Fossil Fuels

• Since the beginning of the Industrial Revolution in the 18th century, fossil fuel usage has substantially increased the levels of carbon dioxide in the earth's atmosphere. 2004 statistics showed that consumption of coal, oil and natural gas to supply energy were responsible for up to 56.6 percent of greenhouse gas emissions. Carbon is added to the atmosphere when fossil fuels are burned, adding to what is already produced naturally by things such as rotting vegetation and the normal process of respiration by both humans and animals. Prior to the Industrial Revolution, levels of carbon dioxide were at approximately 280 ppm (parts per million). Since that time, these levels have increased to around 387 ppm today. It has been suggested that at such a rate, the earth's temperature could see an increase of about 1.4 - 5.6 degrees Celsius, between 1990 and 2100.
  
  

Farming

• The Industrial Revolution also brought with it an increase in concentrations of nitrous oxide. The use of certain fertilizers containing nitrogen by farmers has increased steadily over the last century and added to what is already produced through natural microbial processes. Methane is another greenhouse gas, tied to the farming industry. Livestock digestion, use of manure and rice farming have also added increasingly to the concentration of methane as the needs of our growing population rise.
  
  

Deforestation

• Burning of wood or other dead organisms is also a significant contributor to rising carbon dioxide levels. When a forest burns naturally, the new plants and trees that take the place of the old ones will absorb just as much of the carbon dioxide as was released by the burning, bringing the cycle into balance again. However, permanent deforestation does not allow for the replacement of this vegetation. Instead, it can actually increase greenhouse gases significantly. Whether they are cut down through logging, burnt or decompose naturally, they will still emit carbon dioxide. In addition, many of the areas where deforestation occurs are used as grazing land for livestock, which will contribute in the long-run to rising levels of methane and nitrous oxide.
  
  

Cement Manufacturing

• In the manufacturing of cement, carbon dioxide gas is produced when calcium carbonate is subjected to heat. Fossil fuels are also used to generate the heat required for the process, itself. Estimates suggest that 5 percent of man-made carbon dioxide emissions are produced by the cement industry. Approximately 50 percent of this is caused by the process itself, with another 40 percent due to the burning of fuel. For every 1,000 kilograms of cement produced, more than 900 kilograms of carbon dioxide is emitted into the atmosphere.
  
  

Chlorofluorocarbons

• The use of certain aerosols has also contributed greatly to the greenhouse effect. Chlorofluorocarbons (CFCs) were created in the late 1920s to be used as a propellant for aerosols, cleaning solvents and refrigerants. It was discovered over the years that CFCs are capable of destroying the ozone in the earth's atmosphere and as a result, successful global efforts were undertaken to halt their production. In spite of these efforts, the long lifetimes of CFCs suggest that their presence in our atmosphere could continue for over 100 years.