Window unit air conditioners have become so much smaller, lighter, and even cheaper for the amount of heat they move, that in itself is a revolution. An average man or woman with a screw driver can install one now by themselves. The old ones required two people just to get it out of the box or carry it up a flight of stairs.

I've actually helped to build a green roof on a sizable equipment shed at a community garden, and it was beautiful even after the weeds took it over. But by the time you overbuild the structure to support the weight of the dirt and figure in moving water up the plants, the amount of carbon that is ultimately saved is negligible. There are reasons why such a cool thing hasn't been done extensively before.