Windows to the Universe, a project undertaken by the University of Colorado at Boulder (UCB) and University Corporation for Atmospheric Research (UCAR), in a section of its website, www.windows.ucar.edu, titled "The Greenhouse Effect & Greenhouse Gases" (accessed Dec. 3, 2008), wrote the following:
Source: The COMET Program, www.comet.ucar.edu (accessed Dec. 3, 2008)
"These two simple cartoons show the multiple paths sunlight takes as it enters Earth's atmosphere (left) and the basic mechanism of the greenhouse effect (right). The portion of incoming sunlight that is absorbed by Earth is re-emitted as infrared radiation. Some IR [infrared radiation] energy escapes directly to space, but most is absorbed by greenhouse gases in the atmosphere. This warms Earth's atmosphere; our atmosphere would be roughly 30° C (54° F) colder if it contained no greenhouse gases...
What are the main greenhouse gases? Because of all the press coverage it has received in recent years, you may think that carbon dioxide (CO2) is 'the big one'. Though CO2's role is important, water vapor is actually the dominant greenhouse gas in Earth's atmosphere. Water vapor generates more greenhouse effect on our planet than does any other single gas. Water, in gaseous form (as water vapor) and in liquid form (as tiny droplets in clouds), generates somewhere between 66% and 85% of the greenhouse effect...
After water vapor... In rough order of importance and size of effect, the major ones are carbon dioxide (CO2), methane (CH4) and ozone (O3). There are a number of other gases that contribute to the greenhouse effect to a lesser extent... These 'lesser greenhouse gases' include nitrous oxide (N2O), sulfur hexafluoride (SF6), hydrofluorocarbons (HFCs), perfluorocarbons (PFCs) and chlorofluorocarbons (CFCs)..."
The US Environmental Protection Agency (EPA), in a section of its website, www.epa.gov, titled "Climate Change - Greenhouse Gas Emissions" (accessed Dec. 3, 2008), wrote the following:
"Gases that trap heat in the atmosphere are often called greenhouse gases. This section of the EPA Climate Change Site provides information and data on emissions of greenhouse gases to Earth’s atmosphere, and also the removal of greenhouse gases from the atmosphere.
Some greenhouse gases such as carbon dioxide occur naturally and are emitted to the atmosphere through natural processes and human activities. Other greenhouse gases (e.g., fluorinated gases) are created and emitted solely through human activities. The principal greenhouse gases that enter the atmosphere because of human activities are:
Methane (CH4): Methane is emitted during the production and transport of coal, natural gas, and oil. Methane emissions also result from livestock and other agricultural practices and by the decay of organic waste in municipal solid waste landfills.
Nitrous Oxide (N2O): Nitrous oxide is emitted during agricultural and industrial activities, as well as during combustion of fossil fuels and solid waste.
Fluorinated Gases: Hydrofluorocarbons, perfluorocarbons, and sulfur hexafluoride are synthetic, powerful greenhouse gases that are emitted from a variety of industrial processes. Fluorinated gases are sometimes used as substitutes for ozone-depleting substances (i.e., CFCs, HCFCs, and halons). These gases are typically emitted in smaller quantities, but because they are potent greenhouse gases, they are sometimes referred to as 'High Global Warming Potential' gases ('High GWP gases')."
The Energy Information Administration (EIA), a statistical agency of the US Department of Energy, in a brochure available on its website, www.eia.gov, titled "Greenhouse Gases, Climate Change, and Energy" (accessed Dec. 3, 2008), wrote the following:
"Many chemical compounds found in the Earth’s atmosphere act as 'greenhouse gases.' These gases allow sunlight to enter the atmosphere freely. When sunlight strikes the Earth’s surface, some of it is re-radiated back towards space as infrared radiation (heat). Greenhouse gases absorb this infrared radiation and trap the heat in the atmosphere. Many gases exhibit these 'greenhouse' properties. Some of them occur in nature (water vapor, carbon dioxide, methane, and nitrous oxide), while others are exclusively human made (certain industrial gases). Over time, if atmospheric concentrations of greenhouse gases remain relatively stable, the amount of energy sent from the sun to the Earth’s surface should be about the same as the amount of energy radiated back into space, leaving the temperature of the Earth’s surface roughly constant...In computer-based models, rising concentrations of greenhouse gases produce an increase in the average surface temperature of the Earth over time. Rising temperatures may, in turn, produce changes in precipitation patterns, storm severity, and sea level commonly referred to as 'climate change.'
Assessments by the Intergovernmental Panel on Climate Change (IPCC) suggest that the Earth’s climate has warmed between 0.6 and 0.9 degrees Celsius over the past century and that human activity affecting the atmosphere is 'very likely' an important driving factor. The IPCC’s Fourth Assessment Report (Summary for Policymakers) states, 'Most of the observed increase in globally averaged temperatures since the mid-20th century is very likely due to the observed increase in anthropogenic greenhouse gas concentrations.' It goes on to state, 'The observed widespread warming of the atmosphere and ocean, together with ice mass loss, support the conclusion that it is extremely unlikely that global climate change of the past 50 years can be explained without external forcing, and very likely that it is not due to known natural causes alone.'"