development is a broad concept, encompassing socio-economic and environmental dimensions, envisaging long-term permanence and improvement. While long-term effects are more related to resilience – and hence carry co-benefits and synergies with the mitigation of GHG emissions – some short-term milestones were defined by the post-2015 UN Sustainable Development Agenda SDGs, including a specific goal on climate change (SDG 13) and one on making cities inclusive, safe, resilient and sustainable (SDG 11) ( [[#United%20Nations--2015|United Nations 2015]] ). The SDGs and related indicators can be an opportunity to improve cities by using science-based decision-making and engaging a diverse set of stakeholders ( [[#Simon--2016|Simon et al. 2016]] ; [[#Klopp--2017|Klopp and Petretta 2017]] ; [[#Kutty--2020|Kutty et al. 2020]] ).

There are multiple ways that development pathways can be shifted towards sustainability ( [[IPCC:Wg3:Chapter:Chapter-4#4.3.3|Section 4.3.3]] , Cross-Chapter Box 5 in Chapter 4, [[IPCC:Wg3:Chapter:Chapter-17|Chapter 17]] and Figure 17.1). Urban areas can work to redirect development pathways towards sustainability while increasing co-benefits for urban inhabitants. Figure 8.4 indicates that mitigation options for urban systems can provide synergistic linkages across a wide range of SDGs, and some cases where linkages can produce both synergies and trade-offs. While linkages are based on context and the scale of implementation, synergies can be most significant when urban areas pursue integrated approaches where one mitigation option supports the other (Sections 8.4 and 8.6).

Figure 8.4 summarises an evaluation of the synergies and/or trade-offs with the SDGs for the mitigation options for urban systems based on Supplementary Material 8.SM.1. The evaluations depend on the specific urban context, with synergies and/or trade-offs being more significant in certain contexts than others. Urban mitigation with a view of the SDGs can support shifting pathways of urbanisation towards greater sustainability. The feasibility of urban mitigation options is also malleable and can increase with more ‘enabling conditions’ (see Glossary), provided, perhaps, through institutional (i.e., financial or governmental) support ( [[#8.5|Section 8.5]] ). Strengthened institutional capacity that supports the coordination of mitigation options can increase linkages with the SDGs and their synergies. For example, urban land use and spatial planning for walkable and co-located densities, together with electrification of the urban energy system, can hold more benefits for the SDGs than any one of the mitigation options alone (Sections 8.4.2.3, 8.4.3.1 and 8.6).

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'''Figure 8.4: Co-benefits of urban mitigation actions.''' The first column lists urban mitigation options. The second column indicates synergies with the SDGs. The third column indicates both synergies and/or trade-offs. The dots represent confidence levels with the number of dots representing levels from low to high. In the last column, confidence levels for synergies and/or trade-offs are provided separately. A plus sign (+) represents synergy and a minus sign (–) represents a trade-off. Supplementary Material 8.SM.1 provides 64 references and extends the SDG mappings that are provided in [[#Thacker--2019|Thacker et al. (2019)]] and [[#Fuso%20Nerini--2018|Fuso Nerini et al. (2018)]] . Please see Table 17.SM.1 for details and Annex II for the methodology of the SDG assessment.

Evidence on the co-benefits of urban mitigation measures for human health has increased significantly since AR5, especially through the use of health impact assessments, where energy savings and cleaner energy supply structures based on measures for urban planning, heating, and transport have reduced CO 2 , nitrogen oxides (NO x ), and coarse particulate matter (PM 10 ) emissions ( [[#Diallo--2016|Diallo et al. 2016]] ). Some measures, especially those related to land-use planning and transportation, have also increased opportunities for physical activity for improved health ( [[#Diallo--2016|Diallo et al. 2016]] ). In developing countries, the co-benefits approach has been effective in justifying climate change mitigation actions at the local level ( [[#Puppim%20de%20Oliveira--2016|Puppim de Oliveira and Doll 2016]] ). Mixed-use compact development with sufficient land-use diversity can have a positive influence on urban productivity ( [[#8.4.2|Section 8.4.2]] ). Conversely, urban spatial structures that increase walking distances and produce car dependency have negative impacts on urban productivity considering congestion as well as energy costs ( [[#Salat--2017|Salat et al. 2017]] ).

There is increasing evidence that climate mitigation measures can lower health risks that are related to energy poverty, especially among vulnerable groups such as the elderly and in informal settlements ( [[#Monforti-Ferrario--2018|Monforti-Ferrario et al. 2018]] ). Measures such as renewable energy-based electrification of the energy system not only reduce outdoor air pollution, but also enhance indoor air quality by promoting smoke-free heating and cooking in buildings ( [[#Kjellstrom--2013|Kjellstrom and McMichael 2013]] ). The environmental and ecological benefits of electrification of the urban energy system include improved air quality based on a shift to non-polluting energy sources ( [[#Jacobson--2018|Jacobson et al. 2018]] ; [[#Ajanovic--2019|Ajanovic and Haas 2019]] ; [[#Bagheri--2019|Bagheri et al. 2019]] ; [[#Gai--2020|Gai et al. 2020]] ). Across 74 metropolitan areas around the world, an estimated 408,270 lives per year are saved due to air quality improvements that stem from a move to 100% renewable energy ( [[#Jacobson--2020|Jacobson et al. 2020]] ). Other studies indicate that there is potential to reduce premature mortality by up to 7000 people in 53 towns and cities, to create 93,000 new jobs, and to lower global climate costs and personal energy costs, through renewable energy transformations ( [[#Jacobson--2018|Jacobson et al. 2018]] ).

