Greenhouse gas

Greenhouse gases are gaseous constituents of the atmosphere, both natural and anthropogenic, that absorb and emit radiation at specific wavelengths within the spectrum of thermal infrared radiation emitted by the Earth’s surface, the atmosphere itself, and by clouds. This property causes the greenhouse effect. Greenhouse gases are essential to maintaining the temperature of the Earth; without them the planet would be so cold as to be uninhabitable. Natural sources are the Earth's ecosystem, and anthropogenic sources include industrial, transportation, residential, commercial and agricultural processes. Venus, Mars and Titan also have atmospheric gases that cause greenhouse effects.

The greenhouse effect

In order, Earth's most abundant greenhouse gases are:

• water vapor
  
• carbon dioxide
• methane
• nitrous oxide
• ozone
• CFCs

By effect, the most important greenhouse gases are:

• water vapor, which causes about 36–70% of the greenhouse effect on Earth. (Note clouds typically affect climate differently from other forms of atmospheric water.)
• carbon dioxide, which causes 9–26%
• methane, which causes 4–9%
• ozone, which causes 3–7%

Importance is a combination of the strength of the greenhouse effect of the gas and its abundance. For example, methane is a much stronger greenhouse gas than carbon dioxide, but is present in much smaller concentrations.

It is not possible to state that a certain gas causes a certain percentage of the greenhouse effect, because the influences of the various gases are not additive. (The higher ends of the ranges quoted are for the gas alone; the lower ends, for the gas counting overlaps.) Other greenhouse gases include, but are not limited to, nitrous oxide, sulfur hexafluoride, hydrofluorocarbons, perfluorocarbons and chlorofluorocarbons (see IPCC list of greenhouse gases). Some greenhouse gases are present but not often listed. For example, nitrogen trifluoride has a high global warming potential (GWP) but is only present in small quantities.

The major atmospheric constituents (nitrogen, N2 and oxygen, O2) are not greenhouse gases. Nor is the approximately 1% of argon, Ar. This is because homonuclear diatomic molecules such as N2 and O2 and monatomic molecules such as Ar neither absorb nor emit infrared radiation, as there is no net change in the dipole moment of these molecules when they vibrate. Molecular vibrations occur at energies that are of the same magnitude as the energy of the photons on infrared light. Heteronuclear diatomics such as CO or HCl absorb IR; however, these molecules are short-lived in the atmosphere owing to their reactivity and solubility. As a consequence they do not contribute significantly to the greenhouse effect.

Late 19th century scientists experimentally discovered that N₂ and O₂ did not absorb infrared radiation (called, at that time, "dark radiation") and that CO₂ and many other gases did absorb such radiation. It was recognized in the early 20th century that the greenhouse gases in the atmosphere caused the Earth's overall temperature to be higher than it would be without them.

Natural and anthropogenic

400,000 years of CO2

Aside from purely human-produced synthetic halocarbons, most greenhouse gases now have both natural and anthropogenic sources. During the pre-industrial holocene, concentrations of existing gases were roughly constant. In the industrial era, human activities have added greenhouse gases to the atmosphere, mainly through the burning of fossil fuels and clearing of forests.

Gas	Preindustrial Level	Current Level	Increase since 1750	(W/mRadiative forcing2)
Carbon dioxide	280 ppm	384ppm	104 ppm	1.46
Methane	700 ppb	1,745 ppb	1,045 ppb	0.48
Nitrous oxide	270 ppb	314 ppb	44 ppb	0.15
CFC-12	0	533 ppt	533 ppt	0.17

Ice cores provide evidence for variation in greenhouse gas concentrations over the past 800,000 years. Both CO2 and CH4 vary between glacial and interglacial phases, and concentrations of these gases correlate strongly with temperature. Before the ice core record, direct measurements do not exist. Various proxies and modelling suggests large variations; 500 Myr ago CO₂ levels were likely 10 times higher than now. Indeed higher CO2 concentrations are thought to have prevailed throughout most of the Phanerozoic 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 period, about 400 Mya. 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) appears to have been ended suddenly, about 550 Mya, by a colossal volcanic outgassing which 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 1mm 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 only about 1% of emissions from human sources.

Anthropogenic greenhouse gases

Global anthropogenic greenhouse gas emissions broken down into 8 different sectors for the year 2000.

