In a powerful show of unity and urgency, 300 mayors from major cities around the world convened in Rio de Janeiro on November 3, 2025, just days before the United Nations climate conference in Brazil. Their mission? To pledge coordinated and ambitious action against the mounting threat of extreme heat and climate change, a crisis that is increasingly felt in the world’s urban centers. The summit, organized by the C40 network—a coalition of city leaders dedicated to climate solutions—underscored the pivotal role that mayors play in safeguarding their communities against global warming’s harshest impacts.
As reported by the Associated Press, Selwyn Hart, the U.N. special advisor and assistant secretary-general for climate change, set the tone for the gathering. “In the midst of all the geopolitical tensions and divisions, it is truly amazing and inspirational to see what is happening in this room and on the ground in your cities,” Hart told the assembled mayors. He stressed that local leaders are needed “more than ever” as the world enters the second decade of implementing the 2015 Paris Agreement—a pact that aims to limit the average global temperature rise to ideally 2.7 degrees Fahrenheit (1.5 degrees Celsius) above 1850s levels by slashing emissions from burning coal, oil, and gas.
The urgency is hard to overstate. According to World Weather Attribution, Climate Central, and the Red Cross, scientists have determined that from May 2024 to May 2025, a staggering 4 billion people—about half the world’s population—experienced at least one extra month of extreme heat directly caused by human-induced climate change. The effects have been dire: increased illness and death, crop failures, and immense strain on energy and healthcare systems. As Yvonne Aki-Sawyerr, mayor of Freetown, Sierra Leone and C40 co-chair, bluntly stated, “Extreme heat stands now as the deadliest weather related disaster and contributed to an estimated 489,000 deaths each year.” She also warned that “global extreme heat is projected to cause $2.4 trillion of loss productivity by 2030, as it becomes too dangerous for working outdoors,” particularly in sectors like construction and agriculture.
The mayors’ summit was far from just talk. On the same day, the C40 coalition launched the Cool Cities Accelerator, a global initiative involving 33 cities—among them Austin, Boston, Buenos Aires, Freetown, London, Nairobi, Phoenix, Paris, and Singapore. The goal? To prepare urban centers for a hotter future by collaborating, sharing best practices, and reporting progress on emergency measures such as early warning systems and equitable access to cooling projects. Within five years, the coalition aims to improve building standards, expand urban tree cover and shade, and future-proof critical infrastructure. The initiative is backed by support from organizations like the ClimateWorks Foundation, Robert Wood Johnson Foundation, Z Zurich Foundation, Danish Ministry of Foreign Affairs, and a contribution of just under $1 million from the Rockefeller Foundation to help cities jumpstart the effort.
“Mayors see these challenges very closely,” Emilia Carrera, the Rockefeller Foundation’s director of health, explained. “They have a fresher perspective and an ability to respond more quickly.” Cities will report back every two years on their progress, tracking key metrics such as the establishment of cooling centers, the design of cooler urban spaces, and updated building codes.
Local leadership is proving vital, especially as national governments sometimes falter or even backtrack on climate commitments. The summit was punctuated by pointed criticism of recent U.S. climate policy. London Mayor Sadiq Khan didn’t mince words: “Mayors have long been climate doers, while too many nations and states have been the climate delayers or, indeed, climate deniers. Now, it is an existential fight between the climate wreckers and climate defenders. Among the wreckers is the president of the United States of America, someone who stood up at the U.N. only a matter of weeks ago and called the climate crisis a scam.”
Despite such setbacks at the national level, American city leaders remain undeterred. Phoenix Mayor Kate Gallego brought “good news from the United States,” sharing with the audience that “we have 50 cities from the U.S. that are here, all committed to ambitious climate action. So, while our national government goes backward, these cities go forward.” Her remarks were met with applause, a testament to the resolve of local governments to act where higher authorities may hesitate.
Phoenix’s challenges are emblematic of the broader crisis. The city, notorious for its blistering summers, reached a sweltering 118 degrees Fahrenheit—nearly 48 Celsius—not once, but twice in 2025. Gallego detailed Phoenix’s response: “This year, we hit 118 degrees — nearly 48 Celsius — not once, but twice.” The city’s plan includes planting more trees, installing shade structures, piloting cool-surface technology, and providing heat-relief training for first responders like paramedics. These steps are not just about comfort—they are about saving lives and protecting livelihoods as the mercury rises ever higher.
Brazil’s climate change chief and CEO of COP30, Ana Toni, made it clear that the world’s climate goals hinge on the engagement of city leaders. “It is you, mayors, who have to make very hard decisions in daily life, together with people,” Toni told the summit. The sentiment was echoed by urbanist Hannah Machado, who noted that while cities have a long way to go to fully adapt to the changing climate, they have already demonstrated a remarkable ability to reduce emissions—five times faster than national governments, according to the C40 group.
The Cool Cities Accelerator is designed to harness this momentum. The coalition’s five-year roadmap calls for cities to improve building codes to withstand extreme heat, expand urban greenery to provide natural cooling, and ensure that critical infrastructure—such as hospitals and transportation systems—can function during heatwaves. The coalition will also focus on social equity, ensuring that vulnerable populations have access to cooling centers and relief measures.
Catherine McKenna, Canada’s former climate minister and a former U.N. chair of Net-Zero Emissions Commitments, summed up the stakes: “In this year of major geopolitical shifts, that’s an understatement, city leadership has become even more important.”
The road ahead is daunting, but the resolve among city leaders is palpable. With extreme heat now the deadliest weather disaster worldwide and billions at risk, the mayors’ coordinated action may well be the difference between adaptation and catastrophe for millions. As cities step up, the world will be watching—and, perhaps, following their lead.
