Bikes are parked on a street in Copehhagen, Denmark, August 2018.
Thomson Reuters Foundation/Lin Taylor
More than 70 cities worldwide have pledged to become "carbon neutral" by 2050, meaning they will produce no more climate-changing emissions than they can offset, such as by planting carbon-absorbing trees. Each is going about achieving the goal in its own way.
But because cities account for about three-quarters of the world's carbon dioxide emissions, whether they succeed or fail will have a huge impact on whether the world's climate goals are met.
Read all the stories in our special series on zero-carbon cities: