[Underground coal fires contribute as much to global warming as all the cars in the US](http://freakonomics.blogs.nytimes.com/2007/10/22/from-bagels-to-coal-fires-an-unorthodox-economist-keeps-pushing-for-change/).
One of the real problems in designing the long run policy on this stuff is deciding how to assign property rights to emissions like these. A price imposing scheme (be it a tax or permit system) needs these property rights to work fully.