TheInfoList

A greenhouse gas (GHG or GhG) is a
gas Gas is one of the four fundamental states of matter (the others being solid, liquid A liquid is a nearly incompressible fluid In physics, a fluid is a substance that continually Deformation (mechanics), deforms (flows) under an appl ...

that absorbs and emits
radiant energy In physics Physics (from grc, φυσική (ἐπιστήμη), physikḗ (epistḗmē), knowledge of nature, from ''phýsis'' 'nature'), , is the natural science that studies matter, its Motion (physics), motion and behavior through Sp ...
within the
thermal infrared Infrared (IR), sometimes called infrared light, is electromagnetic radiation In physics Physics (from grc, φυσική (ἐπιστήμη), physikḗ (epistḗmē), knowledge of nature, from ''phýsis'' 'nature'), , is the natur ...
range, causing the
greenhouse effect The greenhouse effect is the process by which radiation from a planet's atmosphere warms the planet's surface to a temperature above what it would be without this atmosphere. Radiatively active gases (i.e., greenhouse gases) in a planet's at ...

. The primary greenhouse gases in
Earth's atmosphere File:Atmosphere gas proportions.svg, Composition of Earth's atmosphere by volume, excluding water vapor. Lower pie represents trace gases that together compose about 0.043391% of the atmosphere (0.04402961% at April 2019 concentration ). Number ...
are
water vapor (99.9839 °C) , - , Boiling point The boiling point of a substance is the temperature at which the vapor pressure of a liquid equals the pressure surrounding the liquid and the liquid changes into a vapor. The boiling point of a liquid v ...
(),
carbon dioxide Carbon dioxide (chemical formula ) is a colorless gas with a density about 53% higher than that of dry air. Carbon dioxide molecules consist of a carbon atom covalent bond, covalently double bonded to two oxygen atoms. It occurs naturally in At ...

(),
methane Methane (, ) is a chemical compound with the chemical formula A chemical formula is a way of presenting information about the chemical proportions of atom An atom is the smallest unit of ordinary matter In classical physics an ...
(),
nitrous oxide Nitrous oxide, commonly known as laughing gas or nitrous, is a chemical compound, an Nitrogen oxide, oxide of nitrogen with the Chemical formula, formula . At room temperature, it is a colourless Flammability#Definitions, non-flammable gas, ...
(), and
ozone Ozone (), or trioxygen, is an inorganic molecule File:Pentacene on Ni(111) STM.jpg, A scanning tunneling microscopy image of pentacene molecules, which consist of linear chains of five carbon rings. A molecule is an electrically neutral gro ...

(). Without greenhouse gases, the average temperature of
Earth's surface Earth is the third planet A planet is an astronomical body orbiting a star or Stellar evolution#Stellar remnants, stellar remnant that is massive enough to be Hydrostatic equilibrium, rounded by its own gravity, is not massive enough to c ...

would be about , rather than the present average of . The atmospheres of
Venus Venus is the second planet from the Sun. It is named after the Venus (mythology), Roman goddess of love and beauty. As List of brightest natural objects in the sky, the brightest natural object in Earth's night sky after the Moon, Venus can c ...

,
Mars Mars is the fourth planet A planet is an astronomical body orbiting a star or Stellar evolution#Stellar remnants, stellar remnant that is massive enough to be Hydrostatic equilibrium, rounded by its own gravity, is not massive enough to ...
and
Titan Titan most often refers to: * Titan (moon), the largest moon of Saturn * Titans, a race of deities in Greek mythology Titan or Titans may also refer to: Arts and entertainment Fictional entities Fictional locations * Titan in fiction, fictional ...
also contain greenhouse gases. Human activities since the beginning of the
Industrial Revolution The Industrial Revolution was the transition to new manufacturing processes in Great Britain, continental Europe Mainland or continental Europe is the contiguous continent of Europe, excluding its surrounding islands. It can also be ...
(around 1750) have increased the atmospheric concentration of carbon dioxide by almost 50%, from 280 ppm in 1750 to 419 ppm in 2021. The last time the atmospheric concentration of carbon dioxide was this high was over 3 million years ago. This increase has occurred despite the absorption of more than half of the emissions by various natural carbon sinks in the
carbon cycle The carbon cycle is the biogeochemical cycle by which carbon Carbon (from la, carbo "coal") is a chemical element Image:Simple Periodic Table Chart-blocks.svg, 400px, Periodic table, The periodic table of the chemical elements In ...