The projected temperature increase for a range of greenhouse gas stabilization scenarios (the coloured bands). The black line in middle of the shaded area indicates 'best estimates'; the red and the blue lines the likely limits. From the work of IPCC AR4, 2007.

Per capita anthropogenic greenhouse gas emissions by country for the year 2000 including land-use change.

Since about 1750 human activity has increased the concentration of carbon dioxide and of some other greenhouse gases. Natural sources of carbon dioxide are more than 20 times greater than sources due to human activity, but over periods longer than a few years natural sources are closely balanced by natural sinks such as weathering of continental rocks and photosynthesis of carbon compounds by plants and marine plankton. As a result of this balance, the atmospheric concentration of carbon dioxide remained between 260 and 280 parts per million for the 10,000 years between the end of the last glacial maximum and the start of the industrial era.

Some of the main sources of greenhouse gases due to human activity include:

• burning of fossil fuels and deforestation leading to higher carbon dioxide concentrations. Land use change (mainly deforestation in the tropics) account for up to one third of total anthropogenic CO2 emissions.
• livestock enteric fermentation and manure management, paddy rice farming, land use and wetland changes, pipeline losses, and covered vented landfill emissions leading to higher methane atmospheric concentrations. Many of the newer style fully vented septic systems that enhance and target the fermentation process also are sources of atmospheric methane.
• use of chlorofluorocarbons (CFCs) in refrigeration systems, and use of CFCs and halons in fire suppression systems and manufacturing processes.
• agricultural activities, including the use of fertilizers, that lead to higher nitrous oxide concentrations.

The seven sources of CO₂ from fossil fuel combustion are (with percentage contributions for 2000–2004):

1. Solid fuels (e.g. coal): 35%
2. Liquid fuels (e.g. gasoline): 36%
3. Gaseous fuels (e.g. natural gas): 20%
4. Flaring gas industrially and at wells: <1%
5. Cement production: 3%
6. Non-fuel hydrocarbons: <1%
7. The "international bunkers" of shipping and air transport not included in national inventories: 4%

The U.S. EPA ranks the major greenhouse gas contributing end-user sectors in the following order: industrial, transportation, residential, commercial and agricultural. Major sources of an individual's GHG include home heating and cooling, electricity consumption, and transportation. Corresponding conservation measures are improving home building insulation, compact fluorescent lamps and choosing energy-efficient vehicles.

Carbon dioxide, methane, nitrous oxide and three groups of fluorinated gases (sulfur hexafluoride, HFCs, and PFCs) are the major greenhouse gases and the subject of the Kyoto Protocol, which came into force in 2005.

Although CFCs are greenhouse gases, they are regulated by the Montreal Protocol, which was motivated by CFCs' contribution to ozone depletion rather than by their contribution to global warming. Note that ozone depletion has only a minor role in greenhouse warming though the two processes often are confused in the media.

Role of water vapor

Increasing water vapor at Boulder, Colorado.

Water vapor is a naturally occurring greenhouse gas and accounts for the largest percentage of the greenhouse effect, between 36% and 66%. Water vapor concentrations fluctuate regionally, but human activity does not directly affect water vapor concentrations except at local scales (for example, near irrigated fields).

The Clausius-Clapeyron relation establishes that warmer air can hold more water vapor per unit volume. Current state-of-the-art climate models predict that increasing water vapor concentrations in warmer air will amplify the greenhouse effect created by anthropogenic greenhouse gases while maintaining nearly constant relative humidity. Thus water vapor acts as a positive feedback to the forcing provided by greenhouse gases such as CO2.

Greenhouse gas emissions

Measurements from Antarctic ice cores show that just before industrial emissions started, atmospheric CO2 levels were about 280 parts per million by volume (ppm; the units µL/L are occasionally used and are identical to parts per million by volume). From the same ice cores it appears that CO2 concentrations stayed between 260 and 280 ppm during the preceding 10,000 years. However, because of the way air is trapped in ice and the time period represented in each ice sample analyzed, these figures are long term averages not annual levels. Studies using evidence from stomata of fossilized leaves suggest greater variability, with CO2 levels above 300 ppm during the period 7,000–10,000 years ago, though others have argued that these findings more likely reflect calibration/contamination problems rather than actual CO₂ variability.

