My first advice to graduate students is always that they should follow their own motivation. That is, in my view, the key to success for the students and the main driver for scientific progress: only motivated researchers will make a difference, in academia and in society. But of course environment and development economics is a good choice: it is very relevant and intellectually challenging. Alleviating poverty and preserving a balance between the human sphere and the natural environment are the major issues of our world. The parts of the world that are still developing are also the parts of the world where important choices will be made on preserving this balance. This makes the field very relevant.
de Zeeuw, A. J. (2014). Twenty years of environment and development economics: Advice to graduate students. Environment and Development Economics, 19(Special Issue 03), 303-304. https://doi.org/10.1017/S1355770X14000230