• Human acitivity has added extra greenhouse gases to the atmosphere leading to global warming.Henrik Weis/Photodisc/Getty Images
  
  Greenhouse gases trap heat in the atmosphere and warm the planet. At naturally occurring levels, greenhouse gases keep the planet's temperature within range for life as we know it. However, according to the U.S. Environmental Protection Agency (EPA), human activities such as combustion of fossil fuels and certain agricultural processes have added extra greenhouse gases to the atmosphere which are thought to contribute to global warming and climate change. These gases have both natural and man-made sources.
  
  

Carbon Dioxide

• Natural sources of carbon dioxide include volcanic eruptions, ocean-atmosphere exchange and animal and plant respiration. This carbon dioxide is part of a natural cycle. When this cycle is in balance, the amount of carbon dioxide emitted into the air is roughly equal to the amount absorbed by plants and the ocean.
  
  Man-made sources of carbon dioxide include fossil fuel combustion, industrial production and deforestation. According to the EPA, the largest source of CO2 in the United States is electricity generation, followed by heavy industry; residential and commercial uses and transportation. Deforestation contributes to the problem, because the carbon dioxide stored in the trees is released when the trees are cut down and isn't reabsorbed by new trees.
  
  

Fluorinated Gases

• Fluorinated gases are the only major type of greenhouse gas that doesn't occur naturally. They are synthetic gases mainly used as a substitute for ozone-depleting chemicals such as CFCs, HCFCs and halons. These gases are emitted in extremely limited quantities; however, they are potent greenhouse gases sometimes referred to as High Global Warming Potential gases (High GWP gases). Besides substitution of ozone-depleting substances, sources of fluorinated gases include electrical power transmissio, aluminum production, magnesium production and the manufacture of the refrigerant HCFC-22.
  
  

Methane

• Natural sources of methane include livestock, fires, geological emissions, bodies of water, termites and wetlands. For example, wetlands provide a home to bacteria that produce methane as they break down organic materials in the wetlands. Man-made sources of methane include fossil fuel production, livestock production and decomposition of organic waste in landfills. Methane is the main component of natural gas and is released into the air when gas is pumped to the surface. Methane trapped underground is released from the earth during coal mining. Cattle and other ungulates produce methane as their stomachs break down their food. This methane is released when the animals exhale. More methane is produced when their manure is collected and begins to decompose in holding tanks and lagoons.
  
  

Nitrous Oxide

• Nitrous oxide occurs naturally through the breakdown of nitrogen stored in the soil and ocean. According to the EPA, this activity accounts for over 90 percent of naturally occurring nitrous oxide. This natural process has been heavily impacted by human activity such as farming. Farming practices such as the use of artificial and manure-based fertilizers add nitrogen to the soil which is broken down by bacteria into nitrous oxide. Extra nitrogen also ends up in the water from farm runoff. Other sources of nitrous oxide include manure, sewage, fossil fuel combustion and nitric acid production. A tiny amount is also produced by production of adipic acid from synthetic fibers, lubricants, urethane foam, plastics, nylon and similar products.