Across 146 signatories of a city climate network, local energy-saving measures led to 6596 avoided premature deaths and 68,476 years of life saved due to improved air quality ( [[#Monforti-Ferrario--2018|Monforti-Ferrario et al. 2018]] ). Better air quality further reinforces the health co-benefits of climate mitigation measures based on walking and bicycling since evidence suggests that increased physical activity in urban outdoor settings with low levels of black carbon improves lung function ( [[#Laeremans--2018|Laeremans et al. 2018]] ). Physical activity can also be fostered through urban design measures and policies that promote the development of ample and well-connected parks and open spaces, and can lead to physical and mental health benefits ( [[#Kabisch--2016|Kabisch et al. 2016]] ) ( [[#8.4.4|Section 8.4.4]] and Figure 8.18).

Cities in India, Indonesia, Vietnam, and Thailand show that reducing emissions from major sources (e.g., transport, residential burning, biomass open burning, and industry) could bring substantial co-benefits of avoided deaths from reduced PM 2.5 (fine inhalable particulates) emissions and radiative forcing from black carbon ( [[#Pathak--2016|Pathak and Shukla 2016]] ; [[#Dhar--2017|Dhar et al. 2017]] ; [[#Permadi--2017|Permadi et al. 2017]] ; [[#Karlsson--2020|Karlsson et al. 2020]] ), reduced noise, and reduced traffic injuries ( [[#Kwan--2016|Kwan and Hashim 2016]] ). Compact city policies and interventions that support a modal shift away from private motor vehicles towards walking, cycling, and low-emission public transport delivers significant public health benefits ( [[#Creutzig--2016|Creutzig 2016]] ; [[#Ürge-Vorsatz--2018|Ürge-Vorsatz et al. 2018]] ). Trade-offs associated with compact development include the marginal health costs of transport air pollution (Lohrey and [[#Creutzig--2016|Creutzig 2016]] ) and stress from traffic noise ( [[#Gruebner--2017|Gruebner et al. 2017]] ) ( [[#8.4.2.3|Section 8.4.2.3]] ).

Urban green and blue infrastructure – a subset of nature-based solutions (NBS) – acts as both climate mitigation and adaptation measures by reducing heat stress ( [[#Kim--2018|Kim and Coseo 2018]] ; [[#Privitera--2018|Privitera and La Rosa 2018]] ; [[#Herath--2021|Herath et al. 2021]] ), improving air quality, reducing noise ( [[#Scholz--2018|Scholz et al. 2018]] ; [[#De%20la%20Sota--2019|De la Sota et al. 2019]] ), improving urban biodiversity ( [[#Hall--2017b|Hall et al. 2017b]] ), and enhancing well-being, including contributions to local development ( [[#Lwasa--2015|Lwasa et al. 2015]] ). Health benefits from urban forestry and green infrastructure include reduced cardiovascular morbidity, improved mental health ( [[#van%20den%20Bosch--2017|van den Bosch and Ode Sang 2017]] ; [[#Vujcic--2017|Vujcic et al. 2017]] ; [[#Al-Kindi--2020|Al-Kindi et al. 2020]] ; [[#Sharifi--2021|Sharifi et al. 2021]] ), raised birth weight ( [[#Dzhambov--2014|Dzhambov et al. 2014]] ), and increased life expectancy ( [[#Jonker--2014|Jonker et al. 2014]] ). Urban agriculture, including urban orchards, rooftop gardens, and vertical farming contribute to enhancing food security and fostering healthier diets ( [[#Cole--2018|Cole et al. 2018]] ; [[#Petit-Boix--2018|Petit-Boix and Apul 2018]] ; [[#De%20la%20Sota--2019|De la Sota et al. 2019]] ) ( [[#8.4.4|Section 8.4.4]] , Figure 8.18 and Box 8.2).

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=== 8.2.2 Economic Development, Competitiveness, and Equity ===

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Sustainable management of urban ecosystems entails addressing economic growth, equity, and good governance. In total, 102 SDG targets (99 synergies and 51 trade-offs) are identified with published evidence of relationships with urban ecosystems – out of the 169 in the 2030 Agenda ( [[#Maes--2019|Maes et al. 2019]] ). The targets require action in relation to urban ecosystem management, environmental improvements, equality related to basic services, long-term economic growth, economic savings, stronger governance, and policy development at multiple scales.

Mitigation measures related to different sectors can provide co-benefits and reduce social inequities. Transport-related measures, such as transportation demand management, transit-oriented development (TOD), and promotion of active transport modes provide economic co-benefits through, for example, reducing health care costs linked with pollution and cardiovascular diseases, improving labour productivity, and decreasing congestion costs (including waste of time and money) ( [[#Sharifi--2021|Sharifi et al. 2021]] ). As a case-in-point, data from cities such as Bangkok, Kuala Lumpur, Jakarta, Manila, Beijing, Mexico City, Dakar, and Buenos Aires indicate that economic costs of congestion account for a considerable share of their gross domestic product (GDP), ranging from 0.7% to 15.0% ( [[#Dulal--2017|Dulal 2017]] ) ( [[#8.4.2|Section 8.4.2]] ).

