France passed a law that requires commercial buildings to either have solar panels or living things on their roofs. Both use solar power, one to generate electricity, one to mitigate urban heating and storm runoff. Conventional roofs actually raise the cooling bills of the buildings around them, including the building itself. Soaking up photons with photovoltaic cells produces electricity, which can help power the building while also reflecting some of the sun’s heat. Soaking up photons with photosynthesis cells produces plant growth, which absorbs carbon dioxide, releases oxygen, and mitigates storm water runoff, while reducing urban heating. The cost is the thing. Solar power is an easier sell because electricity has a market value. Gardens on the roof are a bit tougher.