. At current
greenhouse gas emission Greenhouse gas emissions are emissions of greenhouse gases created from a range of human activities that cause climate change Climate change includes both global warming driven by human emissions of greenhouse gases and the resulting ...
rates, temperatures could increase by 2  (3.6 ), which the
United Nations The United Nations (UN) is an intergovernmental organization that aims to maintain international peace and international security, security, develop friendly relations among nations, achieve international cooperation, and be a centre for har ...

'
Intergovernmental Panel on Climate Change The Intergovernmental Panel on Climate Change (IPCC) is an intergovernmental body of the United Nations The United Nations (UN) is an intergovernmental organization that aims to maintain international peace and international security, s ...
(IPCC) says is the upper limit to avoid "dangerous" levels, by 2050. The vast majority of
anthropogenic Anthropogenic ("human" + "generating") is an adjective that may refer to: * Anthropogeny, the study of the origins of humanity Counterintuitively, anthropogenic may also refer to things that have been generated by humans, as follows: * Human imp ...
carbon dioxide emissions come from
combustion Combustion, or burning, is a high-temperature exothermic In thermodynamics Thermodynamics is a branch of physics that deals with heat, Work (thermodynamics), work, and temperature, and their relation to energy, radiation, and physical pro ...
of
fossil fuel A fossil fuel is a hydrocarbon In organic chemistry, a hydrocarbon is an organic compound , CH4; is among the simplest organic compounds. In chemistry, organic compounds are generally any chemical compounds that contain carbon-hydrogen che ...
s, principally
coal Coal is a combustible black or brownish-black sedimentary rock, formed as rock strata (Argentina Argentina (), officially the Argentine Republic ( es, link=no, República Argentina), is a country located mostly in the southern half o ...

,
petroleum Petroleum (), also known as crude oil and oil, is a #Latent heat of vaporization, naturally occurring, yellowish-black liquid found in geological formations beneath the Earth, Earth's surface. It is commonly refined into various types of fuels. ...

(including
oil An oil is any nonpolar chemical substance A chemical substance is a form of matter In classical physics and general chemistry, matter is any substance that has mass and takes up space by having volume. All everyday objects that can b ...

) and
natural gas Natural gas (also called fossil gas; sometimes just gas) is a naturally occurring hydrocarbon gas mixture consisting of methane and commonly including varying amounts of other higher alkanes, and sometimes a small percentage of carbon dioxide ...

deforestation deforestation in 1750-2004 (net loss) showing anthropogenic modification of remaining forest. File:MODIS (2020-08-01).jpg, 300px, Dry seasons, exacerbated by climate change, and the use of slash-and-burn methods for clearing tropical forest ...

and other changes in land use.

# Gases in Earth's atmosphere

## Non-greenhouse gases

The major constituents of Earth's atmosphere,
nitrogen Nitrogen is the chemical element with the Symbol (chemistry), symbol N and atomic number 7. It was first discovered and isolated by Scottish physician Daniel Rutherford in 1772. Although Carl Wilhelm Scheele and Henry Cavendish had independentl ...

() (78%),
oxygen Oxygen is the chemical element with the chemical symbol, symbol O and atomic number 8. It is a member of the chalcogen Group (periodic table), group in the periodic table, a highly Chemical reaction, reactive nonmetal, and an oxidizing a ...

() (21%), and
argon Argon is a chemical element Image:Simple Periodic Table Chart-blocks.svg, 400px, Periodic table, The periodic table of the chemical elements In chemistry, an element is a pure substance consisting only of atoms that all have the same num ...

(Ar) (0.9%), are not greenhouse gases because molecules containing two atoms of the same element such as and have no net change in the distribution of their electrical charges when they vibrate, and
monatomic In physics Physics (from grc, φυσική (ἐπιστήμη), physikḗ (epistḗmē), knowledge of nature, from ''phýsis'' 'nature'), , is the natural science that studies matter, its Motion (physics), motion and behavior through Spa ...
gases such as Ar do not have vibrational modes. Hence they are almost totally unaffected by
infrared radiation Infrared (IR), sometimes called infrared light, is electromagnetic radiation In physics Physics (from grc, φυσική (ἐπιστήμη), physikḗ (epistḗmē), knowledge of nature, from ''phýsis'' 'nature'), , is the natur ...

