Hi Deepasna !
See first you have to decide that which subjects you would like to take up if you enter teaching line. Environment is definitely not a major part in DU , but Env Eco has a lot of welfare economics involved. So , if you are keen on teaching at DU , then Public Economics could be a suitable subject for you. If not DU , then all esteemed universities/institutes have an environment department ( like IIT , JNU etc ). And if not there , then you may try your luck at TERI , they pay really well.
As far as MSE is concerned , most of their teachers have a specialization in environment. See if you are specializing in environment , then it doesn't mean that there is nothing else left for you to study. You can always make fusions ( environment , development , agriculture , apply some econometrics for estimating your model , game theory etc etc ) - that all depends how innovative you can be. And you are eligible to teach anything , like econometrics , game theory , public economics , provided you have a good hold on them.
Somewhere , I guess , we are sailing in the same boat. It was not easy for me to come to a firm decision , with minimal knowledge of the future prospects at my disposal. But , a big thanks to @Anirudh and @Akshay , who helped and edified me in coming to a conclusion. Rest , I have really stopped bothering myself with any further tension.
"Dekha Jaayega" !!
"Once you eliminate the impossible, whatever remains, no matter how improbable, must be the truth."