Greenhouse gas

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Top: Increasing atmospheric CO₂ levels as measured in the atmosphere and ice cores. Bottom: The amount of net carbon increase in the atmosphere, compared to carbon emissions from burning fossil fuel.

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Greenhouse gases (GHGs) are gaseous components of the atmosphere that contribute to the "greenhouse effect". Although uncertainty exists about exactly how earth's climate responds to these gases, global temperatures are rising. ^[1] Some greenhouse gases occur naturally in the atmosphere, while others result from human activities. Naturally occurring greenhouse gases include water vapor, carbon dioxide, methane, nitrous oxide, and ozone. Certain human activities, however, add to the levels of most of these naturally occurring gases. ^[2]

1 The "Greenhouse Effect"
2 Anthropogenic greenhouse gases
3 The role of water vapor
4 Increase of greenhouse gases
5 Removal from the atmosphere and global warming potential
6 Related effects
7 See also
8 References
9 External links
- 9.1 Carbon dioxide emissions
- 9.2 Methane emissions

[edit] The "Greenhouse Effect"

Main article: Greenhouse effect

When sunlight reaches the Earth's surface, some is absorbed and warms the earth. Because the earth is much cooler than the sun, it radiates energy at much longer wavelengths than the sun (see Black body radiation and Wien's displacement law); some of these longer wavelengths are absorbed by greenhouse gases in the atmosphere before they are lost to space. The absorption of this longwave radiant energy warms the atmosphere (the atmosphere also is warmed by transfer of sensible and latent heat from the surface). Greenhouse gases also emit longwave radiation both upward to space and downward to the surface. The downward part of this longwave radiation emitted by the atmosphere is the "greenhouse effect." The term is in fact a misnomer, as this process is not the primary mechanism that warms greenhouses.

The major natural greenhouse gases are water vapor, which causes about 36-70% of the greenhouse effect on Earth (not including clouds); carbon dioxide, which causes 9-26%; methane, which causes 4-9%, and ozone, which causes 3-7%. Note that it is not really possible to assert 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.)^[3] ^[4]

Other greenhouse gases include, but are not limited to, nitrous oxide, sulfur hexafluoride, perfluorocarbons and chlorofluorocarbons (see IPCC list of greenhouse gases).

The major atmospheric constituents (N₂ and O₂) are not greenhouse gases, because homonuclear diatomic molecules (e.g. N₂, O₂, H₂) neither absorb nor emit infrared radiation as there is no net change in the dipole moment of these molecules.

[edit] Anthropogenic greenhouse gases

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

The concentrations of several greenhouse gases have increased over time.^[5] Human activity raises levels of greenhouse gases primarily by releasing carbon dioxide, but human influences on other gases, e.g., methane, are not negligible.^[6] 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;
livestock and 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 major sources of atmospheric methane;
use of CFCs in refrigeration systems, and use of CFCs and halons in fire suppression systems and manufacturing processes.

According to the global warming trend, greenhouse gases from industry and agriculture have played a major role in the recently observed global warming. Carbon dioxide, methane, nitrous oxide and three groups of fluorinated gases are the subject of the Kyoto Protocol, which entered into force in 2005. Methane, nitrous oxide and ozone-depleting gases are also taken into account in the international agreements, but not ozone. Note that ozone depletion has only a minor role in greenhouse warming, though the two processes often are confused in the popular media.

[edit] The role of water vapor

Increasing water vapor at Boulder, Colorado.

Water vapor is a natural greenhouse gas and accounts for the largest percentage of the greenhouse effect. Water vapor concentrations fluctuate regionally, but human activity does not directly affect water vapor concentrations except at very local scales.

In climate models an increase in atmospheric temperature caused by the greenhouse effect due to anthropogenic gases will in turn lead to an increase in the water vapor content of the troposphere, with approximately constant relative humidity. The increased water vapor in turn leads to an increase in the greenhouse effect and thus a further increase in temperature; the increase in temperature leads to still further increase in atmospheric water vapor; and the feedback cycle continues until equilibrium is reached. Thus water vapor acts as a positive feedback to the forcing provided by human-released greenhouse gases such as CO₂ (but has never, so far, acted on Earth as part of a runaway feedback). Changes in the water vapor may also have indirect effects via cloud formation.

Most scientists agree that the overall effect of the direct and indirect feedbacks caused by increased water vapor content of the atmosphere significantly enhances the initial warming that caused the increase - that is, it is a strong positive feedback.([2], see B7).

Water vapor is a definite part of the greenhouse gas equation even though not under direct human control: Intergovernmental Panel on Climate Change (IPCC) TAR chapter lead author Michael Mann considers citing "the role of water vapor as a greenhouse gas" to be "extremely misleading" as water vapor can not be controlled by humans [3]; see also [4] and [5]. The IPCC discusses the water vapor feedback in more detail [6].

[edit] Increase of greenhouse gases

Based on measurements from Antarctic ice cores, it is widely accepted that just before industrial emissions began, atmospheric CO₂ levels were about 280 µL/L (note the units µL/L are identical to parts per million by volume). From the same ice cores it appears that CO₂ concentrations have stayed between 260 and 280 µL/L during the preceding 10,000 years. Some studies^[7], using evidence from stomata of fossilized leaves, have found greater variability with CO₂ levels above 300 µL/L during the period 7-10 kyr ago, though others have argued that these findings more likely reflect calibration/contamination problems rather than actual CO₂ variability^[8]^[9].