. Some molecules containing just two atoms of different elements, such as
carbon monoxide Carbon monoxide (chemical formula CO) is a colorless, odorless, tasteless, flammable gas that is slightly less dense than air. Carbon monoxide consists of one carbon atom and one oxygen atom. It is the simplest molecule of the oxocarbon family. In ...

(CO) and
hydrogen chloride The compound Compound may refer to: Architecture and built environments * Compound (enclosure), a cluster of buildings having a shared purpose, usually inside a fence or wall ** Compound (fortification), a version of the above fortified with d ...

(HCl), do absorb infrared radiation, but these molecules are short-lived in the atmosphere owing to their reactivity or
solubility In chemistry Chemistry is the science, scientific study of the properties and behavior of matter. It is a natural science that covers the Chemical element, elements that make up matter to the chemical compound, compounds composed of atom ...

. Therefore, they do not contribute significantly to the greenhouse effect and often are omitted when discussing greenhouse gases.

## Greenhouse gases

Greenhouse gases are those that absorb and emit infrared radiation in the wavelength range emitted by Earth. Carbon dioxide (0.04%), nitrous oxide, methane, and ozone are trace gases that account for almost 0.1% of Earth's atmosphere and have an appreciable greenhouse effect. The most abundant greenhouse gases in Earth's atmosphere, listed in decreasing order of average global
mole fraction In chemistry Chemistry is the scientific discipline involved with Chemical element, elements and chemical compound, compounds composed of atoms, molecules and ions: their composition, structure, properties, behavior and the changes they underg ...
, are: *
Water vapor (99.9839 °C) , - , Boiling point The boiling point of a substance is the temperature at which the vapor pressure of a liquid equals the pressure surrounding the liquid and the liquid changes into a vapor. The boiling point of a liquid v ...
() *
Carbon dioxide Carbon dioxide (chemical formula ) is a colorless gas with a density about 53% higher than that of dry air. Carbon dioxide molecules consist of a carbon atom covalent bond, covalently double bonded to two oxygen atoms. It occurs naturally in At ...

() *
Methane Methane (, ) is a chemical compound with the chemical formula A chemical formula is a way of presenting information about the chemical proportions of atom An atom is the smallest unit of ordinary matter In classical physics an ...
() *
Nitrous oxide Nitrous oxide, commonly known as laughing gas or nitrous, is a chemical compound, an Nitrogen oxide, oxide of nitrogen with the Chemical formula, formula . At room temperature, it is a colourless Flammability#Definitions, non-flammable gas, ...
() *
Ozone Ozone (), or trioxygen, is an inorganic molecule File:Pentacene on Ni(111) STM.jpg, A scanning tunneling microscopy image of pentacene molecules, which consist of linear chains of five carbon rings. A molecule is an electrically neutral gro ...

() *
Chlorofluorocarbon Chlorofluorocarbons (CFCs) and hydrochlorofluorocarbons (HCFCs) are fully or partly halogenated paraffin hydrocarbons that contain only carbon (C), hydrogen (H), chlorine (Cl), and fluorine (F), produced as volatile derivative of methane, et ...
s (CFCs and
HCFCs Chlorofluorocarbons (CFCs) and hydrochlorofluorocarbons (HCFCs) are fully or partly halogenated paraffin hydrocarbons that contain only carbon Carbon (from la, carbo "coal") is a chemical element Image:Simple Periodic Table Chart-bloc ...
) *
Hydrofluorocarbons Hydrofluorocarbons (HFCs) are man-made organic compounds that contain fluorine and hydrogen atoms, and are the most common type of organofluorine compounds. Most are gases at room temperature and pressure. They are frequently used in air condition ...
(HFCs) *
Perfluorocarbons Fluorocarbons, sometimes referred to as perfluorocarbons or PFCs, are organofluorine compounds with the formula CxFy, i.e., they contain only carbon Carbon (from la, carbo "coal") is a chemical element Image:Simple Periodic Table Char ...
( ,

, etc.), , and

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). in 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.

Oxidation of CO to directly produces an unambiguous increase in
radiative forcing Radiative forcing is the change in energy flux in the atmosphere caused by Climate variability and change, natural or Human impact on the environment#Impacts on climate, anthropogenic factors of climate change as measured by watts / metre2. It is ...
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 (
wavelength In physics, the wavelength is the spatial period of a periodic wave—the distance over which the wave's shape repeats. It is the distance between consecutive corresponding points of the same phase (waves), phase on the wave, such as two adjac ...