Since policy interventions can result in negative impacts or trade-offs with other objectives, fostering accessibility, equity, and inclusivity for disadvantaged groups is essential ( [[#Viguié--2012|Viguié and Hallegatte 2012]] ; [[#Sharifi--2020|Sharifi 2020]] ; [[#Pörtner--2021|Pörtner et al. 2021]] ). Anti-sprawl policies that aim to increase density, or the introduction of large green areas in cities could increase property prices, resulting in trade-offs with affordable housing and pushing urban poor further away from cities ( [[#Reckien--2017|Reckien et al. 2017]] ; [[#Alves--2019|Alves et al. 2019]] ). Deliberate strategies can improve access of low-income populations to jobs, and gender-responsive transport systems that can enhance women’s mobility and financial independence ( [[#Viguié--2012|Viguié and Hallegatte 2012]] ; [[#Lecompte--2017|Lecompte and Juan Pablo 2017]] ; [[#Reckien--2017|Reckien et al. 2017]] ; [[#Priya%20Uteng--2019|Priya Uteng and Turner 2019]] ).

Low-carbon urban development that triggers economic decoupling and involves capacity-building measures could have a positive impact on employment and local competitiveness ( [[#Dodman--2009|Dodman 2009]] ; [[#Kalmykova--2015|Kalmykova et al. 2015]] ; [[#Chen--2018b|Chen et al. 2018b]] ; [[#García-Gusano--2018|García-Gusano et al. 2018]] ; [[#Hu--2018|Hu et al. 2018]] ; [[#Shen--2018|Shen et al. 2018]] ). Sustainable and low-carbon urban development that integrates issues of equity, inclusivity, and affordability while safeguarding urban livelihoods, providing access to basic services, lowering energy bills, addressing energy poverty, and improving public health, can also improve the distributional effects of existing and future urbanisation ( [[#Friend--2016|Friend et al. 2016]] ; [[#Claude--2017|Claude et al. 2017]] ; [[#Colenbrander--2017|Colenbrander et al. 2017]] ; [[#Ma--2018|Ma et al. 2018]] ; [[#Mrówczyńska--2018|Mrówczyńska et al. 2018]] ; [[#Pukšec--2018|Pukšec et al. 2018]] ; [[#Wiktorowicz--2018|Wiktorowicz et al. 2018]] ; [[#Ramaswami--2020|Ramaswami 2020]] ).

Depending on the context, green and blue infrastructure can also offer considerable economic co-benefits. For example, green roofs and facades and other urban greening efforts such as urban agriculture and greening streets can improve microclimatic conditions and enhance thermal comfort, thereby reducing utility and health care costs. The presence of green and blue infrastructure may also increase the economic values of nearby properties ( [[#Votsis--2017|Votsis 2017]] ; [[#Alves--2019|Alves et al. 2019]] ) ( [[#8.4.4|Section 8.4.4]] and Figure 8.18).

Studies in the UK show that beneficiaries are willing to pay (WTP) an additional fee (up to 2% more in monthly rent) for proximity to green and blue infrastructure, with the WTP varying depending on the size and nature of the green space ( [[#Mell--2013|Mell et al. 2013]] , 2016). Urban agriculture can not only reduce household food expenditure, but also provide additional sources of revenue for the city ( [[#Ayerakwa--2017|Ayerakwa 2017]] ; [[#Alves--2019|Alves et al. 2019]] ). Based on the assessed literature, there is ''high agreement'' on the economic co-benefits of green and blue infrastructure, but supporting evidence is still limited ( [[#8.7|Section 8.7]] ).

Implementing waste management and wastewater recycling measures can provide additional sources of income for citizens and local authorities. Wastewater recycling can minimise the costs associated with the renewal of centralised wastewater treatment plants ( [[#Bernstad%20Saraiva%20Schott--2015|Bernstad Saraiva Schott and Cánovas 2015]] ; [[#Gharfalkar--2015|Gharfalkar et al. 2015]] ; [[#Gonzalez-Valencia--2016|Gonzalez-Valencia et al. 2016]] ; [[#Herrero--2018|Herrero and Vilella 2018]] ; [[#Matsuda--2018|Matsuda et al. 2018]] ; [[#Nisbet--2019|Nisbet et al. 2019]] ). Waste management and wastewater recycling is also a pathway for inclusion of the informal sector into the urban economy with ''high agreement'' and ''medium evidence'' ( [[#Sharifi--2021|Sharifi 2021]] ). Additionally, authorities can sell energy generated from wastewater recycling to compensate for the wastewater management costs ( [[#Colenbrander--2017|Colenbrander et al. 2017]] ; [[#Gondhalekar--2017|Gondhalekar and Ramsauer 2017]] ). Another measure that contributes to reducing household costs is the promotion of behavioural measures such as dietary changes that can decrease the demand for costly food sources and reduce health care costs through promoting healthy diets ( [[#Hoppe--2016|Hoppe et al. 2016]] ) (Sections 8.4.5 and 8.4.6).

