Greenhouse gases are gases in an atmosphere that absorb and emitradiation within the thermal infraredrange. This process is the fundamental cause of the greenhouse effect.[1] The main greenhouse gases in the Earth's atmosphere are water vapor, carbon dioxide, methane, nitrous oxide, andozone. In our solar system, the atmospheres of Venus, Mars and Titanalso contain gases that cause greenhouse effects. Greenhouse gases greatly affect the temperature of the Earth; without them, Earth's surface would be on average about 33°C (59°F) colder than at present.[2][3][4][5]
Human activities since the start of the industrial era around 1750 have increased the levels of greenhouse gases in the atmosphere. The 2007 assessment report compiled by the IPCC noted that "changes in atmospheric concentrations of greenhouse gases and aerosols, land cover and solar radiation alter the energy balance of the climate system", and concluded that "increases in anthropogenic greenhouse gas concentrations is very likely to have caused most of the increases in global average temperatures since the mid-20th century".[6]
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[edit]Greenhouse effects in Earth's atmosphere
In order, Earth's most abundant greenhouse gases are:
The contribution to the greenhouse effect by a gas is affected by both the characteristics of the gas and its abundance. For example, on a molecule-for-molecule basis methane is about eight times stronger greenhouse gas than carbon dioxide[7], but it is present in much smaller concentrations so that its total contribution is smaller. When these gases are ranked by their contribution to the greenhouse effect, the most important are:[8]
- water vapor, which contributes 36–72%
- carbon dioxide, which contributes 9–26%
- methane, which contributes 4–9%
- ozone, which contributes 3–7%
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.[9][10] The major non-gas contributor to the Earth's greenhouse effect, clouds, also absorb and emit infrared radiation and thus have an effect on radiative properties of the greenhouse gases.[10][9]
In addition to the main greenhouse gases listed above, other greenhouse gases include sulfur hexafluoride, hydrofluorocarbons and perfluorocarbons (see IPCC list of greenhouse gases). Some greenhouse gases are not often listed. For example, nitrogen trifluoride has a high global warming potential (GWP) but is only present in very small quantities.[11]
Although contributing to many other physical and chemical reactions, the major atmospheric constituents, nitrogen (N2), oxygen(O2), and argon (Ar), are not greenhouse gases. This is becausemolecules containing two atoms of the same element such as N2and O2 and monatomic molecules such as Ar have no net change in their dipole moment when they vibrate and hence are almost totally unaffected by infrared light. Although molecules containing two atoms of different elements such as carbon monoxide (CO) orhydrogen chloride (HCl) absorb IR, 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 and are not often included when discussing greenhouse gases.
Late 19th century scientists experimentally discovered that N2 and O2 did not absorb infrared radiation (called, at that time, "dark radiation") and that water as a vapour and in cloud form, 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.
[edit]Natural and anthropogenic
Aside from purely human-produced synthetic halocarbons, most greenhouse gases have both natural and human-caused 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.[12][13]
| Gas | Preindustrial Level | Current Level | Increase since 1750 | Radiative forcing (W/m2) |
|---|---|---|---|---|
| Carbon dioxide | 280 ppm | 387ppm | 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 data does not exist. However, various proxies and modelling suggests large variations; 500 million years ago CO2levels were likely 10 times higher than now.[14] Indeed higher CO2concentrations are thought to have prevailed throughout most of thePhanerozoic 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 Ma.[15][16][17] The spread of land plants is thought to have reduced CO2 concentrations during the late Devonian, and plant activities as both sources and sinks ofCO2 have since been important in providing stabilising feedbacks.[18] 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 which raised the CO2concentration 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.[19] 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.[19][20]
[edit]Anthropogenic greenhouse gases
Since about 1750 human activity has increased the concentration of carbon dioxide and other greenhouse gases. Measured atmospheric concentrations of carbon dioxide are currently 100 ppmv higher than pre-industrial levels.[21] Natural sources of carbon dioxide are more than 20 times greater than sources due to human activity,[22]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.[23]
It is likely that anthropogenic warming, such as that due to elevated greenhouse gas levels, has had a discernible influence on many physical and biological systems. Warming is projected to affect various issues such as freshwater resources, industry, food and health.[24]
The main sources of greenhouse gases due to human activity are:
- burning of fossil fuels anddeforestation 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.[23]
- livestock enteric fermentation and manure management,[25] paddy ricefarming, 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 (N2O) concentrations.
