In their model, based on observation of real cities using satellite and temperature data, the researchers found that when the area of the impervious surfaces reaches 35% of the total (the other 65% being vegetation cover), things go haywire. Up until that point, the urban area stays at a constant 1.3°C above the surrounding area. Above it, that difference increases as the vegetation is stripped away, "reaching 1.6°C warmer by 65% urbanization." That might not sound like much, but one degree is enough to push up air-conditioning use by up to 20%.
Noteworthy is the fact that fancier, tree-lined neighborhoods with gardens and well-tended lawns can be a couple of degrees lower than less-wooded areas. The effect is so strong that in cities built in the desert, "the urban area actually has a cooling effect because of irrigated lawns and trees that wouldn't be there without the city."
As our cities grow, and green spaces are replaced with more impervious surfaces, their temperatures rise too. The answer is pretty straightforward: more trees and plants means cooler cities. And more trees also mean a more pleasant city environment for those living there. We just have to figure out how to build cities and plant trees at the same time.