San Francisco is requiring solar panels on all new buildings. But here’s a much greener idea.

One of the greenest, most environmentally friendly moves that big cities like New York or San Francisco or Chicago can make is to increase housing density and allow more people to live in them. And yet, bizarrely, few people seem to think of this as an “environmental” policy.

But it is! And it’s hugely significant. In fact, we can illustrate just how significant by taking a closer look at San Francisco’s new solar law.

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 it’s a modest step for climate change.1 But now let’s look at what would happen if we boosted San Francisco’s housing density…