Gizmag and Science Daily wrote on a new article in the journal Environmental Science & Technology that argues green roofs covering an urban area containing one million people would capture more than 55,000 tons of C02. The amount of C02 sequestered would be “similar to removing more than 10,000 mid-sized SUV or trucks off the road a year.” According to Science Daily, Kristin Getter and her colleagues at Michigan State University based their research on measurements of carbon levels in plant and soil samples collected from 13 green roofs in Michigan and Maryland over a two-year period.
Getter and her colleagues noted the 13 green roofs examined were composed primarily of Sedum species, with substrate depths ranged from 2.5 to 12.7 cm. “On average, these roofs stored 162 g C·m−2 in aboveground biomass.” To test the effects of different species of plants and substrate, the researchers created a substrate-only plot, as well as plots with different species of Sedum (S. acre, S. album, S. kamtshaticum, or S. spurium). “Species and substrate depth represent typical extensive green roofs in the United States.”
According to the researchers, results varied from species to species. “Results at the end of the second year showed that aboveground plant material storage varied by species, ranging from 64 g C·m−2 (S. acre) to 239 g C·m−2 (S. album), with an average of 168 g C·m−2. Belowground biomass ranged from 37 g C·m−2 (S. acre) to 185 g C·m−2 (S. kamtschaticum) and averaged 107 g C·m−2. Substrate carbon content averaged 913 g C·m−2, with no species effect, which represents a sequestration rate of 100 g C·m−2 over the 2 years of this study. The entire extensive green roof system sequestered 375 g C·m−2 in above- and belowground biomass and substrate organic matter.”
Studies from the EPA and other sources have long noted that green roofs can provide a range of benefits, including reduced energy usage for building heating and cooling (and therefore reduced energy costs), as well as cleaner, cooler air, which can reduce the urban heat island effect in cities. Green roofs also play an important role in stormwater retention, which can alleviate pressure on stormwater management systems. While Steven Chu, U.S. Energy Secretary, has pushed cool or white roofs, which are more reflective and absorb less heat, recent research on the ability of green roofs to sequester may mean that combinations of cool and green roofs may be ideal (see earlier post on reflective roofs).
Also, check out an article from Inhabitat outlining how architects and landscape architects are integrating green roofs into the built environment and creating new models of green infrastructure. In discussing a new project in Brooklyn that features a combined ecological green roof and golf course on top of a massive water filtration plant, TreeHugger quoted Ken Smith, ASLA: “The distinction here is it’s not just a green roof, but a performative green roof that needs to provide all these functions. I think we’re pushing both the design of the green roof and the design of the golf course in new directions.”
Image credit: Inhabitat / Grimshaw Partners