15 microns, or
wavenumber In the physical sciences, the wavenumber (also wave number or repetency) is the spatial frequency of a wave, measured in cycles per unit distance or radians per unit distance. Whereas temporal frequency can be thought of as the number of waves per ...
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 , which requires reactions with the OH radical, produces an instantaneous reduction in radiative absorption and emission since is a weaker greenhouse gas than methane. However, the oxidations of CO and 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 compound Non-methane volatile organic compounds (NMVOCs) are a large variety of chemically different compounds, such as benzene Benzene is an organic chemical compound A chemical compound is a chemical substance composed of many identical molecule ...
s (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 . The main chemical that reacts with methane in the atmosphere is the
hydroxyl radical The hydroxyl radical is the diatomic molecule . The hydroxyl radical is very stable as a dilute gas, but it decays very rapidly in the condensed phase. It is pervasive in some situations. Most notably the hydroxyl radicals is produced from the de ...

(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 File:Stratosphere Temperature Trend.jpg, This image shows the temperature trend in the lower stratosphere as measured by a series of satellite-based instruments between January 1979 and December 2005. The lower stratosphere is centered around 18 ...

. CO and NMVOCs produce 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 . The same process that converts NMVOCs to carbon dioxide can also lead to the formation of tropospheric ozone.
HalocarbonHalocarbon compounds are chemicals in which one or more carbon atoms are linked by covalent bonds with one or more halogen atoms (fluorine, chlorine, bromine or iodine – ) resulting in the formation of organofluorine compounds, organochlorine ...
s have an indirect effect because they destroy stratospheric ozone. Finally,
hydrogen Hydrogen is the chemical element Image:Simple Periodic Table Chart-blocks.svg, 400px, Periodic table, The periodic table of the chemical elements In chemistry, an element is a pure substance consisting only of atoms that all have the same ...

can lead to ozone production and increases as well as producing stratospheric water vapor.

## Contribution of clouds to Earth's greenhouse effect

The major non-gas contributor to Earth's greenhouse effect,
clouds In meteorology Meteorology is a branch of the atmospheric sciences which includes atmospheric chemistry and atmospheric physics, with a major focus on weather forecasting. The study of meteorology dates back millennia, though significan ...
, also absorb and emit infrared radiation and thus have an effect on greenhouse gas radiative properties. Clouds are water droplets or
ice crystal Ice is water Water is an Inorganic compound, inorganic, Transparency and translucency, transparent, tasteless, odorless, and Color of water, nearly colorless chemical substance, which is the main constituent of Earth's hydrosphere and ...
s suspended in the atmosphere.

## Role of water vapor

Water vapor (99.9839 °C) , - , Boiling point The boiling point of a substance is the temperature at which the vapor pressure of a liquid equals the pressure surrounding the liquid and the liquid changes into a vapor. The boiling point of a liquid v ...
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 and . Water vapor responds to and amplifies effects of the other greenhouse gases. The
Clausius–Clapeyron relation The Clausius–Clapeyron relation, named after Rudolf Clausius Rudolf Julius Emanuel Clausius (; 2 January 1822 – 24 August 1888) was a German physicist A physicist is a scientist A scientist is a person who conducts Scientific method ...
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 Humidity is the concentration of water vapor present in the air. Water vapor, the gaseous state of water, is generally invisible to the human eye. Humidity indicates the likelihood for precipitation (meteorology), precipitation, dew, or fog to be ...

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 Positive feedback (exacerbating feedback, self-reinforcing feedback) is a process that occurs in a feedback loop which exacerbates the effects of a small disturbance. That is, the effects of a perturbation on a system include an increase in th ...
" that amplifies the original warming. Eventually other earth processes offset these positive feedbacks, stabilising the global temperature at a new equilibrium and preventing the loss of Earth's water through a Venus-like
runaway greenhouse effect A runaway greenhouse effect occurs when a planet's atmosphere contains greenhouse gas A greenhouse gas (sometimes abbreviated GHG) is a gas that Absorption (electromagnetic radiation), absorbs and Emission (electromagnetic radiation), emits r ...
.

# Impacts on the overall greenhouse effect

The contribution of each gas to the 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 Carbon sequestration or carbon dioxide removal (CDR) is the long-term removal, capture or sequestration of atmospheric carbon dioxide, carbon dioxide from the atmosphere to slow or reverse atmospheric CO2 pollution and to Climate change mitigat ...

