CAROUSEL Environment Science & Environment

December 11, 2009

Cooling the Asphalt Jungle

As green roofs proliferate to cool and cheer cityscapes, might they also suck up and store some of the carbon urban life pumps out?

By Enrique Gili

The asphalt jungle is due for a makeover as tar beach becomes a sanctuary for native plants, wildflowers and winged pollinators. Like mushrooms after a spring rain, “green roofs” are proliferating on rooftops across the United States and throughout Europe, gaining adherents among sustainable design advocates intent on creating more livable and greener cities.

While rooftop gardens have been a part of city life since the 19th century (if not earlier), their environmental benefits are just beginning to be fully realized. As global temperatures creep upwards, scientists are glancing at the skyline, looking for ways to cool down concrete-bound cities and the planet. One proposal has been to install white roofs, which would reflect solar heat and require less energy to cool urban areas. Another idea is to absorb — or sequester — heat-trapping gases like carbon dioxide by using rooftops as yards.

In a first-of-its-kind study, researchers at Michigan State University have calculated the carbon sequestration benefits extensive green roofs can provide. Findings from horticulturalists Kristen Getter and Brad Rowe in October’s Environmental Science & Technology revealed green roofs’ potential as carbon sinks.

During photosynthesis, plants remove carbon dioxide from the atmosphere and store CO2 in the leaves, soil and root system, converting sunshine into carbon-based compounds such as carbohydrates and sugar. According to Environmental Protection Agency statistics, U.S. forests sequestered 637 million metric tons of the carbon dioxide emitted by made-sources such as coal, fuel and natural gas. Urban forests sequestered on average an additional 74 million metric tons. (All told, the U.S. offsets about an eighth of the carbon it produces, and the vast majority of the offset comes from forests.)

Currently the job of large-scale carbon sequestration is performed in the vast storehouse of the Earth’s ocean and forest ecosystems that play an integral part in regulation of the temperature of the atmosphere.

While there have been studies on how much heat green roofs might fend off, how well a green roof would store carbon had been uncertain up until Getter and Rowe’s study.

Two experiments were run to measure the potential of storing carbon in green roofs. The first involved eight green roofs in Michigan and four in Maryland ranging from 1 to 6 years of age. The second involved planting an extensive green roof of 20 1-square-meter plots at MSU’s campus in East Lansing.

All the green roofs were planted with Sedum, a genus of leafy succulent known for its hardiness and often used as ground cover. “We planted what we knew would grow,” said Getter.

Over a two-year period, the plants on the East Lansing campus were periodically harvested. Leafy parts stored on average 168 grams of carbon per square meter, the roots and the soil respectively stored 160 and 300 grams on average. Combined, each plot had the capacity to store 375 grams of C02 per square meter.

The researchers estimated the city of Detroit has 219 acres of roof space available for conversion. If black tar roofs were retrofitted, 55,000 tons of CO2 could be removed from the air — enough CO2 to offset the carbon emissions of 10,000 mid-sized SUVs or trucks for an entire year, they calculated.

“Implementing a green roof strategy would definitely be one way of managing the sequestration of carbon,” said Getter. And yet … in order to offset man-made carbon emissions it would require a Texas-sized green roof to make a significant contribution to carbon sequestration.

“Green roofs certainly don’t store the kind of carbon that a forest or productive grassland stores, but a traditional roof is essentially a wasteland – no carbon storage whatsoever,” Getter wrote via e-mail.

Jim Mumford, a horticulturalist turned entrepreneur, is dubious about the amount of carbon sequestration a green roof provides. Perceiving it as an added but minor benefit, “it’s debatable about how much of a carbon sink it really is,” he said.

Still, he’s in total agreement there has to be more green and less tar on city rooftops. In 2007, he retrofitted his office with a 478-square-meter green roof, the first of its kind in San Diego, if not the state.

As the founder of GreenScaped Building, he’s completed construction on nine green roofs in San Diego County and has several more projects under proposal throughout California and the United States.

Instead of talking meters and grams, Mumford is faced with the challenge of turning data into a business plan. He believes retrofitting commercial space with green roofs in Southern California is cost-prohibitive, especially in the current business climate. For the time being, “building has ground to a halt,” he said.

Once the market revives, the next challenge he faces will be building large-scale green roofs at a price real estate developers and building owners will pay. Green roofs cost typically twice as much to install as conventional roofs.

Nonetheless, he’s intrigued. “The more I looked at it the more excited I got,” he said. Mumford envisions a day when installing a green roof will be part of a comprehensive plan to create buildings that are both energy efficient and conserve water.

He’s assembled living walls and demonstrated water-caching systems on the grounds of his business complex, transforming his workplace into an “open laboratory” for water and energy conservation strategies, and seeking cost-effective ways to capture rainwater rather than rely on drinking water to irrigate green roofs.

Burnishing a building with a living skin has several environmental advantages. Most notably, it cuts down on storm water runoff, reducing the energy costs associated with heating and cooling buildings, and extends the material lifespan of roofs exposed to the elements.

Inside his office, Mumford has noticed a marked difference. White noise has been reduced. He’s saved 23 percent on his electric bills. And rather than redoing his roof every 10 to 20 years, he believes his green roof can last up to 60 years if maintained properly.

A study by the Berkeley Lawrence Lab found that if 15 percent of the buildings in Los Angeles installed reflective or green roofs, daytime temperatures would be reduced by 3 degrees Celsius — saving Los Angeles half to 1 gigawatt of power during peak-use hours.

“There is a tremendous movement in other cities,” said Mumford as he rattled off the names of cities where interest in green roofs has grown exponentially. That has to be good news for keeping cities cooler and green-thumbed entrepreneurs.

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