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by Sarah Stevens

It’s sweltering outside. With the summer’s longest and strongest heat wave in full swing, our AC’s are cranked up and the even sunset doesn’t bring much relief.

These summer heat waves only increase the urban heat island effect — hotter temperatures in compact urban areas. As we have developed our cities and replaced open land and vegetation with hard, impermeable, and dry surfaces, these urban areas trap heat, increasing temperatures of urban areas well above those of surrounding, more rural areas. Studies have found that dry, exposed surfaces such as roofs and pavements can be 50-90 degrees hotter than the air, whereas surface temperatures in shaded and more rural areas are closer to the air temperature (Berdahl P. and S. Bretz). These dramatically higher surface temperatures mean more energy is spent on cooling, thereby exacerbating air pollution and greenhouse gas emissions from power plants.

Cities across the world are using green roofs to reduce the heat island effect as well as increase park land within the constraints of limited urban space.

In North America, Chicago’s $7 billion city redevelopment plan has transformed almost 5.5 million square feet into over 359 green roofs. Increasing green space and vegetation is a key component of the City’s plan to battle the heat island effect. The City Hall green roof is one of the many green roofs naturally cooling the city and providing environmental benefits. The 23,000 square foot green roof is 12 stories high, has over 100 plant species, and uses a rainwater irrigation system that reduces stormwater runoff. The green roof provides a natural cooling system as the plants absorb the heat, which reduces the air temperature, keeps City Hall cooler, and reduces energy needs. After completing the green roof, studies demonstrated the dramatic drop in the air temperature as a result of the green roof. Half the roof remained as the typical black roof, and the air temperature was almost 80 degrees hotter than the green side! The City estimates that the green roof has directly resulted in approximately $3,600 in heating and cooling savings. An added bonus, the green roof also provides research and educational outreach for the Midwestern climate.

Another notable green roof is the Fukuoka Prefectural International Hall in Japan. This 15-story, tiered green roof in the city center was designed by architect Emilio Ambasz. The innovative design reconciles often contradictory goals of urban development of how to keep parkland and add new development. Ambasz found a way to create a new building without removing the city center’s last remaining green space – a two-block park. The building’s design seamlessly blends urban and natural environments. Despite the space limitations, the 15 terraced gardens include 37,000 plants, and rainwater flow mimics the natural environment, flowing from the top terraces down. In addition to the environmental benefits, including reduced stormwater runoff, the green roof helps keep the building temperature down.

The Croton Water Filtration Plant will have the nation’s largest green roof. Constructing the filtration plant approximately 80 feet below ground will reduce the impact on an already densely populated urban environment and creates an opportunity for an above ground amenity on the nine-acre green roof.

Green roofs are just one example of how green infrastructure can add ecological, economic, and social value to our cities. A new focus on urban greening projects is transforming cities by integrating urban and natural environments. Gone are the urban concrete jungles.



Berdahl P. and S. Bretz. 1997. Preliminary survey of the solar reflectance of cool roofing materials. Energy and Buildings 25:149-158.

De Melker, Saskia. “How to Build a Cooler City.” PBS. PBS, 9 Oct. 2012