In addition to cost savings, various measures such as stormwater management and urban greening can enhance social equity and environmental justice. For example, the thermal comfort benefits provided by green and blue infrastructure and passive design measures can address issues related to energy poverty and unaffordability of expensive air conditioning systems for some social groups ( [[#Sharma--2018|Sharma et al. 2018]] ; [[#He--2019|He et al. 2019]] ). To achieve such benefits, however, the costs of integrating green and blue infrastructure and passive design measures into building design would need to be minimised. Another example is the flood mitigation benefits of stormwater management measures that can reduce impacts on urban poor who often reside in flood-prone and low-lying areas of cities ( [[#Adegun--2017|Adegun 2017]] ; [[#He--2019|He et al. 2019]] ). Generally, the urban poor are expected to be disproportionately affected by climate change impacts. Carefully designed measures that reduce such disproportionate impacts by involving experts, authorities and citizens would enhance social equity ( [[#Pandey--2018|Pandey et al. 2018]] ; [[#He--2019|He et al. 2019]] ; [[#Mulligan--2020|Mulligan et al. 2020]] ).

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=== 8.2.3 Coupling Mitigation and Adaptation ===

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There are numerous synergies that come from coupling urban adaptation and mitigation. A number of studies have developed methods to assess the synergies between mitigation and adaptation strategies, as well as their co-benefits ( [[#Solecki--2015|Solecki et al. 2015]] ; [[#Buonocore--2016|Buonocore et al. 2016]] ; [[#Chang--2017|Chang et al. 2017]] ; [[#Helgenberger--2017|Helgenberger and Jänicke 2017]] ). Co-benefits occur when implementing mitigation (or adaptation) measures that have positive effects on adaptation (or mitigation) ( [[#Sharifi--2021|Sharifi 2021]] ). In contrast, the trade-offs emerge when measures aimed at improving mitigation (adaptation) undermine the ability to pursue adaptation (mitigation) targets ( [[#Sharifi--2020|Sharifi 2020]] ). The magnitude of such co-benefits and trade-offs may vary depending on various factors. A systematic review of over 50 climate change articles provides evidence that mitigation can contribute to resilience – especially to temperature changes and flooding – with varying magnitudes, depending on factors such as the type of mitigation measure and the scale of implementation ( [[#Sharifi--2019|Sharifi 2019]] ).

Measures from different sectors that can provide both mitigation and adaptation benefits involve urban planning ( [[#8.4.2|Section 8.4.2]] ), buildings (Sections 8.4.3.2 and 8.4.4), energy ( [[#8.4.3|Section 8.4.3]] ), green and blue infrastructure ( [[#8.4.4|Section 8.4.4]] ), transportation ( [[#8.4.2|Section 8.4.2]] ), socio-behavioural aspects ( [[#8.4.5|Section 8.4.5]] ), urban governance ( [[#8.5|Section 8.5]] ), waste ( [[#8.4.5.2|Section 8.4.5.2]] ), and water ( [[#8.4.6|Section 8.4.6]] ). In addition to their energy-saving and carbon-sequestration benefits, many measures can also enhance adaptation to climate threats, such as extreme heat, energy shocks, floods, and droughts ( [[#Sharifi--2021|Sharifi 2021]] ). Existing evidence is mainly related to urban green infrastructure, urban planning, transportation, and buildings. There has been more emphasis on the potential co-benefits of measures, such as proper levels of density, building energy efficiency, distributed and decentralised energy infrastructure, green roofs and facades, and public/active transport modes. Renewable-based distributed and decentralised energy systems improve resilience to energy shocks and can enhance adaptation to water stress considering the water-energy nexus. By further investment on these measures, planners and decision makers can ensure enhancing achievement of mitigation/adaptation co-benefits at the urban level ( [[#Sharifi--2021|Sharifi 2021]] ).

As for trade-offs, some mitigation efforts may increase exposure to stressors such as flooding and the urban heat island (UHI) effect (see Glossary), thereby reducing the adaptive capacity of citizens. For instance, in some contexts, high-density areas that lack adequate provision of green and open spaces may intensify the UHI effect ( [[#Pierer--2019|Pierer and Creutzig 2019]] ; [[#Xu--2019|Xu et al. 2019]] ). There are also concerns that some mitigation efforts may diminish adaptive capacity of urban poor and marginalised groups through increasing costs of urban services and/or eroding livelihood options. Environmental policies designed to meet mitigation targets through phasing out old vehicles may erode livelihood options of poor households, thereby decreasing their adaptive capacity ( [[#Colenbrander--2017|Colenbrander et al. 2017]] ). Ambitious mitigation and adaptation plans could benefit private corporate interests resulting in adverse effects on the urban poor ( [[#Chu--2018|Chu et al. 2018]] ; [[#Mehta--2019|Mehta et al. 2019]] ).

Urban green and blue infrastructure such as urban trees, greenspaces, and urban waterways can sequester carbon and reduce energy demand, and provide adaptation co-benefits by mitigating the UHI effect ( [[#Berry--2015|Berry et al. 2015]] ; [[#Wamsler--2016|Wamsler and Pauleit 2016]] ; WCRP 2019) ( [[#8.4.4|Section 8.4.4]] , Figure 8.18 and Box 8.2).

