C40 is a global network of mayors united in a commitment to climate change action. Since its inception in 2005, C40 has grown to include nearly 100 of the world’s cities, maintaining high standards that focus on inclusivity, collaboration and science-based approaches to combat climate change. We interviewed members of the C40 organization, including mayors of its member cities, to ask about the history, success and challenges of C40, and their plans for future action.What was the idea at the core of C40 during its inception?David Miller: C40 is actually a really radical idea. It started with Ken Livingstone, the mayor of London, and his deputy mayor, Nicky Gavron. Ken had a powerful insight that the voices and actions of the mayors of the world’s major cities could help the world avoid climate breakdown at a time when national governments were not acting fast enough to address the issue. It was radical, because there was no organization of mayors at the time, and because the purpose of it was to solve a global problem.Mark Watts: In 2005, with Hurricane Katrina and with Al Gore starting his public talks, climate change was very much on the international agenda. But on the other hand, it was very nascent, and city governments were definitely not seen as having a responsibility around climate. Mayor Livingston had a long background in environmentalism, and had got very concerned about climate change from reading Al Gore’s book. Ken’s attention started to turn to the things that London could do that could have a bigger impact in the world.Climate change was an emerging issue, but we didn’t really know that much. C40 came out of a desire to do something in London, and a recognition that we just didn’t have the knowledge and skills, so we needed to look out into the world and find others who were ahead of us.IntervieweesYvonne Aki-Sawyerr, OBE, has served as mayor of Freetown, Sierra Leone, since 2018 and is the current co-chair of C40, alongside Mayor Sadiq Khan.Sadiq Khan has served as mayor of London, UK, since 2016, and is the current C40 co-chair, alongside Mayor Aki-Sawyerr. He served as C40 chair from 2021 to 2023.David Miller is the former mayor of Toronto (2003–2010), a founding member of C40 and served as C40 chair 2008–2010. He is now managing director of the C40 Centre for City Climate Policy and Economy (https://go.nature.com/46Nf5fZ).Shruti Narayan is the C40 managing director, regions and mayoral engagement, and regional director, South and West Asia. She has previously worked in finance, sustainable design and education.Eduardo Paes has served as the mayor of Rio de Janeiro from 2009 to 2016 and 2021–present. He served as chair of C40 from 2014 to 2016 and is now a vice chair for Latin America.Mark Watts is the C40 executive director. He previously worked as senior advisor to the mayor of London and was involved in the first Climate Action in Megacities Executive Summary during his time at Arup.How did an idea for city climate action develop into a collaboration?MW: First, we did things the normal bureaucratic way: we asked consultants to come and tell us how we could put together a strong climate plan for London. But we didn’t think any of the consultants knew any more than we did, so we didn’t hire anyone.But we thought that somebody out there must be doing something good, and got the sense that the Scandinavians were ahead. So I went to visit Copenhagen and Stockholm, and then connected with Toronto, where David Miller had an Air Quality Division, and then with San Francisco and the young mayor Gavin Newsom. He called an Urban Environment conference, and that helped us realize that we also had something to offer from London. What we had done on congestion pricing, and the way in which we managed to massively increase public transport usage and democratize it, that was something that other cities were interested in. At the same time, we could learn a lot from them.DM: We saw early on that collaboration between cities can spread actions and ideas quickly. These cities have the financial, public service and political heft to try new things. And when they make a breakthrough, that idea can spread rapidly to other C40 cities. By sharing ideas, things that were at one point considered radical — like electrifying your bus fleet — can quickly become routine because one city took a risk.What were some key challenges in the early years of C40?MW: Initially, it just wasn’t a normal thing that mayors would have an international organization. So the first hurdle was to understand that there was a benefit to talking to each other. And then there was constant pushback of climate being the responsibility of national governments; that cities should get on with managing the waste and the buses. I think that helped stimulate the mayors to work together. They saw that they were as much chief executive as political leader in their role as mayor; they were running services, and people stopped them on the street. They were actually in a better place to move fast on doing something about climate than national leaders, who were much more about setting long-term policies and weren’t willing to take a step forward themselves if their competitor countries weren’t willing to go as far.What were some important drivers of success in the early years?DM: The