- Coalition of 33 cities unite to protect residents and transform urban spaces to prepare for a hotter future
Rio de Janeiro, Brazil (3 November 2025) – A new global coalition of cities has today pledged urgent, coordinated action to tackle one of the most dangerous impacts of the climate crisis: extreme heat.
Announced on the first day of the C40 World Mayors Summit in Rio de Janeiro, the Cool Cities Accelerator will help urban leaders protect residents, safeguard economies, and redesign cities for a hotter future. The initiative brings together 33 founding cities representing over 145 million people from every region of the world, including Austin, Boston, Buenos Aires, Freetown, London, Nairobi, Phoenix, Paris, Singapore and others, all committed to protecting residents and transforming urban spaces for a hotter future by 2030.
C40 established the Accelerator with support from The Rockefeller Foundation and with implementation support from ClimateWorks Foundation, Robert Wood Johnson Foundation, Z Zurich Foundation, Danish Ministry of Foreign Affairs and IBM.
Extreme heat is already the deadliest weather-related hazard worldwide, responsible for nearly half a million deaths each year. Without decisive action, the number of people exposed to life-threatening urban heat is projected to increase fivefold by 2050. These 33 cities are committed to taking that decisive action.
The vast majority of deaths caused by extreme heat are preventable through timely access to cooling, hydration, medical care, public health interventions, and improved infrastructure. These preventable deaths typically occur when individuals – especially vulnerable populations such as the elderly, outdoor workers, or those without access to air conditioning – are not adequately protected during periods of dangerously high temperatures.
The Cool Cities Accelerator provides a science-based, practical framework for cities to take both immediate and long-term action. Participating cities will collaborate, share best practices, and issue progress reports on:
- Protecting residents now by establishing clear heat leadership, strengthening early warning systems, and ensuring access to cooling during emergencies within two years.
- Transforming cities for the future by improving building standards, expanding urban tree cover and shade, and future-proofing critical infrastructure within five years.
This Accelerator answers the call to action on extreme heat from the United Nations Secretary-General and supports the COP30 Beat the Heat action agenda, a flagship effort to implement solutions that speed up local responses to extreme heat and promote sustainable cooling for cities together with national governments.
With this partnership, C40 has supported and will continue to support more than a dozen cities, including Accra, Austin, Bengaluru, Durban (Ethekwini), Freetown, Jakarta, London, Milan, Nairobi, New York City, Quezon City, Rio de Janeiro, and Tshwane, with technical assistance to achieve the heat resilience goals of the Accelerator.
Haris Doukas, the Mayor of Athens, said “Athens is one of the most severely heat-impacted cities in Europe with the number of heatwave days expected to double by 2050.
“In response to this crisis, Athens is leading a comprehensive, cross-agency effort built on the three pillars of awareness, preparedness and redesign.
“Our city has already made great strides in meeting the ambitious goals set by the Cool Cities Accelerator through our innovative heat governance structure, including the appointment of a Chief Heat Officer, and our existing and planned actions around heat risk communication, cool shelters, tree planting initiatives and boosting urban greening infrastructure, with the creation of a first micro-forest as well as advancing a major regeneration project in the hottest and most vulnerable part of Athens.
“Over the next five years, we will continue to transform Athens in alignment with the accelerator, and in collaboration with other leading cities from Europe and all over the world, and make it cooler, greener and safer, especially for those who need it the most.”
Elizabeth Yee, Executive Vice President of The Rockefeller Foundation, said: “Extreme heat is no longer a distant threat—it’s a daily reality affecting the lives and livelihoods of millions around the world.
“Through the Cool Cities Accelerator, we’re proud to support mayors who are investing in bold, science-based solutions to future-proof health systems, ensuring they can withstand 21st-century challenges.
“Local leaders are not just responding to the climate crisis today—they’re redesigning urban life to protect people, strengthen economies, and build a cooler, safer future for all.”
Mark Watts, Executive Director of C40 Cities, said: “Extreme heat is a silent killer and an increasingly urgent global threat.
“The number of days that major capitals experience temperatures above 35°C has increased 54% over the past twenty years. Cities are showing real leadership by taking practical steps to protect communities, safeguard economies, and create more liveable urban environments.
“By aligning with the UN Secretary-General’s call to action on extreme heat, these cities are helping to set a global standard for what bold, collective climate leadership looks like.”
Jessica Brown, Senior Director of Adaptation & Resilience at ClimateWorks Foundation, said: “Extreme heat is becoming a more intense challenge for people’s health and their economic security.
“We are proud to have mobilized $50 million in global adaptation and resilience funding for communities facing climate risks, and we are thrilled to help transform cities through the Cool Cities Accelerator. Local and community organizations hold the trust and insight needed to drive meaningful change and chart a more resilient future.”
In support of the Cool Cities Accelerator, The Rockefeller Foundation is providing a grant of approximately USD 1 million to develop the targets for heat adaptation and provide technical assistance for cities to implement solutions that mitigate the dangerous effects of extreme heat.
The Cool Cities Accelerator is part of C40 Cities’ broader mission to support bold, science-based climate action in the world’s largest and most influential urban areas. By sharing strategies and scaling proven solutions, cities can act faster and more effectively than they could alone.
Together, participating cities represent a global movement to save lives, strengthen resilience, and create thriving, heat-resilient cities for generations to come.