“My office felt like a steamer on Monday morning,” wrote Chinese influencer Bi Dao in a social media post in August. He fetched a drink from a supposed cold-water dispenser – it was 40.8°C (105°F). Bi, who lives in Hangzhou, a regional capital on China’s east coast, decided to roam the city with a temperature gun, pointing it at things to find out exactly how hot they had got. “The ground was 72.6°C, the seat of a sharing bike was 56.5°C, the handrail in the metro station was 45°C, even the tree bark was 38.7°C,” he wrote. He ended his post by thanking Willis Carrier for inventing the air conditioner.

Hangzhou is known for its beautiful lake, large pagoda and rolling green tea farms – not for heat. But what Bi witnessed was only one of the 60 “high-temperature days” – ones that topped 35°C (95°F) – that grilled the city and its 12.5 million inhabitants this year. Hangzhou isn’t alone. Many cities worldwide are feeling the heat. Things are getting so bad that growing numbers of people face temperatures that are beyond human endurance.

Already, such conditions kill around half a million each year. That will inevitably rise as climate change increases the number and intensity of heatwaves around the globe. Cities are on the front line of this unfolding crisis. And China’s vast, densely packed megacities are leading the way. As well as providing a glimpse of what we are in…