. 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) argues that the contribution to climate change from methane is at least double previous estimates as a result of this effect. When ranked by their direct contribution to the greenhouse effect, the most important are: In addition to the main greenhouse gases listed above, other greenhouse gases include
sulfur hexafluoride Sulfur hexafluoride (SF6) or sulphur hexafluoride (British spelling Despite the various English dialects spoken from country to country and within different regions of the same country, there are only slight regional variations in Englis ...
,
hydrofluorocarbon Hydrofluorocarbons (HFCs) are man-made organic compounds that contain fluorine and hydrogen atoms, and are the most common type of organofluorine compounds. Most are gases at room temperature and pressure. They are frequently used in air condition ...
s and
perfluorocarbon Fluorocarbons, sometimes referred to as perfluorocarbons or PFCs, are organofluorine compounds with the formula CxFy, i.e., they contain only carbon and fluorine. The terminology is not strictly followed and many fluorine-containing organic compou ...
s (see IPCC list of greenhouse gases). Some greenhouse gases are not often listed. For example,
nitrogen trifluoride Nitrogen trifluoride () is an inorganic, colorless, non-flammable , Germany Image:Tu braunschweig 750 grad ofen.jpg, 250px, Germany, German test apparatus for determining combustibility at Technische Universität Braunschweig A combustible ...

has a high
global warming potential Global warming potential (GWP) is the heat absorbed by any greenhouse gas A greenhouse gas (sometimes abbreviated GHG) is a gas that Absorption (electromagnetic radiation), absorbs and Emission (electromagnetic radiation), emits radiant energ ...

(GWP) but is only present in very small quantities.

## Proportion of direct effects at a given moment

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.

Aside from
water vapor (99.9839 °C) , - , Boiling point The boiling point of a substance is the temperature at which the vapor pressure of a liquid equals the pressure surrounding the liquid and the liquid changes into a vapor. The boiling point of a liquid v ...
, which has a
residence time The residence time of a fluid parcelIn fluid dynamics In physics and engineering, fluid dynamics is a subdiscipline of fluid mechanics that describes the flow of fluids—liquids and gases. It has several subdisciplines, including aerodynamics ...
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 $\tau$ of an atmospheric
species In biology Biology is the natural science that studies life and living organisms, including their anatomy, physical structure, Biochemistry, chemical processes, Molecular biology, molecular interactions, Physiology, physiological mechanis ...
X in a one- box model as the average time that a molecule of X remains in the box. Mathematically $\tau$ can be defined as the ratio of the mass $m$ (in kg) of X in the box to its removal rate, which is the sum of the flow of X out of the box ($F_$), chemical loss of X ($L$), and
deposition Deposition may refer to: * Deposition (law), taking testimony outside of court * List of deposed politicians, Deposition (politics), the removal of a person of authority from political power * Deposition (university), a widespread initiation ritual ...
of X ($D$) (all in kg/s): $\tau = \frac$. If input of this gas into the box ceased, then after time $\tau$, 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 Image:Plot-exponential-decay.svg, upright=1.5, A quantity undergoing exponential decay. Larger decay constants make the quantity vanish much more rapidly. This plot shows decay for decay constant (λ) of 25, 5, 1, 1/5, and 1/25 for x from 0 to 5. A ...
.
Carbon dioxide Carbon dioxide (chemical formula ) is a colorless gas with a density about 53% higher than that of dry air. Carbon dioxide molecules consist of a carbon atom covalent bond, covalently double bonded to two oxygen atoms. It occurs naturally in At ...

has a variable atmospheric lifetime, and cannot be specified precisely. Although more than half of the CO2 emitted is removed from the atmosphere within a century, some fraction (about 20%) of emitted CO2 remains in the atmosphere for many thousands of years.See also: See also: 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.

