Isaiah said, "For thus saith the Lord that created the heavens, God himself that formed the Earth and made it, he created it not in vain, he formed it to be inhabited..." (Isa 45:18)
So I guess the earth's purpose it to be inhabited, and since we don't know exactly how long it is going to be serving that purpose until the second coming, it would make sense that we not leave for our children a world that is inhabitable, if there are things we can do. Certainly there is debate over if the current state of the environment is man's fault or nature's course, and also whether or not man's efforts can alter the changes. Also, there is some debate over IF there are actually changes. We know that when God created man, part of his charge over the earth was to "keep" it. "And the Gods took the man and put him in the Garden of Eden, to dress it and to keep it." (Abr 5:11) I'm not sure exactly what that entails, but it sounds like taking care of the earth, if I'm not mistaken. The Lord also said in D&C 42:41, "And let all things be done in cleanliness before me." If there are different ways of doing things that keep the earth in more cleanliness before God, then I am interested in learning about them, trying new things that fit with my beliefs about the earth's and my own purpose.