This week, San Francisco became the first major US city to require solar panels on all new buildings that have 10 floors or less. (Larger buildings are exempt for now.) Analysts estimate that the resulting solar installations could help avoid 26,000 metric tons of carbon dioxide annually. If we use the EPA’s handy greenhouse gas calculator, that’s the equivalent of taking 5,500 cars off the road. . . So if San Francisco relaxed its restrictions and enabled, say, an additional 10,000 people to move from elsewhere in the Bay Area to the city, we could expect that to cut 79,000 metric tons of CO2 per year (to a first, crude approximation). This is three times as much CO2 as the solar panel law would save.