Carbon Is Constantly Being Recycled
Carbon is the key to greenhouse effect - exists in the atmosphere as carbon dioxide gas and is also present in other greenhouse gases.
1) Carbon on earth moves in big cycle.
2) Respiration, Combustion and Decay of plants, animals add to carbon dioxide and add to oxygen.
3) Photosynthesis does the opposite - it removes carbon dioxide and adds oxygen.
4) These processes should balance out out. However, it looks like humans have upset natural carbon cycle, which has affected the balance of gases in the atmosphere.
Human Activity Affects The Composition Of Air
1) Human population is increasing. This means more people respiring - giving out more carbon dioxide.
2) More people = more energy needed for lightig, heating, cooking transport and so on. And peoples lifestyles are changing too. More and more countries are becoming more industrial and well-off.