Natural greenhouse gases include carbon dioxide, methane, water vapor, and nitrous oxide. Water vapor is the largest greenhouse gas, it occurs naturally through evaporation of water from the water cycle. Carbon dioxide is produced naturally from the oceans and the terrestrial biosphere. Also, carbon dioxide is produced through forest and rush fires. Carbon dioxide is absorbed by plants through the carbon cycle. Methane is naturally produced in swamplands and in rice production. Both carbon dioxide and methane have increased its numbers due to human’s existence.