Deep underneath Singapore’s northeastern district of Punggol, a 5km (three-mile) network of metal pipes roars as it pumps chilled water to cool offices and classrooms overhead. The 140-year-old concept known as district cooling uses less electricity than centralised air conditioners – a major advantage for a resource-starved tropical island-nation that has to import nearly all its energy and where temperatures are rising twice as fast as the global average. The city state has laid pipes beneath.