A formal definition of greenhouses gases is as follows: "Gaseous constituents of the atmosphere, both natural and anthropogenic, that absorb and emit radiation at specific wavelengths within the spectrum of radiation emitted by the Earth’s surface, by the atmosphere itself, and by clouds. This property causes the greenhouse effect.": 2233 The radiation emitted by the Earth’s surface, the atmosphere and clouds is called thermal infrared or longwave radiation.: 2251
The most abundant greenhouse gases in Earth's atmosphere, listed in decreasing order of average global mole fraction, are:
Water vapor is a potent greenhouse gas but not one that humans are directly adding to. It is therefore not one of the drivers of climate change that the IPCC (Intergovernmental Panel on Climate Change) is concerned with, and therefore not included in the IPCC list of greenhouse gases. Changes in water vapor is a feedback that impacts climate sensitivity in complicated ways (because of clouds mostly).
Infrared active gases
Gases which can absorb and emit thermal infrared radiation, are said to be infrared active.
Most gases whose molecules have two different atoms (such as carbon monoxide, CO), and all gasses with three or more atoms (including H2O and CO2), are infrared active and act as greenhouse gases. Technically, this is because an asymmetry in the molecule's electric charge distribution allows molecular vibrations to interact with electromagnetic radiation.
Gasses with only one atom (such as argon, Ar) or with two identical atoms (such as nitrogen, N 2, and oxygen, O 2) are not infrared active. They are transparent to thermal radiation, and, for practical purposes, do not absorb or emit thermal radiation.
The major constituents of Earth's atmosphere, nitrogen (N 2) (78%), oxygen (O 2) (21%), and argon (Ar) (0.9%), are not infrared active and so are not greenhouse gases. These gases make up more than 99% of the dry atmosphere.
Most greenhouse gases have both natural and human-caused sources. An exception are purely human-produced synthetic halocarbons which have no natural sources. During the pre-industrial Holocene, concentrations of existing gases were roughly constant, because the large natural sources and sinks roughly balanced. In the industrial era, human activities have added greenhouse gases to the atmosphere, mainly through the burning of fossil fuels and clearing of forests.
Water vapor accounts for the largest percentage of the greenhouse effect, between 36% and 66% for clear sky conditions and between 66% and 85% when including clouds. Water vapor concentrations fluctuate regionally, but human activity does not directly affect water vapor concentrations except at local scales, such as near irrigated fields. Indirectly, human activity that increases global temperatures will increase water vapor concentrations, a process known as water vapor feedback. The atmospheric concentration of vapor is highly variable and depends largely on temperature, from less than 0.01% in extremely cold regions up to 3% by mass in saturated air at about 32 °C. (See Relative humidity#Other important facts.)
The average residence time of a water molecule in the atmosphere is only about nine days, compared to years or centuries for other greenhouse gases such as CH 4 and CO2. Water vapor responds to and amplifies effects of the other greenhouse gases. The Clausius–Clapeyron relation establishes that more water vapor will be present per unit volume at elevated temperatures. This and other basic principles indicate that warming associated with increased concentrations of the other greenhouse gases also will increase the concentration of water vapor (assuming that the relative humidity remains approximately constant; modeling and observational studies find that this is indeed so). Because water vapor is a greenhouse gas, this results in further warming and so is a "positive feedback" that amplifies the original warming. Current estimates (as of 2000) suggest that water vapor feedback has a "gain" coefficient of about 0.4; a gain coefficient must be 1 or greater to create an unstable feedback loop of the sort that could stimulate runaway warming. Thus, although water vapor feedback amplifies the impact of temperature changes caused by other factors, there is no indication that Earth is involved in a runaway greenhouse effect of the sort that could lead to Venus-like conditions.