advocacy role of mayors is an incredibly important founding principle, because they’re really significant politically in their relevant countries.MW: Ken was the most famous mayor in the world at the time, and one of the biggest politicians in Britain. We used all of the weight and kudos of that.G20 wasn’t quite yet formed, but we invited the mayors of the G20 capital cities to come to London, and 18 of them did. It wasn’t that there was a huge amount of interest, but more ‘London is asking us to come’. And then Paris said they would come, and then New York. We had no sense of what we would do next, we just asked ‘would anyone like to host the next meeting’. And New York put their hand up.There was this very good, positive tension between collaboration and competition. We knew that New York had hired consultants to work out why London was thriving so much and why financial businesses were moving their headquarters to London. I think there was the sense of, if London thinks this is important, then we better have a look at it, and we have to equal that.It’s a good thing with cities: when you get that competition, it doesn’t descend into a physical war or a trade war. It’s a race to the top. You copy the idea, and you make it your own, and you improve it.Credit: A.P.S. (UK) / AlamyA key principle of C40 is ‘high standards, no fees’. How does this work and is it successful?MW: We wanted political leaders who were willing to go as far as the science demanded, not start from what do they think is politically possible or they can negotiate with others. We didn’t want membership to be constrained by who was willing to pay, and we wanted to have the right to rescind membership if cities really weren’t delivering. I think that’s been our greatest strength.Eduardo Paes: In my time, in line with Mayor Bloomberg’s legacy, we strengthened the emphasis on data-driven governance. We standardized the measurement and reporting of emissions across cities and introduced ‘participation standards’, which required cities to demonstrate science-based climate action.MW: When Mayor Paes was chair, we decided to align City Climate Action Plans with the Paris Agreement to limit warming to 1.5 °C. At the time, even the big, wealthy cities didn’t all have internationally compliant emissions inventories, let alone plans. But that was what the science said was needed. It was far reaching, but looking at where we are now in 2025, most countries will not have a Paris Agreement plan going into COP30, but almost all C40 cities do.EP: As C40 celebrates its 20th anniversary, it is clear that cities are leading the climate movement. Three-quarters of C40 cities are reducing emissions faster than their national governments, validating the foresight behind C40’s founding and the critical role cities play in climate action.National governments are increasingly recognizing the need to collaborate with subnational governments to meet climate goals. With 77 countries now pledging to work with cities through the Coalition for High Ambition Multilevel Partnerships (CHAMP) for Climate Action (https://go.nature.com/3VIGeLE), this collaboration is gaining momentum. Cities are also playing a central role in discussions like COP30, where local leadership will be crucial in shaping global climate strategies.Given the lack of fees, how does C40 fund itself?DM: From the outset, partnerships with philanthropic organizations were important for ensuring the success of the organization. The initial one was with the Clinton Foundation, who paid for a number of city advisors to C40 cities who could help steer things.MW: Now, there are a large number of philanthropic funders, in different amounts, including governments, and a good set of commercial partners.And how do cities afford their climate action?MW: Ultimately, the best mayors are not constrained either by the formal powers or by the formal budgets, they find ways to attract investment and to get things done. That remains true today, but — just like everyone trying to cut emissions and improve resilience fast enough to keep up with the pace of climate breakdown — there’s a huge gap between available finance and commitment.EP: To help address the issues in securing financing — particularly in the global south — we launched the C40 Cities Finance Facility (CFF) in 2016. The CFF has since helped dozens of cities access funding for critical projects in renewable energy, clean mobility and nature-based solutions. By 2030, CFF-supported initiatives are expected to leverage over US$1 billion in new investments.Yvonne Aki-Sawyerr: Freetown has embedded climate action across both adaptation and mitigation by mainstreaming it into core governance and financial systems. This includes integrating climate priorities into the city budget, ensuring that resources are allocated to resilience initiatives; applying project design guidelines that align new developments with sustainability goals; and advancing sustainable capital investment planning to secure long-term, climate-smart infrastructure.While successes arising from Freetown’s participation in C40 include international recognition and resource mobilization, challenges remain in terms of financing, implementation capacity and balancing ambitious climate commitments with the immediate social and economic needs