Cities could hold the key to slowing and eventually stopping global warming. Most greenhouse gas emissions are generated from producing the goods and services used by middle- and upper-income urban consumers. Keeping global warming within safe limits demands far more energy-efficient urban buildings and production systems and urban lifestyles that are far less carbon-intensive. It is up to high-income nations — the biggest contributors to greenhouse gas emissions past and present — to show how such a transformation can be combined with high living standards. However, urgent action is also needed in the urban areas of low- and middle-income countries, both through mitigation to curb greenhouse gas emissions, and adaptation to the serious risks that climate change brings.