Contingent Valuation and Existence Value

I enjoyed reading Portney's article about existence value and contingent valuation. I always believed that there was a value in knowing that there are fish in the ocean or that there will always be clean air to breathe. I am willing to pay to make sure these things never go away. Contingent valuation is an important component in protecting our environment because it is the only, even if artificial, way to assess punitive damages to corporations who would destroy the environment without a care. Portney uses the Exxon Valdez spill as an example of environmental destruction. I also understand Portney's point that contingent valuation is difficult to obtain because it most likely will have to come in the nature of voluntary contributions and those who voluntarily pay will likely use the parks or wildlife areas they support. This, according to Portney makes it difficult to provide and creates the tendency to "underprovide on account of free riding."