## 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 In thermodynamics, heat is energy in transfer to or from a thermodynamic system, by mechanisms other than Work (thermodynamics), thermodynamic work or Mass transfer, transfer of matter. The various mechanisms of energy transfer that define he ...
. Earth's surface temperature depends on this balance between incoming and outgoing energy. If this energy balance is shifted, Earth's surface becomes warmer or cooler, leading to a variety of changes in global climate.. A number of natural and man-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, and therefore can affect Earth's energy balance over a long period.
Radiative forcing Radiative forcing is the change in energy flux in the atmosphere caused by Climate variability and change, natural or Human impact on the environment#Impacts on climate, anthropogenic factors of climate change as measured by watts / metre2. It is ...
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.
The Annual Greenhouse Gas Index (AGGI) is defined by atmospheric scientists at
NOAA The National Oceanic and Atmospheric Administration (NOAA ) is an American scientific and regulatory agency within the United States Department of Commerce The United States Department of Commerce is an executive department The United ...

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 The Industrial Revolution was the transition to new manufacturing processes in Europe and the United States, in the period from about 1760 to sometime between 1820 and 1840. This transition included going from hand production methods to machi ...
). 1990 is chosen because it is the baseline year for the
Kyoto Protocol The Kyoto Protocol is an international treaty which extends the 1992 United Nations Framework Convention on Climate Change (UNFCCC) that commits state parties to reduce greenhouse gas A greenhouse gas (sometimes abbreviated GHG) is a g ...

, 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 Global warming potential (GWP) is the heat absorbed by any greenhouse gas A greenhouse gas (sometimes abbreviated GHG) is a gas that Absorption (electromagnetic radiation), absorbs and Emission (electromagnetic radiation), emits radiant energ ...

(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 and evaluated for a specific timescale. Thus, if a gas has a high (positive)
radiative forcing Radiative forcing is the change in energy flux in the atmosphere caused by Climate variability and change, natural or Human impact on the environment#Impacts on climate, anthropogenic factors of climate change as measured by watts / metre2. It is ...
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 its GWP will increase when the timescale is considered. Carbon dioxide is defined to have a GWP of 1 over all time periods.
Methane Methane (, ) is a chemical compound with the chemical formula A chemical formula is a way of presenting information about the chemical proportions of atom An atom is the smallest unit of ordinary matter In classical physics an ...
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 , 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 Methane (, ) is a chemical compound with the chemical formula A chemical formula is a way of presenting information about the chemical proportions of atom An atom is the smallest unit of ordinary matter In classical physics an ...
is degraded to water and through chemical reactions in the atmosphere. Examples of the atmospheric lifetime and relative to for several greenhouse gases are given in the following table: The use of CFC-12 (except some essential uses) has been phased out due to its properties. The phasing-out of less active HCFC-compounds will be completed in 2030.

# Natural and anthropogenic sources

Aside from purely human-produced synthetic halocarbons, most greenhouse gases have both natural and human-caused sources. During the pre-industrial
Holocene The Holocene ( ) is the current Geologic time scale, geological epoch. It began approximately 11,650 radiocarbon calibration, cal years Before Present, before present, after the last glacial period, which concluded with the Holocene glacial re ...
, 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. The 2021
IPCC Sixth Assessment Report The Sixth Assessment Report (AR6) of the United Nations (UN) Intergovernmental Panel on Climate Change (IPCC) is the sixth in a series of reports which assess scientific, technical, and socio-economic information concerning climate change. Three Wo ...
noted that "From a physical science perspective, limiting human-induced global warming to a specific level requires limiting cumulative CO2 emissions, reaching at least net zero CO2 emissions, along with strong reductions in other greenhouse gas emissions. Strong, rapid and sustained reductions in CH4 emissions would also limit the warming effect resulting from declining aerosol pollution and would improve air quality." ''Abbreviations used in the two tables below: ppm = parts-per-million; ppb = parts-per-billion; ppt = parts-per-trillion; W/m2 =
watt The watt (symbol: W) is a unit of power Power typically refers to: * Power (physics) In physics, power is the amount of energy transferred or converted per unit time. In the International System of Units, the unit of power is the watt, equal ...