Since the beginning of the Industrial Revolution, the concentrations of many of the greenhouse gases have increased. The concentration of CO2 has increased by about 100 ppm (i.e., from 280 ppm to 380 ppm). The first 50 ppm increase took place in about 200 years, from the start of the Industrial Revolution to around 1973; the next 50 ppm increase took place in about 33 years, from 1973 to 2006. Many observations are available online in a variety of Atmospheric Chemistry Observational Databases. The greenhouse gases with the largest radiative forcing are:

Relevant to radiative forcing

Gas	Current (1998) Amount by volume	Increase over pre-industrial (1750)	Percentage increase	Radiative forcing (W/m²)
Carbon dioxide	365 ppm {383 ppm(2007.01)}	87 ppm {105 ppm(2007.01)}	31% {37.77%(2007.01)}	1.46 {~1.532 (2007.01)}
Methane	1,745 ppb	1,045 ppb	150%	0.48
Nitrous oxide	314 ppb	44 ppb	16%	0.15

Global anthropogenic Carbon emissions 1751–2004.

Relevant to both radiative forcing and ozone depletion; all of the following have no natural sources and hence zero amounts
pre-industrial

Gas	Current (1998) Amount by volume	Radiative forcing (W/m²)
CFC-11	268 ppt	0.07
CFC-12	533 ppt	0.17
CFC-113	84 ppt	0.03
Carbon tetrachloride	102 ppt	0.01
HCFC-22	69 ppt	0.03

(Source: IPCC radiative forcing report 1994 updated (to 1998) by IPCC TAR table 6.1 ).

Recent rates of change and emission

Greenhouse gas intensity in 2000 including land-use change

The sharp acceleration in CO₂ emissions since 2000 of >3% y⁻¹ (>2 ppm y⁻¹) from 1.1% y⁻¹ during the 1990s is attributable to the lapse of formerly declining trends in carbon intensity of both developing and developed nations. Although over 3/4 of cumulative anthropogenic CO2 is still attributable to the developed world, China was responsible for most of global growth in emissions during this period. Localised plummeting emissions associated with the collapse of the Soviet Union have been followed by slow emissions growth in this region due to more efficient energy use, made necessary by the increasing proportion of it that is exported. In comparison, methane has not increased appreciably, and N2O by 0.25% y−1.

The direct emissions from industry have declined due to a constant improvement in energy efficiency, but also to a high penetration of electricity. If one includes indirect emissions, related to the production of electricity, CO2 emissions from industry in Europe are roughly stabilized since 1994.

Asia

Atmospheric levels of CO₂ have set another new peak, partly a sign of the industrial rise of Asian economies led by China. Over the 2000-2010 interval China is expected to increase its carbon dioxide emissions by 600 Mt, largely because of the rapid construction of old-fashioned power plants in poorer internal provinces.

United States

The United States emitted 16.3% more GHG in 2005 than it did in 1990. According to a preliminary estimate by the Netherlands Environmental Assessment Agency, the largest national producer of CO₂ emissions since 2006 has been China with an estimated annual production of about 6200 megatonnes. China is followed by the United States with about 5,800 megatonnes. However the per capita emission figures of China are still about one quarter of those of the US population.

Major greenhouse gas trends

Relative to 2005, China's fossil CO₂ emissions increased in 2006 by 8.7%, while in the USA, comparable CO₂ emissions decreased in 2006 by 1.4%. The agency notes that its estimates do not include some CO2 sources of uncertain magnitude. These figures rely on national CO2 data that do not include aviation. Although these tonnages are small compared to the CO2 in the Earth's atmosphere, they are significantly larger than pre-industrial levels.

Long-term trend

Year-to-year increase of atmospheric CO₂

Atmospheric carbon dioxide concentration is increasing at an increasing rate. In the 1960s, the average annual increase was only 37% of what it was in 2000 through 2007. People with asthma and even active healthy adults, such as construction workers, can experience a reduction in lung function and an increase in respiratory symptoms (chest pain and coughing) when exposed to low levels of ozone during periods of moderate exertion.

Removal from the atmosphere and global warming potential

Aside from water vapor, which has a residence time of days, most greenhouse gases 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.

