How is a "green tax" not in the constitution? The constitution doesn't say specifically what cannot be taxed and what can - down to that level of detail. It only says what KIND of taxes can and cannot be made.
For example, corporate income tax isn't specifically listed in the constitution - but it is accepted as being constitutional, because it is an appropriate KIND of tax.
Gasoline tax is another great example - Ron Paul approves this kind of tax, although he is dismayed that it is not being spent to fund what it was supposed to - our highway system.
A green tax is an EXCELLENT idea - especially if it was justified by taking the money earned and SPENDING it on environmental cleanup. Makes PERFECT sense to me really...