Abstract
A key issue in a cost–benefit assessment of an expansion of an airport is its impact on emissions of greenhouse gases. Both taxes and tradable permits can be used to put a price tag on emissions, but practitioners disagree on how to handle permits. Therefore, the paper offers a section clarifying how to handle permits in cost–benefit analysis, with a more formal treatment in the Appendix. The paper also discusses the impact on the outcome of the evaluation of alternative assumptions regarding how greenhouse gases are internalised. Both optimal Pigouvian taxes and tradable permits are considered.