Several TMNs are present at the worldwide level. This manuscript focuses on the most famous ones, due to the availability of data both in the literature and online (see Table 1). The selection foresaw four TMNs: the C40, ICLEI, Global and European Covenant of Mayors, and the 100 Resilient Cities. Appendix A includes a table (Table A1) for addressing the geographical coverage of these initiatives.

All these initiatives can be considered TMNs as their objectives are to: (i) create networks and link together cities across Europe or the world; (ii) support cities in working toward climate change-related goals; (iii) provide some types of directions, instruments and/or counseling activities; and (iv) being directly addressed to cities.

The most famous initiative is the Covenant of Mayors for Climate and Energy (CoM). Launched in 2008 by the European Commission, it has been one of the reference networks for European cities, with “the objective of engaging and supporting mayors to commit in reaching the EU climate and energy targets” [36]. The reference to the main target, mayors, is evident not only because it is part of the name of the initiatives, but also for the attention put into the act of “signing” the participation. A high level of political commitment marks membership in this initiative. CoM started in Europe but was soon extended worldwide, with the name of Global Covenant of Mayors for Climate and Energy in 2016, when the Compact of Mayors joined it [37]. This is a peculiar trait of the CoM: its ability to embed other stand-alone initiatives such as the Compact of Mayors and the Mayors Adapt in 2015 [36]. Its key message is summarised by the first part of the manifesto: “We, Mayors from all over Europe, hereby step up our climate ambitions and commit to delivering action at the pace that science dictates, in a joint effort to keep global temperature rise below 1.5 °C—the highest ambition of the Paris Agreement”. The vision set in the manifesto is to become lighthouses in acting toward decarbonization. However, CoM objectives changed according to the global contingencies, from having a 2020 horizon, they shifted to 2030 and, eventually, to 2050. Although CoM mainly tackles mitigation actions (CO₂ reduction and renewable energies), it expanded towards adaptation measures, such as resilience and general adverse consequences of climate change, thanks to the inclusion of the Mayor Adapt initiative.

Any kind of city can join CoM. To support cities in maintaining their commitment, the initiative mainly asks for two things: the redaction of a Sustainable Energy Action Plan (SEAP), which assesses the current situation and sets the starting point, and the monitoring of results on a four-year basis, which is through emission inventories named Baseline Emission Inventory (BEM) and Monitoring Emission inventories (MEI) [36,38]. However, as evidenced by Rivas et al., no final monitoring reports are required [38], causing different shortcomings in the effectiveness of the initiative.

The support provided to cities by the CoM is mainly given by Coordinators: local authorities (e.g., regions, and others) that decide to be part of the initiative by giving support to a selection of signatory cities. In general, they can help cities financially or, mainly, in completing the assessment and the monitoring reports. Finally, the initiative also includes Supporters, which are local associations, networks, agencies that promote and mobilize several types of additional contributions, such as financial, organizational, and knowledge-based, for their cities’ members. According to the official data included on the website, there are more than 10,500 signatories, 230 Coordinators, and 233 Supporters divided into 53 countries. Of this, however, only 6225 SEAPs have been submitted and only 2536 monitoring reports. As stated by Rivas et al. [38], this is a sign that several barriers are still present.

The C40 Cities Climate Leadership Group initiative is a network of mayors “of nearly 100 world-leading cities collaborating to deliver the urgent action needed right now to confront the climate crisis”, born in 2005 [38]. The main difference with the CoM is that the C40 foresees an entry selection: only cities that are frontrunners in climate change actions can enter the network. The C40 Leadership Standards for 2021–2024, required to enter the initiative, are (i) having an updated climate action plan in line with the Paris Agreement (with also the necessity to demonstrate that the plan is periodically revised); (ii) in 2024 (intermediate check) being on track to reach 2030 targets and effectively implement the action plan; (iii) boldly addressing the climate crisis; (iv) innovating and addressing emissions also beyond the direct actions of the city government; and (v) commitment of the mayor and the city in meeting Paris Agreements [39]. To support member cities, the C40 takes several actions such as defining updated Agendas, establishing task forces for special themes or events (for example, a Task Force for COVID-19 recovery), giving advice to cities for the redaction of their Action Plan, providing advocacy and networking for the supply of finance, and creating a relationship with key financial institutions. Finally, it supports cities in building new initiatives and in networking and learning from others through workshops and summits. It also has a Knowledge Hub providing practical knowledge and solutions to cities on adaptation, air quality, buildings, diplomacy, energy, planning methods, transport, food, waste, finance, and others. According to the 2020 annual report [40], 88 cities are part of the full initiative, having committed to climate actions in line with the Paris Agreements and more than 75 cities and regions committed to one or more declarations of the initiative. The dimension of the initiative is worldwide, with a stronger global dimension than the CoM.