of residents.DM: Of course, there’s always the question of cost–benefit, and as a mayor, you owe something to your people.There’s always an investment required to change from a dirty, polluting system to a clean one, whether it’s for a building or for transportation or to change a business process or anything. There’s always an investment required, but that investment pays off over time, because a clean economy or a clean building or a clean transportation system is far cheaper to operate than a dirty one, it takes time.Sometimes, it is a challenge. When I was in office in Toronto my second term, I did what every political consultant in the western world would say not to do. I ran my campaign saying we’re going to build a great city, but we’re going to have to pay for it.A key concern in climate negotiations is the equitable involvement of the global south. How has C40 worked to be a truly global organization?EP: The inclusion of global south leaders in climate negotiations has evolved significantly, with increasing recognition of their role. As the first C40 chair from the global south, I worked to diversify the membership; by the end of my term in 2016, over 50% of C40 cities were from the global south, and today that figure has surpassed 60%.However, challenges remain. While wealthier countries are responsible for the majority of greenhouse gas emissions, it is the global south that suffers the most severe climate impacts. These challenges are compounded by the financial barriers many cities in developing countries face in accessing climate-related funding.YA-S: Global south leaders are being heard more than ever, but their voices still lack the volume in global climate discussions, which are often dominated by wealthier nations. The frontline impacts of climate change such as flooding, heatwaves, rapid urbanization and inadequate infrastructure are most acutely felt in our cities. African cities, which serve as the continent’s economic engines, contribute up to 70% of its gross domestic product (GDP), yet they are grappling with the dual challenges of rapid urbanization and the climate crisis. Projections indicate that Africa’s urban population will nearly triple to 1.5 billion by 2050, with 92% of the continent’s fastest-growing cities already classified as being at extreme climate risk. Therefore, urgent investment in climate-resilient infrastructure and housing is essential.Progress has been made through platforms like C40, where African mayors are co-chairing discussions, influencing agendas and integrating their lived experiences into critical dialogues. Currently, 26 million people are benefiting from C40-backed adaptation programmes, with 78% of these beneficiaries located in the global south.To amplify these efforts, it is crucial to ensure global south leaders are not only invited to the table but are also empowered to set the agenda. This includes enhancing direct access to climate finance for cities rather than funding them only through national governments. African countries currently face estimated GDP losses of 2–5% per year due to extreme weather events. Investing in adaptation is both crucial and fiscally responsible, with the World Bank suggesting that every dollar spent on adaptation now could yield up to four dollars in avoided future climate-related damages and losses.Credit: Stefan Becker / AlamyKey mayoral moments in C40 historyKen Livingstone, then mayor of London, UK, convenes representatives from 18 megacities in 2005 to agree to cooperatively combat climate impacts. The group grows to 40 cities, and adopts the name C40 by 2006. New York hosts a second C40 Summit.In 2008, the second C40 chair David Miller, then mayor of Toronto, Canada, brings C40 out into the international stage. Coinciding with but juxtaposing the ill-fated Copenhagen climate talks of 2009, mayors of the world meet to agree to act on climate action.Michael R. Bloomberg, then mayor of New York, USA, becomes chair in 2010, and moves C40 into a more firmly data-driven era. There is a focus on optimizing peer-to-peer sharing, utilizing urban planning powers for maximum impact and developing relationships with private investors. C40 is formally registered as a non-profit.From 2013, Eduardo Paes, mayor of Rio de Janeiro, Brazil, the first C40 chair from the global south, develops C40 into a truly international organization, welcoming megacities from Africa, Latin America and Asia. He unveils the second Climate Action in Megacities and pledges C40 cities to develop city action plans consistent with 1.5 °C risk.As chair from 2016, Anne Hidalgo, mayor of Paris, France, focuses on the social and economic consequences of climate action and inaction, and a strong need to lead using the data and science on climate. She creates the Women for Climate programme, and a rule ensuring gender balance within the C40 steering committee is set.Eric Garcetti, mayor of Los Angeles, USA, chairs C40 from 2019 through the COVID-19 pandemic, establishing best-practice information sharing and establishing the Mayoral Task Force for a Green and Just Recovery from COVID-19, which recognizes the vital climate–health link. C40 focuses on equity and sustainable futures through the establishment of the Global Green New Deal and the Cities Race to Zero.As chair from 2021, Sadiq