A greenhouse gas (sometimes abbreviated GHG) is a gas in an atmosphere that absorbs and emits radiation within the thermal infrared range. This process is the fundamental cause of the greenhouse effect.[1] The primary greenhouse gases in Earth's atmosphere are water vapor, carbon dioxide, methane, nitrous oxide, and ozone. Without greenhouse gases, the average temperature of Earth's surface would be about 15 °C (27 °F) colder than the present average of 14 °C (57 °F).[2][3][4] In the Solar System, the atmospheres of Venus, Mars and Titan also contain gases that cause a greenhouse effect.

Human activities since the beginning of the Industrial Revolution (taken as the year 1750) have produced a 40% increase in the atmospheric concentration of carbon dioxide, from 280 ppm in 1750 to 400 ppm in 2015.[5][6] This increase has occurred despite the uptake of a large portion of the emissions by various natural "sinks" involved in the carbon cycle.[7][8] Anthropogenic carbon dioxide (CO2) emissions (i.e. emissions produced by human activities) come from combustion of carbon-based fuels, principally wood, coal, oil, and natural gas.[9]

It has been estimated that if greenhouse gas emissions continue at the present rate, Earth's surface temperature could exceed historical values as early as 2047, with potentially harmful effects on ecosystems, biodiversity and the livelihoods of people worldwide.[10]

Greenhouse gases

Greenhouse gases are those that absorb and emit infrared radiation in the wavelength range emitted by Earth.[1] In order, the most abundant greenhouse gases in Earth's atmosphere are:

Water vapor  (H2O) Carbon dioxide  (CO2) Methane  (CH4) Nitrous oxide  (N2O)

Ozone  (O3) Chlorofluorocarbons  (CFCs)

Atmospheric concentrations of greenhouse gases are determined by the balance between sources (emissions of the gas from human activities and natural systems) and sinks (the removal of the gas from the atmosphere by conversion to a different chemical compound). [11] The proportion of an emission remaining in the atmosphere after a specified time is the "airborne fraction" (AF). More precisely, the annual AF is the ratio of the atmospheric increase in a given year to that year's total emissions. For CO2 the AF over the last 50 years (1956–2006) has been increasing at 0.25 ± 0.21%/year.

Indirect radiative effectsSome gases have indirect radiative effects (whether or not they are greenhouse gases themselves). This happens in two main ways. One way is that when they break down in the atmosphere they produce another greenhouse gas. For example, methane and carbon monoxide (CO) are oxidized to give carbon dioxide (and methane oxidation also produces water vapor; that will be considered below). Oxidation of CO to CO2 directly produces an unambiguous increase in radiative forcing although the reason is subtle. The peak of the thermal IR emission from Earth's surface is very close to a strong vibrational absorption band of CO2 (667 cm−1). On the other hand, the single CO vibrational band only absorbs IR at much higher frequencies (2145 cm

Global warming potentialThe global warming potential (GWP) depends on both the efficiency of the molecule as a greenhouse gas and its atmospheric lifetime. GWP is measured relative to the same mass of CO2 and evaluated for a specific timescale. Thus, if a gas has a high (positive) radiative forcing but also a short lifetime, it will have a large GWP on a 20-year scale but a small one on a 100-year scale. Conversely, if a molecule has a longer atmospheric lifetime than CO2 its GWP will increase with the timescale considered. Carbon dioxide is defined to have a GWP of 1 over all time periods.

Methane has an atmospheric lifetime of 12 ± 3 years. The 2007 IPCC report lists the GWP as 72 over a time scale of 20 years, 25 over 100 years and 7.6 over 500 years. A 2014 analysis, however, states that although methane's initial impact is about 100 times greater than that of CO2, because of the shorter atmospheric lifetime, after six or seven decades, the impact of the two gases is about equal, and from then on methane's relative role continues to decline. [35] The decrease in GWP at longer times is because methane is degraded to water and CO2 through chemical reactions in the atmosphere.

Examples of the atmospheric lifetime and GWP relative to CO2 for several greenhouse gases are given in the following table.

Impacts on the overall greenhouse effect

The contribution of each gas to the greenhouse effect is affected by the characteristics of that gas, its

abundance, and any indirect effects it may cause. For example, the direct radiative effect of a mass of

methane is about 72 times stronger than the same mass of carbon dioxide over a 20-year time

frame[20] but it is present in much smaller concentrations so that its total direct radiative effect is smaller,

in part due to its shorter atmospheric lifetime. On the other hand, in addition to its direct radiative

impact, methane has a large, indirect radiative effect because it contributes to ozone formation.

Shindell et al. (2005)[21] argue that the contribution to climate change from methane is at least double

previous estimates as a result of this effect.[22]

When ranked by their direct contribution to the greenhouse effect, the most important are: [16]

Compound 

Formula 

Contribution(%)

Water vapor and clouds H2O

36–72%  

Carbon dioxide CO2 9–26%

MethaneCH

44–9%  

OzoneO

33–7%  

In addition to the main greenhouse gases listed above, other greenhouse gases include sulfur hexafluoride, hydrofluorocarbons andperfluorocarbons (see IPCC list of greenhouse gases). Some greenhouse gases are not often listed. For example, nitrogen trifluoride has a high global warming potential (GWP) but is only present in very small quantities.

The Greenhouse Effect in and of it self was not meant to be a bad thing. When the greenhouse gases trap sunlight it makes the earth warmer, great so not everyone freezes to death. The bad side is that due to our excess emissions of greenhouse gases, through cars, factories, etc., the greenhouse effect has been trapping too much heat. Though a pro is new jobs can be created to reduce emissions and the growing seasons would be longer (so good for agriculture), the big problem is global warming, acid rain, an overall imbalance in the environmental systems. The climate change that is inevitably caused by the greenhouse gases is a major con and the whole process can alter everything from the oceans patterns to animal and plant life and in the end humans.

The molecular properties of green house gasses allow them to absorb and reflect IR radiation. Without any greenhouse gasses in the atmosphere the earth would be too cold to live on. If the concentration becomes too high, however, too much radiation is bounced back to the earth causing a rise in temperatures, which could lead to serious consequences in the future.

Perfluorocarbons are a group of human-made chemicals composed of carbon and fluorine only. Perfluorocarbons are powerful greenhouse gases that were introduced as alternatives to ozone depleting substances. PFCs replace chlorofluorocarbons (CFCs) in manufacturing semiconductors.

Hydrofluorocarbons, or "super greenhouse gases," are gases used for refrigeration and air conditioning, and known as super greenhouse gases because the combined effect of their soaring use and high global warming potential could undercut the benefits expected from the reduction of other greenhouse gases such as carbon dioxide. Used as refrigerants, they were introduced by the chemical industry to replace ozone destroying CFCs (chlorofluorocarbons) which have (almost) been phased out by the Montreal Protocol. However, HFCs production is rising by 15% per year. HFCs are 3,830 times more potent than CO2 with a lifetime of 14 years.

More than 10,000 tons of SF6 are produced per year, most of which (over 8,000 tons) is used as a gaseous dielectric medium in the electrical industry. Other main uses include an inert gas for the casting of magnesium, and as an inert filling for insulated glazing windows.