This article appeared in Choices a few years back.
It is well worth the read.
As someone who does valuation as part of my job, I often encounter opposition to the very idea of placing monetary values on environmental and natural resources. We've seen some of that right here on the blog, and no doubt there are others that share the viewpoints of those who are brave enough to be vocal in expressing their opinion. Loomis does a great job of pointing out the merits and shortcomings of non-market valuation.