Long ago,the world's environment is very good,there are many trees.They can release oxygen and carbon dioxide inhalation.But since humans appeared,the number of trees are dwindling.Because people's living needs a lot of wood,so they cut down trees ceaselessly.Time for a long time,many forests have disappeared.Thus caused soil erosion.The fertile land into the desert.Many countries of the world have water crisis of difficulty.And the cities are often sandstorms hit.The life of people are a serious threat.While the other creatures and threatened.