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Green Rooftops Help Chicago Stay Cool

Agence France-Presse in Chicago – Updated on Mar 17, 2008

Nestled atop Chicago’s neoclassical city hall lies a secret garden hidden to all but those peering out of the windows of neighbouring office towers.

Dozens more dot the rooftops of shops, restaurants, businesses and city-owned buildings in a patchwork of green aimed at cooling Chicago’s concrete jungle.

About 370,000 square metres of rooftop gardens have been planted on public and private buildings in the seven years since the first plants were placed atop city hall as part of a broader effort to reduce the Illinois city’s carbon footprint.

Inspired by similar programmes in Europe, Chicago, the largest city in the United States’ Midwest, now has one of the most extensive rooftop garden programmes in the world.

Corporate America is joining the trend, planting gardens atop a Chicago McDonald’s restaurant and an Apple store, while smaller businesses and land owners are planting green roofs with the help of city grants.

“Chicago is at the head of the pack,” said Amy Malik, regional director of the non-profit group International Council for Local Environmental Initiatives.

Concrete surfaces – especially those coated with dark tar – both absorb and radiate heat, which significantly increases a building’s heating and cooling costs and contributes to raising urban air temperatures.

The cooling effect of the gardens is dramatic. Thermal images taken of the city hall rooftop on a cloudy summer day found it was the same temperature as the air: 22 degrees Celsius. The black tar roof next door was a scalding 66 degrees.

“There are more than just aesthetic benefits,” said Chicago’s environment commissioner Suzanne Malec-McKenna.

In addition to helping cool buildings, the plants also filter the air, reducing pollution and improving surrounding air quality. The rooftops also “stress sewers less by gathering rain water and using it”, and a green roof can also extend the life of a roof by protecting it from the elements, Ms Malec-McKenna said.

Authorities do not generally open the 36 city-owned rooftop gardens to the public because of safety concerns. But dozens of bird species gather amid the 20,000 plants on the city hall garden alone.

It is a 1,860 square metre oasis perched on top of an 11-storey building in the heart of the central business district that hosts more than 150 species of plants. And honey from the beehives kept in two of the city gardens is sold to raise money for after-school programmes.

“A market has been built around this,” Ms Malec-McKenna said. “The economics of building green roofs have gotten much better. Now we have more than two dozen contractors across the Chicago region who know how to do this.”

Organic grocery shop owner Paula Companio received a US$5,000 grant from the city in 2006 to grow produce on her roof which she hopes to sell in her store below. The garden covers half of the roof of her shop and produces a small crop of onions, potatoes, herbs and tomatoes.

Ms Companio estimates that her building has been 15 to 20 per cent warmer in the winter, and “noticeably” cooler in the summer since the garden was planted.

“The experience became such a community project – everyone asked about it. It showed people it’s possible that they can do it too.”

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