The seven sources of CO2 from fossil fuel combustion are (with percentage contributions for 2000–2004):[26]
- Solid fuels (e.g., coal): 35%
- Liquid fuels (e.g., gasoline, fuel oil): 36%
- Gaseous fuels (e.g., natural gas): 20%
- Flaring gas industrially and at wells: <1%
- Cement production: 3%
- Non-fuel hydrocarbons: <>
- 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.[27] Major sources of an individual's GHG include home heating and cooling, electricity consumption, and transportation. Corresponding conservation measures are improving home building insulation, installing geothermal heat pumps andcompact 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.[28]
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.
[edit]Role of water vapor
Water vapor accounts for the largest percentage of the greenhouse effect, between 36% and 66% for water vapor alone, and between 66% and 85% when factoring in clouds.[9] Water vapor concentrations fluctuate regionally, but human activity does not significantly affect water vapor concentrations except at local scales, such as near irrigated fields.
The Clausius-Clapeyron relationestablishes that air can hold more water vapor per unit volume when it warms. 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.
When a warming trend results in effects that induce further warming, the process is referred to as a "positive feedback"; this amplifies the original warming. When the warming trend results in effects that induce cooling, the process is referred to as a "negative feedback"; this reduces the original warming. Because water vapor is a greenhouse gas and because warm air can hold more water vapor than cooler air, the primary positive feedback involves water vapor. This positive feedback does not result in runaway global warming because it is offset by other processes that induce negative feedbacks, which stabilizes average global temperatures. The primary negative feedback is the effect of temperature on emission of infrared radiation: as the temperature of a body increases, the emitted radiation increases with the fourth power of its absolute temperature.[29]
Other important considerations involve water vapor being the only greenhouse gas whose concentration is highly variable in space and time in the atmosphere and the only one that also exists in both liquid and solid phases, frequently changing to and from each of the three phases or existing in mixes. Such considerations include clouds themselves, air and water vapor density interactions when they are the same or different temperatures, the absorption and release of kinetic energy as water evaporates andcondenses to and from vapor, and behaviors related to vapor partial pressure. For example, the release of latent heat by rain in the ITCZ drives atmospheric circulation, clouds vary atmospheric albedo levels, and the oceans provide evaporative cooling that modulates the greenhouse effect down from estimated 67 °C surface temperature.[5][30]
- See also water, water (molecule).
[edit]Greenhouse gas emissions
Measurements from Antarctic ice cores show that before industrial emissions started atmospheric CO2levels were about 280 parts per million by volume (ppmv), and stayed between 260 and 280 during the preceding ten thousand years.[31] One study using evidence from stomata of fossilized leaves suggests greater variability, with carbon dioxide levels above 300 ppm during the period seven to ten thousand years ago[32], though others have argued that these findings more likely reflect calibration or contamination problems rather than actual CO2 variability.[33][34] 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.
Since the beginning of the Industrial Revolution, the concentrations of most of the greenhouse gases have increased. For example, the concentration of carbon dioxide has increased by about 36% to 380 ppmv, or 100 ppmv over modern pre-industrial levels. The first 50 ppmv increase took place in about 200 years, from the start of the Industrial Revolution to around 1973; however the next 50 ppmv increase took place in about 33 years, from 1973 to 2006.[35]
Recent data also shows 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.[36]
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.
| Gas | Current (1998) Amount by volume | Increase over pre-industrial (1750) | Percentage increase | Radiative forcing (W/m²) |
|---|---|---|---|---|
| Carbon dioxide | ||||
| Methane | ||||
| Nitrous oxide |
| Gas | Current (1998) Amount by volume | Radiative forcing (W/m²) |
|---|---|---|
| CFC-11 | ||
| CFC-12 | ||
| CFC-113 | ||
| Carbon tetrachloride | ||
| HCFC-22 |
(Source: IPCC radiative forcing report 1994 updated (to 1998) by IPCC TAR table 6.1[37][38] ).
