Greenhouse Gases

• A greenhouse gas is defined as any gas that effectively holds and stores heat energy. Water vapor and carbon dioxide are the two most common examples. Both are produced in large quantities by human industrial processes. In the natural environment, both of these substances move through a natural cycle that limits their concentrations. Human activity has upset that balance. Methane, nitrous oxide and chlorofluorocarbons are also powerful greenhouse gases, though they exist in much lower quantities.
  
  

Land Over-Use

• Deforestation reduces the ecosystem's ability to process carbon dioxide.Jupiterimages/Photos.com/Getty Images
  
  The exploitation of natural resources indirectly contributes to climate change in several ways. Trees draw carbon dioxide out of the air, so cutting down forests increases carbon dioxide levels. Wetlands perform a similar function, but they are often drained to make way for human activity. Every time a natural ecosystem is replaced by a factory or highway, the ability of the Earth's climate to naturally moderate itself is impaired.
  
  

Volcanoes

• Volcanoes don't produce enough carbon dioxide to significantly raise temperatures.Photos.com/Photos.com/Getty Images
  
  Large volcanic eruptions can emit a variety of greenhouse gases. However, the ability of dust and other compounds to reflect sunlight is more powerful than any greenhouse effect. Because eruptions are brief events in geological terms, the effects on Earth's climate are temporary. Short periods of intense volcanism have been linked with cooler periods in Earth's climate history. Global air circulation eventually disperses and removes the particulate matter released by these powerful eruptions.
  
  

Solar Energy

• A stronger sun can mean rising temperatures.Comstock/Comstock/Getty Images
  
  If the sun were to radiate a greater amount of energy, logically the average temperature on Earth would rise. Scientists have linked ancient warm and cool periods to solar energy output. In modern times, however, the sun's energy has remained fairly stable and so cannot account for the Earth's warming. Measurements indicate that only the lower atmosphere is getting warmer. If the sun were responsible, then the entire atmosphere would warm equally.