Amid tumultuous global politics and pervasive climate catastrophe, mayors from around the world have a clear message: Cities are taking up the mantle of climate action and international cooperation, regardless of whether their national governments choose to do so.

From intensified rains and flash flooding in New York City to fatally hot temperatures in Paris, cities are often on the front lines of climate change. In recognition that metropolitan areas are both uniquely vulnerable to these impacts and crucial laboratories for innovative solutions, climate was a core theme at the 2026 Urban 20 Summit (U20) cohosted and cochaired by Los Angeles and New York City from September 19 to 21.

The Summit, organized by C40 Cities and United Cities and Local Governments, brought together mayors and senior leaders from more than 30 cities to discuss the pressing and deeply interconnected challenges of climate change, migration, and housing. The Summit occurred on the heels of High-Level Week at the United Nations and Climate Week NYC, both of which will see national governments discuss the future of climate action in a world already surpassing the 1.5-degree warming limit enshrined in the Paris Agreement.

With the shadow of these upcoming national government dialogues looming, mayors and city leaders made clear that climate action remains a priority for local leaders, and that tackling the climate crisis cannot be done in isolation from other interconnected social issues or without international cooperation.

Their unity and commitment represented a sharp contrast to the nationalist, xenophobic, and anti-science rhetoric of a surging far-right politics. Under the right-wing Trump administration, the US has withdrawn for the second time from the leading global climate treaty, the Paris Agreement, and undertaken a seismic environmental deregulation. Climate skepticism has also become a staple of right-wing parties in the United Kingdom and Europe. Meanwhile, New Zealand recently passed a law shielding fossil fuel polluters and the government from climate liability.

Yet, in a powerful display of global solidarity, the Summit made clear that many cities are working to move in the opposite direction. “Climate change does not stop at our borders, which makes collaboration all the more important, and as the mayors…said, we are more powerful when we work together,” Louise Yeung, chief climate officer of New York City, told attendees.

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In the spirit of collaboration, mayors shared their experiences of piloting programs to mitigate and adapt to wide-ranging climate impacts. These solutions ranged from helping communities establish decentralized, renewable energy systems in Amsterdam to electrifying buses in Jakarta to expanding tree canopy and public-private sector electric vehicle partnerships in Miami. “Cities learning from each other is going to be more important…[as] climate change is accelerating,” said Eileen Higgins, mayor of Miami.

While national governments have significant tools at their disposal to reduce emissions, “they don’t transform a street, renovate a building, or plant a tree,” said Emmanual Grégoire, mayor of Paris. “Cities are where climate policies become a tangible reality for residents. In cities, climate change ceases to be a global phenomenon and becomes a firsthand experience.”

As cities undertake these bold climate initiatives, the U20 Communique ratified at the Summit asks national government leaders to support them. It calls for the G20 to support cities with climate finance, include them in national road maps for climate resilience and just energy transitions, integrate climate action with housing and economic development, give cities access to critical environmental and public health data, and strengthen public trust in climate science and communication.

“More and more, [people] experience [government] through the heat wave that keeps them up at night and the storm that floods their street,” said New York City Mayor Zohran Mamdani. “That is why mayors from around the world came to New York to say that cities need the power and resources to protect and deliver for the people we serve.”

The Summit and the Communique also highlighted the importance of equity in the climate conversation. As Grégoire and Jan van Zanen, mayor of The Hague, noted, climate change impacts often reveal and exacerbate underlying inequalities in cities. A commitment to climate action means a commitment to climate justice, and addressing climate change as an integral part of migration and housing strategy. Mayor Yvonne Aki-Sawyerr of Freetown spoke of creating good green jobs for young people in the sanitation sector, while Mayor Roberto Gualtieri of Rome emphasized the importance of requirements for housing that included green design features.

The Summit further affirmed that many cities maintain their support for multilateral institutions, which have been central to global climate action, at a moment when these institutions, as New York City mayor’s chief of staff Elle Bisgaard-Church remarked, are “under duress.” The UN, for instance, faces a significant liquidity crisis, and the international rule of law is being tested by multiple wars in the Middle East and Europe. Several UN representatives spoke at the Summit about the importance of making space for cities at the organization and municipalities’ value in implementing climate policy and Sustainable Development Goals on the ground.

Simultaneously, mayors and city leaders made clear that their climate action would not only rely on traditional multilateral processes. “Climate action isn’t [just] something we’re going to talk about at COP,” said Aki-Sawyerr, referencing the upcoming annual UN Climate Change Conference, where key international climate negotiations take place. “It’s something we’re going to be doing everyday.”