Forbidden or compulsory -- take your pick.

This enjoyable TigerHawk post discusses the concept of painting all city rooftops white, to increase their albedo and thereby slow (or reverse?) global warming; he cites some evidence that doing this would raise earth's albedo from 0.29 to 0.30, which could lower global temp by as much as 1º C, which would effectively undo all the global warming since the beginning of the industrial revolution. The post notes that this is exactly the sort of easy, painless thing anyone with a shred of environmental consciousness should be willing to do, but that many US zoning restrictions and community covenants prohibit this. The best part of the post:

The obvious libertarian answer is to pass a federal law that declares that any state or local law or restrictive covenant may not be enforced to prevent the owner of property from painting his roof or driveway to raise its albedo. The obvious Gorean answer is to require that all building owners paint their roofs (and, for that matter, driveways and parking lots) white by 2010, with an extensive government program to specify the type of paint to be used, an inspection regime to be sure that it has happened, and subsidies to be paid for by a new tax on the "rich."

What a charming synopsis of the problem that undermines attempts at coalition-building, even where both sides are in agreement as to the fundamental goals, as enviros and scientifically-minded libertarians often are on this.