Human activities are causing major greenhouse gases such as carbon dioxide to build up in the atmosphere. Each time we burn gasoline, oil, coal, or even natural gas, more carbon dioxide is added to the atmosphere. By cutting down the forest trees, we allow air pollution to set in. Now that there are no trees to help filter out pollution, we are allowing more damage to the atmosphere causing global warming. At the present rate of destruction it will be gone by the middle of the next century.