When I talk about environmental consulting and my grad school work, lots of folks think it's really cool that I do environmental work outside. Am I an environmentalist? Do I get all starry-eyed about how I'm going to save the planet?
My thesis is about as earth friendly as can be. But the real reason I enjoy it is that I like working at the intersection of 3 disciplines, one of which was a close second for a major (I fell in love with it late in my college career, and geology has better field trips). Geology (and that other stuff I do) is so cool to me because of its complexity - the more you know, the deeper you get into the processes, the more you realize is actually going on. It's like the world's biggest logic problem you're trying to put together. And every new site has its own set of "rules" that you have to figure out. So I'm here pretty much for the love of science.
The other problem with being a scientist in the environmental field is that you quickly realize that there are very few easy answers. Reduce our dependence on fossil fuels by using biofuels, and you encourage the destruction of habitat for corn/sugarcane/soy monoculture (and don't get me started on the fertilizers corn requires). Use cloth napkins instead of paper ones, and you trade tree-killing for needless water and detergent-using. Super high efficiency lightbulbs contain mercury (I know, a tiny amount), causing disposal problems. Shut down dirty industry here, and ship it off to some poor country elsewhere. Etc etc.
So, I recycle, keep my energy and water usage down, and generally try not to make an excessive impact. Being a cheapskate helps. But I'm not making much of an effort to save the earth other than my own (very small) contributions to Science.