It's quite sustainable when it's readily accessible as a renewable source. In situations such as a landfill or a waste water treatment plant or a dairy, where many of those emissions are already being concentrated, or in many cases by regulation have to be captured, it's very sustainable. Once you get a little further out and you start to look at capturing waste material and gasifying it and whatnot, it becomes more challenging to determine how that's going to play out in the marketplace.
But as energy prices increase, there are going to be more and more market forces that drive people to look at those alternatives that seem very far out of reach today. I'm sure capturing methane from a dairy farm 15 or 20 years ago seemed widely out of reach, yet it's an economically feasible way to fuel a vehicle today.