PARIS, FRANCE - MARCH 29: Mayor of London, Sadiq Khan delivers a speech next to the Mayor of Paris and Head of C40 (a grouping of 90 megacities that want to act for the climate), Anne Hidalgo (not pictured) and Mayor of Seoul, Park Won soon (not pictured) at the City Hall on March 29, 2017 in Paris, France. The Three Mayors announced the launch of Air'volution, a vehicle rating system based on their actual emissions of particulate pollutants, to allow citizens to know the environmental impact of the vehicles they buy and drive. (Photo by Aurelien Morissard/IP3/Getty Images)
