Heat Islands are a weather phenomenom caused by the centralized heat produced by our cities through vehicle exhaust, air conditioners, and massive quantities of heat-aborbing asphalt and concrete. 

Green Roofs and Walls directly reduce this effect.  When moisture evaporates from greenery, there is a significant amount of heat consumed in the evaporation process.   Indirectly, Green Roofs and Walls reduce A/C requirements in buildings reducing energy consumption and heat production.