On September 7, the British newspaper The Guardian published an op-ed signed by three mayors: Emmanuel Grégoire of Paris, Giuseppe Sala of Milan, and Kate Gallego of Phoenix. The three leaders highlighted a shared challenge: extreme heatwaves are becoming more frequent and severe, threatening the lives and well-being of millions. They pointed to the recent summer in Europe, which saw over 14,000 deaths in June alone, as a stark reminder of the growing risks posed by rising temperatures. The mayors emphasized that cities must work together to address this global issue, as adapting to extreme heat is no longer just a local concern but a planetary one. The mayors argued that cities have historically adapted to various challenges by learning from one another. They cited examples of shared innovations in transport, housing, and public health policies. However, they warned that time is running out for cities to act independently. As heatwaves become more common, they stressed the need for collaboration and shared knowledge to develop effective responses. Historic European cities like Paris and Milan face unique challenges because their streets, buildings, and public spaces were designed for a different climate and era. In contrast, Phoenix has long treated heat protection as a central policy. The city established one of the first municipal departments dedicated to addressing extreme heat, focusing on both immediate relief for vulnerable residents and long-term strategies to build resilience. Phoenix’s approach includes measures such as creating shaded areas and promoting cooling technologies. Paris has taken inspiration from Phoenix, ensuring that every resident lives within seven minutes of a "cool island"—a designated area offering relief from the heat. The city currently has 1,400 such spaces. Similarly, Milan plans to expand its cool islands by planting more trees, inspired by initiatives in Paris and Barcelona. However, the mayors cautioned that no single model can be directly copied. Each city must adapt solutions to its own geography, history, and political context to ensure they are effective and sustainable.