Scientists in the US and in Europe (ie University of Athens) have identified the so called "heat-island-effect", describing the fact that ambient temperatures can be up to 10°C higher in areas of high building density then in the surrounding green hinterland. 

Streets, buildings and vehicles absorb solar energie and radiate heat - the darker the object the more heat it will produce. This leads to massive heat accumulations in large cities, which presents a health hazard mainly to old people and leads to even more energy consumption due to air conditioning. A vicious circle.

The US administration has ordered the use of "cool roofs" for all state-owned buildings as a countermeasure, whereever technically and economically feasible. In some states in the US and Australia "cool roofs" are already mandatory parts of local building regulations.

Cool Roofs are reflecting a big part of the solar energy and do not heat up - thereby significantly reducing the heat-island effect. At the same time these roofs save a lot of energy for air conditioning in the building underneath and hence reduce the emission of greenhouse-gases (mainly carbon dioxide) from electricity production.