Khan, mayor of London, UK, prioritizes inclusivity in spending and resourcing, maximizing that for lower- and middle-income countries, and in 2023 announces C40’s new co-chair model requiring one mayor each from the global north and south. In the UN Climate Action Summit address in 2023, he focused on cities as the ‘climate titans’ tackling the climate crisis and accelerating the green transition. He recognizes that strengthening C40 cities, through unity, would empower them to defeat the current politics of fear and division through practical policies and climate action that improve global health and well-being.Yvonne Aki-Sawyerr, mayor of Freetown, Sierra Leone, joined in 2023. She focuses on youth empowerment, and developing innovative, youth-led solutions to the climate crisis. Her joint tenure with Mayor Khan has seen formal recognition of the role of subnational and city governments at COP28, the Local Climate Action Summit, and through the creation of the Coalition for High Ambition Multilevel Partnerships (CHAMP) for Climate Action, which bridges the gap between local action and national policy.What are some examples of C40 success stories for your cities and beyond?DM: The C40 principle is very broadly applicable. Use city powers. Use them creatively. For example, in Toronto, we didn’t have authority over building code, but we used our planning powers to set a green building by-law, which ratcheted up over time the environmental standards for new taller buildings. I spoke about our Better Buildings Partnership at a C40 conference, and the mayor of Melbourne [Australia] came over and asked about it, and we shared information, and Melbourne created a Better Buildings Partnership. And then Sydney created one because they’re competitive with Melbourne. That friendly rivalry, in all seriousness, really matters.EP: In Rio, we’ve made significant progress over the past decade. One key example is our Bus Rapid Transit system, launched in 2012 and inspired by other C40 cities. With 150 kilometres of dedicated lanes, it now carries around 620,000 passengers daily, cuts travel times by up to half and avoids an estimated 107,000 tonnes of CO2 emissions each year. We’ve also transformed the port area through Porto Maravilha, removing an elevated highway to open up pedestrian-friendly streets, green spaces and fountains.Our climate resilience strategy is equally comprehensive. Our Center of Operations integrates data from over 50 agencies to respond swiftly to extreme weather. We’ve installed sirens in vulnerable communities, built underground reservoirs to reduce flooding and delivered six new parks in heat-prone neighbourhoods. In addition, Rio pioneered the use of Power Purchase Agreements, and since 2023 — with support from C40 and Mayor Bloomberg — most municipal buildings have been powered by renewable energy, saving millions and avoiding substantial CO2 emissions.Sadiq Khan: The expansion of the London Clean Air Zone has driven down harmful pollution, removed thousands of the most polluting vehicles from our streets and brought cleaner air to millions of Londoners. The greatest benefits have been felt in our most deprived communities, where exposure to illegal levels of pollution has fallen by up to 80%. This is protecting health, supporting children’s lung growth and reducing the risk of asthma, lung cancer and other illnesses linked to toxic air. We are rolling out the largest zero-emission bus fleet in western Europe, building the most extensive electric vehicle charging network of any European city, and rewilding our capital by restoring rivers, creating habitats and reintroducing species like beavers to Enfield. Through C40, London is sharing how ambitious, evidence-led action can cut emissions, improve health and build more resilient cities. By combining regulation with investment and putting fairness at the heart of policy, we are showing the world that climate action works — for people and for the planet.YA-S: Freetown’s engagement within C40 has been great. With support from the C40 CFF, Freetown was able to advance a feasibility study for its cable car project, a climate-smart transport solution aimed at reducing congestion and emissions. C40 also played a key role in supporting the research and articulation of Freetown’s Climate Action Plan, which set out a clear, evidence-based pathway toward resilience and carbon reduction. At the leadership level, Freetown has provided technical input into the co-chairs’ priorities, shaping the global agenda from the perspective of a rapidly growing African city. The city has further extended its impact by participating in critical networks such as the Global Methane Hub (https://www.globalmethanehub.org/) and Breathe Cities (https://breathecities.org/), linking waste management efforts and local air quality to global campaigns.Shruti Narayan: Working in India, having the city of Mumbai develop the first ever climate budget in the global south was hugely successful, and signals that such a thing can be done in the global south. The city brought together different departments to understand and shift how they spend their money in the context of on-the-ground targets.After Mumbai demonstrated the success of climate budgeting, C40 organized a pure learning session with other cities in the region, and