Complex sentence: It is certainly encouraging to realize that the environment will continue long after humanity is gone, but, at the same time, the discouragement comes upon the realization that we may have already outlived our usefulness to it.
Split into simpler sentences: It is certainly encouraging to realize that the environment will continue long after humanity is gone. However, at the same time, the discouragement comes upon the realization that we may have already outlived our usefulness to it.