You can definitely teach full-time environmental science (my school of about 1,000 students has 2 full time environmental teachers), but there is no environmental science MTEL you could take for licensure. You would need to take either the bio or chem MTEL to get your science license.

While you should probably go for whichever subject you're better at, keep in mind that bio, by far, has the most applicants for any open job. Chem is much, much harder to find someone for. The last time we were hiring for a chem teacher, there were just two licensed teachers who applied. The last bio position we hired for had over 30 licensed applicants.