Whilst the global average temperature has increased by one degree Fahrenheit since 1900, city temperatures have risen by considerably more. The temperature in Tokyo, for instance, has risen by 5.2 degrees, reports the New York Times. With little opportunity of respite from the heat, residents have turned to air conditioning, which expels even more heat into the atmosphere.
Central Tokyo has only 14% of greenery, less than New York, London or Berlin, says the newspaper. However, a city survey calculated that there are 11.5 square miles (nearly 30 square kilometres) of rooftops that could be converted into gardens, which could help both clean the air and cool temperatures.
In order to encourage the use of roof gardens, the city authorities recently passed a law stating that all new medium-size buildings – those on plots over one-quarter of an acre (1012 square metres) – should dedicate at least 20% of the roof surface to a garden. There are also a variety of tax breaks being put into effect to encourage roof gardens.
However, roof gardens are also being planted for prestige and aesthetics. The Imperial Hotel has recently opened a 5,700-square-foot (530 square metres) lawn on the roof of its 17-storey building. Workmen are also currently planting a one-quarter-acre roof garden on the Tokyo Metropolitan Assembly Hall to be opened this autumn.
In the past, roof gardens have been avoided due to their weight and the tendency of water and roots to percolate into the structures. However, new materials and technology have solved these problems, and living grass is now even thought to protect roofs from sun damage and save money through lower heating and cooling costs.