Climate change impacts can be especially large in cities. Several large cities are taking climate change into account in long-term strategies, for which it is important to have information on the costs and benefits of adaptation. Studies on climate change impacts in cities mostly focus on a limited set of countries and risks, for example sea-level rise, health and water resources. Most of these studies are qualitative, except for the costs of sea-level rise in cities. These impact estimates do not take into account that large cities will experience additional warming due to the urban heat island effect, that is, the change of local climate patterns caused by urbanization. Here we provide a quantitative assessment of the economic costs of the joint impacts of local and global climate change for all main cities around the world. Cost-benefit analyses are presented of urban heat island mitigation options, including green and cool roofs and cool pavements. It is shown that local actions can be a climate risk-reduction instrument. Furthermore, limiting the urban heat island through city adaptation plans can significantly amplify the benefits of international mitigation efforts.