Adding greenery to the rooftops in a city can help fight climate change.
Credit: iStockphoto
SYDNEY: Replacing traditional roofs with gardens could help fight climate change, by acting as a carbon sink and cooling cities, say U.S. scientists.
If all rooftops in an urban area with a population of about one million (roughly the size of Adelaide or Detroit), grew these rooftop gardens, it would be equivalent to eliminating a year’s worth of carbon dioxide emitted by 10,000 large cars and trucks, say the scientists, based in Michigan.
“Widespread use of [rooftop gardens] would definitely be improving our management of carbon, especially in large cities where rooftops cover a great deal of land area,” said researcher Kristin Getter, a horticulturist at Michigan State University, in East Lansing, whose study was published in the journal Environmental Science & Technology.
Roofs could become carbon sinks
The soil depth and plant types used in rooftop gardens varies, and some can even support trees, however due to building weight restrictions and costs the gardens usually have a shallow layer of soil and small succulent plants.
This strategy to combat global warming draws on the natural process of photosynthesis in plants, which removes carbon dioxide from the atmosphere and stores the carbon in plant biomass and soil. Utilising the space on empty roofs to grow plants has the potential to be an effective carbon sink.
Two experiments were conducted during study. The first experiment involved eight green roofs in Michigan and four in Maryland, ranging from one to six years in age. All 12 roofs were composed primarily of Sedum species (hardy and low growing, shrubby plants) and had soil depths from 2.5 to 12.7 cm.
The second experiment installed an extensive green roof on the Plant and Soil Sciences Building on the campus of Michigan State University. The green roof consisted of six centimetres of soil and four of the most commonly used plants in green roofs in the region.
375 g of carbon per square metre
Plant and soil samples were randomly harvested every month during the growing season for two years. These harvested samples were then analysed for the amount of carbon they contained.
On average, each green roof system had the capacity to sequester 375 g of carbon per square metre of roof.
In addition to absorbing carbon from the environment, Getter said green roofs offer other benefits such as improving stormwater management, conserving energy, air and noise pollution mitigation, and increasing urban biodiversity.
“These results may [also] allow building owners with green roofs to trade in carbon-based commodities,” said Getter, who is currently studying the effectiveness of growing fruits and vegetables on green roofs as a means of growing local produce in dense urban populations
Real impact is cooling effect on city
However, Dickson Despommier, an environmental health scientist with Columbia University, in New York City, does not believe green roofs will have a significant impact on carbon levels.
“Too little green, too much carbon dioxide," he said. Re-forestation of vast areas of tropical regions would be needed to have any real imapact, said Despommier.
Derek Eamus, environmental scientist with the University of Technology in Sydney, Australia, said the shallow soil depths and low lying small garden plants used in the study do not provide long storage times for carbon.
Unlike large woody plant species that provide long storage periods, the plants used in the study will release the carbon back into the atmosphere within a year or two so the impact is fairly low.
“The real impact of large-scale green roofs is likely to be in the cooling effect it has on the house, suburb or city and so the energy used in air-conditioning will be much reduced,” said Eamus.
“Also, green roofs absorb sunlight [through the leaves of the plant] so less is reflected back into the atmosphere and this is also beneficial to the energy balance of the lower atmosphere,” he added.
Follow Cosmos on Twitter!
twitter.com/cosmosmagazine