tens of cities wanted to do their own budgeting. The ripple effect of knowledge and action has been very successful. Sometimes, the national governments also recognize and can replicate the actions. I get so many calls and e-mails on a regular basis from cities that are not our members, who want to know either how they can join or how they can try something that they have heard about from C40.How do you share knowledge within and beyond C40 cities?SN: Cities and governments are realizing that inaction is not a possibility. But I think what they may struggle with firstly, is not knowing exactly what to do.To help share knowledge, C40 is structured as a matrix organization. We have seven regions, broadly defined based on geographical identity. And then we have policy and technical teams that weave through this: air quality, transport, finance, diplomacy, for example. Each region is led by a regional director, whose job it is to ensure that we’re able to create a cohesive understanding of climate ambition, and deliver the accelerated speed of implementation that’s needed, while ensuring we don’t lose the contexts and priorities of different cities, regions and mayors.C40 staff help break through silos, for example working across vertical governance of bureaucrats and subnational and national governments. We connect the dots, ensuring that everyone knows what’s happening, but also handhold on certain issues and work to maximize climate ambition.Cities really want to learn from each other, and C40 has probably one of the most powerful peer learning platforms where we bring in city leadership, but also the technical staff in different focus areas together to learn from each other, to understand not only what worked, but what didn’t work. We have a platform called the Knowledge Hub (https://www.c40knowledgehub.org/) where we post a lot of outcome stories, which is accessible to anyone around the world, so you’re not reinventing the wheel, and you’re saving a lot of time and money.If you look at that opportunity of five mayors of key cities just sitting across the table having a coffee and talking, they can share information that would otherwise take five years to get. It’s that power of knowledge, and knowledge sharing, that drives C40’s success.One of C40’s newer commitments is fighting disinformation. What does this involve, and what is C40’s broader role in the current political context?Credit: robertharding / AlamyDM: The reality is that the voices that push disinformation online are very small but very loud. All over the world, the vast majority of people understand the science of climate change. They understand it’s human caused, and they want action. But they believe that their neighbours don’t want action; and that is fertile ground for inaction.One of the things we do to combat misinformation is to pull together the research on these issues (https://go.nature.com/46kfSVb). We know that people trust the messenger, and often a mayor is a trusted messenger. We know that we’ve got to tell stories, not just facts. To fight disinformation, you have to initially assess whether it’s really material, because if it’s not, you ignore it. But you also have to do ‘pre-bunking’ — which involves having trusted messengers with powerful stories talking to people before the misinformation starts. And we’re getting better at that.SK: In the global political context of 2025, organizations like C40 are more important than ever. We are driving climate action in defiance of those who delay or deny strengthening city resilience to extreme weather and improving the air that millions of people breathe every day. By equipping mayors with science-based strategies and by pressing for international agreements that focus on annual, action-oriented delivery, C40 ensures cities remain at the heart of global progress rather than waiting for national governments to act. In a time of deep uncertainty, with populist rhetoric rising on the world stage, we cannot allow this to derail climate progress or weaken our drive to build a fairer world. Mayors now have a powerful voice internationally, and those who are tackling the climate emergency while putting people first are increasingly being recognized and given the space to lead. The climate crisis cannot be solved without cities, and we will not let national inaction hold us back.After 20 years of action, what do you hope for the coming year or two for C40 and its cities?EP: In November 2025, Rio will host the C40 World Mayors Summit, just days before COP30 begins in Belém, Brazil. The Summit will also launch the Local Leaders Forum, co-organized by Bloomberg Philanthropies and the COP30 Presidency, positioning subnational voices at the centre of the COP30 process and highlighting bold, scalable solutions from mayors and local leaders.At COP30, success will depend on securing the financial backing needed for projects aligned with sustainable development goals, especially in urban areas, where most of the world’s population resides. Bridging the financial gap between the global north and south will be essential.SN: My hope is that cities will start to move into the future with a shifted, more holistic approach to the energy transition. For example, if considering mobility, to undertake a paradigm shift to understand that cities