Closest to the Earth is a filmy blanket of atmosphere called the troposphere. It contains most of the atmosphere’s water vapor and is where weather takes place. Although most of our air is composed of nitrogen and oxygen, a number of other gases, including those referred to as greenhouse gases, affect air quality. Carbon dioxide, methane, and water vapor are naturally occurring gases in the atmosphere that are referred to as greenhouse gases (GHG) because they let sunlight enter the atmosphere, warm the Earth’s surface and then selectively block some of the heat at infrared wavelengths from being reemitted back into space.*
This natural regulation of Earth’s temperature is referred to as the atmospheric greenhouse effect. Without the greenhouse effect the Earth’s average surface temperature would be 60 degrees F cooler than it is today—and life as we know it would not exist.
For as far back as scientists can gather evidence, the Earth has had many changes in its average temperature, most over a very long time. These were caused by variations in the sun’s intensity, alterations in the earth’s orbit, changes in the position of the continents, and natural catastrophes such as volcanic eruptions and asteroid impacts. Since 1900 the Earth’s average temperature has increased 1.2 to 1.4 degrees F. Ten of the warmest years in the 20th century all occurred from 1985 to 1999. The warmest years on record have been 1998, 2002, 2003, 2005 and 2006 according to the Earth Observatory of the National Aeronautics and Space Administration.
Most scientists agree that there is a strong correlation between the increase in temperature and the increased concentration of greenhouse gases in the atmosphere resulting from human activity. These scientists believe the increased concentrations of greenhouse gases enhance the atmospheric greenhouse effect, resulting in increased temperatures. The human activity that makes the largest contribution to the buildup of greenhouse gases is the burning of fossil fuels to generate electricity and power automobiles. Another important human factor is deforestation, which releases sequestered carbon dioxide back into the air.
Carbon Dioxide (CO2)
Animals exhale CO2 and plants take in CO2 for photosynthesis, the process by which plants make their own food. Humans have tipped this natural balance by burning fossil fuels, including petroleum and coal, to operate motor vehicles, factories and power plants. They’ve cleared forest and natural areas to produce goods, adding even more CO2 to the air, and decreasing the Earth’s natural long-term containment of CO2.
Studies of ice cores extracted from the earth’s poles have determined that CO2 levels have risen by a third since the pre-industrial era and each year we add seven billion metric tons of carbon into the atmosphere. CO2 added today remains in the atmosphere for 100 years or more, which means the atmosphere is thrown more and more out of balance each year.
Methane (CH4)
Methane is a hydrocarbon gas that is the main component of natural gas. It is produced from landfills, coal mining, oil and gas production and distribution, and livestock, as well as natural wetlands and rice fields. Methane is 23 times more effective than carbon dioxide at trapping heat in the atmosphere. The concentration of methane in the atmosphere is now about 2.4 times higher than it was in 1750 or at any time in the previous 400 thousand years.
Nitrous Oxide (N20)
Nitrous oxide is another gas emitted by human activity. It comes from nitrogen-containing fertilizers for agriculture, and to a lesser degree, from burning fossil fuels and biofuels. While its warming effect is less than CO2, nitrous oxide stays in the atmosphere a long time. Summed over 100 years, one unit of N2O has 296 times more impact on global warming than does one unit of CO2.
Hydrofluorocarbons (HFCs)
These inert, non-toxic gases are used in refrigeration, packaging, insulation, or as solvents and aerosol propellants. HFCs are 111 times more efficient than CO2.
Perfluorocarbons (PFCs)
PFCs have specific uses in eye surgery and ultrasound, and are also used in refrigerating units, solvent cleaning, aluminum smelting and fire extinguishers. Extremely potent greenhouse gases, they have a lifetime up to 50,000 years. In a 2003 study, the most abundant atmospheric PFC was tetrafluoromethane. The greenhouse warming potential (GWP) of tetrafluoromethane is 6,500 times that of carbon dioxide and the GWP of hexafluoroethane is 9,200 times that of carbon dioxide.
Sulfur hexafluoride (SF6)
SF6 is used by the electrical industry (in transmission and distribution), aluminum recycling, thermal and sound insulation, airplane tires, and in ultrasound imaging. The Intergovernmental Panel on Climate Change stated that SF6 is the most potent greenhouse gas it has evaluated. It has a global warming potential of 22,200 times that of CO2 over a 100 year period.
Because greenhouse gases are a common occurrence in the way we live, work and play, it’s difficult for people to accept that global warming is happening. We experience only the climate in which we live. Often normal weather patterns seem to fluctuate more than long-term predictions. In the United States (population 300 million), each person produces 6.6 tons of greenhouse gases each year – a tremendous load for the atmosphere to carry.
*For a more thorough explanation of "The Greenhouse Effect" visit the University Corporation for Atmospheric Research (UCAR) at: www.ucar.edu/learn/1_3_1.htm.
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