Role in heat transport and radiative forcing
Effects on air and surface
Absorption and emission of thermal radiation by greenhouse gases plays a role in heat transport in the air and at the surface:
Atmospheric cooling: Greenhouse gases emit more thermal radiation than they absorb, and so have an overall cooling effect on air.: 139 
Inhibition of radiative surface cooling: Greenhouse gases limit radiative heat flow away from the surface and within the lower atmosphere. Greenhouse gases exchange thermal radiation with the surface, reducing the overall rate of upward radiative heat transfer.: 139 
Naming these effects contributes to a full understanding of the role of greenhouse gases. However, these effects are of secondary importance when it comes to understanding global warming. It is important to focus on top-of-atmosphere energy balance in order to correctly reason about global warming. It has been argued that the surface budget fallacy, in which focus on the surface energy budget leads to faulty reasoning, constitutes a common fallacy when thinking about the greenhouse effect and global warming.: 413
Effect at top-of-atmosphere (TOA)
At the top of the atmosphere (TOA), absorbing and emission of thermal radiation by greenhouse gases leads to inhibition of radiative cooling to space, which means the amount of thermal radiation reaching space is reduced, relative to what is emitted by the surface. The change in TOA energy balance leads to the surface accumulating thermal energy and warming until TOA energy balance is achieved.
Radiative forcing is a metric that characterizes the impact of an external change in a factor that influences climate, e.g., a change in the concentration of greenhouse gases, or the effect of a volcanic eruption. The radiative forcing associated with a change is calculated as the change in the top-of-atmosphere (TOA) energy balance that would be caused by the external change, if one imagined that the change could be made without giving the troposphere or surface time to respond to reduce the imbalance. A positive forcing indicates more energy arriving than leaving.: 2245 The term radiative forcing has been used inconsistently in the scientific literature.
Increasing the concentration of greenhouse gases is associated with a positive radiative forcing. Increasing the concentration of greenhouse gases tends to increase the TOA energy imbalance, leading to additional warming.
The major non-gas contributor to Earth's greenhouse effect, clouds, also absorb and emit infrared radiation and thus have an effect on greenhouse gas radiative properties. Clouds are water droplets or ice crystals suspended in the atmosphere.
Chemical process contributions to radiative forcing
Some gases contribute indirectly to altering the TOA radiative balance through participation in chemical processes within the atmosphere.
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 (wavelength 15 microns, or wavenumber 667 cm−1). On the other hand, the single CO vibrational band only absorbs IR at much shorter wavelengths (4.7 microns, or 2145 cm−1), where the emission of radiant energy from Earth's surface is at least a factor of ten lower. Oxidation of methane to CO2, which requires reactions with the OH radical, produces an instantaneous reduction in radiative absorption and emission since CO2 is a weaker greenhouse gas than methane. However, the oxidations of CO and CH 4 are entwined since both consume OH radicals. In any case, the calculation of the total radiative effect includes both direct and indirect forcing.
A second type of indirect effect happens when chemical reactions in the atmosphere involving these gases change the concentrations of greenhouse gases. For example, the destruction of non-methane volatile organic compounds (NMVOCs) in the atmosphere can produce ozone. The size of the indirect effect can depend strongly on where and when the gas is emitted.
Methane has indirect effects in addition to forming CO2. The main chemical that reacts with methane in the atmosphere is the hydroxyl radical (OH), thus more methane means that the concentration of OH goes down. Effectively, methane increases its own atmospheric lifetime and therefore its overall radiative effect. The oxidation of methane can produce both ozone and water; and is a major source of water vapor in the normally dry stratosphere. CO and NMVOCs produce CO2 when they are oxidized. They remove OH from the atmosphere, and this leads to higher concentrations of methane. The surprising effect of this is that the global warming potential of CO is three times that of CO2. The same process that converts NMVOCs to carbon dioxide can also lead to the formation of tropospheric ozone. Halocarbons have an indirect effect because they destroy stratospheric ozone. Finally, hydrogen can lead to ozone production and CH 4 increases as well as producing stratospheric water vapor.
K&T (1997) used 353 ppm CO2 and calculated 125 W/m2 total clear-sky greenhouse effect; relied on single atmospheric profile and cloud model. "With Clouds" percentages are from Schmidt (2010) interpretation of K&T (1997). Schmidt (2010) used 1980 climatology with 339 ppm CO2 and 155 W/m2 total greenhouse effect; accounted for temporal and 3-D spatial distribution of absorbers.
It is not possible to state that a certain gas causes an exact percentage of the greenhouse effect. This is because some of the gases absorb and emit radiation at the same frequencies as others, so that the total greenhouse effect is not simply the sum of the influence of each gas. The higher ends of the ranges quoted are for each gas alone; the lower ends account for overlaps with the other gases. In addition, some gases, such as methane, are known to have large indirect effects that are still being quantified.
