Pollution

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http://en.wikipedia.org/wiki/Pollution
http://www.greenstudentu.com/encyclopedia/pollution
http://www.botany.uwc.ac.za/sci_ed/grade10/ecology/conservation/poll.htm
http://readanddigest.com/what-are-different-types-of-pollution/

Global warming
http://en.wikipedia.org/wiki/Global_warming

Natural disasters
https://en.wikipedia.org/wiki/Natural_disaster