"If we can unlock the full potential of our cities, we can minimize the price the planet pays for hosting us in our growing numbers," says Marvin Rees, the Mayor of Bristol, UK, who understands deeply how cities can help (or hurt) the environment. Rees notes that while sustainable infrastructure already exists in many parts of the world -- like electric buses in Colombia and freshwater reserves in Singapore -- major investments could make similar innovations more far-reaching and successful. He highlights the pivotal role of city mayors in advocating for a "worldwide network of efficient decarbonized cities" that will bring the world closer to its climate goals.