you might want to look into the earth institute at columbia (jeff sachs and his cronies), which seems to be developing some interesting projects related to environmental science.
right now columbia is going through its golden age. your "investment" in penn would probably just about pay off (although arguably you wouldn't currently receive the same quality of education.. then again i may or may not be biased), but equally you would be going to columbia at a great time in its history. president of us, richest man in the world, attorney general, governor of new york, all columbia alums. recently got a massive injection of financial aid from the billionaire entrepreneur john kluge. with its most competitive admissions in history in recent years, columbia is on an upward trajectory as well right now.
and as others mentioned, NYC might also help sway your decision. if not for the great social life and entertainment options, then for the numerous internship opportunities available literally at your doorstep (columbia has great ties with many big institutions), throughout the school year and over your summers.