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=== Cross-Working Group Box 2: Cities and Climate Change ===

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'''Authors:''' Xuemei Bai (Australia), Vanesa Castán Broto (Spain/United Kingdom), Winston Chow (Singapore), Felix Creutzig (Germany), David Dodman (Jamaica/United Kingdom), Rafiq Hamdi (Belgium), Bronwyn Hayward (New Zealand), Şiir Kılkış (Turkey), Shuaib Lwasa (Uganda), Timon McPhearson (the United States of America), Minal Pathak (India), Mark Pelling (United Kingdom), Diana Reckien (Germany), Karen C. Seto (the United States of America), Ayyoob Sharifi (Iran/Japan), Diana Ürge-Vorsatz (Hungary)

'''Introduction'''

This Cross-Working Group Box on Cities and Climate Change responds to the critical role of urbanisation as a megatrend impacting climate adaptation and mitigation. Issues associated with cities and urbanisation are covered in substantial depth within all three Working Groups (including WGI Box TS.14, WGII [[IPCC:Wg3:Chapter:Chapter-6|Chapter 6]] ‘Cities, Settlements and Key Infrastructure’, WGII regional chapters, WGII Cross-Chapter Paper ‘Cities and Settlements by the Sea’, and WGIII [https://www.ipcc.ch/report/ar6/wg3/chapter/chapter-8 Chapter 8] ‘Urban Systems and Other Settlements’). This Box highlights key findings from WGII and III and substantial gaps in literature where more research is urgently needed relating to policy action in cities. It describes methods of addressing mitigation and adaptation in an integrated way across sectors and cities to advance sustainable development and equity outcomes and assesses the governance and finance solutions required to support climate-resilient responses.

'''Urbanisation: A megatrend driving global climate risk and potential for low-carbon and re''' '''silient futures'''

Severe weather events, exacerbated by anthropogenic emissions, are already having devastating impacts on people who live in urban areas, on the infrastructure that supports these communities, as well as people living in distant places ( ''high confidence'' ) ( [[#Cai--2019|Cai et al. 2019]] ; [[#Folke--2021|Folke et al. 2021]] ). Between 2000 and 2015, the global population in locations that were affected by floods grew by 58–86 million ( [[#Tellman--2021|Tellman et al. 2021]] ). The direct economic costs of all extreme events reached USD210–268 billion in 2020 ( [[#Aon--2021|Aon 2021]] ; [[#Munich%20RE--2021|Munich RE 2021]] ; [[#WMO--2021|WMO 2021]] ) or about USD0.7 billion per day; this figure does not include knock-on costs in supply chains ( [[#Kii--2020|Kii 2020]] ) or lost days of work, implying that the actual economic costs could be far higher. Depending on RCP, between half (RCP2.6) and three-quarters (RCP8.5) of the global population could be exposed to periods of life-threatening climatic conditions arising from coupled impacts of extreme heat and humidity by 2100 ( [[#Mora--2017|Mora et al. 2017]] ; [[#Huang--2019|Huang et al. 2019]] ) (see WGII [[IPCC:Wg3:Chapter:Chapter-6#6.2|Section 6.2]] .2.1, WGII Figure 6.3, and WGIII Sections 8.2 and 8.3.4).

Urban systems are now global, as evidenced by the interdependencies between infrastructure, services, and networks driven by urban production and consumption; remittance flows and investments reach into rural places, shaping natural resource use far from the city and bring risk to the city when these places are impacted by climate change (WGIII [[#8.4|Section 8.4]] and Figure 8.15). This megatrend ( [[#Kourtit--2015|Kourtit et al. 2015]] ) amplifies as well as shapes the potential impacts of climate events and integrates the aims and approaches for delivering mitigation, adaptation, and sustainable development ( ''medium evidence'' , ''high agreement'' ) ( [[#Dawson--2018|Dawson et al. 2018]] ; [[#Tsavdaroglou--2018|Tsavdaroglou et al. 2018]] ; [[#Zscheischler--2018|Zscheischler et al. 2018]] ). For cities facing flood damage, wide-ranging impacts have been recorded on other urban areas near and far ( [[#Carter--2021|Carter et al. 2021]] ; [[#Simpson--2021|Simpson et al. 2021]] ) as production and trade is disrupted ( [[#Shughrue--2020|Shughrue et al. 2020]] ). In the absence of integrated mitigation and adaptation across and between infrastructure systems and local places, impacts that bring urban economies to a standstill can extend into supply chains and across energy networks causing power outages.

Urban settlements contribute to climate change, generating about 70% of global CO 2 -eq emissions ( ''high confidence'' ) (see WGI Box TS.14, WGII Sections 6.1 and 6.2, and WGIII [[#8.3|Section 8.3]] ). This global impact feeds back to cities through the exposure of infrastructure, people, and business to the impacts of climate-related hazards. Particularly in larger cities, this climate feedback is exacerbated by local choices in urban design, land use, building design, and human behaviour ( [[#Viguié--2020|Viguié et al. 2020]] ) that shape local environmental conditions. Both the local and global combine to increase hazardousness. Certain configurations of urban form and their elements can add up to 2°C to warming; concretisation of open space can increase run-off, and building height and orientation influences wind direction and strength (see WGII [[IPCC:Wg3:Chapter:Chapter-6#6.3|Section 6.3]] and WGIII [[#8.4.2|Section 8.4.2]] ).

