While green roofs certainly won't solve the global warming problem, their ability to sop up greenhouse gases — even just a little bit — bolsters the case for planting them on city buildings, despite extra costs on the front end, said lead researcher Kristin Getter, of Michigan State University in East Lansing.
"The key to fighting global warming is capturing carbon from the atmosphere and storing it in new reservoirs that weren't storing carbon before," Getter said. "In the whole scheme of things, green roofs are not the one answer to sequestering carbon, but they will certainly help."
Green roofs offer a long list of known benefits. They lower air-conditioning costs in the summer by absorbing and reflecting heat. They lower heating costs in the winter by adding extra insulation.
Green roofs appeal to cities because they soak up rainwater, making excess stormwater less likely to flood sewage systems and inflate sewage treatment costs. Plant-filled rooftops make urban areas less likely to become heat islands. They reduce air pollution and noise pollution. And vegetation, even when it's several stories up, provides habitat for animals.
Like any forested or vegetation-covered area, a patch of green on top of a roof should theoretically lower levels of carbon dioxide in the air, as well. Plants breathe in the greenhouse gas like we breathe in oxygen, and they store carbon in their leaves and other tissues. Until now, however, no one had measured how much carbon a green roof could actually take in.
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