s per
square metre The square metre ( international spelling as used by the International Bureau of Weights and Measures The International Bureau of Weights and Measures (french: Bureau international des poids et mesures, BIPM) is an intergovernmental organis ...
''
Ice core An ice core is a core sample that is typically removed from an ice sheet or a high mountain glacier. Since the ice forms from the incremental buildup of annual layers of snow, lower layers are older than upper, and an ice core contains ice form ...
s provide evidence for greenhouse gas concentration variations over the past 800,000 years (see the following section). Both CO2 and 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 CO2
mole fraction In chemistry Chemistry is the scientific discipline involved with Chemical element, elements and chemical compound, compounds composed of atoms, molecules and ions: their composition, structure, properties, behavior and the changes they underg ...
s 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
Phanerozoic The Phanerozoic Eon is the current geologic eon in the geologic time scale The geologic time scale (GTS) is a system of chronological dating that classifies Geology, geological strata (stratigraphy) in time. It is used by geologists, paleontolo ...
Eon, 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 The Devonian ( ) is a geologic period and system of the Paleozoic The Paleozoic (or Palaeozoic) Era ( ; from the Greek ''palaiós'' (), "old" and ''zōḗ'' (), "life", meaning "ancient life") is the earliest of three geologic eras of the P ...
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 stabilising feedbacks. Earlier still, a 200-million year period of intermittent, widespread glaciation extending close to the equator (
Snowball Earth The Snowball Earth hypothesis proposes that, during one or more of Earth's Greenhouse and icehouse Earth, icehouse climates, the Earth's surface, planet's surface became entirely or nearly entirely frozen. It is believed that this occurred someti ...
) appears to have been ended suddenly, about 550 Ma, by a colossal volcanic outgassing that raised the concentration of the atmosphere abruptly to 12%, about 350 times modern levels, causing extreme greenhouse conditions and carbonate deposition as
limestone Limestone is a common type of carbonate rock, carbonate sedimentary rock. It is composed mostly of the minerals calcite and aragonite, which are different Polymorphism (materials science), crystal forms of calcium carbonate (). Limestone forms ...

at the rate of about 1 mm per day. This episode marked the close of the
Precambrian The Precambrian (or Pre-Cambrian, sometimes abbreviated pꞒ, or Cryptozoic) is the earliest part of History of the Earth, Earth's history, set before the current Phanerozoic Eon. The Precambrian is so named because it preceded the Cambrian, the ...

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 tonnes of per year, whereas humans contribute 29 billion tonnes of each year.

## Ice cores

Measurements from Antarctic ice cores show that before industrial emissions started atmospheric CO2 mole fractions were about 280
parts per million In science Science (from the Latin word ''scientia'', meaning "knowledge") is a systematic enterprise that Scientific method, builds and Taxonomy (general), organizes knowledge in the form of Testability, testable explanations and predictio ...
(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 File:LeafUndersideWithStomata.jpg, The underside of a leaf. In this species (''Tradescantia zebrina'') the guard cells of the stomata are green because they contain chlorophyll while the epidermal cells are chlorophyll-free and contain red pigme ...

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.

## Changes since the Industrial Revolution

Since the beginning of the
Industrial Revolution The Industrial Revolution was the transition to new manufacturing processes in Great Britain, continental Europe Mainland or continental Europe is the contiguous continent of Europe, excluding its surrounding islands. It can also be ...
, the concentrations of many of the greenhouse gases have increased. For example, the mole fraction of carbon dioxide has increased from 280 ppm to 415 ppm, or 120 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. Total cumulative emissions from 1870 to 2017 were 425±20 GtC (1539 Gt) from
fossil fuels A fossil fuel is a hydrocarbon In organic chemistry, a hydrocarbon is an organic compound , CH4; is among the simplest organic compounds. In chemistry, organic compounds are generally any chemical compounds that contain carbon-hydrogen che ...
and industry, and 180±60 GtC (660 Gt) from land use change. Land-use change, such as deforestation, caused about 31% of cumulative emissions over 1870–2017,
coal Coal is a combustible black or brownish-black sedimentary rock, formed as rock strata (Argentina Argentina (), officially the Argentine Republic ( es, link=no, República Argentina), is a country located mostly in the southern half o ...

32%, oil 25%, and gas 10%. Today, the stock of carbon in the atmosphere increases by more than 3 million tonnes per annum (0.04%) compared with the existing stock. This increase is the result of human activities by burning fossil fuels, deforestation and forest degradation in tropical and boreal regions. (webpage has a translation button) The other greenhouse gases produced from human activity show similar increases in both amount and rate of increase. Many observations are available online in a variety of
Atmospheric Chemistry Observational Databases Over the last two centuries many environmental chemical observations have been made from a variety of ground-based, airborne Airborne or Airborn may refer to: Arts, entertainment, and media Films * Airborne (1962 film), ''Airborne'' (1962 film) ...
.

