This summer, mayors met in London during the heatwave that ironically threatened to derail London Climate Action Week.

As temperatures broke records across the UK and Europe, leading some climate talks to be suspended or cancelled, we found ourselves seeking shelter and comparing how our cities prepare for extreme heat: how we protect vulnerable residents, communicate risk, and redesign neighbourhoods for hotter summers.

At the same time, Paris was overseeing the city’s crisis response as another intense heatwave swept across France. Across Europe, more than 14,000 people lost their lives due to the heat.

As mayors, we are confronting the same question: how do you protect millions of people when extreme heat is no longer an exceptional event, but an increasingly familiar part of summer?

For Phoenix, that question has shaped city government for decades. For Paris and Milan, it is now becoming one of the defining challenges of urban leadership.

It may strike some people as unusual that two of Europe’s oldest and most prestigious cities are looking to a city in the Arizona desert for ideas. But rising temperatures are now a global challenge, and we don’t have time for each city to figure out what works on its own.

Phoenix has learned, sometimes the hard way, that extreme heat cannot be treated as simply a weather event. It shapes decisions about transport, housing, public health, public space, and emergency planning. Preparing for summer begins long before temperatures start to rise.

Phoenix established one of the world’s first city offices dedicated to heat response and mitigation, reflecting an approach that focuses on both immediate relief from extreme heat, especially for those most vulnerable, and building resilience in the medium-long-term by implementing cooling solutions to make the city safer and more comfortable.

The city’s shade plans require 75% shade coverage in new development in areas with high pedestrian activity, and........

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