California Academy of Sciences Image by Jaymi Heimbuch
There is a lot to like about green roofs; they help keep cities cool, create habitat for birds and insects, provide wonderful amenities and just look so good on google earth. But do they actually sequester any CO2? Of course, any green plant does, but there isn't much to a green roof. Is there enough to matter?
Researchers at Michigan State University have measured it. According to Enrique Gili in Miller-McCune, they studied 10 existing green roofs and planted 20 one-meter square plots. They looked at sedums, the most common, and thinnest green roof. It is also one of the hardiest: "We planted what we knew would grow," said researcher Kristen Getter.
Research at University of Michigan's Green Roof Research Program
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.
That's not very much. But it is better than nothing. Getter notes:
"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."