It’s normal to worry about some of these problems once in a while, but an irrational obsession or incessant thoughts about the world’s problems, especially if these are continuing over a period of time and are interfering with your daily activities or studies, is indicative of a problem. The cause of these stress and depressive symptoms could be anything in the past that is being triggered off by these environmental concerns, any other problem which is manifesting itself in this manner or could be related to your present stress situation. Also someone in your family or immediate environment could also display such extreme concerns; the cause requires some introspection on your side. I would suggest that you should consider seeing a counsellor or a therapist as 3 months is a long duration and the problem may not disappear without intervention.