Contributions to enhanced greenhouse effect
Anthropogenic changes to the greenhouse effect are referred to as the enhanced greenhouse effect.: 2223
The contribution of each gas to the enhanced greenhouse effect is determined 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 84 times stronger than the same mass of carbon dioxide over a 20-year time frame but it is present in much smaller concentrations so that its total direct radiative effect has so far been smaller, in part due to its shorter atmospheric lifetime in the absence of additional carbon sequestration. On the other hand, in addition to its direct radiative impact, methane has a large, indirect radiative effect because it contributes to ozone formation. A publication from 2005 said that the contribution to climate change from methane was at least double previous estimates as a result of this effect.
Radiative forcing and annual greenhouse gas index
Earth absorbs some of the radiant energy received from the sun, reflects some of it as light and reflects or radiates the rest back to space as heat. A planet's surface temperature depends on this balance between incoming and outgoing energy. When Earth's energy balance is shifted, its surface becomes warmer or cooler, leading to a variety of changes in global climate.
A number of natural and human-made mechanisms can affect the global energy balance and force changes in Earth's climate. Greenhouse gases are one such mechanism. Greenhouse gases absorb and emit some of the outgoing energy radiated from Earth's surface, causing that heat to be retained in the lower atmosphere. As explained above, some greenhouse gases remain in the atmosphere for decades or even centuries such as Nitrous oxide and Fluorinated gases, and therefore can affect Earth's energy balance over a long period. Radiative forcing quantifies (in Watts per square meter) the effect of factors that influence Earth's energy balance; including changes in the concentrations of greenhouse gases. Positive radiative forcing leads to warming by increasing the net incoming energy, whereas negative radiative forcing leads to cooling, as with anti-greenhouse effects causing gases like sulfur dioxide.
The Annual Greenhouse Gas Index (AGGI) is defined by atmospheric scientists at NOAA as the ratio of total direct radiative forcing due to long-lived and well-mixed greenhouse gases for any year for which adequate global measurements exist, to that present in year 1990. These radiative forcing levels are relative to those present in year 1750 (i.e. prior to the start of the industrial era). 1990 is chosen because it is the baseline year for the Kyoto Protocol, and is the publication year of the first IPCC Scientific Assessment of Climate Change. As such, NOAA states that the AGGI "measures the commitment that (global) society has already made to living in a changing climate. It is based on the highest quality atmospheric observations from sites around the world. Its uncertainty is very low."
Global warming potential
The 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 when the timescale is considered. Carbon dioxide is defined to have a GWP of 1 over all time periods.
Methane has an atmospheric lifetime of 12 ± 2 years. The 2021 IPCC report lists the GWP as 83 over a time scale of 20 years, 30 over 100 years and 10 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. The decrease in GWP at longer times is because methane decomposes 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:
Atmospheric lifetime and GWP relative to CO2 at different time horizon for various greenhouse gases
Atmospheric concentrations 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 or absorption by bodies of water).
The proportion of an emission remaining in the atmosphere after a specified time is the "airborne fraction" (AF). The annual airborne fraction is the ratio of the atmospheric increase in a given year to that year's total emissions.
As of 2006 the annual airborne fraction for CO2 was about 0.45. The annual airborne fraction increased at a rate of 0.25 ± 0.21% per year over the period 1959–2006.
Aside from water vapor, which has a residence time of about nine days, major greenhouse gases are well mixed and take many years to leave the atmosphere. Although it is not easy to know with precision how long it takes greenhouse gases to leave the atmosphere, there are estimates for the principal greenhouse gases. Jacob (1999) defines the lifetime of an atmospheric species X in a one-box model as the average time that a molecule of X remains in the box. Mathematically can be defined as the ratio of the mass (in kg) of X in the box to its removal rate, which is the sum of the flow of X out of the box
chemical loss of X
and deposition of X
(all in kg/s):
If input of this gas into the box ceased, then after time , its concentration would decrease by about 63%.