[edit]Recent rates of change and emission
The sharp acceleration in CO2emissions since 2000 to more than a 3% increase per year (more than 2 ppm per year) from 1.1% per year during the 1990s is attributable to the lapse of formerly declining trends incarbon 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.[26] In comparison, methane has not increased appreciably, and N2O by 0.25% y−1.[39]
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, emissions from industry in Europe are roughly stabilized since 1994.[40]
[edit]Asia
Atmospheric levels of CO2 continue to rise, partly a sign of the industrial rise of Asian economies led by China.[41]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.[42]
See also: Asian brown cloud
[edit]United Kingdom
The UK set itself a target of reducing carbon dioxide emissions by 20% from 1990 levels by 2010, but according to its own figures it will fall short of this target by almost 4%.[43]
[edit]United States
The United States emitted 16.3% more GHG in 2005 than it did in 1990.[44] According to a preliminary estimate by the Netherlands Environmental Assessment Agency, the largest national producer of CO2emissions 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.
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.[45] 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.
[edit]Relative CO2 emission from various fuels
Pounds of Carbon dioxide emitted per million British thermal units of energy for various fuels[46]:
Fuel name  | CO2 emitted (lbs/106 Btu) |
|---|---|
| Natural gas | 117 |
| Liquefied petroleum gas | 139 |
| Propane | 139 |
| Aviation gasoline | 153 |
| Automobile gasoline | 156 |
| Kerosene | 159 |
| Fuel oil | 161 |
| Tires/tire derived fuel | 189 |
| Wood and wood waste | 195 |
| Coal (bituminous) | 205 |
| Coal (subbituminous) | 213 |
| Coal (lignite) | 215 |
| Petroleum coke | 225 |
| Coal (anthracite) | 227 |
[edit]Removal from the atmosphere and global warming potential
[edit]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).
- as a consequence of chemical reactions within the atmosphere. For example, methane is oxidizedby reaction with naturally occurring hydroxyl radical, OH· and degraded to CO2 and water vapor (CO2from 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.
- as a consequence of a physical exchange between the atmosphere and the other compartments of the planet. An example is the mixing of atmospheric gases into the oceans.
- 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 carbonateions (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).
[edit]Atmospheric lifetime
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.[47] 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)[48] 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 (Fout), 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.[citation needed]
[edit]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.
Examples of the atmospheric lifetime and GWP for several greenhouse gases include:
- Carbon dioxide has a variable atmospheric lifetime, and cannot be specified precisely.[49] 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.[50][51] 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 72 over 20 years, 25 over 100 years and 7.6 over 500 years. The decrease in GWP at longer times is because methane is degraded to water and CO2 through chemical reactions in the atmosphere.
- Nitrous oxide has an atmospheric lifetime of 114 years and a GWP of 289 over 20 years, 298 over 100 years and 153 over 500 years.
- CFC-12 has an atmospheric lifetime of 100 years and a GWP of 11000 over 20 years, 10900 over 100 years and 5200 over 500 years.
- HCFC-22 has an atmospheric lifetime of 12 years and a GWP of 5160 over 20 years, 1810 over 100 years and 549 over 500 years.
- Tetrafluoromethane has an atmospheric lifetime of 50,000 years and a GWP of 5210 over 20 years, 7390 over 100 years and 11200 over 500 years.
- Sulphur hexafluoride has an atmospheric lifetime of 3,200 years and a GWP of 16300 over 20 years, 22800 over 100 years and 32600 over 500 years.
- Nitrogen trifluoride has an atmospheric lifetime of 740 years and a GWP of 12300 over 20 years, 17200 over 100 years and 20700 over 500 years.
Source: IPCC Fourth Assessment Report, Table 2.14.
The use of CFC-12 (except some essential uses) has been phased out due to its ozone depletingproperties.[52] The phasing-out of less active HCFC-compounds will be completed in 2030.[53]
[edit]Airborne fraction
Airborne fraction (AF) is the proportion of a emission (e.g. CO2) remaining in the atmosphere after a specified time. Canadell (2007)[54] 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.[54]
[edit]Negative emissions
See bio-energy with carbon capture and storage, carbon dioxide air capture, geoengineering andgreenhouse gas remediation
There exists a number of technologies which produce negative emissions of greenhouse gases. Most widely analysed are those which remove carbon dioxide from the atmosphere, either to geologic formations such as bio-energy with carbon capture and storage[55][56][57] and carbon dioxide air capture[57], or to the soil as in the case with biochar.[57] It has been pointed out by the IPCC, that many long-term climate scenario models require large scale manmade negative emissions in order to avoid serious climate change. [58]
[edit]Related effects
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 tocarbon dioxide. Carbon monoxide has an atmospheric lifetime of only a few months[59] 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)[60] argue that the contribution to climate change from methane is at least double previous estimates as a result of this effect.[61]
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