Cross-Working Group Box 2

Designing for resilient and low-carbon cities today is far easier than retrofitting for risk reduction tomorrow. As urbanisation unfolds, its legacy continues to be the locking-in of emissions and vulnerabilities ( ''high confidence'' ) ( [[#Seto--2016|Seto et al. 2016]] ; [[#Ürge-Vorsatz--2018|Ürge-Vorsatz et al. 2018]] ) (see WGIII [[#8.4|Section 8.4]] and Figure 8.15). Retrofitting, disaster reconstruction, and urban regeneration programmes offer scope for strategic direction changes to low-carbon and high-resilience urban form and function, so long as they are inclusive in design and implementation. Rapid urban growth means new investment, new buildings and infrastructure, new demands for energy and transport and new questions about what a healthy and fulfilling urban life can be. The USD90 trillion expected to be invested in new urban development by 2030 ( [[#NCE--2018|NCE 2018]] ) is a global opportunity to place adaptation and mitigation directly into urban infrastructure and planning, as well as to consider social policy including education, health care, and environmental management ( [[#Ürge-Vorsatz--2018|Ürge-Vorsatz et al. 2018]] ). If this opportunity is missed, and business-as-usual urbanisation persists, social and physical vulnerability will become much more challenging to address.

The benefits of actions taken to reduce GHG emissions and climate stressors diminish with delayed action, indicating the necessity for rapid responses. Delaying the same actions for increasing the resilience of infrastructure from 2020 to 2030 is estimated to have a median cost of at least USD1 trillion ( [[#Hallegatte--2019|Hallegatte et al. 2019]] ) while also missing the carbon emissions reductions required in the narrowing window of opportunity to limit global warming to 1.5°C (WGI). In contrast, taking integrated actions towards mitigation, adaptation, and sustainable development will provide multiple benefits for the health and well-being of urban inhabitants and avoid stranded assets (see WGII [[IPCC:Wg3:Chapter:Chapter-6#6.3|Section 6.3]] , WGII Chapter 17, Cross-Chapter Box on ‘Feasibility’ in WGII Chapter 18, WGIII Chapter 5, and WGIII [[#8.2|Section 8.2]] ).

'''The policy-action gap: urban low-carbon and climate-resili''' '''ent development'''

Cities are critical places to realise both adaptation and mitigation actions simultaneously with potential co-benefits that extend far beyond cities ( ''medium evidence'' , ''high agreement'' ) ( [[#Göpfert--2019|Göpfert et al. 2019]] ; [[#Grafakos--2020|Grafakos et al. 2020]] ). Given rapid changes in the built environment, transforming the use of materials and the land intensiveness of urban development, including in many parts of the Global South, will be critical in the next decades, as well as mainstreaming low-carbon development principles in new urban development in all regions. Much of this development will be self-built and ‘informal’ – and new modes of governance and planning will be required to engage with this. Integrating mitigation and adaptation now rather than later, through reshaping patterns of urban development and associated decision-making processes, is a prerequisite for attaining resilient and zero-carbon cities (see WGIII Sections 8.4 and 8.6, and WGIII Figure 8.21).

While more cities have developed plans for climate adaptation and mitigation since AR5, many remain to be implemented ( ''limited evidence'' , ''high agreement'' ) ( [[#Araos--2017|Araos et al. 2017]] ; [[#Aguiar--2018|Aguiar et al. 2018]] ; [[#Olazabal--2021|Olazabal and Ruiz De Gopegui 2021]] ). A review of local climate mitigation and adaptation plans across 885 urban areas of the European Union suggests mitigation plans are more common than adaptation plans – and that city size, national legislation, and international networks can influence the development of local climate and adaptation plans with an estimated 80% of those cities with above 500,000 inhabitants having a mitigation and/or an adaptation plan ( [[#Reckien--2018|Reckien et al. 2018]] ).

Integrated approaches to tackle common drivers of emissions and cascading risks provide the basis for strengthening synergies across mitigation and adaptation, and help manage possible trade-offs with sustainable development ( ''limited evidence'' , ''medium agreement'' ) ( [[#Grafakos--2019|Grafakos et al. 2019]] ; [[#Landauer--2019|Landauer et al. 2019]] ; [[#Pierer--2019|Pierer and Creutzig 2019]] ). An analysis of 315 local authority emission-reduction plans reveals that the most common policies cover municipal assets and structures ( [[#Palermo--2020a|Palermo et al. 2020a]] ). Estimates of emission reductions by non-state and sub-state actors in 10 high-emitting economies projected GHG emissions in 2030 would be 1.2–2.0 GtCO 2 -eq yr –1 or 3.8–5.5% lower compared to scenario projections for current national policies (31.6–36.8 GtCO 2 -eq yr –1 ) if the policies are fully implemented and do not change the pace of action elsewhere ( [[#Kuramochi--2020|Kuramochi et al. 2020]] ). The value of integrating mitigation and adaptation is underscored in the opportunities for decarbonising existing urban areas, and investing in social, ecological, and technological infrastructure resilience (WGII [[IPCC:Wg3:Chapter:Chapter-6#6.4|Section 6.4]] ). Integrating mitigation and adaption is challenging ( [[#Landauer--2019|Landauer et al. 2019]] ) but can provide multiple benefits for the health and well-being of urban inhabitants ( [[#Sharifi--2021|Sharifi 2021]] ) (See WGIII [[#8.2.3|Section 8.2.3]] ).

