Mo, let me first of all say that I personally think one should study what one is most interested in. I for example studied IR and still love it. that being said and given your interest I would suggest to study environmental economics (there are courses on this) but with a focus on the economic and environmental theory. After all it is the complexity of theory that forms your mind. You can then start applying it in a job. The Toulouse guys, LERNA, have a good environmental economics program. Hope this helps.