The world’s cities are mushrooming at the rate of around 1 million people a week as the planet’s population heads toward 9 billion people by 2050 from 7 billion now. Urban areas are set to sprawl over an extra area equivalent to most of Europe within 20 years, yet little is being done to prepare for the major challenges that expansion will bring.

Already more than half the world’s population is urbanized – a fraction, they said, that would surge to some two-thirds by midcentury if current projections hold true. “Re-engineering cities is urgently needed for global sustainability,” Shobhakar Dhakal, director of Tokyo’s Global Carbon Project, said during the Planet Under Pressure conference. More than 70% of climate-changing CO2 emissions are generated in cities, but most cities around the world were neither planned for nor intended to hold the vast numbers of people they are projected to house, with, in many cases, aging water, sewage, power and transport systems already struggling to cope.