Bjorn Lomborg looks at how over-heating cities have already figured out how to keep relatively cool:
Cities are hotter than the land around them because they are drier. They lack moist green spaces and have drainage systems that efficiently remove water. In London, the air around the river Thames is cooler than it is a few blocks away in built-up areas. If we plant trees and build water features, we won't just beautify our surroundings, but we'll also cool things down - by upwards of 8C, according to climate models.
Moreover, although it may seem almost comically straightforward, one of the best temperature-reducing approaches is very simple: paint things white. Cities have a lot of black asphalt and dark, heat-absorbing structures. By increasing reflection and shade, a great deal of heat build-up can be avoided. Paint most of a city and you could lower the temperature by 10C.