Another high level TMN is the ICLEI—Local Government for Sustainability network. In contrast with the previous ones, ICLEI is mainly built to create a network of cities around the sustainability topic. Its first aim is to “build connections across levels of government, sectors and stakeholder groups, sparking city-to-city, city-to-region, local-to-global and local-to-national connections. By linking subnational, national and global actors, policies, commitments and initiatives, ICLEI strengthens action at all levels, in support of sustainable urban development” [41]. As stated in their reference reports [18,42], cities are involved in five interconnected thematic pathways: (1) low emission development, (2) nature-based development, (3) circular development, (4) resilient development, and (5) equitable and people centered development. These pathways are expressed as manifesto lines of actions, with key points, stakeholders, and partnerships. Like the others, this network targets cities with the possibility to open to different dimensions, such as regions. It seems, in fact, that more than a strong political commitment based on mayors’ signatures, ICLEI asks for a coral engagement of the government together with the community and the cross-levels. In addition, ICLEI provides a lot of support in policy creation, not only in network constitution. A fee is at the base of the membership, for a network that is global and includes more than 2500 cities, towns, and regions. According to Frantzeskaki et al. [22], ICLEI embeds three core roles as an intermediary: a knowledge role, a game-changing role, and a relational role. From the knowledge perspective, authors evidence how ICLEI is a key intermediary in translating scientific knowledge to policy practices and solutions, making easier the link between these elements and groups of actors. It also provides a policy translation across levels, especially between the local and the global one. Within the knowledge role, authors [22] cite their contribution as educators (of city officers) and as integrators of multiple knowledge forms. The game-changing role is mainly referred to as allowing the co-creation of solutions, policies, and strategies, but also allowing frontier experimentations and learning-by-doing practices. Finally, the third role is the relational one, intended as creating networks among cities and staff as well as mediating across policies’ levels.

The last TMN analyzed is the 100 Resilient Cities (100RC). In contrast with the other TMNs, the 100 Resilient Cities is the only one being properly funded and led, since its birth in 2013, by a private foundation, The Rockefeller Foundation. Even if its future survivance is not confirmed, this experience is important in the TMNs history as it was the first one directly targeting resilience, both in its shocks and stresses, and providing support to cities in defining innovative strategies. As happened for the C40 and the CoM initiatives, the 100RC also required an application and cities needed to go through a first evaluation process to enter the initiative. As reported by Galderisi et al. [43], “the 100RC Initiative was designed to financially and technically support cities all over the world in enhancing their resilience in the face of multiple and complex challenges”. As also reported by the authors, the 100RC partnered with ARUP to provide several instruments to signatory cities, such as a City Resilience Framework and a City Resilience Index [44]. This initiative provided cities with a high degree of support, especially in the beginning phases. Indeed, while selected, a Chief Resilience Officer was appointed to support the city in building its Resilience Strategy. This strategy was generally based, as for the CoM, on a baseline assessment of resilience and then on the proper definition of the Resilience Strategy. As for the C40 Climate Action Plan, the 100RC Resilience Strategy was also intended as a living document, to be updated through time [45,46].

It is increasingly common to observe citizens—especially young citizens—organizing to bring institutional attention to a specific urban climate goal that is not yet included in policy [48,49]. In a phase of reflection and uncertainty about the future, between coexistence and overcoming the pandemic, as well as awareness of the challenges of climate change [50], it is urged not to ignore these emerging forces from activists of the urban scene. Despite the great analytical effort to read these forms of movements from the [6,51,52,53] bottom-up, what still seems to be neglected is the identification of their operational capacity: the understanding of the reasons why the practices materialize, whether in an integrated way, in conflict, or by filling gaps, thus acting in subsidiarity towards the transition; what these “critical agents of change towards resilient future” [54] are saying, who leads them, and in what relationship they enter in relation to the city; and, finally, the institutional role to be entrusted to these subjects, in terms of their power to convey the message to their targets. For the purpose of this analysis, this study takes into consideration a specific field of bottom-up initiatives against climate change: grassroots movements for climate action. This choice is motivated primarily by the exponential growth of the climate movement that arose in the world in the years 2018 and 2019, becoming one of the most widespread environmental social movements in history [51].