The atmospheric lifetime of a species therefore measures the time required to restore equilibrium following a sudden increase or decrease in its concentration in the atmosphere. Individual atoms or molecules may be lost or deposited to sinks such as the soil, the oceans and other waters, or vegetation and other biological systems, reducing the excess to background concentrations. The average time taken to achieve this is the mean lifetime.
Carbon dioxide has a variable atmospheric lifetime, and cannot be specified precisely.: 2237 Similar issues apply to other greenhouse gases, many of which have longer mean lifetimes than CO2, e.g. N2O has a mean atmospheric lifetime of 121 years.
Since the beginning of the Industrial Revolution, the concentrations of many of the greenhouse gases have increased. For example, the mole fraction of carbon dioxide has increased from 280 ppm to 421 ppm, or 140 ppm over modern pre-industrial levels. The first 30 ppm increase took place in about 200 years, from the start of the Industrial Revolution to 1958; however the next 90 ppm increase took place within 56 years, from 1958 to 2014.
Recent data also shows that the concentration is increasing at a higher rate. In the 1960s, the average annual increase was only 37% of what it was in 2000 through 2007.
Measurements from ice cores over the past 800,000 years
Ice cores provide evidence for greenhouse gas concentration variations over the past 800,000 years (see the following section). Both CO2 and CH 4 vary between glacial and interglacial phases, and concentrations of these gases correlate strongly with temperature. Direct data does not exist for periods earlier than those represented in the ice core record, a record that indicates CO2mole fractions stayed within a range of 180 ppm to 280 ppm throughout the last 800,000 years, until the increase of the last 250 years. However, various proxies and modeling suggests larger variations in past epochs; 500 million years ago CO2 levels were likely 10 times higher than now. Indeed, higher CO2 concentrations are thought to have prevailed throughout most of the PhanerozoicEon, with concentrations four to six times current concentrations during the Mesozoic era, and ten to fifteen times current concentrations during the early Palaeozoic era until the middle of the Devonian period, about 400 Ma. The spread of land plants is thought to have reduced CO2 concentrations during the late Devonian, and plant activities as both sources and sinks of CO2 have since been important in providing stabilizing feedbacks.
Earlier still, a 200-million year period of intermittent, widespread glaciation extending close to the equator (Snowball Earth) appears to have been ended suddenly, about 550 Ma, by a colossal volcanic outgassing that raised the CO2 concentration of the atmosphere abruptly to 12%, about 350 times modern levels, causing extreme greenhouse conditions and carbonate deposition as limestone at the rate of about 1 mm per day. This episode marked the close of the Precambrian Eon, and was succeeded by the generally warmer conditions of the Phanerozoic, during which multicellular animal and plant life evolved. No volcanic carbon dioxide emission of comparable scale has occurred since. In the modern era, emissions to the atmosphere from volcanoes are approximately 0.645 billion tons of CO2 per year, whereas humans contribute 29 billion tons of CO2 each year.
Measurements from Antarctic ice cores
show that before industrial emissions started atmospheric CO2 mole fractions were about 280 parts per million (ppm), and stayed between 260 and 280 during the preceding ten thousand years. Carbon dioxide mole fractions in the atmosphere have gone up by approximately 35 percent since the 1900s, rising from 280 parts per million by volume to 387 parts per million in 2009. One study using evidence from stomata of fossilized leaves suggests greater variability, with carbon dioxide mole fractions above 300 ppm during the period seven to ten thousand years ago, though others have argued that these findings more likely reflect calibration or contamination problems rather than actual CO2 variability. Because of the way air is trapped in ice (pores in the ice close off slowly to form bubbles deep within the firn) and the time period represented in each ice sample analyzed, these figures represent averages of atmospheric concentrations of up to a few centuries rather than annual or decadal levels.
a physical change (condensation and precipitation remove water vapor from the atmosphere).
a chemical reaction within the atmosphere. For example, methane is oxidized by reaction with naturally occurring hydroxyl radical (OH·) and converted to CO2 and water vapor (CO2 from the oxidation of methane is not included in the methane Global warming potential). Other chemical reactions include solution and solid phase chemistry occurring in atmospheric aerosols.
a physical exchange between the atmosphere and the other components of the planet. An example is the mixing of atmospheric gases into the oceans.