Since the beginning of the Industrial Revolution, the concentrations of many of the greenhouse gases have increased. Most of the increase in carbon dioxide occurred after 1945. Those 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 {381 ppm(2006)}	87 ppm {103 ppm(2006)}	31% {37.05%(2006)}	1.46 {~1.524 (2006)}
Methane	1,745 ppb	1,045 ppb	150%	0.48
Nitrous oxide	314 ppb	44 ppb	16%	0.15

Global carbon dioxide emissions 1751–2000.

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 [7][8]).

[edit] Removal from the atmosphere and global warming potential

Major greenhouse gas trends

Aside from water vapor near the surface, which has a residence time of days, most greenhouse gases take a very long time to leave the atmosphere. It is not easy to know with precision how long, because the atmosphere is a very complex system. However, there are estimates of the duration of stay, i.e., the time which is necessary so that the gas disappears from the atmosphere, for the principal ones. 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 GWP). 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 atoms from N₂ which then react with O₂ to form NO₂.

Two scales can be used to describe the effect of different gases in the atmosphere. The first, the atmospheric lifetime, describes how long it takes to restore the system to equilibrium following a small increase in the concentration of the gas in the atmosphere. Individual molecules may interchange with other reservoirs such as soil, the oceans, and biological systems, but the mean lifetime refers to the decaying away of the excess. One may encounter claims that the atmospheric lifetime of CO₂ is only a few years because that is the average time for any CO₂ molecule to stay in the atmosphere before mixing into the ocean, being transformed to oxygen by photosynthesis, etc. 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.

The second scale is global warming potential (GWP). The 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:

CO₂ has a variable atmospheric lifetime (approximately 200-450 years for small perturbations). 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.^[10]^[11] 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 associated with longer times is associated with the fact that the 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(100) of 10600.
HCFC-22 has an atmospheric lifetime of 12.1 years and a GWP(100) of 1700.
Tetrafluoromethane has an atmospheric lifetime of 50,000 years and a GWP(100) of 5700.
Sulfur hexafluoride has an atmospheric lifetime of 3,200 years and a GWP(100) of 22000.

Source : IPCC, table 6.7.

[edit] Related effects

MOPITT 2000 global carbon monoxide

Carbon monoxide has an indirect radiative forcing effect by elevating concentrations of methane and tropospheric ozone through chemical reactions with other 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 concentrations are both short-lived in the atmosphere and spatially variable.

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)^[12] argue that the contribution to climate change from methane is at least double previous estimates as a result of this effect[9].

One of the related effects of global warming is that as the level of carbon dioxide in the atmosphere increases, so does the acidity of the oceans.

One of the more alarming potential correlations with Greenhouse gases and Global Warming is the notion of Global dimming which seems to have masked the effect of Global Warming due to the Earth getting cooler through Global Dimming.

[edit] See also

Global Warming

Subtopics

[edit] References

^ EPA's Clean Air Markets - Climate Change
^ EPA's Clean Air Markets - Climate Change
^ Kiehl, J. T., Kevin E. Trenberth (February 1997). "Earth’s Annual Global Mean Energy Budget" (PDF). Bulletin of the American Meteorological Society 78 (2): 197-208. Retrieved on 2006-05-01.
^ Water vapour: feedback or forcing?. RealClimate (6 Apr 2005). Retrieved on 2006-05-01.
^ Climate Change 2001: Working Group I: The Scientific Basis: C.1 Observed Changes in Globally Well-Mixed Greenhouse Gas Concentrations and Radiative Forcing. Retrieved on 2006-05-01.
^ Climate Change 2001: Working Group I: The Scientific Basis: figure 6-6. Retrieved on 2006-05-01.
^ Friederike Wagner, Bent Aaby and Henk Visscher (2002). "Rapid atmospheric CO2 changes associated with the 8,200-years-B.P. cooling event". PNAS 99 (19): 12011-12014. DOI:10.1073/pnas.182420699.
^ Andreas Indermühle, Bernhard Stauffer, Thomas F. Stocker (1999). "Early Holocene Atmospheric CO2 Concentrations". Science 286 (5446): 1815. DOI:10.1126/science.286.5446.1815a. Early Holocene Atmospheric CO2 Concentrations. Science. Retrieved on May 26, 2005.
^ H.J. Smith, M Wahlen and D. Mastroianni (1997). "The CO₂ concentration of air trapped in GISP2 ice from the Last Glacial Maximum-Holocene transition". Geophysical Research Letters 24 (1): 1-4.
^ Archer, David (2005). "Fate of fossil fuel CO₂ in geologic time". Journal of Geophysical Research 110, C09S05. DOI:10.1029/2004JC002625.
^ Caldeira, Ken and Wickett, Michael E. (2005). "Ocean model predictions of chemistry changes from carbon dioxide emissions to the atmosphere and ocean". Journal of Geophysical Research 110, C09S04. DOI:10.1029/2004JC002671.
^ Shindell, Drew T.; Faluvegi, Greg; Bell, Nadine; Schmidt, Gavin A. "An emissions-based view of climate forcing by methane and tropospheric ozone", Geophysical Research Letters, Vol. 32, No. 4 [1]