Researchers at Columbia University have demonstrated that a layer of plants and earth can cut the rate of heat absorption through the roof of a building in summer by 84%.  While conventional roofs can reach temperatures of 80 °C at 1.00 p.m. even outside of high summer, green roofs always stay closer to ambient temperatures. 

Researchers say that "I think the way we're going to cool things down in cities is going to be a combination of vegetation and brighter surfaces," ....... "There's going to be a contribution from both.".

Read more here: http://www.guardian.co.uk/environment/2010/sep/02/green-roof-urban-heat-island