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).
• as a consequence of chemical reactions within the atmosphere. This is the case for methane. It is oxidized by reaction with naturally occurring hydroxyl radical, OH· and degraded to CO2 and water vapor at the end of a chain of reactions (the contribution of the CO2 from the oxidation of methane is not included in the methane Global warming potential). This also includes solution and solid phase chemistry occurring in atmospheric aerosols.
• as a consequence of a physical interchange at the interface between the atmosphere and the other compartments of the planet. An example is the mixing of atmospheric gases into the oceans at the boundary layer.
• as a consequence of a chemical change at the interface between the atmosphere and the other compartments of the planet. This is the case for CO₂, which is reduced by photosynthesis of plants, and which, after dissolving in the oceans, reacts to form carbonic acid and bicarbonate and carbonate ions (see ocean acidification).
• as a consequence of a photochemical change. Halocarbons are dissociated by UV light releasing Cl· and F· as free radicals in the stratosphere with harmful effects on ozone (halocarbons are generally too stable to disappear by chemical reaction in the atmosphere).
• as a consequence of dissociative ionization caused by high energy cosmic rays or lightning discharges, which break molecular bonds. For example, lightning forms N anions from N2 which then react with O2 to form NO2.

Per capita responsibility for current anthropogenic atmospheric CO₂

Atmospheric lifetime

Jacob (1999) defines the lifetime τ of an atmospheric species X in a one-box model as the average time that a molecule of X remains in the box. Mathematically τ 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_out), chemical loss of X (L), and deposition of X (D) (all in kg/sec):

The atmospheric lifetime of a species therefore measures the time required to restore equilibrium following an increase 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. The atmospheric lifetime of CO₂ is often incorrectly stated to be only a few years because that is the average time for any CO₂ molecule to stay in the atmosphere before being removed by mixing into the ocean, photosynthesis, or other processes. However, this ignores the balancing fluxes of CO₂ into the atmosphere from the other reservoirs. It is the net concentration changes of the various greenhouse gases by all sources and sinks that determines atmospheric lifetime, not just the removal processes.

Examples of the atmospheric lifetime and GWP for several greenhouse gases include:

• CO2 has a variable atmospheric lifetime, and cannot be specified precisely. Recent work indicates that recovery from a large input of atmospheric CO₂ from burning fossil fuels will result in an effective lifetime of tens of thousands of years. Carbon dioxide is defined to have a GWP of 1 over all time periods.
• Methane has an atmospheric lifetime of 12 ± 3 years and a GWP of 62 over 20 years, 23 over 100 years and 7 over 500 years. The decrease in GWP at longer times is because methane is degraded to water and CO₂ by chemical reactions in the atmosphere.
• Nitrous oxide has an atmospheric lifetime of 120 years and a GWP of 296 over 100 years.
• CFC-12 has an atmospheric lifetime of 100 years and a GWP of 10600 over 100 years.
• HCFC-22 has an atmospheric lifetime of 12.1 years and a GWP of 1700 over 100 years.
• Tetrafluoromethane has an atmospheric lifetime of 50,000 years and a GWP of 5700 over 100 years.
• Sulfur hexafluoride has an atmospheric lifetime of 3,200 years and a GWP of 22000 over 100 years.

Source: IPCC, table 6.7.

The use of CFC-12 (except some essential uses) has been phased out due to its ozone depleting properties. The phasing-out of less active HCFC-compounds will be completed in 2030.

Airborne fraction

Airborne fraction (AF) is the proportion of a emission (e.g. CO₂) remaining in the atmosphere after a specified time. Canadell (2007) define the annual AF as the ratio of the atmospheric CO₂ increase in a given year to that year’s total emissions, and calculate that of the average 9.1 PgC y^-1 of total anthropogenic emissions from 2000 to 2006, the AF was 0.45. For CO₂ the AF over the last 50 years (1956-2006) has been increasing at 0.25±0.21%/year.

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 molecule has a high GWP on a short time scale (say 20 years) but has only 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 time.

Related effects

MOPITT 2000 global carbon monoxide

Carbon monoxide has an indirect radiative effect by elevating concentrations of methane and tropospheric ozone through scavenging of atmospheric constituents (e.g., the hydroxyl radical, OH) that would otherwise destroy them. Carbon monoxide is created when carbon-containing fuels are burned incompletely. Through natural processes in the atmosphere, it is eventually oxidized to carbon dioxide. Carbon monoxide has an atmospheric lifetime of only a few months and as a consequence is spatially more variable than longer-lived gases.

Another potentially important indirect effect comes from methane, which in addition to its direct radiative impact also contributes to ozone formation. Shindell et al (2005) argue that the contribution to climate change from methane is at least double previous estimates as a result of this effect.