Since there is rarely a market for the actual easements their value is computed by figuring the value of the property at its highest and best use before the easement and after the easement.
In his 2008 book, A Question of Balance, Weighing the Options on Global Warming Policies, Professor Nordhaus provided the computed discounted costs and benefits for a variety of policies, assuming the IPCC central value for warming due to increased atmospheric CO2 (3 degrees C for doubling of CO2).