Excess carbon dioxide released into the atmosphere from burning fossil fuels such as oil, coal and gas. A certain amount of carbon dioxide is released in the air in the natural carbon cycle–but this carbon dioxide is re-absorbed by plants and trees. However, at the rate fossil fuels are being burned for industry, urban needs and transportation, so much carbon dioxide is being released that trees and plants are not able to re-absorb them. These excess carbon emissions have contributed significantly to global warming.