Chicago’s City Hall is one of the city’s most prominent green roofs.Photo courtesy of Center for Neighborhood Technology via FlickrSkyscrapers and other enormous buildings are environmental horror shows. Countless tons of steel, concrete, and glass are required to build them. Endless streams of energy keep them warm enough in winter and cool enough in summer. Lights and computers are kept on all night long. And just imagine the water that goes into flushing all those toilets. So you might think that Chicago -- birthplace of the skyscraper and home to nine of the world's 100 tallest buildings -- is like …