The term "greenhouse gas" throws a lot of people off. Its so: Say what?
You have to understand the purpose of a greenhouse an entire building made of glass or other transparent material. This allows sunlight to enter, but then a funny thing happens: The heat in the sunlight is trapped inside the building. This makes the air in a greenhouse warmer than air outside -- great for starting young plants early in the spring.
The same thing is going on all the time with the earths atmosphere, which acts like a greenhouse. And its a good thing, too without this greenhouse effect, Earth would be icy and devoid of human life.
This greenhouse effect is enhanced because humans unnaturally have increased the concentration of some gases in the atmosphere "greenhouse gases."
You hear the most about carbon dioxide, or CO2, because its the greenhouse gas that humans produce most prolifically. Since 1750, the concentration of the gas in our atmosphere has increased from about 280 parts CO2 per million parts of air, to about 375 parts CO2 today per million parts of air. CO2 also stays aloft for more than a century, so it will be a big focus of work to slow global warming.
Nitrous oxide, which also can stay aloft for more than a century, comes from burning fossil fuels, use of some fertilizers, and chemical practices of industry, as well as some natural processes.
Methane is an extremely potent greenhouse gas. It is produced naturally, but then taken up by the earth except when humans upset the balance by doing things such as raising cattle, which burp out methane constantly. Methane also comes from garbage dumps, rice paddies, burning natural gas and mining coal.
Ozone is a greenhouse gas when its found in the troposphere, the first six to 12 miles of the atmosphere above Earth. Its presence there is enhanced by air pollution.
Fluorocarbons are gases used for cooling. Chlorofluorocarbons were banned by international treaty when they were found to be creating a hole in the ozone layer. But they were replaced largely by hydrofluorocarbons, which are also a greenhouse gas, although a less powerful one.
Water vapor is the greenhouse gas most responsible for the greenhouse effect, and its been around for long before humans. Scientists fear that more will be produced as the earth warms, exacerbating global warming.
For more info:
-Department of Energy primer: http://www.eia.doe.gov/oiaf/1605/ggccebro/chapter1.html
-National Oceanic and Atmospheric Administration primer: http://lwf.ncdc.noaa.gov/oa/climate/gases.html
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