In 1818, Luke Howard published The Climate of London, in which he described an emerging problem: urban development was affecting the local weather. The early 1800s was a period of rapid growth for London, and Howard observed that temperatures in the city were gradually becoming higher than those in rural areas. We now refer to these areas as Urban Heat Islands. The temperature difference is usually larger at night, and the phenomenon occurs in both winter and summer. Experts attribute this to urban development, in which open green spaces are replaced with asphalt roads and tall brick or concrete buildings.

These materials retain heat from the Sun during the day and release it through the night. In Atlanta, in the US, this has even led to thunderstorms occurring in the morning rather than, as is more common, in the afternoon. Officials there are advising builders to use light-colored roofs in a bid to reduce the problem.

Large cities around the world are adopting strategies to address this issue, and plants are now often grown on the roofs or along the walls of large buildings. In Singapore, the government has pledged to turn the country into a 'city within a garden', and in 2006 it held an international competition for entries to develop a master plan toward that goal. One outcome was the creation of 19 “Supertrees.”

These metal constructions are made to resemble very tall trees and range in height from 25m to 50m. Each one is a vertical freestanding garden that holds exotic plants and ferns. Their structure allowed the designers to create an immediate rainforest canopy without having to wait for trees to reach such heights. They contain solar panels used to light the trees at night and also containers to collect rainwater, making them truly self-sufficient.