Carbon dioxide enters the atmosphere via natural processes and human activities. A natural source of carbon dioxide is the result of respiration--living things release carbon dioxide as a waste product of cellular respiration. Volcanic activity releases carbon dioxide when volcanoes erupt. Sometimes there are fires as a result of dry conditions and when grasslands or forests burn, carbon dioxide is released. Combustion of fossil fuels such as coal, oil and gas release the bulk of carbon dioxide into the atmosphere. These are used to generate electricity in power plants and for fuels for transportation. Activities such as deforestation cause the levels to increase because plants fix carbon dioxide into glucose, thus removing it from the atmosphere and cutting down trees removes the filter thus increasing carbon dioxide to the atmosphere.