Tim Sigsworth

Picture the scene. You have just made it through the door from work, although not by car because private vehicles no longer exist. You change out of your work clothes into something more comfortable, perhaps one of three new items of clothing you are allowed to buy every year.

Then it is downstairs for dinner, since all this virtue is hungry work. But don't forget that meat and dairy are off the menu, so instead you might like to daydream about getting away from it all – only to remember that you used up your quota of one short-haul return flight every three years last summer.

This is the radical vision of a net zero future dreamed up by C40, a global collective of city mayors chaired by Sadiq Khan, which advocates extreme measures to halve greenhouse gas emissions by 2030 and limit global temperature increases to 1.5C.

The Mayor of London is, of course, no stranger to pushing the dial on climate change. His unrelenting expansion of the Ulez ultra low emissions zone in August faced down major criticism from affected businesses, disadvantaged citizens and vigilante vandals.

Khan is showing no signs of slowing down: this week, plans were unveiled to lower the speed limit to 20mph on a further 40 miles of roads in London, the capital's largest-ever rollout to date.