# Removal from the atmosphere

## Natural processes

Greenhouse gases can be removed from the atmosphere by various processes, as a consequence of: * a physical change (condensation and precipitation remove water vapor from the atmosphere). * a chemical reaction within the atmosphere. For example, methane is
oxidized (mild reducing agent) are added to powdered potassium permanganate (strong oxidizing agent), a violent redox reaction accompanied by self-ignition starts. Redox (reduction–oxidation, pronunciation: or ) is a type of chemical reaction A ...

by reaction with naturally occurring
hydroxyl radical The hydroxyl radical is the diatomic molecule . The hydroxyl radical is very stable as a dilute gas, but it decays very rapidly in the condensed phase. It is pervasive in some situations. Most notably the hydroxyl radicals is produced from the de ...

, OH· and degraded to and water vapor ( from the oxidation of methane is not included in the methane
Global warming potential Global warming potential (GWP) is the heat absorbed by any greenhouse gas A greenhouse gas (sometimes abbreviated GHG) is a gas that Absorption (electromagnetic radiation), absorbs and Emission (electromagnetic radiation), emits radiant energ ...

). 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. * a chemical change at the interface between the atmosphere and the other components of the planet. This is the case for , which is reduced by
photosynthesis File:Photosynthesis equation.svg, upright=1.8, Overall equation for the type of photosynthesis that occurs in plants Photosynthesis is a process used by plants and other organisms to Energy transformation, convert light energy into chemical energ ...

of plants, and which, after dissolving in the oceans, reacts to form
carbonic acid In chemistry, carbonic acid is a dibasic acid with the chemical formula A chemical formula is a way of presenting information about the chemical proportions of atom An atom is the smallest unit of ordinary matter In classical ...

and
bicarbonate In inorganic chemistry#REDIRECT Inorganic chemistry features unusual bonding B: Caesium chloride Caesium chloride or cesium chloride is the inorganic compound with the formula Caesium, CsChloride, Cl. This colorless salt is an important sourc ...

and
carbonate In chemistry, a carbonate is a salt Salt is a mineral composed primarily of sodium chloride (NaCl), a chemical compound belonging to the larger class of Salt (chemistry), salts; salt in its natural form as a crystallinity, crystalline min ...

ions (see
ocean acidification File:Acidifiedupwelledwater.jpg, upright=1.5, NOAA provides evidence for the upwelling of "acidified" water onto the Continental Shelf. In the figure above, note the vertical sections of (A) temperature, (B) aragonite saturation, (C) pH, (D) DIC ...
). * a photochemical change.
HalocarbonsHalocarbon compounds are chemicals in which one or more carbon atoms are linked by covalent bonds with one or more halogen atoms ( fluorine, chlorine, bromine or iodine – ) resulting in the formation of organofluorine compounds, organoch ...
are dissociated by light releasing Cl· and F· as
free radical A daughter category of ''Ageing'', this category deals only with the biological aspects of ageing. Ageing Ailments of unknown cause Biogerontology Biological processes Causes of death Cellular processes Gerontology Life extension Metabol ...
s in the
stratosphere File:Stratosphere Temperature Trend.jpg, This image shows the temperature trend in the lower stratosphere as measured by a series of satellite-based instruments between January 1979 and December 2005. The lower stratosphere is centered around 18 ...

with harmful effects on
ozone Ozone (), or trioxygen, is an inorganic molecule File:Pentacene on Ni(111) STM.jpg, A scanning tunneling microscopy image of pentacene molecules, which consist of linear chains of five carbon rings. A molecule is an electrically neutral gro ...

(halocarbons are generally too stable to disappear by chemical reaction in the atmosphere).

## Negative emissions

A number of technologies remove greenhouse gases emissions from the atmosphere. Most widely analysed are those that remove carbon dioxide from the atmosphere, either to geologic formations such as
bio-energy with carbon capture and storage Bioenergy with carbon capture and storage (BECCS) is the process of extracting bioenergy from biomass and capturing and storing the carbon, thereby removing it from the atmosphere. The carbon in the biomass comes from the greenhouse gas ...
and carbon dioxide air capture, or to the soil as in the case with biochar. The IPCC has pointed out that many long-term climate scenario models require large-scale man-made negative emissions to avoid serious climate change. in

# History of scientific research

In the late 19th century, scientists experimentally discovered that and 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 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 opinion on climate change, 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 Effects of global warming, consequences for the Physical impacts of climate change, environment and for Effects of global warming on human health, human health.

* * (pb: ) *