Greenhouse Gases

Greenhouse gases (GHGs) are gases , both natural and anthropogenic,  that trap heat in the atmosphere. Radiation from the sun reaches the earth in the form of short wave radiation (ultraviolet light) and visible light. The Earth absorbs the radiation and then re-emits it in the form of longwave radiation (heat). Greenhouse gases trap a percentage of the heat emitted by the Earth, preventing it from radiating back into space. This process is the fundamental cause of the greenhouse effect. Greenhouse gases are essential to maintaining the current temperature of the Earth; without them this planet would be so cold as to be uninhabitable.

The most abundant greenhouse gas is water vapor (H2O), constituting 1-4% of the atmosphere.  Human activities have not had a significant impact on the abundance of water vapor in the atmosphere. Ozone (O3) is also an abundant greenhouse gas. International attention is focused on "anthropogenic greenhouse gases", gases that are being produced in significant quantities through human activities and are contributing to anthropogenic global warming.  Carbon dioxide is the most significant greenhouse gas in terms of emissions. However, there are other gases which exert a similar effect on the climate. According to the influence exerted by each relative to carbon dioxide, they are assigned global warming potentials (GWP). There are seven major anthropogenic greenhouse gases:

• Carbon dioxide: (CO2) the most important greenhouse gas, results from burning fossil fuels, land use changes, and cement production. Since 1750, before the industrial revolution, global atmospheric concentrations of CO2 have increased from 280 parts per million (ppm) to 379 ppm in 2005, by far exceeding atmospheric levels any time during the past 650,000 years.¹
• Methane: (CH4) is a result of fossil fuel use and production, agricultural practices (including livestock raising and rice growing) and waste dumps. Since 1750 atmospheric concentrations have increased from 715 parts per billion (ppb) to 1774 ppb in 2005. ²Though methane concentrations are much lower than CO2, its global warming potential, or its ability to trap heat per molecule, is 23 times that of CO2.³
• Nitrous oxide: (N₂O) is a result of fertilizers, industrial processes and combustion. Nitrous oxide has a global warming potential 296 times that of CO2⁴ and N2O concentrations have increased from 270 ppb in pre-industrial times to 319 ppb in 2005.⁵
• 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 substance (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”).

These gases are formed from industrial processes. The last three groups of gases were not present in the atmosphere before the industrial age.