Effective climate strategies combine mitigation and adaptation responses, including through linking adaptive urban land use with GHG emission reductions ( ''medium evidence'' , ''high agreement'' ) ( [[#Xu--2019|Xu et al. 2019]] ; [[#Patterson--2021|Patterson 2021]] ). For example, urban green and blue infrastructure can provide co-benefits for mitigation and adaptation ( [[#Ürge-Vorsatz--2018|Ürge-Vorsatz et al. 2018]] ) and is an important entry point for integrating adaptation and mitigation at the urban level ( [[#Frantzeskaki--2019|Frantzeskaki et al. 2019]] ) (see WGIII [[#8.4.4|Section 8.4.4]] and WGIII Figure 8.18). Grey and physical infrastructure, such as sea defences, can immediately reduce risk, but also transfer risk and limit future options. Social policy interventions including social safety nets provide financial security for the most at-risk and can manage vulnerability determined by specific hazards or independently.

Cross-Working Group Box 2

Hazard-independent mechanisms for vulnerability reduction – such as population-wide social security – provide resilience in the face of unanticipated cascading impacts or surprise and novel climate-related hazard exposure. Social interventions can also support or be led by ambitions to reach the SDGs ( [[#Archer--2016|Archer 2016]] ). Climate-resilient development invites planners to develop interventions and monitor the effectiveness of outcomes beyond individual projects and across wider remits that consider sustainable development. Curbing the emission impacts of urban activities to reach net-zero emissions in the next decades, while improving the resilience of urban areas, necessitates an integrated response now.

Key gaps in knowledge include: urban-enabling environments; the role of smaller settlements, low-income communities, and informal settlements, as well as those in rental housing spread across the city; and the ways in which actions to reduce supply chain risk can be supported to accelerate equitable and sustainable adaptation in the face of financial and governance constraints ( [[#Birkmann--2016|Birkmann et al. 2016]] ; [[#Shi--2016|Shi et al. 2016]] ; [[#Rosenzweig--2018|Rosenzweig et al. 2018]] ; [[#Dulal--2019|Dulal 2019]] ).

'''Enabling action'''

Innovative governance and finance solutions are required to manage complex and interconnected risks across essential key infrastructures, networks, and services, as well as to meet basic human needs in urban areas ( ''medium confidence'' ) ( [[#Colenbrander--2018a|Colenbrander et al. 2018a]] ; [[#Moser--2019|Moser et al. 2019]] ). There are many examples of ‘ready-to-use’ policy tools, technologies, and practical interventions for policymakers seeking to act on adaptation and mitigation ( [[#Bisaro--2018|Bisaro and Hinkel 2018]] ; [[#Keenan--2019|Keenan et al. 2019]] ; [[#Chirambo--2021|Chirambo 2021]] ) (see WGIII [[#8.5.4|Section 8.5.4]] ). Tax and fiscal incentives for businesses and individuals can help support city-wide behaviour change towards low-carbon and risk-reducing choices. Change can start where governments have most control – often in public sector institutions and investment – but the challenge ahead requires partnership with private sector and community actors acting at scale and with accountability. Urban climate governance and finance needs to address urban inequalities at the forefront if the urban opportunity is to realise the ambition of the SDGs.

Increasing the pace of investments will put pressure on governance capability, transparency, and accountability of decision-making ( ''medium confidence'' ) (see WGII [[IPCC:Wg3:Chapter:Chapter-6#6.4.5|Section 6.4.5]] ). Urban climate action that actively includes local actors is more likely to avoid unintended, negative maladaptive impacts and mobilise a wide range of local capacities. In the long run, this is also more likely to carry public support, even if some experiments and investments do not deliver the intended social benefits. Legislation, technical capacity, and governance capability are required to be able to absorb additional finance.

In recent years, about USD384 billion of climate finance has been invested in urban areas per year. This remains at about 10% of the annual climate finance that would be necessary for low-carbon and resilient urban development at a global scale ( [[#Negreiros--2021|Negreiros et al. 2021]] ). Rapid deployment of funds to stimulate economies in the recovery from COVID-19 has highlighted the pitfalls of funding expansion ahead of policy innovation and capacity building. The result can be an intensification of existing carbon-intensive urban forms – exactly the kinds of ‘carbon lock-in’ (see WGIII Glossary and WGIII [[#8.4.1|Section 8.4.1]] ) that have contributed to risk creation and its concentration amongst those with little public voice or economic power.

Iterative and experimental approaches to climate adaptation and mitigation decision-making grounded in data and co-generated in partnership with communities can advance low-carbon climate resilience ( ''medium evidence'' , ''high confidence'' ) ( [[#Culwick--2019|Culwick et al. 2019]] ; [[#Caldarice--2021|Caldarice et al. 2021]] ; [[#van%20der%20Heijden--2021|van der Heijden and Hong 2021]] ). Conditions of complexity, uncertainty, and constrained resources require innovative solutions that are both adaptive and anticipatory. Complex interactions among multiple agents in times of uncertainty makes decision-making about social, economic, governance, and infrastructure choices challenging and can lead decision-makers to postpone action. This is the case for those balancing household budgets, residential investment portfolios, and city-wide policy responsibilities. Living with climate change requires changes to business-as-usual design-making. Co-design and collaboration with communities through iterative policy experimentation can point the way towards climate-resilient development pathways ( [[#Ataöv--2021|Ataöv and Peker 2021]] ). Key to successful learning is transparency in policymaking, inclusive policy processes, and robust local modelling, monitoring, and evaluation, which are not yet widely undertaken ( [[#Sanchez%20Rodriguez--2018|Sanchez Rodriguez et al. 2018]] ; [[#Ford--2019|Ford et al. 2019]] ).