On Friday, 20 August 2018, then-16-year-old Swedish Greta Thunberg refuses to go to school and begins protesting in front of the Swedish Parliament building. With her handwritten “school climate strike” sign, she accuses the government of failing to meet the Paris Agreement goals and calls for radical action to prevent global warming [33].

The initiative repeats every Friday, eventually gaining numerous followers through social media coverage, and soon goes viral, spawning the Friday for Future (F4F) movement. On 15 March 2019, the first global climate strike gathers more than 1.5 million young people, in over 2000 locations, expressing their resistance to the failure to act on the climate emergency [55,56]. In addition to strikes, the movement has evolved to create different formats to initiate dialogue with schools, universities, politicians, city councils, media, and businesses.

The movement now has a participant base of approximately 7.6 million young people around the world, as also indicated in the map that is updated in real time on their website, which also includes also other green movements [57]. Their demands are varied and can be summarised in the urgency of action to accelerate the efforts of power groups to reduce greenhouse gas emissions and respect the Paris Agreements. Their message proclaims a sense of urgency: in the present, society is called for immediate radical action to interrupt the insurgency of heavy impacts of climate change. The targets of F4F are mainly politicians, political and global leaders throughout the world. Their message is mostly conveyed during international meetings (e.g., World Summits, COPs), where powerful speeches are communicated by the main leaders of the movements, often using radical expressions and wording, to clearly sentence the gravity of the climatic situation, but also to call both leaders and peer to action. The catastrophic tone and rhetoric of climate movements are also recognized as a way of recruiting participants through self-identification in a common cause.

F4F’s experience started a snowball effect of other small or large activist networks, which over time organized to demonstrate against climate challenges and to raise awareness of local governments for action. Earth Uprising is one of them, a network of about 50 active members around the world, protesting through strikes and motivational speeches, but at the same time offering educational programs on environmental issues aimed at young people. On this last point, they act in collaboration with educational institutions, but also with the support of sponsors to support microgrant funds for educational projects and expenses of the protesters.

Shortly thereafter, other parts of the population began to show sympathy and solidarity to these youth movements: Parents for Future [58], Teachers for Future [59], Artists for Future [60], Farmers for Future [61], and Scientists for Future [62] all pledged to build a broad social alliance for climate policy and bold politics. Besides a large digital network of exchanges and knowledge provision, these movements reaffirm the need to strengthen the physical-relational dimension of governance experiences, which in the current contingency is likely to remain in the background compared to the digital dimension. A clear example of this claim is the Extinction Rebellion initiative.

Extinction Rebellion (XR) is an international movement that uses nonviolent actions of civil disobedience to push institutional actors and the media to act and communicate urgently towards the current climate and ecological crisis [34]. The founders of Extinction Rebellion are 15 activists who came up with the idea in April 2018 during the protests of the RisingUp! Movement. The movement, born in October 2018 in London, defines itself as “a-political”, even though the government is the target of its demands, and “decentralized”, having hundreds of volunteers and 650 autonomous local groups in 45 countries [63]. The group is composed of various age groups, professionals and representatives of different communities, including Christian branches. Extinction Rebellion presents three main statements: that climate change is declared a national emergency, that the loss of biodiversity be stopped by reducing greenhouse gas emissions to zero by 2025, and that a citizens’ assembly is created to monitor progress. At the heart of the project is the desire to make environmental protests more aggressive and concrete, to attract more attention and better communicate the urgency of the problem. City and strategic infrastructures shutdowns, hunger strikes, and mass arrests are some of the tactics that have been used by XR to get attention from the public on the urgency of climate issues [63]. For example, the November 2018 week of protests (https://www.theguardian.com/environment/2018/nov/17/thousands-gather-to-block-london-bridges-in-climate-rebellion (accessed on 10 December 2021) was a great communication success despite, or thanks to, the 85 people arrested, and was instrumental in the UK Parliament declaring an environmental and climate emergency.

In response to the solicitations, several cities have declared themselves to be in a climate emergency status (first Great Britain, then Ireland, rising to 12 states worldwide, including 4 in Europe). The declaration of climate emergency recognizes locally the need for action against climate change. In addition, it introduces the tool of climate impact assessment to assess the integration of measures to combat climate change in local policies. This declaration became the worldwide tangible output of the protests, leading to different outcomes and degrees of implementation throughout the countries.