should be designed for movement of people, and not buses and cars, and to consider how we can use the tools of urban planning to lock in our blue and green infrastructure. This is particularly important in global south cities, where many of our large cities are yet to be created.We must mainstream climate so that every decision that the city is making is with the lens of climate, and no dollar is being spent without that lens. We must integrate our understanding of climate from a perspective of livelihood, creation, equity and health, because we cannot look at these things separately. Finally, we need to consider adaptation and resilience: building from the perspective of withstanding climate impacts, but also ensuring that we look at mitigation and adaptation together, in a holistic way.YA-S: I see C40 continuing to support cities to move from ambition to measurable delivery. Our strength truly lies in showing that climate action is not abstract. My hope is that C40 will deepen its support for global south cities, particularly by unlocking direct access to climate finance opportunities and building capacity where the need is greatest. For the immediate future, mayors should collectively continue to demonstrate that even in times of economic and political challenge, cities are leading the way. For Freetown, and for many others, by standing together we can scale solutions faster, ensuring that equity is central to climate action.DM: On the short term, we need to say ‘we’re going to do this next year, and we’re coming back next year to tell you our progress’.What do you see for the next 20 years of C40?DM: I think at this moment, what’s required is building a coalition of institutions, entities, people, businesses, governments, who are acting on climate the same way we’ve built with cities. The next step is to build a similar kind of coalition of willing national governments, state and provincial governments, businesses and institutions — like hospitals, universities, churches, mosques, temples. Everybody who can take action on climate to a meaningful level.And the gift of C40 is that we collaborate without a requirement for unanimity — it’s okay for some to take bold actions that others don’t feel ready for. We need to replicate that model at a bigger scale.SN: My hope is that — working along with our city leaders and other partners — we will actually manage the trajectory of how warming is happening. To deliver on our commitment of halving fossil fuel use, and creating a model for cities to thrive and grow, which are equitable, healthy but also resilient to climate, and working with nature.YA-S: C40 has become a formidable global advocate for city-led climate action. In the next two decades, cities will continue to act as innovation laboratories, testing scalable, equitable and resilient solutions, with the global south taking a leading role. By 2030, C40 aims to reduce fossil fuel use in cities by 50% through initiatives like clean transportation, large-scale building retrofits and human-centred urban redesign. I envision a network that enhances city resilience by investing in green infrastructure and adaptive strategies to shield communities from extreme weather events. Beyond mitigation and adaptation, we must also focus on transforming urban economies by improving air quality and creating millions of green jobs, ensuring that climate action promotes inclusive and sustainable growth.SK: My vision is for C40 to remain a powerful force for change, rooted in equity and justice. We must accelerate the transition away from fossil fuels and build stronger urban resilience against extreme weather. I want C40 to grow in influence and impact, showing the world that cities are not just innovators but a united community of action driving a sustainable future. In a world facing enormous challenges, I hope that through C40 every mayor feels they are not standing alone, but part of a global network committed to delivery. At COP30, we have the chance to show that the most urgent and effective climate solutions are already being implemented in our cities and that local leadership is lighting the path forward.EP: Over the past two decades, C40 has grown to connect 97 of the world’s largest cities across 51 countries, representing around 920 million people and 23% of the global economy. Looking ahead, the solution to the climate crisis lies in deeper collaboration. Our focus will be on strengthening city resilience to extreme weather through green infrastructure, improving air quality and creating millions of green jobs to drive inclusive, sustainable urban economies.As Brazil prepares to host COP30 in Belém, we are at a pivotal moment for the climate agenda. A decade after the Paris Agreement, this COP must mark the start of a new era — an era of implementation. With rising geopolitical tensions and growing pressure on multilateralism, cities have become even more crucial as engines of stability, cooperation and tangible progress. Local governments are focused on delivering practical solutions, and in an increasingly divided world, well-supported cities offer the most reliable path to unity and action.COP30 must be a COP of delivery, not delay. It’s time to empower cities, unlock climate finance and make climate justice a lived reality, not just rhetoric.