In the late 19th century, scientists experimentally discovered that N 2 and O 2 do not absorb infrared radiation (called, at that time, "dark radiation"), while water (both as true vapor and condensed in the form of microscopic droplets suspended in clouds) and CO2 and other poly-atomic gaseous molecules do absorb infrared radiation. In the early 20th century, researchers realized that greenhouse gases in the atmosphere made Earth's overall temperature higher than it would be without them. During the late 20th century, a scientific consensus evolved that increasing concentrations of greenhouse gases in the atmosphere cause a substantial rise in global temperatures and changes to other parts of the climate system, with consequences for the environment and for human health.
^"Climate Change Indicators: Greenhouse Gases". United States Environmental Protection Agency. 16 December 2015. Carbon dioxide's lifetime cannot be represented with a single value because the gas is not destroyed over time, but instead moves among different parts of the ocean–atmosphere–land system. Some of the excess carbon dioxide is absorbed quickly (for example, by the ocean surface), but some will remain in the atmosphere for thousands of years, due in part to the very slow process by which carbon is transferred to ocean sediments.
^Betts (2001). "6.3 Well-mixed Greenhouse Gases". Chapter 6 Radiative Forcing of Climate Change. Working Group I: The Scientific Basis IPCC Third Assessment Report – Climate Change 2001. UNEP/GRID-Arendal – Publications. Archived from the original on 29 June 2011. Retrieved 16 October 2010.
Ehhalt, D.; et al., "Table 4.1", Atmospheric Chemistry and Greenhouse Gases, archived from the original on 3 January 2013, in IPCC TAR WG1 (2001), pp. 244–45. Referred to by: Blasing (2013). Based on Blasing (2013): Pre-1750 concentrations of CH4,N2O and current concentrations of O3, are taken from Table 4.1 (a) of the IPCC Intergovernmental Panel on Climate Change, 2001. Following the convention of IPCC (2001), inferred global-scale trace-gas concentrations from prior to 1750 are assumed to be practically uninfluenced by human activities such as increasingly specialized agriculture, land clearing, and combustion of fossil fuels. Preindustrial concentrations of industrially manufactured compounds are given as zero. The short atmospheric lifetime of ozone (hours-days) together with the spatial variability of its sources precludes a globally or vertically homogeneous distribution, so that a fractional unit such as parts per billion would not apply over a range of altitudes or geographical locations. Therefore a different unit is used to integrate the varying concentrations of ozone in the vertical dimension over a unit area, and the results can then be averaged globally. This unit is called a Dobson Unit (D.U.), after G.M.B. Dobson, one of the first investigators of atmospheric ozone. A Dobson unit is the amount of ozone in a column that, unmixed with the rest of the atmosphere, would be 10 micrometers thick at standard temperature and pressure.
^Because atmospheric concentrations of most gases tend to vary systematically over the course of a year, figures given represent averages over a 12-month period for all gases except ozone (O3), for which a current global value has been estimated (IPCC, 2001, Table 4.1a). CO2 averages for year 2012 are taken from the National Oceanic and Atmospheric Administration, Earth System Research Laboratory, web site: www.esrl.noaa.gov/gmd/ccgg/trends maintained by Pieter Tans. For other chemical species, the values given are averages for 2011. These data are found on the CDIAC AGAGE web site: http://cdiac.ornl.gov/ndps/alegage.htmlArchived 21 January 2013 at the Wayback Machine or the AGAGE home page: http://agage.eas.gatech.eduArchived 7 January 2015 at the Wayback Machine.
^IPCC AR4 WG1 (2007), p. 140:"The simple formulae ... in Ramaswamy et al. (2001) are still valid. and give an RF of +3.7 W m–2 for a doubling in the CO2 mixing ratio. ... RF increases logarithmically with mixing ratio" Calculation: ln(new ppm/old ppm)/ln(2)*3.7
^ abcdThe first value in a cell represents Mace Head, Ireland, a mid-latitude Northern-Hemisphere site, while the second value represents Cape Grim, Tasmania, a mid-latitude Southern-Hemisphere site. "Current" values given for these gases are annual arithmetic averages based on monthly background concentrations for year 2011. The SF 6 values are from the AGAGE gas chromatography – mass spectrometer (gc-ms) Medusa measuring system.