The diversity of cities’ experiences of climate mitigation and adaptation strategies brings an advantage for those city governments and other actors willing to ‘learn together’ ( ''limited evidence'' , ''high confidence'' ) ( [[#Bellinson--2019|Bellinson and Chu 2019]] ; [[#Haupt--2019|Haupt and Coppola 2019]] ). While contexts are varied, policy options are often similar enough for the sharing of experiments and policy champions. Sharing expertise can build on existing regional and global networks, many of which have already placed knowledge, learning, and capacity building at the centre of their agendas. Learning from innovative forms of governance and financial investment, as well as strengthening co-production of policy through inclusive access to knowledge and resources, can help address mismatches in local capacities and strengthen wider SDGs and COVID-19 recovery agendas ( ''limited evidence'' , ''medium agreement'' ). Perceptions of risk can greatly

Cross-Working Group Box 2

influence the reallocation of capital and shift financial resources ( [[#Battiston--2021|Battiston et al. 2021]] ). Coupling mitigation and adaptation in an integrated approach offers opportunities to enhance efficiency, increases the coherence of urban climate action, generates cost savings, and provides opportunities to reinvest the savings into new climate action projects to make all urban areas and regions more resilient.

Local governments play an important role in driving climate action across mitigation and adaptation as managers of assets, regulators, mobilisers, and catalysts of action, but few cities are undertaking transformative climate adaptation or mitigation actions ( ''limited evidence'' , ''medium confidence'' ) ( [[#Heikkinen--2019|Heikkinen et al. 2019]] ). Local actors are providers of infrastructure and services, regulators of zoning, and can be conveners and champions of an integrated approach for mitigation and adaptation at multiple levels ( ''limited evidence'' , ''high confidence'' ). New opportunities in governance and finance can enable cities to pool resources together and aggregate interventions to innovate ways of mobilising urban climate finance at scale ( [[#Colenbrander--2019|Colenbrander et al. 2019]] ; [[#Simpson--2019|Simpson et al. 2019]] ; [[#White--2019|White and Wahba 2019]] ). However, research increasingly points towards the difficulties faced during the implementation of climate financing in situ, such as the fragmentation of structures of governance capable of managing large investments effectively ( [[#Mohammed--2019|Mohammed et al. 2019]] ) (see WGIII [[#8.5|Section 8.5]] and WGIII Chapter 13).

Scaling up transformative place-based action for both adaptation and mitigation requires enabling conditions, including land-based financing, intermediaries, and local partnerships ( ''medium evidence'' , ''high agreement'' ) ( [[#Chu--2019|Chu et al. 2019]] ; Chaudhuri, 2020) supported by a new generation of big data approaches. Governance structures that combine actors working at different levels with a different mix of tools are effective in addressing challenges related to implementation of integrated action while cross-sectoral coordination is necessary ( [[#Singh--2020|Singh et al. 2020]] ). Joint institutionalisation of mitigation and adaptation in local governance structures can also enable integrated action ( [[#Göpfert--2020|Göpfert et al. 2020]] ; [[#Hurlimann--2021|Hurlimann et al. 2021]] ). However, the proportion of international finance that reaches local recipients remains low, despite the repeated focus of climate policy on place-based adaptation and mitigation ( [[#Manuamorn--2020|Manuamorn et al. 2020]] ). Green financing instruments that enable local climate action without exacerbating current forms of inequality can jointly address mitigation, adaptation, and sustainable development. Climate finance that also reaches beyond larger non-state enterprises (e.g., small and medium-sized enterprises, local communities, or non-governmental organisations (NGOs)), and is inclusive in responding to the needs of all urban inhabitants (e.g., disabled individuals, or citizens of different races or ethnicities) is essential for inclusive and resilient urban development ( [[#Colenbrander--2019|Colenbrander et al. 2019]] ; [[#Gabaldón-Estevan--2019|Gabaldón-Estevan et al. 2019]] ; [[#Frenova--2021|Frenova 2021]] ). Developing networks that can exert climate action at scale is another priority for climate finance.

The urban megatrend is an opportunity to transition global society. Enabling urban governance to avert cascading risk and achieve low-carbon, resilient development will involve the co-production of policy and planning, rapid implementation and greater cross-sector coordination, and monitoring and evaluation ( ''limited evidence'' , ''medium agreement'' ) ( [[#Di%20Giulio--2018|Di Giulio et al. 2018]] ; [[#Grafakos--2019|Grafakos et al. 2019]] ). New constellations of responsible actors are required to manage hybrid local-city or cross-city risk management and decarbonisation initiatives ( ''limited evidence'' , ''medium agreement'' ). These may increasingly benefit from linkages across more urban and more rural space as recognition of cascading and systemic risk brings recognition of supply chains, remittance flows, and migration trends as vectors of risk and resilience. Urban governance will be better prepared in planning, prioritising, and financing the kind of measures that can reduce GHG emissions and improve resilience at scale when they consider a view of cascading risks and carbon lock-ins globally, while also acting locally to address local limitations and capacities, including the needs and priorities of urban citizens ( [[#Colenbrander--2018a|Colenbrander et al. 2018a]] ; [[#Rodrigues--2019|Rodrigues 2019]] ).

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