Almost everyone agrees that black roofs should be replaced. (Some say that they are better in the north). However a new study is making headlines; Fast Company leads with "White Roofs Are 3 Times More Efficient Than Green Roofs" and subheads with "Rooftop vegetation looks awesome, but isn't the most environmentally friendly choice out there."
And indeed, the abstract of the behind-a-paywall study says ". Owners concerned with global warming should choose white roofs, which are three times more effective than green roofs at cooling the globe." It goes on to suggest that "Owners concerned with local environmental benefits should choose green roofs, which offer built-in stormwater management and a “natural” urban landscape esthetic.
In the Berkely Lab Report, Arthur Rosenfield gives a bit more of an explanation.
Unlike white roofs, green roofs do not offset climate change. White roofs are more reflective than green roofs, reflecting roughly three times more sunlight back into the atmosphere and therefore absorbing less sunlight at earth’s surface. By absorbing less sunlight than either green or black roofs, white roofs offset a portion of the warming effect from greenhouse gas emissions.
“Both white and green roofs do a good job at cooling the building and cooling the air in the city, but white roofs are three times more effective at countering climate change than green roofs,” said Rosenfeld.
Now Fast Company changed the word to efficient, which is not used in the abstract or the article, and which I think has a completely different meaning. They were only talking about the effectiveness at reflecting heat back into the atmosphere. The study authors also admit that they didn't look at all the factors involved, and note the limitations of their study.
....while the economic results are interesting, it also highlights the need to include factors such health and environment in a more comprehensive analysis. “We’ve recognized the limitations of an analysis that’s only economic,” Mandel said. “We would want to include these other factors in any future study.”
Those other factors include how much sunlight is absorbed and converted by photosynthesis into CO2 eating plants; how much is absorbed in the transpiration of moisture from those plants; how much energy is saved because of the extra insulation and thermal mass of the green roof. The white roof reflects more heat, but the green roof soaks it up. Nor is all of that reflected light going straight up; a lot goes on to other buildings and increases the air conditioning load.
It is simply not true that "green roofs do not offset climate change." They are made of plants. There are a lot more things going on here than the Fast Company title suggests.