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This is the VOA Special English ENVIRONMENT REPORT.

Officials in Chicago, Illinois, are hoping to help the environment by planting gardens on the tops of buildings. They say plants and trees have the ability to clean the air and decrease the temperature. They say rooftop gardens can keep buildings cooler, save energy and extend the useful life of a roof.

Chicago City Hall

Almost half of the streets, parking areas for cars, and buildings in Chicago have dark-colored surfaces. More than sixty percent of Chicago's rooftops are dark in color. During the summer, dark-colored surfaces take in and trap heat from the sun. This causes the temperature to rise higher in the city than in surrounding areas.

This is known as the urban heat island effect. It is felt most in the summer when temperatures are already high. More energy is needed to cool buildings as a result of the temperature increase. The heat island effect also increases air pollution.

Not all cities experience the heat island effect. It depends on the weather and the condition of streets, buildings and other man-made structures. It also depends on the number of natural areas with plants and trees, such as parks and gardens. In addition to Chicago, several North American cities experience the heat island effect. They include Atlanta, Georgia; Baton Rouge, Louisiana, and Toronto, Canada.

The city of Chicago's Department of Environment wants building owners to do what they can to reduce the heat island effect. City officials say one way to do this is by planting gardens on the roofs of buildings. One example is the rooftop garden on Chicago's City Hall. Workers planted trees and other plants on the roof. They chose native plants that need less water. Many kinds of insects and birds have made their homes in the rooftop garden. Workers also replaced surfaces with light-colored materials. They say this has helped reduce energy use inside the building to keep the building cooler.

Officials say the Chicago City Hall rooftop garden also helps prevent rainwater from overflowing in the streets. The water is taken in by the plants, trees and soil. Officials say the overflow of rainwater would be reduced if enough buildings in the city had rooftop gardens.

This VOA Special English ENVIRONMENT REPORT was written by Cynthia Kirk.