In this series on a Biblical view of the environment, I've pointed out a couple of things. First, that God cares about creation ergo we should too. Second, that God has (at least) four roles in regards to creation: sovereign creator, sustainer, redeemer and owner.
Today I'd like to address the purpose of creation, specifically, our environment. In some respects, this goes to the question of "why are we here?" as the answer to that question is similar to one of the answers I'll provide regarding the environment. So it's not really a small question.
But I think the answers are fairly clear, and not terribly complex to grasp. First, the environment is obviously here to provide the resources necessary for us to live. Since God has found a reason to create us (the "why are we here?" question), we need a place to live. We also need resources to sustain our lives, and a physical context in which to carry out our lives. The earth and environment are here to support us, physically.
Second, and oddly enough more importantly, the environment is here for God's glory. I say "oddly enough" because it seems somewhat odd that our survival is secondary - but in some respects it is. We are here for God's glory too, not our own, and while our lives are very important to Him, He is above us in all things. God's glory is in all of the earth, and all we do is to glorify Him.
Finally (for the purposes of this post - I'm sure there are other purposes we could discover for the environment), the environment is here to teach us about God. God is revealed by His creation, including our environment. His care for us is evident by the way He cares for plants and animals, for instance. He even used an (allegedly stubborn) animal to instruct a stubborn prophet.
God has a purpose in everything He does. Creating a world and atmosphere to sustain our lives is no exception - He knew what He was doing, and He did it for a reason. Ultimately, our world is here for His glory. That fact should drive our response, and responsibility to the planet we share.