I watched this webinar (I missed the live version) which included some of the key players in the (future) formation of a carbon market that can function at scale. The market is very diffuse right now -- there are 64 different markets! -- but demand for carbon offset credits is escalating at such a steep rate (might I say exponential?) that these guys think there will be a functioning worldwide carbon market (i.e. something akin to Brent Crude or WTI) within 2-5 years.
One interesting factoid from the webinar: from wellhead to burner, a single BTU of natural gas gets traded an average of 10 times.